Platinum Metals Review - Johnson Matthey Technology Review

VOLUME 52 NUMBER 2 APRIL 2008

Platinum Metals Review

www.platinummetalsreview.com E-ISSN 1471–0676

E-ISSN 1471–0676

PLATINUM METALS REVIEW A Quarterly Survey of Research on the Platinum Metals and of Developments in their Application in Industry www.platinummetalsreview.com

VOL. 52 APRIL 2008 NO. 2

Contents Safer, Faster and Cleaner Reactions Using Encapsulated Metal Catalysts and Microwave Heating

64

By M. R. Pitts

Practical New Strategies for Immobilising Ruthenium Alkylidene Complexes: Part I

71

By Ileana Dragutan and Valerian Dragutan

“Green Chemistry and Catalysis”

83

A book review by Duncan Macquarrie

A Disordered Copper-Palladium Alloy Used as a Cathode Material

84

By Philippe Poizot, Lydia Laffont-Dantras and Jacques Simonet

Dalton Discussion 10: Applications of Metals in Medicine and Healthcare

96

A conference review by Christian G. Hartinger

Platinum as a Reference Electrode in Electrochemical Measurements

100

By Kasem K. Kasem and Stephanie Jones

Faraday Discussion 138: Nanoalloys – From Theory to Applications

107

A conference review by Geoffrey C. Bond

Challenges in Catalysis for Pharmaceuticals and Fine Chemicals

110

A conference review by Chris Barnard

The Periodic Table and the Platinum Group Metals

114

By W. P. Griffith

Frederick A. Lewis

120

An appreciation by Ted B. Flanagan

“Fuel Cell Today Industry Review 2008”

123

Abstracts

124

New Patents

127

Final Analysis: Crystallite Size Analysis of Supported Platinum Catalysts by XRD

129

By Tim Hyde

Communications should be addressed to: The Editor, Barry W. Copping, Platinum Metals Review, [email protected]; Johnson Matthey Public Limited Company, Orchard Road, Royston, Hertfordshire SG8 5HE, U.K.

DOI: 10.1595/147106708X292526

Safer, Faster and Cleaner Reactions Using Encapsulated Metal Catalysts and Microwave Heating PERFORMANCE ENHANCEMENT OF PALLADIUM, PLATINUM AND OSMIUM CATALYSTS By M. R. Pitts* Reaxa Ltd., Hexagon Tower, Blackley, Manchester M9 8ZS, U.K.; E-mail: [email protected]

The combination of focused microwave heating and encapsulated metal promoters (EnCatTM) offers a safer, cleaner and more cost-effective solution to a wide range of catalyst-mediated reactions, some of which are not widely accessible to the bench chemist due to high hazard ratings. These include the palladium-catalysed Sonogashira cross-coupling, palladiumcatalysed transfer hydrogenation, platinum-mediated hydrogenation and osmium tetroxidecatalysed dihydroxylation.

Microwave heating has developed as an important tool for research chemists, enabling reactions to be carried out and optimised more quickly than using traditional heating methods (1–3). Direct irradiation of the reaction mixture produces a more uniform and homogeneous heating profile than does, for example, an oil bath. In most cases the observed increase in rate can be explained by the extremely efficient energy transfer and homogeneous heating effect. This can lead to superheating of the reaction mixture (4): indeed, even microwave heating of an open vessel can achieve temperatures several degrees higher than the boiling point of the solvent (5). In certain cases the presence of elements that strongly absorb microwave energy and release it efficiently as heat can cause localised ‘hotspots’ tens of degrees higher than the bulk temperature, generating significant rate enhancements (6–8). This effect can be exploited to heat materials of low microwave absorbance by the use of ‘passive heating elements’ (9). Non-polar and poorly absorbing solvents can also be superheated by adding small amounts of a strongly absorbing cosolvent such as an ionic liquid (10–13). The application of this selective heating can be particularly striking when the element is a heterogeneous

catalyst (14–16). A localised increase in temperature at a catalyst surface over the bulk temperature, or a selective absorption of microwave energy by catalytic species or organometallic intermediates on a reaction pathway, can lead to increased selectivity for the catalytic process while unwanted (thermally driven) side reactions are minimised by a relatively low bulk temperature (17). A synergistic advantage between microwave heating and platinum group metal catalysis can therefore be demonstrated (18). The use of commercially available focused (monomode) microwave units (19–21) enhances the safety and reproducibility of reactions. The standard integration of monomode units into many laboratory environments has expanded the armoury of techniques available to chemists, allowing ready access to previously difficult-to-achieve chemistries. These include high-temperature reactions such as Ullmann couplings (22); some heterocycle preparations previously requiring metal baths (23, 24); the use of near-critical water as solvent (25–29); and shortening the reaction time on slow processes such as cycloadditions (30) to practically useful timescales, including replacing the need for autoclaves (31); and automated peptide synthesis (32, 33).

*Present address: Chemistry Innovation KTN, The Heath, Runcorn WA7 4QZ, U.K.; E-mail: [email protected]

Platinum Metals Rev., 2008, 52, (2), 64–70

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For the reasons discussed, metal-catalysed reactions work particularly well under microwave irradiation; however safety and isolation issues still arise from their use. Elemental metal can deposit from reaction mixtures onto the side of the glass tube, causing localised superheating of the glass and explosive rupture of the vessel (34). This can occur with both homogeneous and heterogeneous catalysts. It can also be difficult to remove metal species selectively from the product on completion of the reaction. The EnCatTM range of encapsulated metal catalysts were designed to address these issues of purification and reuse. Unlike other immobilised homogeneous catalysts such as FibreCatTM, where phosphine ligands are attached to polyethylene fibres (35), the homogenous catalyst in EnCat is contained within a resin microcapsule. The use of such supported or ‘heterogenised’ catalysts industrially is being driven by regulatory pressures towards lower residual levels of metal catalysts within active pharmaceutical intermediates (APIs) (36, 37). EnCats are prepared by an interfacial micropolymerisation of an organic solution containing the homogeneous metal catalyst, monomers (functionalised isocyanates) and additives, dispersed as a suspension in an aqueous phase. Reactive groups generated at the interface combine to form polymer walls and, as the surrounding matrix forms, the catalyst is entrapped to give spherical microcapsules (38). The individual catalytic species gain additional stabilisation through interaction with the amide functionality of the polyurea matrix, resulting in very low levels of metal leaching. Consequently the catalyst can be recovered efficiently by simple filtration and reused. Examples of catalysts already encapsulated this way include palladium(II) acetate (39, 40) with and without various phosphine ligands (41), palladium(0) nanoparticles (42), platinum(0) (43) and osmium tetroxide (44). Here we describe how EnCats provide a homogeneous catalyst in a more effective form for use with microwave heating.

EnCats in Microwave Heated Reactions EnCats have been shown to be highly compatible with microwave heating (45, 46). Following the excel-

Platinum Metals Rev., 2008, 52, (2)

lent work by Ley and coworkers in demonstrating microwave-enhanced palladium EnCat-catalysed Suzuki couplings in both batch and flow modes (47), we were keen to understand the role of EnCat in heating bulk solution. Ley found that cooling reactions while providing a fixed microwave power equivalent to that required for good conversion in the non-cooled method resulted in cleaner products at similar or better conversions. The lower bulk temperature in the case of cooling may explain the reduction in side reactions, with the temperature ‘inside’ the EnCat beads potentially much higher. It is known that Pd/C preferentially absorbs microwave energy when suspended in a virtually microwave-transparent solvent, and ‘passively’ heats the surroundings (48). To investigate whether EnCat acts in the same way, a 5 cm3 sample of anhydrous toluene, with various additives, was irradiated at a constant power of 200 W for 5 minutes and the temperature recorded (Figure 1). Adding 250 mg of Pd EnCat had a negligible effect on the heating profile, as did the addition of ‘blank’ EnCat beads containing no metal. Addition of an equivalent amount of homogeneous palladium acetate (27 mg) also had no effect on the heating behaviour, whereas 50 mg of palladium (5%) on carbon caused a significantly increased rate of heating. These results suggest Pd EnCat does not cause superheating of the bulk solution, and behaves more like homogeneous palladium acetate than palladium on carbon.

Palladium(II) for Cross-Coupling Reactions Considerable effort has been focused on the use of Pd EnCat to facilitate cross-coupling reactions (41). The extremely low leaching of metal species and ease of handling of EnCat beads greatly simplify purification of these reactions. Many examples have been published regarding the use of EnCats with microwave heating for the acceleration of specific reactions (49–52). An important advantage, often not considered, is improved safety when using EnCats in a microwave reactor. Deposition of a film of elemental metal on the glass walls of microwave tubes by precipitation from solution is a common problem with conventional metal catalysts. This has been shown not to occur with Pd EnCat (53). Where a

65

180 160

Temperature, temperatureºC (C)

140 5%Pd/C toluene PdEnCat(0.48mmol Pd g–1 ) blank beads

120 100 80

Blankbeads 0.48 mmol/g Pd EnCat Toluene palladium acetate

60

Palladiumacetate 5% Pd/C

40 20 0 0 0

62 124 186 248000 248 310000 310 372000 372 434000 434 62000 124000 186000 Time, 1000 points) data points time (data

Fig. 1 Rate of heating of toluene containing various dopants under microwave irradiation

film is deposited, it absorbs microwave energy strongly, and hotspots can form, resulting in vessel failure. With modern microwave reactor designs such ruptures are well contained; however the release of vapours and subsequent decontamination pose serious issues. These can lead to restrictions on the use of particularly hazardous reagents. By way of example, a useful palladium-mediated microwave process is the carbonylation of aryl halides with solid sources of carbon monoxide (54). Molybdenum hexacarbonyl has been shown to be an effective carbon monoxide releasing agent (55, 56), however it is a very toxic substance with relatively high volatility (57). The risk of vessel rupture in such procedures can be greatly reduced by substituting Pd EnCat for the traditional palladium catalyst. The reaction proceeds with quantitative conversion as shown in Scheme I. EnCats have been applied in flow chemistry with the beads packed in simple columns and

reagents passed over them. The initial work in this area is extremely promising for the processintensification of homogeneous catalytic reactions (47, 58–60). A low degree of leaching of the catalytic species is vital in a continuous process, in order to avoid rapid deactivation and resulting contamination of the product flow stream. Certain substrates are known to induce leaching of palladium from EnCat resins, with aryl iodides and alkynes showing a high propensity. Indeed, running the microwave-assisted Sonogashira reaction in Scheme II with Pd EnCat 30 resulted in product with a palladium content of 83 ppm. The triphenylphosphine-entrapped Pd EnCat (polyTPP30) resin demonstrates an extremely high retention of both the palladium and phosphorus ligand, and has been used to great effect in the same reaction (Scheme II). Using Pd EnCat polyTPP30 as the catalyst, the residual palladium concentration in the product was only 14 ppm. O

Pd EnCat 30 Mo(CO)6

I + H2N Me

Ph

DBU, THF Microwave 120ºC 30 min

N

Ph

H H

Scheme I

Me Yield 98%

DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene


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O

Pd EnCat 30 or polyTPP30

O Me

+ Ph I

Me Scheme II

CuI, Et3N, THF Microwave 140ºC 20 min

Ph

Yield 99%

Palladium(0) for Hydrogenation Reactions

Platinum(0) for Hydrogenation and Reduction Reactions

The nanoparticulate palladium(0) EnCat catalyst has been demonstrated as a highly chemoselective hydrogenation and transfer hydrogenation catalyst (61, 62). In addition to the improved selectivity shown by Pd EnCat NP30, a superior safety profile and ease of handling make it a powerful alternative to palladium on charcoal. Transfer hydrogenation with Pd EnCat NP30 is easily performed in the microwave, allowing reactions in minutes rather than hours. A recent paper by Quai and coworkers demonstrated the efficiency of microwaveassisted transfer hydrogenation for O-benzyl deprotection (Scheme III) (63). The use of EnCat was recommended to improve the safety of the process and reduce palladium contamination of the products. Scheme IV shows a representative example of an aromatic nitro reduction. These reactions are conventionally carried out at ambient temperature overnight (64). However, the microwave transfer hydrogenation procedure gave a quantitative conversion to the final product in only 5 minutes.

To complement the palladium(0) EnCat range, a platinum(0) EnCat has recently been developed, offering the same benefits over its carbon-supported equivalents as the palladium version: improved safety profile, ease of handling and low metal leaching. Pt(0) EnCat 40 performs similarly to Pt/C in hydrogenation reactions, and is particularly useful in selective reductions in the presence of aryl chlorides. The reaction shown in Scheme V gave 3-chloroaniline with > 98% selectivity at room temperature under an atmosphere of hydrogen after one hour (65). Microwave-assisted hydrogenations have recently been investigated (66), and equipment to run them in the laboratory is becoming commercially available (67, 68). With microwave reactors designed to meter pressures up to 15 bar and run at them, such technology offers the bench chemist simple, safe access to hydrogenation. The microwave-assisted hydrogenation of 3chloronitrobenzene shown in Scheme V was run using a standard microwave vial. A hydrogen atmosphere (at slight positive pressure) was introduced via a needle and manifold cycled between vacuum and hydrogen from a lecture

R Ph

Pd(0) EnCat NP30 HCOONH4, DMF

R

Microwave (cooled) 80ºC 10 min

O

Scheme III

HO

R = NH2, NHMe, COOH, CN, COR, heterocycle, etc. NO2 HO


Pd(0) EnCat NP30 HCOONH4, EtOH Microwave 80ºC 5 min

NH2 Scheme IV HO Yield > 99%

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Cl

NO2

Pt(0) EnCat 40 H2, EtOH

Scheme V

Microwave 30 W 13 min

Ph Ph

Yield 85%

Os EnCat NMO H2O/acetone Microwave 80ºC 20 min

NH2

Cl

OH Ph Ph

Scheme VI

OH Yield 91%

NMO = N-methylmorpholine N-oxide

bottle. Following irradiation at a constant power (30 W) for 13 minutes all the starting material was consumed, giving 3-chloroaniline in 85% yield. With equipment designed to charge gas to a given pressure and monitor the pressure drop, it is to be expected that this reaction could be optimised to higher selectivities.

Encapsulated Osmium Tetroxide for Dihydroxylation Reactions The osmium tetroxide-catalysed dihydroxylation reaction is Nobel Prize-winning chemistry (69); however the routine use of osmium in the laboratory is avoided where possible due to its toxicity, the likelihood of contact due to its volatility and its propensity to cause burns (70). Os EnCat 40 is an encapsulated osmium tetroxide that is safer to handle because no osmium tetroxide vapour can escape the polymer matrix (44). The EnCat acts as a reservoir of osmium tetroxide, releasing catalytic amounts under oxidation reaction conditions, but retaining sufficient activity for recycling (71). Following the reaction only very low levels of residual osmium are detectable in the reaction media. Os EnCat 40 has been successfully applied to asymmetric dihydroxylation reactions (72). To demonstrate the application of Os EnCat 40 under microwave conditions, the simple dihydroxylation in Scheme VI was carried out at 80ºC and was complete in 20 minutes. The corresponding reaction at ambient temperature, when allowed to proceed overnight, gave the product in 86% yield (73). With the reaction performed in a sealed microwave tube, the contents could be removed


via syringe with a fine filter fitting, minimising contact and potential hazards, and allowing routine, safe use of such chemistry.

Conclusions Microwave heating has expanded the arsenal of synthetic methods available to the bench chemist. The use of encapsulated platinum group metal catalysts coupled with the inherently safe design of modern microwave apparatus enables safe access to an even greater range of useful transformations. Such a synergistic combination of technologies enables reactions to be performed that furnish clean products with very low levels of residual metal, thus simplifying the preparation of complex molecules.

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51 52 53 54 55 56 57

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63 64

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um, Alfa Aesar GmbH & Co. KG, 08.02.2008: http://www.avocadochem.com/daten_msds/GB/ 13057_-_GB.pdf C. K. Y. Lee, A. B. Holmes, S. V. Ley, I. F. McConvey, B. Al-Duri, G. A. Leeke, R. C. D. Santos and J. P. K. Seville, Chem. Commun., 2005, 2175 G. A. Leeke, R. C. D. Santos, B. Al-Duri, J. P. K. Seville, C. J. Smith, C. K. Y. Lee, A. B. Holmes and I. F. McConvey, Org. Process Res. Dev., 2007, 11, (1), 144 I. R. Baxendale and M. R. Pitts, Chem. Today, 2006, 24, (3), 41 S. V. Ley, C. Mitchell, D. Pears, C. Ramarao, J.-Q. Yu and W. Zhou, Org. Lett., 2003, 5, (24), 4665 S. V. Ley, A. J. P. Stewart-Liddon, D. Pears, R. H. Perni and K. Treacher, Beilstein J. Org. Chem., 2006, 2:15 M. Quai, C. Repetto, W. Barbaglia and E. Cereda, Tetrahedron Lett., 2007, 48, (7), 1241 Results from Reaxa laboratories are available in ‘Pd(0) EnCatTM NP30 Hydrogenation & Transfer Hydrogenation User Guide’, Reaxa Ltd., April 2006: http://www.reaxa.com/images/stories/reaxa_pd0_ encat_30np_user_guide_2006.pdf Results from Reaxa laboratories are available in

66 67 68 69 70

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‘Pt(0) EnCatTM 40 User Guide’, Reaxa Ltd., March 2007: http://www.reaxa.com/images/stories/Reaxa%20 Pt(0)%20EnCatT%20User%20Guide_mar_07.pdf G. S. Vanier, Synlett, 2007, 131 C. M. Kormos and N. E. Leadbeater, Synlett, 2006, 1663 E. Heller, W. Lautenschläger and U. Holzgrabe, Tetrahedron Lett., 2005, 46, (8), 1247 K. B. Sharpless, Angew. Chem. Int. Ed., 2002, 41, (12), 2024 Material Safety Data Sheet, Osmium(VIII) Oxide, Alfa Aesar GmbH & Co. KG, 08.02.2008: http://www.avocadochem.com/daten_msds/GB/ 12103_-_GB.pdf D. C. Whitehead, B. R. Travis and B. Borhan, Tetrahedron Lett., 2006, 47, (22), 3797 A.-L. Lee and S. V. Ley, Org. Biomol. Chem., 2003, 1, 3957 Results from Reaxa laboratories are available in ‘User Guide – Catalytic Oxidations with Os EnCatTM Microencapsulated Osmium Tetroxide Catalysts’, Reaxa Ltd.: http://www.reaxa.com/images/stories/reaxaosencatu serguide.pdf

The Author Mike Pitts obtained his first degree at Loughborough University, U.K., in 1997. Zeneca sponsored a project on dioxirane chemistry in his final year, following a successful industrial placement year as part of the degree. He then moved to the University of Exeter, U.K., to obtain a Ph.D. with Professor Chris Moody on ‘Selective Reductions with Indium Metal’. A postdoctoral stay with Professor Johann Mulzer at the University of Vienna, Austria, followed, where he completed a formal total synthesis of laulimalide as part of a European Network focused on antitumour natural products. Mike returned to the U.K. in 2002 to work for StylaCats Ltd., a start-up company from the University of Liverpool, where he initiated and developed a microwave research platform. In September 2005 he moved to Reaxa Ltd. in Manchester, a technology spin-out from Avecia, to develop microwave processes with their proprietary catalysts. In August 2007 he took up his current position managing Sustainable Technologies at the Chemistry Innovation Knowledge Transfer Network.


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DOI: 10.1595/147106708X297477

Practical New Strategies for Immobilising Ruthenium Alkylidene Complexes: Part I IMMOBILISATION VIA PHOSPHANE, ALKYLIDENE AND N-HETEROCYCLIC CARBENE LIGANDS By Ileana Dragutan* and Valerian Dragutan** Institute of Organic Chemistry “Costin D. Nenitescu”, Romanian Academy, 202B Spl. Independentei, PO Box 35-108, 060023 Bucharest, Romania; E-mail: *[email protected]; **[email protected]

The paper critically presents various routes for immobilising ruthenium alkylidene complexes through their ligands. This part (Part I) describes immobilisation via coordinating/actor ligands (phosphane/alkylidene), and established ancillary ligands such as N-heterocyclic carbenes. Other ligands commonly encountered in immobilisation protocols, such as Schiff bases, arenes, anionic ligands and specifically tagged (ionic liquid tag, fluoro tag) substituents will be the topic of Part II. Selected applications of some of these ruthenium complexes in olefin metathesis reactions are highlighted where they are particularly advantageous.

1. Introduction Compelling environmental and health-and-safety demands are presently driving fundamental change in the design of chemical processes, especially those involving catalytic and/or highly hazardous reactions. The last few years have seen substantial progress in designing and implementing novel, clean and sustainable technologies, but considerable challenges remain for future academic and industrial research. In this regard, the immobilisation of well defined homogeneous catalytic complexes has proved a beneficial strategy, combining the advantages of homogeneous and heterogeneous catalytic systems (1–7). This technique offers multiple benefits for organic synthesis, such as simplification of the reaction scheme, greater control of process selectivity, better removal of the catalyst from the reaction products, the recycling of expensive catalysts, the possibility of designing continuous-flow processes on a large scale and, in polymer synthesis, the precise control of polymer morphology and bulk density in high polymers (8–14). However, immobilisation shares with heterogeneous catalysis the major drawback of a diminished catalytic performance as compared with that of the homogeneous counterpart. This effect is often attributed to non-uniform local concentration of the catalyst, limited access of reactants to the active sites and, in certain cases, to opposing groups on


the heterogeneous support or to steric effects of the latter. The commonly applied methodology to transform a homogeneous catalytic reaction into a heterogeneous process involves anchoring the active catalyst on a solid support possessing a large surface area (15, 16). This procedure should not unduly affect the intrinsic catalytic properties of the complex, and the system should benefit effectively from the characteristics of both the deposited catalyst and the solid support. Recently, the coordination and organometallic chemistry of ruthenium complexes has seen unprecedented development, due to the emergence of the increasing potential of this class as efficient promoters of versatile catalytic processes (17–23). Most of these complexes possess an appropriate balance between the electronic and steric properties within the ligand environment and, as a result, exhibit attractive catalytic properties; in particular enhanced activity, chemoselectivity and stability in targeted chemical transformations (24–29). Olefin metathesis, a most efficient transition metal mediated reaction for forming C–C bonds, has proved to be a powerful synthetic strategy for obtaining fine chemicals, pharmaceuticals and biologically active compounds, structurally complex assemblies, novel materials and functionalised

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polymers tailored for specific uses. Examples of applications for the latter include sensors, semiconductors and microelectronic devices (30–36). Procedures such as ring-closing metathesis (RCM), ring-opening metathesis (ROM), cross-metathesis (CM), enyne metathesis and ring-opening metathesis polymerisation (ROMP), are sometimes combined in tandem with non-metathetical processes. This has resulted in broad diversification towards progressive technologies and new perspectives for industrial applications (37–42). Advances have mainly been due to the discovery of a wide range of functional group-tolerant ruthenium alkylidene complexes, resistant to air and moisture, bearing appropriate ancillary ligands such as phosphanes (1 and 2, R = phenyl (Ph) or cyclohexyl (Cy)), N-heterocyclic carbenes (3 and 4), Schiff bases (5 and 6) or arene groups (7) (Scheme I). Although some of these complexes exhibit a good selectivity profile and activity in the free state, immobilising them on organic or inorganic supports has emerged as an improvement in their capability for ‘green’ metathesis chemistry, enhancing their potential as clean, recyclable and highly efficient catalysts (43–47). Most frequently, the ruthenium complexes 1–7 have been immobilised by binding one of their stable ligands to the support (48–51). Both anionic and neutral ligands

PR33 Cl Cl Ru Ru Cl Cl PR33

Ph Ph

Cl Cl

Ph Ph

Cl Cl

1

PR PR33 Ru Ru PR PR33

Ph Ph HH

have so far been employed. Table I summarises currently well developed methods for immobilising ruthenium metathesis catalysts.

2. Immobilisation via the Phosphane Ligand Since the first well defined and widely applied homogeneous ruthenium metathesis catalysts incorporated phosphines as ligands, it was not surprising that immobilisation through the phosphane was tried first. It was obvious that while performance of the resulting catalyst depends on release of the active species into solution, its recyclability is strongly affected by the poor ability of the bound phosphine to recapture the ruthenium. Consequently, disadvantages associated with this mode of immobilisation were to be expected. An early report on the immobilisation of a metathesis catalyst was by Nguyen and Grubbs (52), who anchored the homogeneous Ru vinylcarbene complex 1 (R = Ph or Cy) on a polystyrene support through both its phosphane ligands, obtaining the well defined immobilised complexes 8–10 (Scheme II). Despite the apparent practical advantages of applications of precatalysts 8–10 in metathesis of cis-2-pentene and polymerisation of norbornene, the activity of these precatalysts was found to be at least two orders of magnitude less than that of the

N N

Mes Mes

Cl

2

Ru Ru

N N Cl

Cl Cl

Ph

N

Mes Mes

Ru

PCy33 PCy

O

3

4

Br

Br

N Cl O Ru

Mes Mes

N N Mes Cl Cl

N Cl

Ph

O Ru

Cl Cl Ru Ru PCy PCy33

Ph Ph

PF66 PF

Ph Ph

PCy3 5

6

7

Scheme I Homogeneous metathesis ruthenium complexes suitable for immobilisation on solid supports


72

Table I

Approaches for Immobilisation of Ruthenium Metathesis Catalysts Mode of immobilisation

Section†

References

Immobilisation via the phosphane ligand

I/2

52, 53 (Part I)

Immobilisation via the alkylidene ligand

I/3

54–63 (Part I)

Immobilisation via the N-heterocyclic carbene (NHC) ligand

I/4

64–84 (Part I)

Immobilisation via the Schiff base ligand

II/1

Part II

Immobilisation via the arene ligand

II/2

Part II

Immobilisation via anionic ligands

II/3

Part II

Tagged ruthenium alkylidene complexes

II/4

Part II

†

I = Part I (this paper); II = Part II, to be published in a future issue of Platinum Metals Review

PPh 2 PPh

Ph Ph

Cl Ru

PPh PPh22

nn

PCy22 PCy

Cl Ru

Ph Ph PCy PCy22

Cl

Ph Ph

Scheme II Immobilised vinylcarbene ruthenium complexes 8–10

Ph Ph

Cl

n 8

9

CH22

PCy22

Cl Ru

CH22

PCy22

Ph Ph Ph

Cl

n n 10

homogeneous complex 1. This result was rationalised in terms of the detrimental effect of the two chelated phosphane ligands on the dissociative reaction pathway, and the need for the substrate to diffuse into the polymer cavities. Subsequently, immobilisation of complex 2, through only one of its phosphane ligands, to give complexes 11 and 12, was reported by Verpoort et al. (53). A phosphinated mesoporous aluminosilicate matrix (P-MCM-41) was used as the solid support (Scheme III). Gratifyingly, the immobilised catalysts 11 and 12 displayed good activity in norbornene polymerisation (yield up to 70%) and very high activity in RCM of diallylamine and diethyl diallylmalonate (yield up to 100%). Moreover, catalyst 12 was active even in an aqueous environment. Since, by contrast with complexes 8–10, in catalysts 11 and


12 the Ru-alkylidene entity is grafted onto the support through only one phosphane ligand, the dissociative mechanism of the metathesis reaction is favoured in this case.

3. Immobilisation via the Alkylidene Ligand A remarkable innovation came with the design of the so-called ‘boomerang’ catalyst 13 (54), in which the ruthenium complex is anchored onto the vinyl polystyrene support (vinyl-PS resin) through its alkylidene ligand (Scheme IV). Polymer-supported catalyst 13 was readily obtained by CM of vinyl polystyrene with the ruthenium complex 2, and was isolated as an orange-brown solid, after filtration and washing. Catalyst 13 was found to be effective in RCM, its activity being comparable with that of the homo-

73

OH

EtO

Scheme III Synthesis of zeolite-supported ruthenium complexes 11 and 12

O

OH + EtO Si (CH2)xx PR2 OH EtO

O

Si (CH2)xx PR2

O

Cl Cl R3P Ru PR3 O O

O

Ph

O

Si (CH2)xx PR2

Cl Cl Si (CH2)xx PR2 Ru PR3

O

O

Ph

11 (x = 2) 12 (x = 3)

Ph

PCy 3 Cl Ru Cl PCy 3

CH 2Cl2, 1–2 1 - 2hh

Vinyl-PS resin Vinyl-P S Resin

geneous catalyst 2. It was suggested that during the initial reaction step with the diene substrate the active catalytic species becomes detached from the vinyl polystyrene support, acts then as a homogeneous RCM catalyst in solution and, after all of the diene has been consumed, reattaches itself to the vinyl polystyrene support. Under these conditions, the inhibiting necessity for reactants to diffuse to the active sites of the immobilised complex is fully eliminated, and the advantages of a homogeneous catalytic system are enjoyed. Catalyst 13 could be recycled several times by simple filtration, and the residual ruthenium in the product mixture was considerably reduced, as compared with the case of the homogeneous catalyst 2 (55). Improved immobilised ruthenium alkylidene complexes have subsequently been reported by Nolan (56–58) and Barrett (50). The increased strength of the coordinative Ru–O bond in catalyst 4 (of the Hoveyda type) could render such catalysts even more suitable for immobilisation. Indeed, a highly efficient polymerbound, recyclable catalyst 14 has been prepared by Blechert et al. (59) via ROMP of the norbornene


PCy 3 Cl Ru Cl PCy 3

Scheme IV Synthesis of the immobilised ruthenium ‘boomerang’ complex 13

13

derivative 15 in the presence of complex 3 (Scheme V). The procedure has been further extended to the synthesis of the supported catalyst 16, where an oxanorbornene benzoate comonomer was employed in conjunction with 15 and the ruthenium complex 3 (59) (Scheme VI). Excellent conversions have been obtained in RCM of a variety of diene substrates, leading readily to five-, six-, seven- and higher-ring carboand heterocyclic compounds. It is important to note that the recyclability of catalysts such as 16 in metathesis reactions is remarkable. Catalyst 16 affords high conversions of diallyl tosyl amide to 1-tosylpyrroline (> 98%), even after seven reaction cycles, and complete recovery of the catalyst was possible (59). The synthesis and olefin metathesis activity in protic solvents of a new, phosphine-free ruthenium alkylidene 17, bound to a hydrophilic PEGA resin support (PEGA = polyethylene glycol amine), has been reported by Connon and Blechert (60) (Scheme VII). This heterogeneous catalyst promotes relatively efficient RCM and CM reactions in both methanol and water.

74

i r O OiP Pr

n(x + y)

O O

O

O O

O O

N M es N N Mes Mes Mes Cll C Ru Ru n nx C Cll Ph PCy PCy33 Ph

xx Cll C Cl Cl Ru Ru

Mes Mes

CH CH22Cl Cl22,, CuCl CuCl

i O Pr OiPr

y O O

O O

N N N N

O OiiPr Pr

M es Mes n

15

14 (x:y = 1:99)

Scheme V Synthesis of immobilised NHC ruthenium complex 14

N N

Mes Mes

nn

O O

i OiPr O Pr

+ nz z

y n(x ++ y) O O

Cl Cl Cl Cl

N N

Mes Mes

Ru PCy PCy33

Ph

CH CuCl CH22Cl22, CuCl

O O O O22CPh CPh

15 O

O

O

x Cl Cl

Mes

i

O Pr

O

z

y O

O

O

Ru

N

i

N

O Pr Mes

n 16 (x:y:z = 1:9:30) Scheme VI Synthesis of immobilised NHC ruthenium complex 16

On using an appropriate linker (generated by CM from the styryl ether 18, and allyldimethylchlorosilane), Hoveyda and coworkers (61) bound the resulting isopropoxy benzylidene Ru complex 19 on a monolithic sol-gel, thus preparing in an advantageous ‘one-pot’ procedure a series of highly active and recyclable supported Ru complexes 20–22 (Scheme VIII and Scheme IX). Practically, these supported catalysts provided products in RCM and tandem ROM/CM that are


of excellent purity, even before silica gel chromatography or distillation. They are readily employed in combinatorial synthesis in air and with reagent-grade commercial solvents. An interesting soluble polymer-bound ruthenium alkylidene catalyst 23 was prepared by Lamaty et al. (62) through exchange of the benzylidene unit from the commercially available Grubbs catalyst 3 with the supported ligand 24 (PEG = polyethylene glycol) (Scheme X). This catalyst was fully charac-

75

Mes O

N Cl

O

Cl

Cat 3 3 Catalyst

N

Mes

Ru O N O

O

O

OH

N

PEGA Resin resin PEGA

17 Scheme VII Synthesis of immobilised NHC ruthenium complex 17

N Mes Mes N Cl Cl Cl Cl

O O

N Mes N Mes Ru Ru PCy PCy33

O O O O

Ph Ph

N Mes N N N Mes Mes Mes Cl Cl Ru Ru Cl Cl O O

SiMe22Cl Cl SiMe

O O

CH CH22Cl Cl22, 222ºC, 2 °C, 11 hh

SiMe22Cl Cl SiMe

O O

19

18

OH OH

OH O H

Si Si

Si Si

O O

O O

O O

OH OH O

Si Si

O

Mes Mes

N

N N

Mes Mes

Cl Cl Ru Ru Cl Cl O O O

CH2ClC2H , 40ºC, 5d 2 C l2

O O

40 °C, 5 d

SiMe SiMe22 O O

OR OR

Si Si

Si Si

O

O O

O

OR OR O O

Si Si

O O

20

Scheme VIII Synthesis of the supported NHC ruthenium complex 20

terised by solution nuclear magnetic resonance (NMR) spectroscopy and matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry, and tested in RCM reactions. It proved to be particularly active and could be used in the parallel


synthesis of cyclic amino esters. Most significantly, catalyst 23 could be recovered and recycled; 1H NMR analysis provided key information concerning the recovery of the catalyst at the end of the reaction.

76

NAr NAr ArN ArN

Cl Cl Ru Ru Cl Cl O O

ArN ArN

NAr NAr

H H H H

O O

Ru Ru

Cl Cl Cl Cl

O O

O O

NAr NAr O O

ArN ArN Ru Cl Ru Cl Cl O Cl O

H H

Me22Si Me Si OR OR

O O

Si Si

Si Si

O O

O O

O O

H H

Me22Si Me Si

O O

OR OR

OR OR

O O

Si Si

Si Si

Si Si

O O

O O

21

O O

22

O O

OR OR Si O O Si O O

Scheme IX Supported NHC ruthenium complexes 21 and 22 (Ar = 2,4,6-trimethylphenyl)

N N

PEG O O PEG

+ Cl Cl

N N Cl Cl Ru Ru

CH CH22Cl Cl22 Ph

(CuCl) (CuCl)

N N Cl Cl Ru Ru

Cl Cl

+

O O

PCy33 PCy

Ph Ph

PEG PEG 23

3

24

N N

Scheme X Synthesis of soluble polymer-bound NHC ruthenium complex 23

The synthesis of a highly efficient, fluorine-containing, immobilised metathesis catalyst 25, derived from the Grubbs second-generation ruthenium alkylidene complex 3, has been described by Yao (63) (Scheme XI). The air-stable polymer-bound ruthenium alkylidene complex 25 showed high reactivity in RCM of a broad spectrum of diene and enyne substrates, leading to the formation of

Mes

N

N Cl Mes Ru

O

Cl O

O

4. Immobilisation via the NHC Ligand Fluorous polyacrylate Polyacrylate

25 Scheme XI Fluorine-containing polymer-bound ruthenium alkylidene complex 25


di-, tri-, and tetrasubstituted cyclic olefins in “minimally fluorinated solvent systems” (PhCF3/CH2Cl2, 1:9–1:49 vol./vol.). The catalyst could readily be separated from the reaction mixture by extraction with FC-72 (perfluoro-n-hexane) and repeatedly reused. The practical advantage of recyclability offered by this fluorinated catalyst has been demonstrated by its sequential use in up to five different metathesis reactions (63).

Immobilisation via the NHC ligand capitalises on the NHC’s characteristic of generally forming strong σ-bonds with the metal (64–70); consequently, these ligands have been successfully employed as suitable linkers for anchoring metal complexes onto solid supports. This propensity

77

has been exploited by Blechert (71) to prepare a permanently immobilised and highly active NHC ruthenium benzylidene complex 26, by attaching 2 to a polymeric support through an NHC ligand. The approach consisted in synthesising first a suitably immobilised precursor 27, starting from the diamine A (Scheme XII). Compound A, prepared from 2,3-dibromo-1-propanol and 2,4,6-trimethylaniline, was attached by an ether linkage, after deprotonation of the hydroxyl group, to Merrifield resin (polystyrene crosslinked with 1% divinyl benzene (DVB)), yielding quantitatively the immobilised diamine B; this diamine was cyclised under acidic conditions and, after anion exchange, gave the support-bound 1,3-dimesityl-4,5-dihy-

droimidazolium salt 27. Precursor 27 was converted into the protected carbene 28 (2-tertbutoxy-4,5-dihydroimidazoline), which through in situ deprotection in the presence of the diphosphane ruthenium benzylidene complex 2 (with R = Ph) yielded the support-bound NHC ruthenium complex 26. Immobilised complex 26 proved to be an excellent precatalyst for various metathesis reactions. It cleanly cyclised diallyl or dihomoallyl derivatives to the respective carbocycles and heterocycles, in high yields (90 to 100%). Macrocyclic and dicyclic architectures were also accessible in considerable yields (80 to 100%), starting from the corresponding α,ω-dienes (Scheme XIII). It is remarkable that

PS-DVB PS-DVB

OH

PS-DVB PS-DVB

O O NH NH

a.KOtBu (a) KOtBu b.PS-DVB (b) PS-DVB DMF DMF

NH NH

NH NH

O O

(c) HC(OMe)33,HCO , HCO c.HC(OMe) 2H 2H Toluene Toluene

NH NH

H H

B

A

PS-DVB

O O

N N

N N

27

PS-DVB

Cl Cl

H H

N N

N

(d) HCl/THF d.HCl/THF

Cl Cl

O O

(a) a.TTMSOTf, MSOTf, CH CH22Cl C2l2 RT, 30 min

RT, 30 min

N N

N N

b.K OtBu, tBu,THF THF (b) KO RT, RT, 60 60 min min

H O OttBu Bu H

27

28

PS-DVB

Cl Cl

O O

Cl Cl N N

N N

H O OttBu Bu H 28

PCy PCy33 Ru Ru PCy PCy33

PS-DVB

O

Ph Ph

Toluene, Toluene, 70–80ºC 70-80°C

N Cl Cl Cl Cl

N N Ru Ru Ph Ph PCy PCy33

26

Scheme XII Synthesis of the immobilised NHC ruthenium complex 26 (TMSOTf = trimethylsilyl trifluoromethanesulfonate)


78

enantiomerically pure α,ω-dienes could rearrange quantitatively in the presence of 26 and ethylene into new compounds of high enantiomeric purity (Scheme XIV). In addition to ring closing, some demanding enyne cross-metatheses have readily been performed, to produce functionalised 1,3-dienes in high yield by a simple and efficient atom economical procedure (71) (Scheme XV). In the context of experimental endeavours in ‘green’ chemistry, novel water-soluble rutheniumbased olefin metathesis catalysts (29 and 30), supported via poly(ethylene glycol)-NHC ligands, have recently been introduced by Grubbs and coworkers (72, 73) (Scheme XVI). These soluble

O 7

Scheme XIII Synthesis of macrocyclic and dicyclic compounds via RCM using catalyst 26 (15 = 15-membered ring; Ns = pnitrobenzenesulfonyl)

26 (5 mol%) 3

O

catalysts display greater activity in aqueous RCM and ROMP than do other previously reported (74–77) water-soluble metathesis catalysts. Significantly, RCM and ROMP with 29, in protic solvents (such as methanol), proceeded comparably to reactions with the earlier water-soluble catalysts. It is impressive that catalyst 30 proved highly active in RCM of α,ω-heterodiene salts in water, giving substantial yields (95%) of the corresponding heterocyclic structures (Scheme XVII). Related water-soluble, immobilised ruthenium alkylidene complexes have been devised by Yao (78), Bowden (79) and Gnanou (80) and successfully applied in RCM of dienes and ROMP of norbornene.

15

CH2Cl2, 45ºC 45 oC Yield 80% 80%

Ns

Ns

N

N

+

O O

26 (5 mol%)

E/Z E / Z = 1.4

Ns

Ns

N

N +

CH2Cl2, 45ºC 45 oC Yield 100% 100%

C2H4

OTBS

H OTBS

26 (5 mol%)

H Ts

Scheme XIV Synthesis of enantiomerically pure compounds using catalyst 26 (Ts = tosyl; TBS = (tert-butyl)dimethylsilyl)

CH2Cl2, 45ºC 45 oC

Ts

Yield 100% 100%

RO 3

SiMe3

+

26 (5 mol%) CH2Cl2, 45ºC 45 oC

(3 equiv.)

SiMe3

+

(3 equiv.)

Yield 100% 100%

26 (5 mol%) o CH2Cl2, 45 45ºC C

RO

SiMe3 3

E/Z E/Z = 1.2

Scheme XV Synthesis of functionalised 1,3-dienes by cross-metathesis using catalyst 26

SiMe3

E/Z E/Z = 1.6

Yield 80% 80%


79

Scheme XVI Water-soluble ruthenium-based olefin metathesis catalysts 29 and 30

O MeO

O

O

N

nn

H

N

29

Cl

O

N Cl Cl

O

H2N

PCy3

Ph

OMe n

N Ru O

Cl

N Cl Cl Ru

30

Cl 30 (5 mol%)

H 2N

H 2O Yield > 95% > 95%

Scheme XVII RCM of α,ω-heterodiene salts in water with immobilised ruthenium complex 30

Immobilisation of a ruthenium complex through its NHC ligand, as in 31, has been achieved by Buchmeiser et al. (81) by an interesting approach using a monolithic support; the latter was modified by ROMP of norbornene, or its functionalised derivatives, in order to be suitable for anchoring the homogeneous complex (Scheme XVIII).

m

nn

O O

Monolithic support support

Ad

31

N

N

Cl Ru Cl PCy3

Ad Ph

Scheme XVIII NHC ruthenium complex immobilised on monolithic support 31 (Ad = adamantyl)


Another well designed strategy, introduced by the same group, employs a silica-based support to create immobilised NHC Ru complexes (82). Various polymer monolithic materials have also been ingeniously applied to heterogenise well defined Ru complexes (83, 84).

Conclusion Overall, this first part of the survey convincingly illustrates that ruthenium alkylidene complexes can be effectively immobilised onto solid and soluble polymers by various routes. These capitalise on beneficial attributes of both the catalysts’ actor/spectator ligands and their supports. This strategy has emerged as an improvement in the catalysts’ capability for ‘green’ metathesis chemistry, enhancing their potential as clean, recyclable and highly efficient catalysts and paving the way for scaling up to industrial applications. The concluding paper of this series, Part II, will be published in a future issue of Platinum Metals Review; see Table I for the projected topics in Part II. Note added in proof : When certain types of immobilised catalyst are used for olefin metathesis, ruthenium byproducts may be removed from the products by simple aqueous extraction (85).

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23 I. Dragutan, V. Dragutan, L. Delaude, A. Demonceau and A. F. Noels, Rev. Roumaine Chim., 2007, 52, (11), 1013 24 V. Dragutan, I. Dragutan and A. T. Balaban, Platinum Metals Rev., 2001, 45, (4), 155 25 I. Dragutan, V. Dragutan, R. Drozdzak and F. Verpoort, in “Metathesis Chemistry: From Nanostructure Design to Synthesis of Advanced Materials”, eds. Y. Imamoglu and V. Dragutan, NATO Science Series II: Mathematics, Physics and Chemistry, Vol. 243, Springer Verlag, Berlin, Heidelberg, 2007, pp. 137–150 26 I. Dragutan, V. Dragutan and P. Filip, ARKIVOC, 2005, (x), 105 27 V. Dragutan, I. Dragutan and A. Demonceau, Platinum Metals Rev., 2005, 49, (3), 123 28 V. Dragutan and I. Dragutan, Platinum Metals Rev., 2004, 48, (4), 148 29 I. Dragutan, V. Dragutan, L. Delaude and A. Demonceau, ARKIVOC, 2005, (x), 206 30 “Handbook of Metathesis”, ed. R.H. Grubbs, in 3 vols., Wiley-VCH, Weinheim, Germany, 2003, Vol. I 31 “Metathesis Chemistry: From Nanostructure Design to Synthesis of Advanced Materials”, eds. Y. Imamoglu and V. Dragutan, NATO Science Series II: Mathematics, Physics and Chemistry, Vol. 243, Springer Verlag, Berlin, Heidelberg, 2007 32 K. J. Ivin and J. C. Mol, “Olefin Metathesis and Metathesis Polymerization”, 2nd Edn., Academic Press, London, 1997 33 V. Dragutan, M. Dimonie and A. T. Balaban, “Olefin Metathesis and Ring-Opening Polymerization of Cycloolefins”, John Wiley & Sons, Chichester, New York, 1985 34 V. Dragutan, I. Dragutan and A. T. Balaban, Platinum Metals Rev., 2000, 44, (2), 58 35 V. Dragutan, I. Dragutan and A. T. Balaban, Platinum Metals Rev., 2000, 44, (3), 112 36 V. Dragutan, I. Dragutan and A. T. Balaban, Platinum Metals Rev., 2000, 44, (4), 168 37 “Handbook of Metathesis”, ed. R. H. Grubbs, in 3 vols., Wiley-VCH, Weinheim, Germany, 2003, Vols. II–III 38 V. Dragutan and I. Dragutan, J. Organomet. Chem., 2006, 691, (24–25), 5129 39 D. E. Fogg and E. N. dos Santos, Coord. Chem. Rev., 2004, 248, (21–24), 2365 40 K. C. Nicolaou, P. C. Bulger and D. Sarlach, Angew. Chem. Int. Ed., 2005, 44, (29), 4490 41 D. E. Fogg and H. M. Foucault, in “Comprehensive Organometallic Chemistry III”, in 13 vols., eds. R. Crabtree and M. Mingos, Elsevier, Amsterdam, 2006, Vol. 11, pp. 623–652 42 V. Dragutan and R. Streck, “Catalytic Polymerization of Cycloolefins – Ionic, ZieglerNatta and Ring-Opening Metathesis Polymerization”, Studies in Surface Science and Catalysis, Vol. 131, Elsevier, Amsterdam, 2000

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43 W. J. Sommer and M. Weck, Adv. Synth. Catal., 2006, 348, (15), 2101 44 F. Michalek, D. Mädge, J. Rühe and W. Bannwarth, Eur. J. Org. Chem., 2006, 3, (3), 577 45 M. R. Buchmeiser, Catal. Today, 2005, 105, (3–4), 612 46 R. van de Coevering, R. J. M. Klein Gebbink and G. van Koten, Progr. Polym. Sci., 2005, 30, (3–4), 474 47 L. Li and J. Shi, Adv. Synth. Catal., 2005, 347, (14), 1745 48 S. J. Connon and S. Blechert, Angew. Chem. Int. Ed., 2003, 42, (17), 1900 49 N. E. Leadbeater and M. Marco, Chem. Rev., 2002, 102, (10), 3217 50 M. Ahmed, T. Arnauld, A. G. M. Barrett, D. C. Braddock and P. A. Procopiou, Synlett, 2000, (7), 1007 51 V. Dragutan and F. Verpoort, Rev. Roumaine Chim., 2007, 52, (8–9), 905 52 S. T. Nguyen and R. H. Grubbs, J. Organomet. Chem., 1995, 497, (1–2), 195 53 K. Melis, D. De Vos, P. Jacobs and F. Verpoort, J. Mol. Catal. A: Chem., 2001, 169, (1–2), 47 54 M. Ahmed, A. G. M. Barrett, D. C. Braddock, S. M. Cramp and P. A. Procopiou, Tetrahedron Lett., 1999, 40, (49), 8657 55 A. G. M. Barrett, S. M. Cramp and R. S. Roberts, Org. Lett., 1999, 1, (7), 1083 56 L. Jafarpour and S. P. Nolan, Org. Lett., 2000, 2, (25), 4075 57 L. Jafarpour and S. P. Nolan, Adv. Organomet. Chem., 2001, 46, 181 58 L. Jafarpour, M.-P. Heck, C. Baylon, H. M. Lee, C. Mioskowski and S. P. Nolan, Organometallics, 2002, 21, (4), 671 59 S. J. Connon, A. M. Dunne and S. Blechert, Angew. Chem. Int. Ed., 2002, 41, (20), 3835 60 S. J. Connon and S. Blechert, Bioorg. Med. Chem. Lett., 2002, 12, (14), 1873 61 J. S. Kingsbury, S. B. Garber, J. M. Giftos, B. L. Gray, M. M. Okamoto, R. A. Farrer, J. T. Fourkas and A. H. Hoveyda, Angew. Chem. Int. Ed., 2001, 40, (22), 4251 62 S. Varray, R. Lazaro, J. Martinez and F. Lamaty, Organometallics, 2003, 22, (12), 2426 63 Q. Yao and Y. Zhang, J. Am. Chem. Soc., 2004, 126, (1), 74

64 K. Öfele, W. A. Herrmann, D. Mihalios, M. Elison, E. Herdtweck, W. Scherer and J. Mink, J. Organomet. Chem., 1993, 459, (1–2), 177 65 W. A. Herrmann, K. Öfele, M. Elison, F. E. Kühn and P. W. Roesky, J. Organomet. Chem., 1994, 480, (1–2), c7 66 W. A. Herrmann, Angew. Chem. Int. Ed., 2002, 41, (8), 1290 67 S. Díez-Gonzáles and S. P. Nolan, Coord. Chem. Rev., 2007, 251, (5–6), 874 68 V. Dragutan, I. Dragutan and A. Demonceau, Platinum Metals Rev., 2005, 49, (4), 183 69 “N-Heterocyclic Carbenes in Synthesis”, ed. S. P. Nolan, Wiley-VCH, Weinheim, 2006 70 “N-Heterocyclic Carbenes in Transition Metal Catalysis”, ed. F. Glorius, Topics in Organometallic Chemistry, Vol. 21, Springer-Verlag, Berlin, 2007 71 S. C. Schurer, S. Gessler, N. Buschmann and S. Blechert, Angew. Chem. Int. Ed., 2000, 39, (21), 3898 72 J. P. Gallivan, J. P. Jordan and R. H. Grubbs, Tetrahedron Lett., 2005, 46, (15), 2577 73 S. H. Hong and R. H. Grubbs, J. Am. Chem. Soc., 2006, 128, (11), 3508 74 D. M. Lynn, S. Kanaoka and R. H. Grubbs, J. Am. Chem. Soc., 1996, 118, (4), 784 75 D. M. Lynn, B. Mohr and R. H. Grubbs, J. Am. Chem. Soc., 1998, 120, (7), 1627 76 D. M. Lynn, B. Mohr, R. H. Grubbs, L. M. Henling and M. W. Day, J. Am. Chem. Soc., 2000, 122, (28), 6601 77 T. A. Kirkland, D. M. Lynn and R. H. Grubbs, J. Org. Chem., 1998, 63, (26), 9904 78 Q. Yao and A. R. Motta, Tetrahedron Lett., 2004, 45, (11), 2447 79 M. T. Mwangi, M. B. Runge and N. B. Bowden, J. Am. Chem. Soc., 2006, 128, (45), 14434 80 D. Quémener, V. Héroguez and Y. Gnanou, J. Polym. Sci. Part A: Polym. Chem., 2006, 44, (9), 2784 81 M. Mayr, B. Mayr and M. R. Buchmeiser, Angew. Chem. Int. Ed., 2001, 40, (20), 3839 82 M. Mayr, M. R. Buchmeiser and K. Wurst, Adv. Synth. Catal., 2002, 344, (6–7), 712 83 M. R. Buchmeiser, New J. Chem., 2004, 28, 549 84 M. R. Buchmeiser, Polymer, 2007, 48, (8), 2187 85 S. H. Hong and R. H. Grubbs, Org. Lett., 2007, 9, (10), 1955

The Authors Ileana Dragutan is a Senior Researcher at the Institute of Organic Chemistry “Costin D. Nenitescu” of the Romanian Academy. Her interests lie in the synthesis of stable organic radicals, EPR spin probe applications in organised systems and biological environments, late transition metal complexes with radical ligands, ruthenium catalysis in organic and polymer chemistry, iminocyclitols and prostaglandin-related prodrugs.


Valerian Dragutan is a Senior Researcher at the Institute of Organic Chemistry “Costin D. Nenitescu” of the Romanian Academy. His research interests are homogeneous catalysis by transition metals and Lewis acids; olefin metathesis and ROMP of cycloolefins; bioactive organometallic compounds; and mechanisms and stereochemistry of reactions in organic and polymer chemistry. He is a member of several national and international chemical societies, and has contributed significant books, book chapters, patents and papers to the scientific literature.

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DOI: 10.1595/147106708X299646

“Green Chemistry and Catalysis” BY ROGER A. SHELDON, ISABEL ARENDS and ULF HANEFELD (Delft University of Technology, The Netherlands), Wiley-VCH, Weinheim, Germany, 2007, 448 pages, ISBN 978-3-527-30715-9, £95.00, €142.50, U.S.$190.00

Reviewed by Duncan Macquarrie Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K.; E-mail: [email protected]

Catalysis is behind virtually every chemical we use today, and is involved in some way in virtually every consumer product on the market. This has been true for a century or more, and will continue to be true for the foreseeable future. The last century saw catalyst scientists and technologists master the conversion of crude oil into an enormous array of chemicals, using solid-phase acids such as zeolites to convert the original oil into a range of more useful products. These could then be elaborated using systems such as redox catalysts, metal-centred coupling catalysts or enzymes, all of which provide excellent control over reactivity and selectivity. As a consequence of the uncertainties over crude oil supply, this century is likely to see a focus on converting renewables, for example biopolymers such as starch and cellulose, into more sophisticated molecules. The C:O ratio of these molecules is typically 1 (compare with crude oil, where it is > 100), and that of the desired products is typically between 3 and 10. This means that the processes required to convert these feedstocks will involve different blends of chemistries, with reduction perhaps becoming more central than oxidation as a conversion technology. Reduction catalysis will be expected to play a very prominent role, with the outlook for palladium, platinum and related reduction catalysts in deoxygenation type chemistry being very interesting. Catalysis will certainly retain its central importance to chemistry. Additionally, the role of chemistry in ‘greening’ existing processes will drive the development of more efficient and selective catalysts, and their more effective use. Improved processing, separation and recovery are key concepts, and reduced energy costs will also be vital. For these reasons, this book is very valuable,

Platinum Metals Rev., 2008, 52, (2), 83

as it pulls together all the main catalytic technologies, with a focus on green chemistry and processing. The book is structured to cover the key areas of catalysis, with chapters on solid acids and bases, oxidation (including ruthenium catalysts), reduction, C–C bond forming (including palladium and ruthenium catalysts) and hydrolysis. Other important aspects are also covered; for example, the chapter on alternative reaction media covers developments in solvent choice. In recent years, many new solvent types have been developed in order to improve the environmental impact of processes, since solvents are typically by far the largest components of a reaction mixture and, given their volatility, are one of the hardest parts to control. The newer solvents include ‘typical’ organic solvents with lower toxicity, plus novel systems such as fluorous solvents, supercritical media and ionic liquids. Biphasic catalysis is also covered here. These newer solvents are the focus of intense research activity and are likely to find uses in industry; a few processes already use them. Biocatalysis is also an emerging theme, and may find itself more suited to transformations of biomolecules (which are given a chapter) than petrochemicals. Process design and integration is also given a chapter, as it can produce significant improvements by intelligent combination of two catalytic steps. The book deals with chemical catalysis and biocatalysis as two parts of the same whole, something which is pleasing to see. The two parts are often seen as competing, whereas they can be combined to great effect. The coverage of the area is well rounded and the chapters are up to date, well written and referenced. Overall, this is an impressive book, and a valuable addition to the field.

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DOI: 10.1595/147106708X292517

A Disordered Copper-Palladium Alloy Used as a Cathode Material THE ONE-ELECTRON CLEAVAGE OF CARBON–HALOGEN BONDS By Philippe Poizot and Lydia Laffont-Dantras LRCS, UMR 6007, Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cedex, France

and Jacques Simonet Laboratoire MaSCE, UMR 6226, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France; E-mail: [email protected]

A novel method of forming a palladised copper (Cu/Pd) interface of well defined structure is described. The CuPd alloy is straightforwardly obtained by immersing a copper substrate in acidic solutions of palladium salts. Depending on the composition of the salt/acid solution, the copper surface is virtually instantly covered with a CuPd deposit. With nitric and sulfuric acid solutions and the corresponding Pd(II)-based salt, the deposit is composed of nanoparticles of disordered CuPd alloy dispersed at the copper interface. The alloy-modified surface was successfully used as an efficient promoter of bond cleavage reactions, especially those of carbon–iodide and carbon–bromide bonds in alkyl halides. The catalytic activity is specifically characterised by a very large shift in potential as between the use of a regular glassy carbon surface and the palladised copper interface. With alkyl halides (RBr and RI), the shift toward less cathodic potentials is so large that it enables the one-electron cleavage of C–I and C–Br bonds. This method should enable the heterogeneous generation of free alkyl radicals as transients in electrochemical reactions. These novel cathodic materials could also be of considerable interest for the disposal of halogenated waste.

For achieving novel reactions in organic electrochemistry the design and construction of an ideal, multi-purpose electrode remains a perennial goal (1). Within the cathodic range, the use of mercury is now banned for environmental reasons (2). Platinum, owing to both its cost and its weak hydrogen over-voltage, is difficult to use over a wide cathodic domain. Various carbon interfaces (such as graphite, glassy carbon and carbon felts) may provide useful working electrodes but they are not always inactive. Their neutrality towards electrochemical insertion of ions as well as their tendency to graft free radicals have been noted (3). Electrodes modified with functionalised conductive polymers have been claimed to be useful in interfacial synthesis since they can mimic some of the mechanisms of organic chemistry on solid supports (4). Consequently, some new insights in electrochemical synthesis may be linked to the


development of solid electrodes with very specific properties. Following this approach, pure silver (5) as well as other solid metal electrodes modified by adatoms (6) could offer interesting prospects when tailored to specific reactions. Two-electron cathodic cleavage reactions involving halides, sulfones, sulfonamides or tosylates are of importance in organic synthesis, since they can be applied to deprotection processes, see for example (7). However, most of these reactions have been reported as slow electrochemical processes, depending on the electrochemical potential necessary to cleave the carbon–heteroatom bonds. Such cleavage reactions have been shown to occur at quite negative potentials – basically lower than –2 V vs. SCE. Thus the use of aqueous or water-wet organic solvents is inappropriate when considering most solid metals for use as cathodes.

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The tailoring of surfaces by means of a specific deposit of catalyst is an alternative approach to novel electrode design. Here, the activation potential may become so large as to transform the nature of the cathodic reactions. Large potential shifts can be observed, which in principle enable the overall reaction process to be fundamentally changed. Following this approach, we have developed a very convenient method to produce a modified electrode based on a copper-palladium alloy (8). Preliminary results have shown that the as-obtained Cu/Pd interface appears particularly efficient in accelerating the cleavage of carbon–halogen bonds. These cleavage reactions have often been noted as possessing a high activation energy to achieve the first electron transfer (9–14). In the present paper, we intend to fully define the characteristics of the copper-palladium layer produced onto copper substrates by displacement reactions from several Pd(II)-based precursors, and to investigate the efficiency of the as-produced surface towards the electrochemical cleavage of several alkyl halides RX (with X = Cl, Br and I). At regular solid metallic electrodes, these cleavages are commonly reported to occur with very large activation energies. A series of organic halides were therefore used as probes to compare their cleavage modes as conventionally observed at glassy carbon electrodes (GCEs) with those at palladised surfaces. The palladised surfaces were prepared by electrochemical deposition (onto platinum or glassy carbon substrates under the same experimental conditions). The advantages of this Cu-Pd surface, such as its great catalytic activity, its stability, and its simplicity of synthesis will be presented. The use and specificity of this new interface are discussed in terms of potential shift values related to its catalytic efficiency. It must be borne in mind that these interfaces specifically promote unexpected one-electron processes, which involve the transient formation of free alkyl radicals. With aryl halides, however, reduction processes retain a two-electron mechanism. This is in agreement with the observed high reactivity of aryl radicals (15, 16).


Experimental Formation of the Palladised Copper Interface The copper/palladium interface was simply prepared by dipping into a fresh acidic solution of Pd(II) for 15 s a copper substrate (grid or sheet) previously cleaned with acetone. Three different solutions were prepared by dissolving a palladium salt Pd(Yn–)2/n (with Y = SO42–, NO3– and Cl–) into the corresponding acid. For example, 1 g of palladium(II) sulfate dihydrate (Pd(SO4)·2H2O) (Alfa Aesar) was dissolved in 100 cm3 of 0.1 N H2SO4 solution. The dipping procedure produced a virtually instant deposit onto the copper surface, due to the displacement of copper by palladium cations, together with the unexpected formation of a palladised copper interface. The shiny layer appeared to be quite stable, and sonication had no visible effect on its adhesion to the copper substrate. To prevent any residue of acidic impurities more or less strongly adsorbed onto the surface, a preliminary cleaning step is recommended; the modified electrode is dipped into a dilute aqueous solution of tetramethylammonium hydroxide, followed by rinsing with water, alcohol and finally acetone before drying with a hot air flow (at about 60ºC). Such electrodes were easily reused, giving coherent data, since they were rinsed regularly following the above procedure.

Texture and Structure Analysis Modified surface samples were first examined by X-ray diffraction (XRD) measurements at room temperature. However, the nanometric scale of the deposits made it difficult to identify the asproduced material correctly. To precisely determine the structure/texture of the copper/palladium interface, investigations were made using high-resolution transmission electron microscopy (HRTEM). Commercially available copper grids for electron microscopy were used as the substrate to study the Cu/Pd interface. Three samples were prepared for TEM investigation by dipping a copper grid into a fresh acidic solution of Pd(Yn–)2/n. Electron-transparent specimens were obtained. The TEM and HRTEM imaging were performed using a FEI Tecnai F20 S-TWIN

85

microscope. The elemental composition was also determined by energy dispersive spectroscopy (EDS) at nanometre resolution. The diffraction patterns were obtained using the selected area electron diffraction (SAED) mode or by Fourier transform of the HRTEM imaging.

Electrochemical Procedures: Salts and Solvents In all the electrochemical experiments, tetra-nbutylammonium tetrafluoroborate (TBABF4) was used as the supporting salt at a fixed concentration of 0.1 M. Its purity (at least 98%, Aldrich) was considered suitable for the experiments; there was no further purification. The dimethylformamide (DMF) solvent (SDS, France) was typically employed without drying. However, if ultra-dry solutions were required, DMF stored over activated alumina was used. Alumina activation was by heating at 340ºC under vacuum overnight. Alumina could be added into the electrode cell if necessary, and this in situ drying technique gave a moisture level well below 100 ppm. It is worth mentioning that the procedures given below do not require extremely dry solutions. If one wishes to reach potentials as low as –2 V vs. SCE, the solution could be dried more efficiently to avoid hydrogen evolution via the reduction of residual water, thereby increasing the electrical yield of the overall organic cleavage. The organic halides (RX) used in the present work were purchased from Aldrich (minimum purity 95%) and used as supplied. All electrochemical experiments were performed under an inert atmosphere (dry argon) using a three-electrode cell with a glass separator, as described elsewhere (8). Potential values given in this study are quoted versus SCE. The electrodes used here had an apparent surface area of S = 0.8 mm2, except for those using copper as a substrate (S = 1.6 or 3.2 mm2). Glassy carbon, pure palladium disc and copper electrodes were always carefully polished with silicon carbide paper (Struer) or with Norton polishing paper (grades 02 and 03). Before use, the conventional working electrodes were rinsed twice with water, then alcohol and finally acetone before drying with a hot air


flow. Palladised electrodes (including those used for comparison purposes) were prepared by a galvanostatic deposit of Pd from a palladium chloride solution onto several types of metallic substrates (platinum, gold or palladium). The plating bath contained 10 g l–1 of PdCl2 (Alfa Aesar) in aqueous 0.1 N HCl. In the present experiments, the charge density for galvanostatic deposition was 4 mC mm–2 throughout, with current densities of the order of a few hundreds of μA cm–2.

Coulometry and Electrolyses Coulometric experiments and electrolyses of organic chlorides, bromides and iodides were carried out using three-electrode cells allowing a total catholyte volume of about 5 to 10 cm3. The anodic compartment was separated by a fritted glass of weak porosity. Substrate volume was about 0.1 mM. In order to avoid disturbance resulting from the possible presence of copper oxide, which could depend on the history of the copper substrate, the solution was always pre-electrolysed before adding the RX compound to the cell. Owing to the high reactivity of the Cu/Pd interface towards impurities (in particular dioxygen), there was efficient argon bubbling in all cases, ensuring a good reproducibility of results, especially in voltammetry.

Results Characterisation of the Palladised Copper Interface A complete TEM study was performed on all the samples in order to determine the texture, the structure and the precise composition of the palladium-based layer. The bright field image (Figure 1(a)) shows a dendritic-like growth of the layer (particle size < 50 nm) obtained with the palladium sulfate solution. The HRTEM image of one part of the bright field image represents one of the 12 nm nanoparticles of which the dendrite was composed. The morphology and dimensions (around 10–15 nm) of these particles are homogeneous, and each is well crystallised. For the two other samples, prepared with palladium chloride and palladium nitrate solutions, the growth of the Pd-based layer is not dendritic (Figures 1(b) and

86

(b)

(a)

5 nm 5 nm

100 nm

50 nm

(c)

160

Intensity, a.u.

140

5 nm

(d)

*

120 100 80

*

60 40

* CuPd (ICDD No. 48–1551) * *

20 50 nm

0

20 40

*

* **

60 80 100 120 140 160 2θ (Cu Kα)

Fig. 1 Bright field images of the palladium-based layer obtained with the solution of: (a) palladium sulfate; (b) palladium chloride; and (c) palladium nitrate, (insets: HRTEM images of CuPd nanoparticles composing these layers); (d) common SAED pattern of these three layers combined with a graph similar to an X-ray diffraction pattern which enables characterisation of the disordered CuPd alloy

1(c), respectively). However, the layer is always composed by the juxtaposition of nanoparticles, the size of which is closely dependent on the precursors and varies from 3 to 5 nm (Figure 1(b)) and from 5 to 8 nm (Figure 1(c)). All these nanoparticles are well crystallised. The SAED patterns obtained from these three samples are identical, as shown in Figure 1(d). They are composed of diffraction circles due to randomly oriented nanoparticles. Thanks to the ‘ProcessDiffraction’ software, a line profile of the electron diffraction pattern may be plotted, similarly to the X-ray diffraction pattern (Figure 1(d)). Hence it was determined that the layer can be related to the CuPd disordered alloy (ICDD card No. 48-1551, space group: Fm3m). The nature and crystallographic properties of this layer are presented and developed in the Discussion section of this article. The EDS analysis of these layers (not given here) systematically indicates a Cu:Pd ratio


near to unity, corroborating the CuPd alloy formation. It was concluded that, for all samples, the Pd-based modified electrodes consisted of a thin layer of the stoichiometric CuPd alloy. Additionally, as mentioned in a previous paper (17), it is worth noting that, when using palladium chloride as the salt in solution, the sparingly soluble compound copper(I) chloride (CuCl) may also be incorporated into the surface electrode layer, in which case Cu(I) is stabilised by the excess chloride as a transient in the redox process.

Primary Alkyl Iodides When using Cu-Pd electrodes, and in particular those obtained with palladium sulfate and nitrate solutions, the results regarding the electrochemical reduction of primary alkyl halides differ from those already described for conventional solid electrodes. Several alkyl iodides such as 1-iodobutane, 1-iodohexane, 1-iodooctane and 1-iodohexadecane

87

were tested (see Figure 2). 1-Iodobutane was found to exhibit a strong activation phenomenon at any Cu-Pd electrode. This activation is quantifiable as a positive shift in potential with respect to results obtained at a GCE, which is supposed to be the ideal type of electrode with zero activity towards alkyl halide molecules. Thus under standard conditions, the half-peak potential of iodobutane (Ep/2 = –1.93 V vs. SCE) has been unambiguously assigned to the classical two-electron reaction step, which is regarded as unchanged by the nature of the interface. By contrast, using a Cu-Pd electrode prepared from a PdSO4 solution yields a cathodic step that is strongly shifted in potential, and of much smaller limiting current (Ep/2 = –1.44 V). This electrochemical process is diffusion-controlled, but strongly irreversible. The transfer coefficient estimated from half-peak width measurements was found to be smaller than 0.2. The catalytic efficiency of the Cu/Pd interface was also compared with those of palladised interfaces such as smooth platinum and polished palladium. –3.0

–2.0

–1.0

E, V/SCE

10 A

2 1

Alkyl Bromides

B

C

i, μA

Fig. 2 Voltammetry of 1-iodohexane (concentration: 9 mM) in 0.1 M TBABF4 using DMF as solvent, recorded at different microelectrodes. Scan rate: 100 mV s–1 (A) Response at a GCE (S = 0.8 mm2) (B) Response at a palladised platinum electrode (S = 0.8 mm2) (C) Response at a Cu-Pd modified electrode prepared from a palladium sulfate solution (S = 1.6 mm2)


As already reported (18) palladised surfaces such as those of Pt/Pd electrodes led to a higher potential shift (Ep/2 = –1.36 V) as compared with that of a GCE. As a general trend, all types of palladised surfaces exhibit highly significant potential shifts with alkyl iodides. Peak currents are halved when using Cu-Pd electrodes, suggesting that the overall electrochemical process has become a one-electron reaction. Coulometric measurements verified this proposal satisfactorily with all the primary alkyl iodides tested. It is worth noting that smooth copper also exhibits a one-electron step with butyl iodide, but located at more negative potentials, typically –1.82 V vs. SCE. Therefore the sole participation of the copper substrate in the overall activation process is vanishingly unlikely. Whatever the formation mode, the one-electron reduction process at Cu-Pd surfaces could be verified by microcoulometric measurements on millimolar amounts of reactant. At low reduction potentials (Er < –1.2 V vs. SCE) the measured charge values are consistently very close to 1 F mol–1. Since nitrones (classically used as spin markers) do not disturb electrochemical reduction reactions of alkyl iodides, complementary investigations were performed with N-tert-butyl-αphenylnitrone (PBN). In all cases, a distinct one-electron process was observed, whereas the formation of paramagnetic nitroxides was demonstrated. Under these conditions, 1-iodobutane produces a six-line ESR signal with the coupling constants aH = 3.16 G and aN = 14.9 G. This remains in good agreement with previous results obtained at regular palladised surfaces (18).

A large suite of long-chain primary alkyl bromides were reduced at different solid electrodes and their voltammetric data were compared. Almost all of this range exhibited a two-electron irreversible step at a GCE at quite strongly reducing potentials (i.e. Ep/2 < –2.5 V vs. SCE). Smooth palladium electrodes also yielded a main reduction step that occurs at very negative potentials (within a comparable potential range < –2.4 V) since the electrolyte was thoroughly dried by adding activated alumina in situ. The reduction of

88

short-chain alkyl bromides at palladised electrodes may be effective at much less negative potentials than –2 V, but currents are generally small. However, these voltammetric steps exhibited a kinetically controlled character, and appeared to vanish completely upon repeated scans with increasing alkyl chain length. At a smooth copper interface, a reduction step was generally observed beyond –1.8 V, together with the possible occurrence of an adsorption-like step attributable to the reduction of copper oxide at moderate potentials. By contrast, Cu-Pd modified electrodes yielded surprising results: much larger reduction steps for alkyl bromides were consistently observable at much higher potentials than –2.0 V (see Figure 3 for the case of 1-bromodecane). The step obtained from the second scan is generally Sshaped, with the overall current indicating a process close to a one-electron transfer. The nature of the step strongly suggests a kind of selfinhibition, probably due to the adsorption at the electrode surface of the free radical produced. Voltammetric experiments have shown that the shape of the reduction step is strongly sensitive to impurities in the solution. Thus, with traces of dioxygen, there is no pre-peak and the main step is clearly shifted. The second scan of an ‘impurityfree’ solution also exhibits such a potential shift, –3.0

–2.0

–1.0

probably underlining that the catalysis is slowed down by the decay of the free active surface. The activated surface can only be regenerated by rinsing the electrode according to the procedure described above. However, a pre-peak of variable height is obtained with a freshly produced microelectrode, depending on the nature and the concentration of the alkyl bromides (see Figure 4 in the case of 1-bromohexadecane). The total height of the overall cathodic step is a linear function of alkyl bromide concentration. With R = n-propyl, n-butyl, n-pentyl, n-hexyl, n-octyl and ndecyl, the total current of the reduction step (found to be diffusion-controlled throughout) is roughly half of the current observed at a GCE. This observation argues in favour of a one-electron reduction. Within the scanned potential range (i.e. –2.5 ≤ E ≤ –0.5 V vs. SCE), there is no appearance of a second step attributable to the reduction of the free radical release by the alkyl halide reduction. Moreover, it has been verified that there is no evidence of a partial reduction of the alkyl halide onto a pure copper cathode at potentials above –2 V. In order to produce cheap and strongly activated electrodes, we attempted to build a Cu/Pd interface onto a GCE. Copper was –3.0

–2.0

–1.0

E, V/SCE

E, V/SCE 10 A

A 2

2

10 1

1

25

B B i, μA Fig. 3 Voltammetry of 1-bromodecane (concentration: 9 mM) in 0.1 M TBABF4 using DMF as solvent, recorded at different microelectrodes. Scan rate: 100 mV s–1 (A) Response at a GCE (S = 0.8 mm2) (B) Response at a Cu-Pd modified electrode prepared from a palladium chloride solution (S = 1.6 mm2). First two sweeps


i, μA

Fig. 4 Voltammetry of 1-bromohexadecane (concentration: 9 mM) in 0.1 M TBABF4 using DMF as solvent, recorded at different microelectrodes. Scan rate: 100 mV s–1 (A) Response at a GCE (S = 0.8 mm2) (B) Response at a Cu-Pd modified electrode prepared from a palladium sulfate solution (S = 1.6 mm2). First two sweeps

89

galvanostatically deposited from a copper nitrate solution prepared by dissolving 0.1 g of the salt in 100 cm3 of 0.1 N HNO3. The charge density was limited to 5 × 10–3 C mm–2 and the current was fixed at 0.5 mA. After obtaining the copper deposit, (estimated average thickness ≈ 0.2 μm), the electrode was briefly dipped into a palladium sulfate solution. The glassy carbon surface emerged shiny. Results from use of the interface as a voltammetric electrode were interesting, since the degree of activation appeared extremely favourable. The ‘pre-peak’ turned out to be the main peak (see Figure 5, curve (C)). If the main reduction step decays during repetitive sweeps, a brief pause at 0 V may regenerate most of the original current. It is likely that a finely divided deposit of the alloy Cu-Pd can be superimposed on the thin copper deposit, producing quite a large activated surface. This procedure has so far only been achieved with a glassy carbon support. Our observations suggest that using Cu-Pd electrodes at much less negative potentials than those –3.0

–2.0

–1.0

E, V/SCE 5 2 1 A B 1 2

C

25

i, μA

Fig. 5 Voltammetry of 1-bromodecane (concentration: 9 mM) in 0.1 M TBABF4 using DMF as solvent, recorded at different microelectrodes. Scan rate: 100 mV s–1 (A) Response at a GCE (S = 0.8 mm2) (B) Response at a freshly made CuPd cathode prepared from a palladium chloride solution (S = 3.2 mm2). First two sweeps (C) Response at a GCE first covered by a galvanostatic deposit of copper (S = 0.8 mm2) and then treated by palladium sulfate solution. First two steps


already reported with conventional electrode materials leads, at least with alkyl bromides, to one-electron processes. To verify this hypothesis, an extended series of coulometric experiments was carried out on a large suite of primary alkyl halides. Cu-Pd electrodes (visible as a bright metallic deposit) formed from palladium sulfate or nitrate solutions could be reused for a large number of experiments without any apparent deterioration in efficiency. This was not the case with electrodes produced from palladium chloride, which turned blue over time, probably due to the oxidation of residual cuprous ions inside the layer. It was found for all alkyl bromides in the series that the Cu-Pd electrode then consistently produced a one-electron process. Finally, the analysis by gas chromatography/mass spectrometry (GC/MS) of the R–Br electrolysis products showed that R–R dimers and/or mixtures of R(H)/R(–H) in equal amounts were obtained with R = C8, C10 and C12. The formation of free alkyl radicals in the cleavage of primary alkyl bromides at Cu-Pd cathodes is strongly corroborated by the spin marker technique. 10–20 mg of the alkyl halide, dissolved in 5 cm3 of DMF, was reduced in the presence of a threefold excess of N-tert-butyl-α-phenylnitrone (PBN) (electrolysis current = 10–15 mA). By way of example, the reduction current for 1-bromoheptane at –1.5 V on a Cu-Pd electrode vanished completely at 1 F mol–1. ESR analysis of the electrolyte in the absence of dioxygen disclosed a strong paramagnetic signal, fully attributable to the trapping of the n-heptyl radical. The nitroxide radical obtained (see Structure 1) displayed a six-ray spectrum with coupling constants aN =14.379 G and aH = 2.614 G. t

H Ph

C CH3(CH2)6

Bu

N Oz

1

6-Bromo-1-hexene, which is known to afford a cyclisable free radical, usable as a ‘radical clock’, gives very similar results. Thus the reduction at a GCE shows Ep/2 = –2.34 V, whereas the use of a

90

Cu-Pd working electrode (still prepared with PdSO4) produces a spectacular shift to Ep/2 = –1.40 V. As shown in Figure 6(a), the presence of nitrone at the reduction led to two paramagnetic transients, with the formation of two parent nitroxides. It is presently premature to assign these two nitroxides to the trapping of the uncyclised and cyclised n-hexenyl radical. Finally, fixed potential electrolyses on alkyl bromides (all exhibiting one-electron processes) led to mixtures of R–R, RH and R(–H). The ratio RH:R(–H) was equal to 1, as shown by GC/MS experiments with C8, C10 and C12 bromides.

n-Alkyl Chlorides It was found possible to reduce 1-chloroalkanes at Cu-Pd cathodes. An appreciable potential shift was also observed. However, in all cases, half-peak potentials were still located at very negative

potentials (E < –2.5 V vs. SCE). This precludes obtaining one-electron reduction processes similar to those observed with alkyl bromides and iodides.

Discussion In order to characterise the electrochemical efficiency of our as-prepared electrodes, whatever the palladium precursor used, the primary objective was to unambiguously identify the structure of the deposited layer. Electron diffraction was an appropriate technique here, since the nanometric scale of the metallic particles made valid identification difficult when using a conventional XRD analysis. For all deposits, the electron diffraction line profile enabled identification of the deposited layer as a disordered CuPd alloy, thanks to a perfect match with the diffraction data given by Nekrasov (19) and more specifically by Zhu et al. (20). The latter performed a thorough study of clusters of Fig. 6 ESR signals obtained from: (a) 6-bromo-1-hexene and (b) phenyl iodide when reduced in 0.1 M TBABF4 using DMF and with dissolved TBPN (threefold excess). In both cases, reductions were completed after a total consumption of 1 F mol–1 based on the halide amount

(a)

Intensity, a.u.

40,000

0

g = 2.00641 aN = 14.161 G aH = 2.397 G

–40,000

3450

3480 3510 Magnetic field, Gauss

(b)

Intensity, a.u.

100,000

0

g = 2.01 aN = 14.692 G aH = 2.564 G

–100,000 3420

3440


3460 3480 3500 Magnetic field, Gauss

3520

3540

91

disordered CuPd (i.e. nanoparticles) via a theoretical approach using the bond order simulation (BOS) model for metals and the corrected effect medium (CEM) theory. The simulation model of Zhu et al. predicts diffraction patterns and relative peak intensities, which are in good agreement with the reported experimental data. Having demonstrated the formation of the disordered CuPd phase via TEM investigations, data regarding the Cu-Pd system must be considered, since the disordered structure is not expected at room temperature. Thermodynamically speaking, below the solidus, the CuPd system is first characterised by a continuous solid solution showing a face-centred cubic (f.c.c.) structure (21) with a lattice spacing ranging from 3.615 Å (pure copper) to 3.892 Å (pure palladium) (22). At the 50:50 atomic composition, the disordered CuPd A1-type alloy (solid solution) has a cell constant close to 3.77 Å (22), and is formed of copper and palladium that randomly occupy, with 50% probability, each site of the f.c.c. structure (Table I). As the temperature decreases (T < 600ºC), ordering of Cu and Pd atoms is energetically favoured (23–25) and the cubic CuPd alloy adopts the b.c.c.-based structure (CsCl-type). This phase, which is also referred to as B2 or CuPd (β), shows an alternation of (001) planes of Cu and Pd (Table I). It is worth noting that the f.c.c.-based L10 ordered superstructure (CuAu-type) with alternating

(001) planes of Cu atoms and (001) planes of Pd atoms does not exist (Table I). The competition between the B2 and L10 ordered phases of CuPd resolves in favour of the former, thanks to a substantially lower energy of formation (for more details see (25)). Moreover, the high stability of the B2-type structure is substantiated empirically by the recent discovery of the corresponding mineral (skaergaardite) (26). Consequently, under our experimental conditions, the layer growth must be kinetically controlled, since it leads to the A1-type alloy, a metastable phase at room temperature. A noteworthy result of this study is that we succeeded in synthesising very straightforwardly nanoparticles of the disordered CuPd alloy by immersing a copper substrate (grid, sheet, Cu electrodeposit) into a fresh acidic solution of a palladium salt. Other methods known to date are very much more complicated, usually involving polyvinylpyrrolidone (PVP) as stabiliser to obtain nanoparticles. Esumi’s method (27) (or adaptations) yields this alloy at nanometric scale by thermal decomposition of mixtures of copper and palladium precursors in high-boiling organic solvents (20, 27–29) or by condensing Pd and Cu atoms at 350ºC under ultra-high vacuum (30, 31). Interestingly, CuPd alloys also show potential as gas-phase catalysts in enhancing the selectivity of hydrogenation of dienes (32) and the reduction of NO by CO (30, 33, 34).

Table I

Crystal Structure Information for the f.c.c. and b.c.c. Copper-Palladium Alloys Phase formula

CuPd

“CuPd(α)”*

CuPd(β)

Pearson symbol

cF4

tP4

cP2

Space group

Fm 3 m

P 4 /mmm

Pm 3 m

Strukturbericht designation

A1

L10

B2

Crystal structure

z Pd c Cu Cu or Pd *Note the L10 structure is given only for comparison since “CuPd(α)” phase does not exist for energetic reasons


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Conclusion It may be concluded that a simple redox displacement reaction between Cu0 and Pd2+, operating in acidic solution at room temperature offers a route to a thin and very stable layer, characterised as a well crystallised nanometric CuPd alloy. The displacement is simply achieved in the presence of Pd(II)-based salts such as sulfate, nitrate and chloride. However, a pure CuPd alloy is formed only with palladium sulfate and nitrate. Electrochemical data obtained from the reduction of a large series of organic halides (mainly iodides and bromides) showed that the use of such alloys as cathode materials very strongly activates the cleavage of the carbon–halide bond, sometimes displaying a +1 V shift in potential. There were no strong passivating phenomena during the electrolyses, even though a moderate decay of the cathodic current could be observed after a few minutes. The deposit was shown to act as a porous material, and its structure may change dramatically with time; this corroborates the assumption that palladium reacts with alkyl halides (Figure 7 depicts the modification in morphology of the Cu-Pd layer during the reduction of alkyl bromides). We have already mentioned (18) the use of palladium deposits as modifier of the cathode surface (for example deposits onto platinum or glassy carbon). The mode of action of palladium probably stems from the finely divided nature of the deposits (nanosized particles). Hitherto it was believed that electrolytic deposition (from Pd2+ in acidic solutions) was a prerequisite for electrocatalytic activity (here quantified mainly in terms of a shift of the main voltammetric step toward much less negative potentials). The mode of catalysis is not yet fully determined, but it is conceivable as the insertion of palladium into the carbon–halide bond, giving a strongly adsorbed chain species such as C–Pd–X. Such an insertion may corroborate the catalytic hypothesis, given the constant regeneration of the copper-palladium alloy (see Scheme I), promoted by the strong electronic interaction between Pd and Cu upon alloying with a specific feature (33, 34). In the process proposed here, the rates of adsorption and insertion of palladium into the


C–halogen bond would be rapid compared with diffusion of the electroactive species. As stressed above, catalysis by the Cu-Pd surface is of very great interest for C–Br bond cleavage reactions. The C–I cleavage reaction is also facilitated, but results are quite similar to those already observed with palladised surfaces. Very often (but not invariably), the potential shift is so large that the cathodic reaction is fundamentally changed, and turns out to be mono-electronic. The method may therefore be seen as an efficient source of free radicals (with a possible coupling reaction outside the cathodic layer) more or less strongly adsorbed at the interface. These results are in full agreement with previous estimates by Lund et al. (35–37) concerning the standard potentials corresponding to the reduction of a large number of free alkyl radicals in DMF between –1.39 and –1.72 V vs. SCE, under very similar experimental conditions.

(a)

200 nm

(b)

200 nm

Fig. 7 SEM images of the Cu-Pd layer before and after reduction of 1-bromodecane (concentration 2 × 10–2 M). The metal layer (shown in (a)) has been obtained after a dipping of a copper sheet into PdSO4 (see Experimental section) for 2 minutes. The structural change (b) of the layer (consecutive to the catalytic reduction of the RBr compound) was obtained by electrolysis at –1.9 V vs. SCE after 2 C cm–2 have passed through the cell. Average current density: 0.5 mA cm–2

93

RBr

Cu-Pdinterface

e– – Br–

[RBr]ads

[R

Pd ≡ Cu

Br]ads

Scheme I ads = adsorbed; sol = solution; ≡ represents that an interaction exists between Cu and Pd atoms in the solid state

R•sol

[R•]ads + Cu-Pd

R–R (coupling) •

R sol R(H) + R(–H) (disproportionation)

Acknowledgements The authors are grateful to Professor Viatcheslav Jouikov (Laboratoire MaSCE) for the ESR measurements and to Michèle Nelson (LRCS) for helpful assistance.

References 1 D. G. Peters, in “Organic Electrochemistry”, 4th Edn., eds. H. Lund and O. Hammerich, Marcel Dekker, New York, Basel, 2001, Chapter 8, p. 341 2 O. R. Brown, in “Physical Chemistry of Organic Solvent Systems”, ed. A. K. Covington and T. Dickinson, Plenum Press, New York, 1973, pp. 747–781 3 E. Coulon, J. Pinson, J.-D. Bourzat, A. Commerçon and J. P. Pulicani, Langmuir, 2001, 17, (22), 7102 4 E. Steckhan, in “Organic Electrochemistry”, eds. H. Lund and O. Hammerich, Marcel Dekker, New York, Basel, 2001, Chapter 27, p. 1103 5 S. B. Rondinini, P. R. Mussini, F. Crippa and G. Sello, Electrochem. Commun., 2000, 2, (7), 491 6 G. Kokkinidis, J. Electroanal. Chem., 1986, 201, (2), 217 7 R. Kossai, J. Simonet and G. Jeminet, Tetrahedron Lett., 1979, 20, (12), 1059 8 J. Simonet, P. Poizot and L. Laffont, J. Electroanal. Chem., 2006, 591, (1), 19 9 J. M. Savéant, J. Am. Chem. Soc., 1987, 109, (22), 6788 10 C. P. Andrieux, I. Gallardo, J. M. Savéant and K. B. Su, J. Am. Chem. Soc., 1986, 108, (4), 638 11 J. M. Savéant, J. Am. Chem. Soc., 1992, 114, (26), 10595 12 J. Grimshaw, J. R. Langan and G. A. Salmon, J. Chem. Soc., Faraday Trans., 1994, 90, (1), 75 13 C. P. Andrieux, I. Gallardo and J. M. Savéant, J. Am. Chem. Soc., 1989, 111, (5), 1620 14 J. M. Savéant, in “Advances in Physical Organic Chemistry”, ed. T. T. Tidwell, Academic Press, New York, 2000, Vol. 35, p. 117 and references therein 15 P. Hapiot, V. V. Konavalov and J. M. Savéant, J. Am. Chem. Soc., 1995, 117, (4), 1428 16 C. P. Andrieux and J. Pinson, J. Am. Chem. Soc., 2003, 125, (48), 14801 17 J. Simonet, Electrochem. Commun., 2005, 7, (6), 619


18 J. Simonet, J. Electroanal. Chem., 2005, 583, (1), 34 19 I. Nekrasov and V. Ustinov, Dokl. Acad. Sci. USSR, Earth Sci. Sect. (Engl. Transl.), 1993, 328, 128 20 L. Zhu, K. S. Liang, B. Zhang, J. S. Bradley and A. E. DePristo, J. Catal., 1997, 167, (2), 412 21 “Binary Alloy Phase Diagrams”, 2nd Edn., eds. T. B. Massalski, H. Okamoto, P. R. Subramanian and L. Kacprzak, in 3 vols., ASM International, Ohio, U.S.A., 1990, Vol. 2, p. 1454 22 W. B. Pearson, “A Handbook of Lattice Spacings and Structure of Metals and Alloys”, Pergamon Press, New York, 1967 23 S. Takizawa, S. Blügel, L. Terakura and T. Oguchi, Phys. Rev. B, 1991, 43, (1), 947 24 Z. W. Lu, S.-H. Wei, A. Zunger, S. Frota-Pessoa and L. G. Ferreira, Phys. Rev. B, 1991, 44, (2), 512 25 G. Bozzolo, J. E. Garcés, R. D. Noebe, P. Abel and H. O. Mosca, Prog. Surf. Sci., 2003, 73, (4–8), 79 26 N. S. Rudashevsky, A. M. McDonald, L. J. Cabri, T. F. D. Nielsen, C. J. Stanley, Yu. L. Kretzer and V. N. Rudashevsky, Mineral. Mag., 2004, 68, (4), 615 27 K. Esumi, T. Tano, K. Torigoe and K. Meguro, Chem. Mater., 1990, 2, (5), 564 28 J. S. Bradley, E. W. Hill, C. Klein, B. Chaudret and A. Duteil, Chem. Mater., 1993, 5, (3), 254 29 N. Toshima and Y. Wang, Langmuir, 1994, 10, (12), 4574 30 S. Giorgio and C. Henry, Microsc. Microanal. Microstruct., 1997, 8, (6), 379 31 S. Giorgio, H. Graoui, C. Chapon and C. Henry, in “Metal Clusters in Chemistry”, eds. P. Braunstein, L. A. Oro and P. R. Raithby, in 3 vols., Wiley-VCH, Weinheim, Germany, 1999, Chapter 2, p. 1194 32 J. Philips, A. Auroux, G. Bergeret, J. Massardier and A. Renouprez, J. Phys. Chem., 1993, 97, (14), 3565 33 Y. Debauge, M. Abon, J. C. Bertolini, J. Massardier and A. Rochefort, Appl. Surf. Sci., 1995, 90, (1), 15

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and references therein 34 A. Rochefort, M. Abon, P. Delichère and J. C. Bertolini, Surf. Sci., 1993, 294, (1–2), 43 35 D. Occhialini, S. U. Pedersen and H. Lund, Acta Chem. Scand., 1990, 44, (7), 715

36 D. Occhialini, J. S. Kristensen, K. Daasbjerg and H. Lund, Acta Chem. Scand., 1992, 46, (5), 474 37 D. Occhialini, K. Daasbjerg and H. Lund, Acta Chem. Scand., 1993, 47, (11), 1100

The Authors Philippe Poizot is presently Assistant Professor at the Department of Chemistry (LRCS, UMR 6007) of the Université de Picardie Jules Verne (Amiens, France) where he studied Chemistry, and completed his Ph.D. in Materials Science in 2001. His research topics are mainly focused on the lithium-ion battery and the synthesis of nanostructured electrode materials using soft chemistry routes such as electrodeposition.

Lydia Laffont-Dantras is Assistant Professor at the Department of Chemistry (LRCS, UMR 6007) of the Université de Picardie Jules Verne (Amiens, France). Her principal interest is the study of organic and inorganic compounds by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Her research work is currently focused on the characterisation (morphology and nanostructure) of electrochemical devices such as electrochromic thin films or lithium-ion batteries by TEM and EELS.

Jacques Simonet is Directeur de Recherche Emérite in the Electrochemistry Group, Université de Rennes 1 (UMR 6226), France. His principal interests are organic electrochemistry, the activation of organic reactions by electron transfer, electro-polymerisation and the formation of redox polymers. He also researches on the reversible cathodic charging of precious metals (platinum and palladium) in superdry conditions, in contact with polar organic solvents containing electrolytes, mimicking Zintl phases for transition metals.


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DOI: 10.1595/147106708X298296

Dalton Discussion 10: Applications of Metals in Medicine and Healthcare APPLICATIONS OF PLATINUM GROUP METALS IN CANCER AND HIV TREATMENT Reviewed by Christian G. Hartinger Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland; and University of Vienna, Institute of Inorganic Chemistry, Währinger Str. 42, A-1090 Vienna, Austria; E-mail: [email protected]

Dalton Discussion 10 on the topic “Applications of Metals in Medicine and Healthcare” was held at the University of Durham, U.K., from 3rd to 5th September 2007 (1). Dalton Discussions represent a conference concept quite different from the norm with a clear focus, as the name implies, on the discussion. Therefore the majority of the presentations were short five minute talks on papers submitted for a Dalton Transactions special issue (2), distributed in advance to all the participants, followed by a discussion of about twenty-five minutes. Additionally, five Keynote lectures, given by experts in the field, and approximately sixty poster presentations were included. The conference was perfectly suited to initiate collaborations, develop ideas or simply discuss. The follow-up meeting “Dalton Discussion 11: The Renaissance of Main Group Chemistry” was announced by Professor Robin Perutz, the President of the Dalton Division Council of the Royal Society of Chemistry, and will take place in 2008 at the University of California, Berkeley, U.S.A. (3). About 100 participants from both academia and industry, including chemists, biologists and clinicians, discussed recent results obtained for metal-based therapeutics and diagnostics. Currently, metal compounds are not the drugs of first choice in clinical application or for companies to develop. As several experts pointed out, there is a need to initiate long-term discussion and interdisciplinary research, and to convince both clinicians and society of the benefit of metal-based drugs. The example of the vanadium compound bis(maltolato)oxovanadium(IV) (BMOV), which has insulin mimetic properties (developed by Chris


Orvig and colleagues), which re-entered clinical trials in 2007 after several years with a lack of interest from drug development companies, was used to underline the fact that patience is required, and that nobody in the field can expect to develop drugs overnight. This is a long term process with an average duration of about ten years, there are many failures and costs are high.

Antitumour and Anti-HIV Applications of PGMs With regard to platinum group metals (pgms), an overwhelming number of presentations at Dalton Discussion 10 focused on their application as antineoplastic agents. Platinum complexes are applied in half of all chemotherapeutic schemes against a wide range of tumours, although they are effective against only a handful of tumourigenic diseases. Keynote presenter Chi-Ming Che (University of Hong Kong, China) et al. reported on their recent developments of platinum(II), ruthenium(II), ruthenium(III) and ruthenium(IV) complexes alongside non-pgm compounds (gold, iron and vanadium) as anticancer and anti-HIV agents (4). Che presented Pt complexes which bind non-covalently to biomolecules, and are capable of binding in an electrostatic or hydrophobic manner as well as via intercalation. Some of the complexes were found to be up to 100 times more potent in vitro than cisplatin. Furthermore, aminoalcohol-platinum complexes were proposed as protein-staining reagents in sodium dodecyl sulfate (SDS)-polyacrylamide gels, due to their high binding affinities to proteins, and protein interaction is also accompanied by an enhancement of the emission. In addition, Ru complexes with quinone-

96

diimine as auxiliary ligands were shown to intercalate into DNA, but were found to exhibit mild cytotoxicities of about 200 μM against epidermal KB-3-1 and KB-V-1 carcinoma cell lines. A ruthenium-oxo oxalato cluster was presented which exhibited promising anti-HIV properties, being about ten times more active than the common HIV-1 RT inhibitor 3'-azido-3'-deoxythymidine-5'phosphate. Nicholas P. Farrell (Virginia Commonwealth University, U.S.A.) and coworkers, who developed the trinuclear Pt compound BBR3464 up to clinical trials, reported on BBR3464 analogues which are not capable of binding covalently to biomolecules (5). They observed by mass spectrometry, circular dichroism and fluorescence studies the pre-association of these compounds with human serum albumin (HSA) at an initial stage. It is thought that non-covalent interaction of these Pt complexes with HSA might circumvent the deactivation of Pt drugs by binding to serum proteins, and this suggests a new mode of action for this compound class. The contribution from Peter J. Sadler’s group, presented by Abraha Habtemariam (University of Warwick, U.K.), was the 106Ru-radiolabelled putative antitumour organometallic compound [(η6-fluorene)RuCl(en)]PF6 (6). Synthesised with the purpose of following and locating the Ru species in vivo, the compound was administered to a nontumour bearing male albino rat. The compound was found to be distributed over the whole animal, with the highest level in liver and kidneys, which illustrates the difficulty in finding drugs that do not accumulate in these organs. The Keynote talk of Simon P. Fricker (AnorMED Inc., Canada), entitled ‘Metal based drugs: from serendipity to design’, was focused on established Pt anticancer agents but also reported on new developments in the field, including compounds such as picoplatin, iproplatin and the orally administerable satraplatin, as well as non-Pt complexes (7). The advantages of second and third generation compounds in comparison to cisplatin were highlighted: carboplatin has lower toxicity, satraplatin is orally bioavailable and picoplatin overcomes resistance. Notably, an interesting road-map


from the presenter’s point of view was given (Figure 1).

Fig. 1 A road-map for the development of drugs (ADME = adsorption, distribution, metabolism and excretion)

Fricker pointed out that the advantages of metalbased drugs are thought to derive from a precise 3D configuration, leading to precise target/drug interaction; a capacity to coordinate to biomolecules, which is also tuneable by modification of the ligand sphere; and capacities to participate in biological redox processes and to undergo ligand exchange reactions. The speaker reviewed progress in the field of non-Pt anticancer drug candidates, notably Ru anticancer agents including the two Ru compounds in clinical trials, indazolium trans-[tetrachlorobis(1Hindazole)ruthenate(III)] (KP1019) and imidazolium trans-[tetrachloro(imidazole)(dimethyl sulfoxide)ruthenate(III)] (NAMI-A) (8). In his Keynote lecture, Trevor W. Hambley (University of Sydney, Australia) described recent developments in metal-based pharmaceuticals (9) and classified metallodrugs into seven classes (see Figure 2). Several pgm compounds are represented in classes (i) and (iii). In particular, the Ru-based glycogen synthase kinase 3β (GSK3β) inhibitor DW1/2, developed in Eric Meggers’s group (Philipps-Universität Marburg, Germany) (10), was mentioned as an example of a class (i) metallodrug. Since most of the known anticancer Pt complexes are believed to exhibit their activity in other than their administered forms, cisplatin and carboplatin as well as Ru(III), Ru–arene and other Pt(II) and Pt(IV) complexes can be considered as representatives of class (iii).

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Other Metals Besides the talks mentioned above, there were numerous fascinating presentations and posters on applications of other metal compounds for medicinal purposes. These include gold, titanium, rhenium and iron complexes for cancer chemotherapy, COreleasing molecules, radio-metallodrugs for diagnosis and therapy (for example, copper complexes for positron emission tomography (PET) imaging and technetium binding to peptides), gadolinium complex-based contrast agents, zinc and copper complexes as probes for in vitro fluorescence imaging and α-emitters such as 213bismuth, 211astatine and 225actinium as therapeutics. Fig. 2 Classification of metallodrugs

Janice R. Aldrich-Wright (University of Western Sydney, Australia) et al. reported on a targeted approach exploiting molecular hosts as drug delivery vehicles (11). Notably, some Pt complexes containing the (1S,2S)-cyclohexanediamine (chxn) moiety were found to be more active in vitro than oxaliplatin analogues with the (1R,2R)-chxn ligand. Loading such Pt(II) complexes onto cucurbit[n]urils resulted in only small modifications of the cytotoxicity of the complexes, indicating only minor influence of the drug delivery system on the activity of the complexes. However, the application of these compounds is limited by their low solubility, therefore other molecular hosts such as cyclodextrins, calix[n]arenes and dendrimers are being evaluated. Finally, the contribution from Paul J. Dyson’s (École Polytechnique Fédérale de Lausanne (EPFL), Switzerland) group dealt with the design of Ru organometallics, the tuning of lipophilicity, which influences the cellular uptake, and altering the compounds’ reactivity towards DNA models and proteins (12). All these efforts were undertaken with a view to improving the efficacy of Ru compounds. Based on these studies, it was concluded that modifications which lead to increased interactions of a drug with DNA at the expense of protein binding are more toxic towards healthy cells, and therefore are likely to exhibit more unwanted side effects in patients.


Concluding Remarks In summary, a broad variety of applications in metals in medicine and healthcare was presented, with Pt, Ru and other metal-based drugs demonstrating the potential to become the major treatments for some common diseases. Dalton Discussion 10 was a very interesting conference at a pleasant venue in the old city of Durham, and had an appropriate size to benefit from the special conference mode with its focus on discussion. All the contributions can be read in the special issue of Dalton Transactions, published in autumn 2007 (2).

References 1 Dalton Discussion 10: Applications of Metals in Medicine and Healthcare, 3rd–5th September, 2007, Durham University, U.K.: http://www.rsc.org/DD10 2 Dalton Trans., 2007, (43), 4873–5092 3 Dalton Discussion 11: The Renaissance of Main Group Chemistry, 23rd–25th June, 2008, University of California, Berkeley, U.S.A.: http://www.rsc.org/DD11 4 R. W.-Y. Sun, D.-L. Ma, E. L.-M. Wong and C.-M. Che, Dalton Trans., 2007, (43), 4884 5 E. I. Montero, B. T. Benedetti, J. B. Mangrum, M. J. Oehlsen, Y. Qu and N. P. Farrell, Dalton Trans., 2007, (43), 4938 6 J. D. Hoeschele, A. Habtemariam, J. Muir and P. J. Sadler, Dalton Trans., 2007, (43), 4974 7 S. P. Fricker, Dalton Trans., 2007, (43), 4903 8 C. G. Hartinger, S. Zorbas-Seifried, M. A. Jakupec, B. Kynast, H. Zorbas and B. K. Keppler, J. Inorg. Biochem., 2006, 100, (5–6), 891 9 T. W. Hambley, Dalton Trans., 2007, (43), 4929

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10 K. S. M. Smalley, R. Contractor, N. K. Haass, A. N. Kulp, G. E. Atilla-Gokcumen, D. S. Williams, H. Bregman, K. T. Flaherty, M. S. Soengas, E. Meggers and M. Herlyn, Cancer Res., 2007, 67, (1), 209 11 N. J. Wheate, R. I. Taleb, A. M. Krause-Heuer, R. L.

Cook, S. Wang, V. J. Higgins and J. R. AldrichWright, Dalton Trans., 2007, (43), 5055 12 C. Scolaro, A. B. Chaplin, C. G. Hartinger, A. Bergamo, M. Cocchietto, B. K. Keppler, G. Sava and P. J. Dyson, Dalton Trans., 2007, (43), 5065

The Reviewer Christian G. Hartinger received his M.S. and Ph.D. in Chemistry in 1999 and 2001, respectively, from the University of Vienna, Austria, under the supervision of Bernhard K. Keppler. Up to 2006, he worked as a research assistant at the same department. He has recently joined the working group of Paul Dyson at the EPFL in Switzerland as a Schrödinger Fellow. His research interests include the development of mono- and multinuclear platinum group metal complexes as anticancer agents, and the elucidation of the transport mechanism and mode of action for such compounds using modern separation and mass spectrometric techniques.


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DOI: 10.1595/147106708X297855

Platinum as a Reference Electrode in Electrochemical Measurements By Kasem K. Kasem* and Stephanie Jones Department of Natural, Information, and Mathematical Science, Indiana University Kokomo, Kokomo, IN, 46904-9003, U.S.A.; *E-mail: [email protected]

The usefulness of platinum as an electrochemical reference electrode was investigated. Well known redox systems with one-electron single or multiple redox waves, and two-electron multiple redox waves were used as test regimes. The effects on electrode performance of variables such as the solvent, the physical state of the electrolyte and its temperature were investigated. Cyclic voltammetry (CV) was used to derive kinetic parameters for comparison with corresponding measurements on traditional reference electrodes. The results indicate that Pt can be used as a reference electrode under specific conditions in which traditional reference electrodes cannot be used.

Traditional reference electrodes used for electrochemical measurements, such as the calomel and silver/silver chloride electrodes, have a limited range of applicability. The liquid junction is problematic with these electrodes, and they cannot be used either with wholly solid-state electrochemical cells or for very high-temperature reactions such as those in molten electrolytes. The use of solid platinum electrodes in molten salts has been reported (1–14), but there are problems associated with the use in molten media of Pt electrodes. Under strongly alkaline conditions, they actually function as oxidation electrodes (2). It has been demonstrated that Pt foil cannot act as a reference electrode in molten electrolytes, since it is neither stable nor depolarised (4). On the other hand, Pt wire immersed in molten NaCl/KCl can maintain a steady electrode potential for more than 12 hours, and shows electrochemical irreversibility. It can therefore can be used as a pseudo-reference electrode for the study of electrode reaction kinetics, with the advantages of simplicity, convenience and ease of operation (7). The Pt reference electrode performs well in geothermal brine solutions at high pressure and temperature (~ 250ºC). Unlike conventional reference electrodes (even when modified for high temperature), the Pt reference electrode is applicable to measurements in complex polluted brines (8).


In certain electrolytes, modification of the Pt surface is important for its stability. Anodised, nonporous Pt has demonstrated its usefulness as a solid-state reference electrode by virtue of its nearNernstian behaviour, low hysteresis and rapid response (15). Modifications to Pt wire may extend its usefulness to more electrochemical systems. The use of polypyrrole (16), poly-1,3-phenylendiamine (15) or polyvinyl ferrocene (17) as a surface modifier can successfully suppress significant interference by any coupled redox systems or contaminants. The fact that a Pt electrode can be modified with nitrogen-based polymers or be incorporated as part of a biosensor assembly (18) indicates its resistance to interference from these compounds. Some of the problems associated with the reference electrode can be solved outside the electrochemical cell. A reference electrode is defined as an ideal non-polarisable electrode; thus its potential does not vary with the current passing. In practice, no electrode follows this ideal behaviour; consequently, the interfacial potential of the counterelectrode in the two-electrode system varies with the flow of current passed through the cell. In order to overcome this problem, a threeelectrode cell can be used. The functions of the counterelectrode (in a three-electrode cell) are divided between the reference and auxiliary electrodes. The passage of current between the

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working and auxiliary electrodes ensures that less current passes through the reference electrode. Furthermore, the three-electrode cell allows the potential between the working and reference electrodes to be controlled. Most electrochemical devices include an operational amplifier of high input impedance for the reference electrode input, to eliminate the possibility of any current passing through the reference electrode. Since no Faradaic process takes place at the reference electrode, its area relative to that of working electrode has no effect on the electrochemical results. The physical form of the Pt reference electrode may contribute to its performance. Studies (19) indicate that the Pt mesh electrode yields very reproducible results, and that it can be used as a convenient reference electrode. On the other hand, Pt sheet or wire has been used in all-solidstate electrochemical cells at room temperature (20–22), and in reactions in frozen agar or frozen aqueous electrolytes (23, 24). Most of these studies involved only one-electron redox systems. In this study, the usefulness of Pt as a reference electrode in electrochemical systems was investigated using CV techniques. Single or multi-electron redox systems involving one- or two-electron redox waves were used in this study. To verify the suitability of Pt as a reference electrode, kinetic parameters were determined for comparison with corresponding measurements on traditional reference electrodes.

cyanoferrate (III)) electrolyte. The two electrodes were connected briefly to the inputs of a Fluke 27 multimeter that has input impedance 200 MΩ. The voltage reading was used to assess the quality and suitability of the Pt reference electrode. Electrochemical experiments were carried out using a 10 cm3 cylindrical cell (Figure 1). The reference electrode, unless otherwise stated, was Pt. CV was performed first using Pt as a reference electrode and then using Ag/AgCl/Cl– as a reference electrode. The counter (auxiliary) electrode was a Pt wire, and the working electrodes were glassy carbon (0.07 cm2) or Pt (0.02 cm2) in disc or microelectrode (10 μm diameter or 7.85 × 10–7 cm2) configuration. Electrodes were positioned in the cell in a similar way. The Pt wire reference electrode was coiled around the Teflon jacket of the working electrode; the counter electrode was placed at a distance from both the reference and working electrodes. The working electrodes were cleaned by polishing with 1 μm α-alumina paste or diamond paste, and rinsed with water and acetone prior to use. A BAS 100B Electrochemical Analyzer (Bioanalytical System, Inc.) was used to Working electrode Reference electrode

Counter electrode

Experimental Details The reagents FeCl3, KCl, K3[Fe(CN)6] and K4[Fe(CN)6] were of analytical grade. A purified agar powder was obtained from Sigma Chemical Co. All other reagents were of at least reagent grade and were used without further purification. Analytical grade nitrogen gas was used to purge oxygen from the electrolyte. Unless otherwise stated, experiments were performed at 25ºC and 1 atm pressure. To test the suitability of Pt as a reference electrode, a Pt wire reference electrode was coupled with a standard Ag/AgCl/Cl– reference electrode in a beaker containing 0.5 M KCl with or without the addition of 2 mM K3Fe(CN)6 (potassium hexa-


Electrode cross-section Fig. 1 Schematic drawing for the electrochemical cell used in this study

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perform the electrochemical studies. For frozen electrolyte experiments, the electrolyte was first frozen at –20ºC and measurements were performed at –5ºC.

Suitability Tests on Platinum Wire Reference Electrodes Prior to use as a reference electrode, Pt wire was mechanically polished using 600 grade sandpapers, followed by 2 μm diamond paste, and rinsed with deionised water. In an alternative method, the mechanically cleaned Pt wire was heated to 1000ºC for 10 minutes. The Pt wires with the different treatments were each coupled with a reference electrode of known potential as described in the Experimental Details section. The voltammetric reading was less than 0.2 mV in each case. This value is much lower than the standard maximum allowed for this test, which is 3 mV. These results indicate that Pt wire is an adequate reference electrode for routine laboratory use. The suitability test showed that stirring or agitation has no effect on the performance of the Pt wire reference electrode. Several Pt wires of different lengths were subjected to the reference electrode suitability test. The results indicate that the surface area of the Pt wire has no effect on performance. One of the most undesirable effects which a reference electrode can cause is a change in potential during the course of an experiment. In CV studies, the quantity ΔEp (the difference between the reduction peak potential Epc and the oxidation peak potential Epa) is very important in the calculation of the charge transfer rate constant kCT when diffusion is the dominant process. The calculation is made using Equation (i) (25):

Ψ=

(DO/DR)α kCT

(i)

(NF π ν DO/RT)

1/2

where: Ψ is a dimensionless rate parameter, the value of which decreases from 20 to 0.1 as ΔEp increases from 0.061 V to 0.212 V (25); DO = diffusion coefficient of oxidation; DR = diffusion coefficient of reduction; N = number of electrons; ν = scan rate (V s–1); F = Faraday constant; T = temperature; R = gas constant; α = transfer


coefficient. The validity of Pt as a reference electrode has been tested in each of the three systems described below.

One-Electron Redox System Liquid aqueous, agar gel and frozen systems containing 5 mM of K3Fe(CN)6 and 100 mM KCl as supporting electrolytes were chosen as model systems for one-electron redox reactions. Figure 2(a) indicates that the use of Pt as a reference electrode shows a clear potential window in a redox-free electrolyte, whereas Figure 2(b) shows only a parallel shift in both Epc and Epa without affecting the value of ΔEp, even in agar medium. Typical characteristics of diffusional redox waves are reported and displayed in Figures 2(c)–2(e). The use of Pt as a reference electrode in a frozen electrolyte shows CV outcomes similar to those generated when Ag/AgCl/frozen Agar (KCl saturated) was used as a reference electrode. The results are displayed in Figure 3. These results were interpreted using a high pressure effect model (24).

Multi One-Electron System 1 mM of TCQN (7,7,8,8-tetracyanoquinodimethane) in acetonitrile containing 0.2 M LiClO4 (lithium perchlorate) was used as a medium to investigate a multi-one electron redox system. Figure 4 displays the CV of this system using a Pt disc working electrode with a Pt wire electrode (solid trace), and Ag/AgCl/KCl (dashed trace) as reference electrodes. Two one-electron redox waves can be identified. It can be observed that increasing the scan rate from 5 mV s–1 (Figure 4(a)) to 50 mV s–1 (Figure 4(b)) generated well defined redox waves. Measured ΔEp for redox waves after ‘IR’ compensation (for the ohmic potential drop) indicates a typical one-electron wave character (amplitude 61 mV) which is very close to the theoretical amplitude of 59 mV. The fact that the TCQN CV shows a consistent behaviour regardless of the reference electrode used indicates that the results are attributable to the redox system and not to the type of reference electrode. This conclusion demonstrates the usefulness of Pt as reference electrode in non-aqueous media for multi oneelectron redox systems.

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(b)

(a)

20 μA

+1.0

0

–1.0

–2.0 E, V

+1.0

+0.5

0

–0.4 E, V

(c) 20 μA +0.4

(d)

0

(e)

50

50

40

40

30

30

ip, μA

ip, μA

–0.4 E, V

20 = ipc = ipa

10 0

0.1 0.2 0.3 0.4 0.5 V½

20 = Epc = Epa

10 0

0.1 V s–1

0.2

Fig. 2 (a) CV at 0.20 V s–1 of: ⎯ GCE; - - - Pt reference electrode, in agar gel containing KCl; (b) CV at 100 V s–1 of Pt electrode in agar gel containing 5 mM K3[Fe(CN)6]/KCl: ⎯ vs. Ag/AgCl (agar saturated KCl) as reference electrode; - - - vs. Pt electrode; (c) CV of Pt electrode (0.02 cm2) in agar gel containing 5 mM K4[Fe(CN)6]/KCl at: 0.020, 0.050, 0.10 and 0.150 V s–1 (inner to outer CV traces, respectively); (d) Plot of ip vs. V½ ( = ipc, = ipa); (e) Plot of ip vs. V sec–1 ( = Epc, = Epa)

Multi Two-Electron Redox System

Conclusions

5 mM H3PMo12O40 (a Keggin heteropolyacid) immobilised in agar gel containing H2SO4/KCl was used as a model for a multi two-electron redox system. Either a glassy carbon microelectrode (10 μm diameter) or a disc electrode (0.07 cm2) was used as a working electrode. The results are displayed in Figure 5(a) (microelectrode) and Figure 5(b) (disc electrode). Figure 5 clearly illustrates the multi two-electron redox waves typical of phosphomolybdic acid. The measured ΔEp for each of the first two redox waves was less than 28 mV. The position of the formal potential of each of these waves was negatively shifted by approximately 600 mV from that observed when a Ag/AgCl/KCl reference electrode was used.

Under certain conditions, such as high temperature or in molten electrolytes, where the usual reference electrodes such as calomel or Ag/AgCl/KCl electrodes cannot be used in electrochemical measurements, this study has demonstrated that Pt is the reference electrode of choice. Our study also shows that, under conditions where traditional reference electrodes are viable, Pt can replace them. Furthermore, in highpressure electrochemical systems the change in the formal potential of the redox system follows Equation (ii):


ΔV° = –(δE°)TNF/δP

(ii)

where δP is the partial derivative of the pressure.

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(a)

(b)

6 μA

+0.6

0

+0.6

–0.6 E, V

0

–0.6 E, V

10 μA

(c)

500 pA

+0.6

+0.3

0

–0.3 E, V

Fig. 3 CV at 100 mV s–1 for 5 mM [Fe(CN)6]3–/4–: (a) Pt disc in aqueous KCl; (b) Pt disc in frozen KCl electrolyte; (c) Pt microelectrode in frozen KCl electrolyte

In liquid electrolytes the change of the volume (ΔV°) originates in the outer sphere molecules in the solvation layer. Traditional reference electrode components contribute to this volume change. The capability of a Pt wire to act alone as a reference electrode without any associated solvated ions, eliminates the error in calculation of ΔV°. Consequently, the measured δE° relates more specifically to the particular redox system. Our studies show a 3.5 mV change in the formal potential (ΔE°) of [Fe(CN)6]–3/–4 when the aqueous liquid electrolyte is frozen. ΔE° was 98 mV when aqueous gel electrolyte is frozen. It has been reported (26) that a change in the formal potential of the [Fe(CN)6]–3/–4 system in a liquid electrolyte of 3.93 × 10–5 V atm–1 took place when the system was subject to pressure. This change in the formal potential is equivalent to applying 89 atm and 2500 atm to frozen aqueous and gel electrolytes of [Fe(CN)6]–3/–4


respectively. The volumes of both the reference and counter electrode components will remain constant, because Pt wires were used as reference and counter electrodes; the change in the formal potential is due to the change in the volume of the redox ion. This is an additional advantage of using Pt as a reference electrode over the use of a traditional reference electrode under these conditions. In conclusion, we have shown that where conventional reference electrodes are not suitable for some electrochemical measurements, Pt wire is a satisfactory reference electrode in various electrochemical systems such as aqueous, non-aqueous, gel or frozen electrolytes, and for measurements under high pressure. Single or multi one- or twoelectron redox systems were studied, with peak separation (ΔEp) indicating that Pt can be used reliably as a reference electrode under a variety of conditions.

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(a)

(a) 4.0 μA

1 nA +0.2

0

–0.2

–0.4

–0.6 E, V

0

(b)

+0.2

–0.5

–1.1 E, V

3.0 μA

0

–0.2

–0.4

(b)

–0.6 E, V

Fig. 4 CV of GCE (0.07 cm2) in agar gel containing 1 mM TCQN (7,7,8,8-tetracyanoquinodimethane) in 0.2 M LiClO4 in acetonitrile: ⎯ Pt reference electrode, - - Ag/AgCl/Cl– reference electrode: (a) at 0.005 V s–1; (b) at 0.50 V s–1

+1.0

0

–1.0 E, V

10 μA

Fig. 5 CV at 0.5 V s–1 in 5 mM H3PMoO40 in agar gel containing H2SO4/KCl for: (a) carbon microelectrode; (b) GCE (0.07 cm2)

References 1 D.-S. Kim, H.-J. Park, H.-M. Jung, J.-K. Shin, P. Choi, J.-H. Lee and G. Lim, Jpn. J. Appl. Phys., 2004, 43, (6B), 3855 2 A. S. Afanas’ev and V. P. Gamazov, Zh. Fiz. Khim., 1964, 38, (12), 2823 3 I. Uchida and H. A. Laitinen, J. Electrochem. Soc., 1976, 123, (6), 829 4 H. C. Gaur and H. L. Jindal, Curr. Sci., 1968, 37, (2), 49 5 P. P. Leblanc and R. A. Rapp, J. Electrochem. Soc., 1992, 139, (3), L31 6 Yu. K. Delimarskii, L. S. Berenblyum and I. N. Sheiko, Zh. Fiz. Khim., 1951, 25, 398 7 Z. Xie and Y. Liu, Zhongguo Youse Jinshu Xuebao, 1998, 8, (4), 668 8 J. A. Sampedro, N. Rosas and B. Valdez, Corros. Rev., 1999, 17, (3–4), 253 9 J. Horiuchi and G. Okamoto, Sci. Pap. Inst. Phys. Chem. Res. (Jpn.), 1936, 28, 231


10 F. G. Bodewig and J. A. Plambeck, J. Electrochem. Soc., 1970, 117, (5), 618 11 B. Popov and H. A. Laitinen, J. Electrochem. Soc., 1970, 117, (4), 482 12 H. Goto and F. Maeda, Nippon Kagaku Zasshi, 1969, 90, (8), 787 13 J.-B. P. F. Lesourd and J. A. Plambeck, Can. J. Chem., 1969, 47, (18), 3387 14 O. A. Songina and A. A. Usvyatsov, Zavod. Lab., 1964, 30, (11), 1419 15 J.-H. Han, S. Park, H. Boo, H. C. Kim, J. Nho and T. D. Chung, Electroanalysis, 2007, 19, (7–8), 786 16 J. Ghilane, P. Hapiot and A. J. Bard, Anal. Chem., 2006, 78, (19), 6868 17 P. J. Peerce and A. J. Bard, J. Electroanal. Chem. Interfacial Electrochem., 1980, 108, (1), 121 18 Y. Hanazato and S. Shiono, Anal. Chem. Symp. Ser., 1983, 17, (Chem. Sens.), 513

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19 E. P. Jacobs and F. Doerr, Ber. Bunsen-Ges., 1972, 76, (12), 1271 20 J. A. Cox, A. M. Wolkiewicz and P. J. Kulesza, J. Solid State Electrochem., 1998, 2, (4), 247 21 P. J. Kulesza and Z. Galus, J. Electroanal. Chem., 1992, 323, (1–2), 261 22 P. J. Kulesza, L. R. Faulkner, J. Chen and W. G. Klemperer, J. Am. Chem. Soc., 1991, 113, (1), 379

23 K. K. Kasem, J. New Mater. Electrochem. Syst., 2005, 8, (3), 189 24 L. S. Books, C. Harris and K. K. Kasem, Am. J. Undergrad. Res., 2007, 5, (4), 25 25 A. J. Bard and L. R. Faulkner, “Electrochemical Methods: Fundamentals and Applications”, 1st Edn., John Wiley & Sons, New York, 1980, p. 231 26 J. I. Sachinidis, R. D. Shalders and P. A. Tregloan, Inorg. Chem., 1994, 33, (26), 6180

The Authors Dr Kasem K. Kasem is a Professor of Chemistry at Indiana University Kokomo, U.S.A. He is interested in developments in the field of applied electrochemistry, especially in physical and analytical applications of chemically-modified electrodes. He also has interests in semiconductor photoelectrochemistry, the electrochemical behaviour of polymeric thin films, activation and metallisation of polymers, and the electrodeposition of metals and alloys.


Stephanie Jones participated in this work during her senior year at Indiana University Kokomo, of which she is now an alumna, having graduated with a bachelor’s degree in chemistry. She works with a chemical company in Bloomington, Indiana, U.S.A.

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DOI: 10.1595/147106708X298278

Faraday Discussion 138: Nanoalloys – From Theory to Applications PRE-EMINENT ROLE OF THE PLATINUM GROUP METALS Reviewed by Geoffrey C. Bond 59 Nightingale Road, Rickmansworth WD3 7BU, U.K.; E-mail: [email protected]

A Faraday Discussion on “Nanoalloys”, organised by the Royal Society of Chemistry, and sponsored by Johnson Matthey and the Collaborative Research Network for Nanotechnology (CRNNT) of the University of Birmingham, was held in the Department of Chemistry, University of Birmingham, U.K., on 3rd to 5th September 2007 (1). It was attended by about sixty participants from the U.K., Europe and the U.S.A., mainly from academia. As usual with Faraday Discussions, preprints were available in advance, and authors had only five minutes to present the highlights of their work. There was therefore ample time for discussion, which never flagged and was at times vigorous and forthright. The term ‘nanoalloy’ has been coined to describe the assembly of a small number of atoms of metallic elements of two (or sometimes more) kinds. Although there is no consensus, the term ‘cluster’ is used for an assembly formed in the gas phase, where the number of atoms is small (e.g. < 100) and countable by mass spectrometry, whereas the term ‘nanoparticle’ describes larger assemblies usually made by chemical routes (e.g. as colloids) and ranging in size from about 1 to 10 nm. Both clusters and nanoparticles can be deposited on supports, and their physical and catalytic properties examined in that state. Such small assemblies are of great current interest because their properties often differ significantly from those of the corresponding bulk materials, and nanoalloys are formed between pairs of elements that do not form homogenous bulk alloys.

Theory and Simulation Of the twenty-five papers presented, seventeen concerned one or more of the platinum


group metals (pgms), the palladium-gold combination being the most popular. Many of the papers involved collaborations between several institutions; the details of these are given in (2). The Discussion was divided into four parts: the first part dealt with ‘Theory and simulation of nanoalloy structures and dynamics’, and need not detain us, as in the main the work presented appeared to be somewhat remote from practical reality, and failed to produce insights into observable properties. In some of the papers, imaginative structures were invented and studied, irrespective of whether they had been or could be prepared. Thus F. Calvo (Université Claude Bernard Lyon 1, France) studied the interchange of palladium and platinum atoms in icosahedral particles containing alternating layers of the two kinds of atoms, and E. P. M. Leiva (Universidad Nacional de Córdoba, Argentina) et al. examined by computer simulation how bimetallic nanoparticles could arise by collision of two clusters, one of each kind. There were no computations of the number of angels that could dance on the point of a nanoalloy particle, but it would not have been surprising if there had been.

Optoelectronic and Catalytic Properties The Discussion returned to earth for the section on ‘Electronic, optical and magnetic properties of nanoalloys’; the term ‘optoelectronic’ covers all three. Condensation controlled by laser vaporisation, reported by M. S. El-Shall (Virginia Commonwealth University, U.S.A.) et al. enabled nanoalloy particles to be made of a number of binary combinations (PdAu, PtAu, PdFe, PtFe, PdNi and PtNi), those involving iron and nickel being superparamagnetic. The magnetism of CoRh nanoparticles was reported

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by G. M. Pastor (Universität Kassel, Germany) and colleagues. The average spin moment per atom is larger than for macroscopic materials, and the likely structures have a rhodium core and a cobalt-rich outer layer. The structure of CoPt particles was investigated theoretically by C. Mottet (Centre de Recherche en Matière Condensée et Nanosciences (CRMC-N), Marseille, France) et al. The six papers in ‘Nanoalloys in Catalysis’ on catalytic properties of nanoalloys having one of the pgms as a component stimulated a lively discussion. G. J. Hutchings (Cardiff University, U.K.) et al. reported their latest results on hydrogen peroxide synthesis by oxidation of hydrogen using catalysts containing palladium and/or gold. With carbon as support, high activities were obtained, and binary compositions contained homogeneous alloy particles, but with titania and alumina supports core-shell structures occurred. P. A. Sermon (University of Surrey, U.K.) et al. advocated using alkane conversions (hydrogenolysis, isomerisation) as a means of characterising the surface of PtAu and PtSn nanoparticles, illustrating their catalytic capability by reference to the reactions of n-hexane and methylcyclopentane. P. Wells (University of Southampton, U.K.) and colleagues showed that the oxygen reduction activity of the Pt3Cr nanoalloy phase was better than that of platinum alone in model fuel cells; this work had input from D. Thompsett of the Johnson Matthey Technology Centre, U.K. Structure-performance relationships in PdRh/γ-Al2O3 catalysts for the NO-CO reaction were described by M. Tromp (University of Southampton, U.K.) and coworkers. Inclusion of palladium prevented dissociative oxidation of rhodium by NO, but did not stop its extensive disruptive oxidation by CO. Sir John Meurig Thomas (University of Cambridge, U.K.) and associates showed that organometallic cluster compounds comprising ruthenium and tin atoms could be decomposed on a silica support to give effective catalysts for the selective hydrogenation of cyclododecatriene to cyclododecene and for other reactions.


Structural Studies The fourth section of the Discussion concentrated on ‘Structural studies of nanoalloys’, the palladium-gold combination proving the most popular. The use of energy dispersive X-ray spectroscopy for investigating the structure of supported PdAu catalysts used for hydrogen peroxide synthesis was explained by C. J. Kiely (Lehigh University, Bethlehem, Pennsylvania, U.S.A.) and colleagues. The principal conclusions of this work were mentioned in connection with the Cardiff University work (see above). Strongly size-controlled synthesis of icosahedral palladium-gold nanoparticles has been accomplished in inert-gas condensation in a sputtering reactor by E. Pérez-Tijerina (Universidad Autónoma de Nuevo Léon, Monterrey, Mexico) and colleagues; particles were homogeneous and did not show core-shell structures. This combination was also studied by C. R. Henry (CRMC-N, Marseille, France) and his associates; they prepared bimetallic particles uniformly dispersed on nanostructured alumina film by sequential condensation of the two kinds of atoms.

Concluding Remarks In conclusion, this Discussion clearly demonstrated the potential for practical applications of small bimetallic particles. What is of particular interest and importance is the fact that small homogeneous alloy particles can be formed from pairs of elements for which the bulk phase diagram shows a miscibility gap. This point did not receive emphasis in this Discussion, and it is unfortunate that theoreticians have not addressed the problem, the nature of which has recently been considered in the context of the platinum-gold pair (3). Discussion on the papers relating to catalysis focused on the utility of physical characterisation of catalysts before (or occasionally after) use in understanding their catalytic performance. It was noted that in many papers much more time and effort appeared to have been spent on the characterising than on the catalytic reaction, the

108

investigation of which was often brief and simple. Failure to consider why a particular structure or composition behaved as it did betrays a lamentable lack of curiosity, and retards the development of basic theory. The discussions, together with the texts of the papers, will be published in full by the Royal Society of Chemistry in 2008 (2); they should make interesting reading.

References 1

2

3

Faraday Discussion 138: Nanoalloys – From Theory to Applications, 3rd–5th September, 2007, University of Birmingham, U.K.: http://www.rsc.org/FD138 “Nanoalloys from Theory to Applications”, Faraday Discussions No 138, RSC Publishing, Cambridge, U.K., 2008: http://www.rsc.org/shop/books/2008/9780854 041190.asp G. C. Bond, Platinum Metals Rev., 2007, 51, (2), 63

The Author Geoffrey Bond held academic posts at Leeds and Hull Universities before joining Johnson Matthey PLC in 1962 as Head of Catalysis Research. In 1970 he was appointed Professor in Brunel University’s Chemistry Department, and is now Emeritus Professor.


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DOI: 10.1595/147106708X298287

Challenges in Catalysis for Pharmaceuticals and Fine Chemicals PLATINUM GROUP METALS IN CATALYTIC PROCESSES Reviewed by Chris Barnard Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, U.K.; E-mail: [email protected]

Around eighty participants with an interest in homogeneous catalysis came together in London on 6th November 2007 to discuss “Challenges in Catalysis for Pharmaceuticals and Fine Chemicals” at a symposium organised by the Society of Chemical Industry Fine Chemicals Group and the Royal Society of Chemistry Applied Catalysis Group. In response to increased focus on environmental issues and sustainability for the preparation of active pharmaceutical ingredients and fine chemicals, industry and academia are collaborating to identify catalytic processes that will reduce waste, energy demand and safety hazards by replacing the use of some stoichiometric reagents. This symposium highlighted some of the areas where progress is being made. A series of seven oral presentations was supported by a poster session where students presented their work on environmental improvements in chemistry. Content involving platinum group metals is described here.

Improvements in Pharmaceutical Manufacturing Peter Dunn (Pfizer, U.K.) reviewed the background to this meeting and in particular the role of the American Chemical Society Green Chemistry Institute Pharmaceutical Roundtable (1). This forum allows an exchange of information between the major pharmaceutical companies with regard to improving the sustainability of manufacturing for active pharmaceutical ingredients. A survey of the reactions used in such preparations (2) provided the background information for identifying those areas where improvements were most needed. The results of these further discussions were also published (3).


The topics were divided between the categories of common reactions requiring better reagents, and more aspirational reactions where new technology is required for industrial application. A priority ranking was used to highlight five examples in each category, for example, amide formation avoiding poor atom economy reagents was the item with the highest priority in the former category. The Roundtable has the capability to offer research grants to meet these objectives, with two programmes already operating (one in hydrogenation of amides and one in C–H activation for biaryl coupling) and further grants are on offer for 2008 and 2009. Rhodium-catalysed asymmetric hydrogenation is one of the most commonly used homogeneous catalysis processes in the pharmaceutical industry. Nevertheless, Johannes de Vries (DSM Pharmaceutical Products and University of Groningen, The Netherlands) noted that there are relatively few examples due to a number of obstacles: long process development time (requiring improved, automated screening); high cost (requiring cheaper metals, simpler ligands and greater catalyst turnover numbers); limited tolerance for substrate variations and lengthy or inefficient routes for substrate preparation. Improvements can be achieved through a modular approach to ligand design, allowing ligand libraries to be generated by automated equipment. Ligand design can thus become part of a wider, automated screening process. The development of chiral monophosphoramidite ligands, 1, (4), which have proved to give enantioselectivities equivalent to or even better than those of the well known chiral bisphosphine ligands in Rhcatalysed asymmetric hydrogenation, was used to illustrate this. Another elegant approach to the

110

1 1

2

R = R = Me (MonoPhos) 1 2 R = R = Et 1 2 R , R = –(CH2)5– (PipPhos) 1 2 R , R = –[(CH2)2]2O (MorfPhos) 1 2 R = H, R = (R)-CH Me Ph 1 2 i R = H, R = Pr

design of bidentate ligands is the use of supramolecular chemistry to bring two molecules together so that the donor atoms are brought into the correct positions for coordination. While much attention has been given to developing catalysis for particular substrates, often insufficient attention has been paid to the impact of preparation of these substrates on the overall process efficiency. Thus the benefits of the catalytic step may be nullified if the synthesis and purification of the substrate add extra steps. For example, the preparation of pure E- or Z-vinyl compounds is often difficult and the purification of imines may present an issue. This issue merits greater attention to prevent subsequent effort on the optimisation of the catalytic step being wasted. The lack of generality in application of catalytic methods can also be a limiting factor. For example, unfunctionalised olefins may give low enantioselectivity, and molecules containing sulfur donors may act as catalyst poisons. No effective catalyst is available for homogeneous hydrogenation of substituted aromatic compounds, and the more reactive heteroaromatic compounds generally give poor enantioselectivity. One area where synthesis of the substrate has a major impact on catalytic chemistry is the use of aryl boronic acid derivatives as coupling partners in palladium-catalysed biaryl synthesis (SuzukiMiyaura reaction). The process would be greatly


simplified if C–H activation of aromatic groups could be achieved directly. This was discussed by Robin Bedford (University of Bristol, U.K.). Significant progress in this area has been made for intramolecular systems, while, with the aid of directing groups, some intermolecular reactions can also be carried out effectively. The initial report of Pd-catalysed cyclisation of 2-substituted halogenoarenes to form dibenzofurans and related heterocycles (intramolecular 5-membered ring formation) appeared in 1984 (5). Statistically it is more difficult to form larger rings, but 7- and 8membered ring formation has now been reported. Bedford, himself, has shown how new rings can be formed by combining sequential Pdcatalysed aryl bromide and C–H activation reactions (6). The major challenges to the implementation of chemistry based on C–H activation are selectivity (both with regard to single versus multiple substitution and regioselectivity), catalyst loading/activity, and the complexity and corresponding high cost of the catalysts. However, progress is being made on each of these issues. Hydroformylation is a well-established chemistry in the bulk preparation of aldehydes. The development of the Rh-catalysed LP OxoSM process was recently reviewed in this Journal (7, 8). However, the reaction is little used in the pharmaceutical industry. Graham Meek (DowpharmaSM, U.K.) described his company’s efforts to make hydroformylation more attractive to the pharmaceutical industry. Having inherited Union Carbide’s expertise following that takeover, Dowpharma have further developed a range of bisphosphite ligands, particularly one known as BIPHEPHOS, 2, (9), that gives satisfactory rates and high yields of linear aldehydes for a wide variety of substrates. Reactions can typically be carried out at 3 bar pressure and 80ºC with a substrate:catalyst ratio of around 1000. The reactivity of the aldehyde product means that under certain circumstances tandem reactions can be carried out. Thus, for example, hydroaminomethylation (the combination of hydroformylation and reductive amination) may be used to synthesise secondary and tertiary amines from olefins.

111

MeO MeO O O O O O O

PP

O O O O P P O O

MeO MeO

BIPHEPHOS

2

For enantioselective hydroformylation, early work studied the reaction of styrene, with BINAPHOS a preferred ligand. More recently, chiral phosphites and diazaphospholanes have been developed by Dowpharma. The enantioselectivity of these reactions is often highly sensitive to minor structural changes in the ligand, so the availability of an extensive ligand library of these compounds is a distinct advantage. The synthesis of amides with minimal waste is a highly desired transformation. Andy Whiting (University of Durham, U.K.) described a range of studies aimed at achieving this from carboxylic acid and amine substrates. Because of the difficulties of this direct approach, various other routes to amides are being developed. For example, the reaction of amines and alcohols to form amides using ruthenium-pincer catalysts has been reported by Milstein (10). So far, the reaction has limited scope for substrates, being sensitive to steric hindrance at the α-position of either partner and not giving the desired product with secondary amines. Amides can also be formed by aminocarbonylation of aryl halides. Buchwald has recently reported (11) that earlier work on this Pd-catalysed reaction of aryl iodides and bromides can be extended to aryl chlorides in the presence of sodium phenoxide. The reaction proceeds via the initial formation of the phenyl ester, which then undergoes a phenoxide-catalysed acyl transfer reaction to form the amide.


The asymmetric addition of –NH and –OH functions to C=C bonds was reviewed by Mimi Hii (Imperial College London, U.K.). Alternative routes to the chiral amine and alcohol products include hydroboration or hydrogenation, but conditions for more direct reaction are desirable. The majority of the research has been carried out on the hydroamination reaction. As usual, the intramolecular reaction was performed first with the intermolecular enantioselective reaction being reported by Togni in 1997 (12) using iridium(I) catalysts, but later work by Hartwig (13) and by Hii (14) has shown that significantly improved results can be obtained with Pd catalysts. Thermodynamics provides a constraint for these conversions, since there is little driving force for the reaction (ΔG ~ 0). Only limited yields of products are possible in some cases. The reaction of activated alkenes is also known as the azaMichael reaction. In this case, reaction of primary aromatic amines has been achieved under mild conditions (< 60ºC in toluene). Compared to hydroamination, the addition of –OH has received little attention. Cyclisation by enantioselective –OH addition (an intramolecular, asymmetric version of the Wacker reaction) was first reported in 1981 (15) with modest enantiomeric excess. More recent work by Hayashi (16) and others has improved these results, but examples of this chemistry are still limited.

Poster Presentations In the poster presentations, Asma Qazi (Queen Mary University, London, U.K.) described various forms of Pd(II) immobilised on silica. While Pd coordination to sulfur donors gave materials with low leaching, they showed low activity in Suzuki coupling (catalyst loading ca. 5 mol%). Better activity was seen for an ethylphosphatrioxaadamantane Pd catalyst that was effective at 0.1 mol%. Direct arylation for heterocycles is often more efficient than for aryl compounds due to the strong directing effect of the heteroatom. The C-5 arylation of thiazoles was reported by Gemma Turner (University of Edinburgh, U.K.). This reaction can be carried out with Pd catalysis in only 12 hours at 60ºC

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using water as solvent. Nathan Owston (University of Bath, U.K.) reported on studies of metal-catalysed amide preparation from alcohols. Catalyst loadings can be significantly reduced by changing from an Ir catalyst to a Ru one, and the methodology has now been adapted to allow formation of a variety of acyl derivatives (e.g. esters). Xiaofeng Wu (University of Liverpool, U.K.) described a monotosylated ethylenediamine Ir(III) complex which can be used as catalyst for hydrogenation of aldehydes in water as solvent (using hydrogen gas or by transfer hydrogenation).

Concluding Remarks It is clear that improved catalytic chemistry will be required to replace wasteful stoichiometric routes for pharmaceuticals and fine chemicals in the future. Improvements to catalytic processes will involve adaptation to more readily available substrates, simpler ligand preparation and the minimisation of metal costs through lower catalyst loadings. It is hoped that developments towards these goals will provide the focus for further meetings on this topic in the future, to be organised by the Society of Chemical Industry and the Royal Society of Chemistry.

References 1 American Chemical Society Green Chemistry Institute Pharmaceutical Roundtable: http://por tal.acs.org/portal/PublicWebSite/greenchem istry/industriainnovation/roundtable/index.htm 2 J. S. Carey, D. Laffan, C. Thomson and M. T. Williams, Org. Biomol. Chem., 2006, 4, (12), 2337 3 D. J. C. Constable, P. J. Dunn, J. D. Hayler, G. R. Humphrey, J. L. Leazer, Jr., R. J. Linderman, K. Lorenz, J. Manley, B. A. Pearlman, A. Wells, A. Zaks and T. Y. Zhang, Green Chem., 2007, 9, (5), 411 4 D. J. Ager, A. H. M. de Vries and J. G. de Vries, Platinum Metals Rev., 2006, 50, (2), 54 5 D. E. Ames and A. Opalko, Tetrahedron, 1984, 40, (10), 1919 6 R. B. Bedford and M. Betham, J. Org. Chem., 2006, 71, (25), 9403 7 R. Tudor and M. Ashley, Platinum Metals Rev., 2007, 51, (3), 116

8 R. Tudor and M. Ashley, Platinum Metals Rev., 2007, 51, (4), 164 9 C. J. Cobley, R. D. J. Froese, J. Klosin, C. Qin and G. T. Whiteker, Organometallics, 2007, 26, (12), 2986 10 C. Gunanathan, Y. Ben-David and D. Milstein, Science, 2007, 317, (5839), 790 11 J. R. Martinelli, T. P. Clark, D. A. Watson, R. H. Munday and S. L. Buchwald, Angew. Chem. Int. Ed., 2007, 46, (44), 8460 12 R. Dorta, P. Egli, F. Zürcher and A. Togni, J. Am. Chem. Soc., 1997, 119, (44), 10857 13 M. Kawatsura and J. F. Hartwig, J. Am. Chem. Soc., 2000, 122, (39), 9546 14 K. Li, P. N. Horton, M. B. Hursthouse and K. K. (M.) Hii, J. Organomet. Chem., 2003, 665, (1–2), 250 15 T. Hosokawa, T. Uno, S. Inui and S. Murahashi, J. Am. Chem. Soc., 1981, 103, (9), 2318 16 Y. Uozumi, K. Kato and T. Hayashi, J. Am. Chem. Soc., 1997, 119, (21), 5063

The Reviewer Chris Barnard is a Scientific Consultant in the Liquid Phase Catalysis Group at the Johnson Matthey Technology Centre, U.K., with interests in homogeneous catalysis employing the platinum group metals. He is also interested in the application of platinum compounds as cancer therapy.


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DOI: 10.1595/147106708X297486

The Periodic Table and the Platinum Group Metals By W. P. Griffith Department of Chemistry, Imperial College, London SW7 2AZ, U.K.; E-mail: [email protected]

The year 2007 marked the centenary of the death of Dmitri Mendeleev (1834–1907). This article discusses how he and some of his predecessors accommodated the platinum group metals (pgms) in the Periodic Table, and it considers the placing of their three transuranic congeners: hassium (108Hs), meitnerium (109Mt) and darmstadtium (110Ds). Over twenty-five years ago McDonald and Hunt (1) wrote an excellent account of the pgms in their periodic context. This account is indebted to that work. The present article introduces new perspectives and shows some of the relevant tables. There are good books on the history of the Periodic Table, e.g. (2, 3) and other texts (4, 5) which provide a fuller picture than it is possible to give here.

Discovery and Early Classification of the Platinum Group Metals Antoine-Laurent Lavoisier (1743–1794) in 1789 defined the element as being “the last point that analysis can reach”, and it was largely this clear statement which brought about the discovery of 51 new elements in the nineteenth century alone. John Dalton’s (1766–1844) recognition in 1803 of the atom as being the ultimate constituent of an element, with its own unique weight, was crucial. Stanislao Cannizzaro (1826–1910), at the celebrated Karlsruhe Congress (1860), published a paper recognising the true significance of Avogadro’s molecular hypothesis and thereby clarified the difference between atomic and molecular weights. From then, reasonably accurate atomic weights of known elements became readily available and greatly helped the construction of useful Periodic Tables. Atomic (or elemental) weights were useful but were not a sine qua non for table construction. A number of tables were produced with incorrect values, or, as Mendeleev later noted, inconsistencies in published atomic weights became apparent from these tables. We have the benefit of hindsight and know that atomic numbers are crucial factors for periodicity. Platinum is a metal of antiquity, but the other five pgms were isolated in the nineteenth century. The bicentenaries of four were marked in this Journal: William Hyde Wollaston’s (1766–1828) discovery of


palladium and rhodium in 1802 and 1804 (6) and Smithson Tennant’s (1761–1815) isolation of iridium and osmium in 1804 (7, 8). Ruthenium was the last to be isolated, by Karl Karlovich Klaus (1796–1864) in 1844 (9–11). Thus, five of the six were known by 1804, and the sixth by 1844, in good time for the development of the Periodic Table. The pgms are now known to fall into two horizontal groups: Ru-Rh-Pd and Os-Ir-Pt, but we benefit from some 200 years of hindsight in this observation. Johann Döbereiner (1780–1849) noted similarities in the chemical behaviour of ‘triads’ of elements, in which the equivalent weight of the middle element lay roughly halfway between those of the other two. In 1829, when Professor of Chemistry at Jena, he used his equivalent weights for these metals (based on oxygen = 100) to demonstrate that Pt-IrOs and Pd-‘pluran’-Rh ‘triads’ existed (12). ‘Pluran’ had been reported together with two other ‘new’ elements in 1827 by Gottfried Osann (1796–1866). It may possibly have contained some ruthenium, but Berzelius was unable to confirm the novelty of these three elements, and Osannn subsequently withdrew his claim (13). In 1853 John Hall Gladstone (1827–1902), then a chemist at St. Thomas’s Hospital, London, noted that the Rh-Ru-Pd triad was related to that of Pt-Ir-Os, while the ‘atomic weights’ (sic) of the latter triad were roughly twice those of the former (14). In

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1857 William Odling (1829–1921), then teaching chemistry at Guy’s Hospital, London, noted the great similarity of Pd, Pt and Ru, that the ‘atomic weight’ (sic) of Pt (98.6) was about twice that of Pd (53.2), and that Pt, Ir and Os were chemically similar (15). The stage was now set for a periodic classification of these and indeed all the elements then known.

The Development of Periodic Classifications In 1862 Alexandre-Emile Béguyer de Chancourtois (1820–1886), Professor at the École des Mines, Paris, devised a ‘vis tellurique’ (telluric screw) (16), a helix on a vertical cylinder on which symbols of the elements were placed at heights proportional to their atomic weights. Although some pgms appeared on it (Rh and Pd on one incline and Ir and Pt on another), no relationships between them are discernible. Karl Karlovich Klaus, then professor of chemistry at the University of Kazan (now in Tatarstan), had discovered Ru in 1844 (9–11) and knew more about the pgms than anyone else. In 1860 he arranged the three most abundant ones in a Principal series (Haupt Reihe), and beneath them placed a Secondary series (Neben Reihe), noting also the chemical similarities of each vertical pair (17–19) (Figure 1 (18)). Klaus’s table shows the correct vertical pairs, but not in the now accepted sequence. The pgms were not set in the context of other elements. In 1864 the analytical chemist John Alexander Raina Newlands (1837–1898) proposed the first of his tables, arranging the known 61 elements in order of ascending atomic weights (20, 21). In his subsequent ‘law of octaves’ he noted that the chemical properties of some elements were repeated after each series of seven, and assigned ordinal numbers to elements in the sequence of their ascending atomic weights: an

early form of the atomic number (e.g. H = 1, Li = 2 etc.) (22). Although the pgms featured in Newlands’s tables they were often out of place. William Odling (born, like Newlands, in Southwark, London), whose pgm triads we have noted above (15), produced in 1864 a table of 61 elements in which the six pgms were grouped together (Ro is rhodium). He was the first to arrange them in a reasonably logical way in a Periodic Table (Figure 2) (23). The stage was now set for two giants of periodicity, Lothar Meyer and, above all, Dmitri Mendeleev. In 1868 Julius Lothar Meyer (1830–1895), Professor of Chemistry at Tübingen arranged 52 elements in an unpublished table with Ru & Pt, Rh & Ir, Pd & Os side-by-side. His slightly later table, published in 1870 (24), places the pgms correctly, but a number of other elements lie in a sequence different from that of modern tables: Mn = 54.8

Ru = 103.5

Os = 198.6?

Fe = 55.9

Rh = 104.1

Ir = 196.7

Co = Ni = 58.6 Pd = 106.2

Pt = 196.7

On 6th March, 1869, Dmitri Mendeleev (1834–1907) produced his first table (25, 26). Mendeleev was born in Tobolsk, Siberia, the last of fourteen children. His father became blind when Dmitri was sixteen, and his indomitable mother, determined that he should be well educated, hitchhiked with him on the 1400 mile journey to the University at Moscow. Here he was refused admittance because he was Siberian; they travelled a further 400 miles to St. Petersburg. There in 1850 Mendeleev got a job as a trainee teacher; his mother died from exhaustion in the same year. In 1866, after a spell of study in Germany (he had attended the 1860 Karlsruhe Congress) and France, Mendeleev became Professor of Chemistry at the University of St. Petersburg. Mendeleev’s interest in periodicity may well have dated from the Karlsruhe Congress and been

Fig. 1 Klaus’s arrangement of the platinum group metals of 1864 (18)


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Fig. 2 William Odling’s table of elements from 1864 (23)

cemented by a textbook on inorganic chemistry, part of which he finished in 1868. More than any of his predecessors in the field of periodicity, he had a remarkable knowledge of the chemistry of the elements. His first published version placed the pgms together but with unusual pairings (25, 26): Rh 104.4

Pt 197.4

Ru 104.4

Ir 198

Pd 106.6

Os 199

The version normally regarded as Mendeleev’s definitive table appeared in 1871, first printed in a Russian journal (27) and then reprinted in Annalen in the same year (Figure 3) (28). By then Mendeleev had seen Lothar Meyer’s paper and almost certainly knew of Newlands’s and Odling’s work, but his table represents a major advance in classification of the elements, for the first time placing the pgms in their modern sequence and in context. The dashes under the Ru-Rh-Pd-Ag listing under Group VIII misled some later workers to think that missing elements were being denoted (13). Acceptance of his table was


partly brought about by his astonishingly accurate predictions of the properties of the then unknown scandium (shown as ‘–- = 44’ in Figure 3), gallium ‘–- = 68’ and germanium ‘–- = 72’. Mendeleev’s predictions also led to the subsequent discovery of other elements including francium, radium, technetium, rhenium and polonium. Other factors such as the successful accommodation or placement of the elements were also important, a topic well discussed in a recent book (3). It is apparent from Mendeleev’s tables that for him (and others) the pgms, some of the transition metals, lanthanides and actinides then known posed a problem; here we concentrate on the pgms. He noted their very similar properties and that there were very small differences between the atomic weights of Ru-Rh-Pd and between those of Os-IrPt (28). He knew that only Ru and Os demonstrated octavalency in Group VIII (‘RO4’; R denotes an element), but includes Rh, Pd, Ir and Pt in Group VIII. Mendeleev also placed iron, cobalt and nickel, and the coinage metals copper, silver and gold in

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Fig. 3 Mendeleev's Periodic Table of 1871 (28)

Group VIII; he additionally accommodated the coinage metals in Group I. His problems with all his Group VIII elements continued to trouble him: as late as 1879 he published two papers in Chemical News which tried to address this difficulty (29, 30). In the first paper he split Groups I–VII into lefthand ‘even’ and right-hand ‘odd’ blocks, with Group VIII in the centre, Cu, Ag and Au being accommodated in both VIII and the ‘odd’ I–VII block (29). In the second paper he ruefully refers to Group VIII as ‘special’ and ‘independent’ (30). Mendeleev published some thirty Periodic Tables and left another thirty unpublished (3), but the 1871 one (Figure 3) (28) is his most successful: it is the definitive Periodic Table of the nineteenth century and the basis of all later ones. As late as 1988, the leading inorganic textbook “Advanced Inorganic Chemistry”, by Cotton and Wilkinson (fifth edition) (31) shows Group VIII as containing the nine elements Fe, Co, Ni and the pgms (Cu, Ag and Au are designated as Group IB). It was only in the sixth edition of 1999 that the modern form (Figure 4), in which the pgm vertical pairs are in Groups 8, 9 and 10, was used (32).

The Transuranic Congeners of the Platinum Group Metals The story now moves forward to the Second World War, when there was discussion as to whether uranium, neptunium and plutonium were


appropriately placed in the fourth row of the transition metals (using 6d orbitals), or were members of a lanthanide-like series, the ‘actinides’, using 5f orbitals. The latter view prevailed (33), and now all the actinides (thorium to lawrencium inclusive) are known. Indeed, elements up to and including 118 are now established, with the exception of element 117 (34). These elements are recognised by the International Union of Pure and Applied Chemistry (IUPAC), although only those up to 111 have ‘official’ names (Figure 4) (35); see also (36). Mendeleev’s table (28) omits most of the lanthanides and actinides and, of course, the noble gases which were not known when he made up his table. However, some 140 years earlier, his version had essentially contained the kernel of our modern Periodic Tables. Recent chemical work on a few very short-lived atoms of each element strongly suggests that elements 104 to 111 are members of a fourth transition metal series involving 6d orbitals. Thus 104 rutherfordium, 105dubnium, 106seaborgium and 107 bohrium have properties analogous to those of hafnium (Group 4), tantalum (Group 5), tungsten (Group 6) and rhenium (Group 7) respectively. The next three elements were all made in the linear accelerator in the city of Darmstadt, Hessen, Germany. Hassium was first made in 1984, and named from the Latin ‘Hassias’ for the state of Hessen. Meitnerium was first made in 1982, and

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Fig. 4 The current Periodic Table (35) based on IUPAC recommendations

named after Lise Meitner (1878–1968), the discoverer of protactinium in 1917. Darmstadtium was first made in 1994, and named after Darmstadt. For any meaningful chemistry to be carried out on a given element, at least four atoms are necessary, of half-life (t½) > 1 second, and a production rate of at least one atom per week is required. The nuclear reactions producing the elements should give only single products. For these three elements the most useful nuclear reactions are (Equations (i)–(iii)): 269, 270 1 Cm + 26 108Hs + 5 or 4 0 n 12Mg →

248 96

Bi + Fe →

209 83

58 26

266 109

1 0

Mt + n

271 1 Pb + 64 28Ni → 110 Ds + 0 n

208 82

(i) (ii) (iii)

Of these, 269Hs and 270Hs have t½ = 14 and 23 s respectively; 266Mt has t½ = 6 × 10–3 s and 271Dt has t½ = 6 × 10–2 s, so at present chemistry can only be carried out on hassium. It is clearly a congener of Os: using just seven atoms it was found to form a volatile tetroxide (37) which in alkaline NaOH gives a species which is probably cisNa2[HsO4(OH)2] (38). For studies on meitnerium and darmstadtium to be made, longer-lived isotopes are essential – they would also be much


more difficult to study chemically, since distinctive volatile Ir and Pt compounds are rare and difficult to synthesise on a very small scale, unlike HsO4, although the fluorides IrF6 and PtF6 are volatile above 60ºC. It seems likely, however, that these three elements are congeners of Os, Ir and Pt, particularly since it has recently been shown that the unnamed (at the time of writing) element 112 is itself volatile. This suggests that it is a congener of mercury (39), as would be expected if elements 104–111 inclusive form a fourth transition metal series.

Conclusions The story of the Periodic Table is convoluted, and this article has concentrated on the pgms. It is clear that they represented a challenge to the makers of the tables, but the problem was finally resolved by Mendeleev some 140 years ago (28). The three man-made congeners of these elements, hassium, meitnerium and darmstadtium, are likely to have chemistries similar to those of osmium, iridium and platinum. At the time of writing it has been possible to demonstrate this only for hassium.

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Acknowledgements I am grateful to Professor Christoph Düllmann (Gesellschaft für Schwerionenforschung mbH,

Darmstadt, Germany) and Dr Simon Cotton (Uppingham School, Rutland, U.K.) for their advice on aspects of transuranium chemistry.

References 1 D. McDonald and L. B. Hunt, “A History of Platinum and its Allied Metals”, Johnson Matthey, London, 1982, p. 333 2 J. W. van Spronsen, “The Periodic System of Chemical Elements: A History of the First Hundred Years”, Elsevier, Amsterdam, 1969 3 E. R. Scerri, “The Periodic Table: Its Story and Its Significance”, Oxford University Press, New York, U.S.A., 2007 4 M. E. Weeks and H. M. Leicester, “Discovery of the Elements”, 7th Edn., Journal of Chemical Education, Easton, Pennsylvania, U.S.A., 1968 5 W. H. Brock, “The Fontana History of Chemistry”, Fontana Press, London, 1992 6 W. P. Griffith, Platinum Metals Rev., 2003, 47, (4), 175 7 W. P. Griffith, Platinum Metals Rev., 2004, 48, (4), 182 8 M. Usselman, in “The 1702 Chair of Chemistry at Cambridge”, eds. M. D. Archer and C. D. Haley, Cambridge University Press, Cambridge, U.K., 2005, Chapter 5, p.113 9 C. Claus, Ann. Phys. Chem. (Poggendorff), 1845, 64, 192 10 C. Claus, Phil. Mag. (London), 1845, 27, 230 11 V. N. Pitchkov, Platinum Metals Rev., 1996, 40, (4), 181 12 J. W. Döbereiner, Ann. Phys. Chem. (Poggendorff), 1829, 15, 301 13 W. P. Griffith, Chem. Brit., 1968, 4, (10), 430 14 J. H. Gladstone, Phil. Mag., 1853, 5, (4), 313 15 W. Odling, Phil. Mag., 1857, 13, (4), 480 16 A. B. de Chancourtois, Compt. Rend. Acad. Sci., 1862, 54, 757, 840 and 967 17 C. Claus, J. Prakt. Chem., 1860, 79, (1), 28 18 C. Claus, J. Prakt. Chem., 1860, 80, (1), 282 19 C. Claus, Chem. News, 1861, 3, 194 and 297 20 J. A. R. Newlands, Chem. News, 1863, 7, 70 21 J. A. R. Newlands, Chem. News, 1864, 10, 59 and 94 22 J. A. R. Newlands, Chem. News, 1865, 12, 83 and 94 23 W. Odling, Quarterly J. Sci., 1864, 1, 642 24 L. Meyer, Ann. Chem. Pharm. (Leipzig), Supplementband VII, 1870, 354 25 D. Mendeleev, Zhur. Russ. Khim. Obshch., 1869, 1, 60 26 D. Mendelejeff, Z. Chem., 1869, 12, 405 27 D. Mendeleev, Zhur. Russ. Khim. Obshch., 1871, 3, 25

28 D. Mendelejeff, Ann. Chem. Pharm. (Leipzig), Supplementband VIII, 1871, 133 29 D. Mendeleef, Chem. News, 1879, 40, 231 30 D. Mendeleef, Chem. News, 1879, 40, 267 31 F. A. Cotton and G. Wilkinson, “Advanced Inorganic Chemistry: A Comprehensive Text”, 5th Edn., John Wiley & Sons, Chichester, U.K., 1988 32 F. A. Cotton, G. Wilkinson, C. A. Murillo and M. Bochmann, “Advanced Inorganic Chemistry”, 6th Edn., John Wiley & Sons, Chichester, U.K., 1999 33 G. T. Seaborg, Chem. Eng. News, 10th December, 1945, 23, (23), 2190 34 S. Cotton, “Lanthanide and Actinide Chemistry”, John Wiley & Sons, Chichester, U.K., 2006 35 Periodic Table, World Wide Web version prepared by G. P. Moss, London, U.K., 2007: http://www.chem.qmul.ac.uk/iupac/AtWt/table. html 36 IUPAC Periodic Table of the Elements, 2007: http://www.iupac.org/reports/periodic_table/ind ex.html 37 Ch. E. Düllmann, W. Brüchle, R. Dressler, K. Eberhardt, B. Eichler, R. Eichler, H. W. Gäggeler, T. N. Ginter, F. Glaus, K. E. Gregorich, D. C. Hoffman, E. Jäger, D. T. Jost, U. W. Kirbach, D. M. Lee, H. Nitsche, J. B. Patin, V. Pershina, D. Piguet, Z. Qin, M. Schädel, B. Schausten, E. Schimpf, H.-J. Schött, S. Soverna, R. Sudowe, P. Thörle, S. N. Timokhin, N. Trautmann, A. Türler, A. Vahle, G. Wirth, A. B. Yakushev and P. M. Zielinski, Nature, 2002, 418, (6900), 859 38 A. von Zweidorf, R. Angert, W. Brüchle, S. Bürger, K. Eberhardt, R. Eichler, H. Hummrich, E. Jäger, H.-O. Kling, J. V. Kratz, B. Kuczewski, G. Langrock, M. Mendel, U. Rieth, M. Schädel, B. Schausten, E. Schimpf, P. Thörle, N. Trautmann, K. Tsukada, N. Wiehl and G. Wirth, Radiochim. Acta, 2004, 92, (12), 855 39 R. Eichler, N. V. Aksenov, A. V. Belozerov, G. A. Bozhikov, V. I. Chepigin, S. N. Dmitriev, R. Dressler, H. W. Gäggeler, V. A. Gorshkov, F. Haenssler, M. G. Itkis, A. Laube, V. Ya. Lebedev, O. N. Malyshev, Yu. Ts. Oganessian, O. V. Petrushkin, D. Piguet, P. Rasmussen, S. V. Shishkin, A. V. Shutov, A. I. Svirikhin, E. E. Tereshatov, G. K. Vostokin, M. Wegrzecki and A. V. Yeremin, Nature, 2007, 447, (7140), 72

The Author Bill Griffith is an Emeritus Professor of Chemistry at Imperial College, London. He has much experience with the platinum group metals, particularly ruthenium and osmium. He has published over 260 research papers, many describing complexes of these metals as catalysts for specific organic oxidations. He has written seven books on the platinum metals, and is currently writing another on oxidation catalysis by ruthenium complexes. He is the Secretary of the Historical Group of the Royal Society of Chemistry.


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DOI: 10.1595/147106708X298836

Frederick A. Lewis AN APPRECIATION

Frederick Alastair Lewis (Courtesy of Donetsk National Technical University, Ukraine)

Frederick Alastair Lewis died in Belfast, Northern Ireland, on 22nd May 2007 after a long illness. He had been a frequent contributor to Platinum Metals Review. Born in Belfast in 1926, Fred served in the Royal Navy during the Second World War. After his naval service (1944–47), he began the study of Chemistry at Queen’s University, Belfast, which culminated in his Ph.D. degree under the direction of Professor A. R. Ubbelohde. He subsequently carried out postdoctoral studies at Imperial College, London, again under the direction of Professor Ubbelohde. While at Imperial College, Fred and Ubbelhohde coauthored the book, “Graphite and Its Crystalline Compounds” (1). In 1956 Fred returned to Queen’s University as a Lecturer and later became a Reader in Inorganic Chemistry.


During his career at Queen’s, Fred carried out research on metal-hydrogen systems and specifically on palladium and its alloys. His 33 contributions to Platinum Metals Review (2–34) included numerous review articles; his first, ‘The Hydrides of Palladium and Palladium Alloys’, was published in 1960 (2) and his last in 2003 (33), ‘Uphill Effects on Hydrogen Diffusion Coefficients in Pd 0.77Ag 0.23 Alloy Membranes’, coauthored with Tong, Cermák and Bell. His paper of 1982, ‘The PalladiumHydrogen System’ (11) is particularly frequently cited. Fred especially enjoyed interacting with colleagues from foreign countries, and collaborating in research with them. During the “Iron Curtain” period, J. Cermák (Institute of Physics, Academy of Science, Prague, Czech Republic), R. Bucur (Institute of Stable Isotopes, Cluj, Romania) and B. Baranowski (Institute of Physical Chemistry, Academy of Science, Warsaw, Poland) collaborated with Fred, at Queen’s and at their home institutions. During that time, scientific exchanges with Eastern European scientists provided valuable contacts for both the Eastern and Western European scientists. Fred also collaborated with scientists from other countries, for example, F. Mazzolai (Department of Physics, University of Perugia, Italy), Y. Sakamoto (Department of Material Science and Engineering, Nagasaki University, Japan), X.-Q. Tong (Department of Materials Science, Tsinghua University, Beijing, China) and K. Kandasamy (Department of Physics, University of Jaffna, Sri Lanka). Fred played a unique role by attending conferences which were often not well attended by other Western European scientists. Some examples were the “First International Workshop on Stress and Diffusion”, Balatonfüred, Hungary,

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1995 (25), “Hypothesis III” symposium, in St. Petersburg, Russia, 1999 (31), “Hydrogen Treatment of Materials”, HTM-2001, DonetskMariupol, Ukraine, 2001 (32), and the “Second International Symposium on Safety and Economy of Hydrogen Transport”, IFSSEHT2003, Sarov, Russia, 2003 (34). Useful reports on these conferences by Fred appeared in Platinum Metals Review (25, 31, 32, 34). Fred participated in the “Hydrogen Treatment of Materials” conferences held in Donetsk, Ukraine, organised triannually since 1995 by Professor V. A. Goltsov of the Donetsk State Technical University, Ukraine (26, 30, 32). Professor Goltsov is a recognised authority on H diffusion in Pd and other metals. Professor Goltsov has referred to the phenomenon of uphill diffusion of H in Pd as the ‘Lewis Effect’ because it was first observed and characterised by Fred Lewis and his coworkers (35). Fred first observed this phenomenon in 1983, he realised its importance, and, with international coworkers, X.-Q. Tong, R. Burcur, B. Baranowski, K. Kandasamy, Y. Sakamoto and others, actively pursued it until his retirement. An official award was given to Fred in 2001 by the International Association for Hydrogen Energy (IAHE) and the Permanent Working International Scientific Committee on Hydrogen Treatment of Materials (PWISC HTM) in recognition of the Lewis Effect (36). The book “The Palladium-Hydrogen System” was written by F. A. Lewis and published in 1967 by Academic Press (37). Palladium alloys and isotopes of hydrogen were also included in the book which has continued

Award of gold medal and diploma to Fred Lewis on behalf of the IAHE and PWISC HTM in 2001 (Courtesy of Donetsk National Technical University, Ukraine)


as a valuable reference forty years after its publication. Fred was very meticulous about citing references properly which makes this book and his many review articles valuable for searches of the literature. In 2000, Fred with Kandasamy and Tong published a book-length review (295 pages), entitled ‘Platinum- and PalladiumHydrogen’, in “Hydrogen in Metal Systems II”, Solid State Phenomena, Scitec Publications, Zürich (38). This constituted a useful update of his earlier book. Fred and A. Aladjem edited the series of reviews, Parts I and II, which included his article with Kandasamy and Tong. Reviews by other scientists in this series covered most metals which absorb hydrogen. In 1985 Fred organised an “International Symposium on Hydrogen in Metals” held in Belfast (39). The Symposium was well attended and considered to be very successful. The proceedings were published in Zeitschrift für Physikalische Chemie Neue Folge (40), and then as a book (41). The editor of the journal at that time was Ewald Wicke, Director of the Institute of Physical Chemistry, University of Münster, Germany. Fred and Professor Wicke collaborated in the editing of the symposium papers for the published proceedings in the Zeitschrift. A long friendship developed between the two scientists as a result of this collaboration. Fred will be missed by his many scientific friends because he was an excellent scientist and a jovial, kind-hearted person. TED B. FLANAGAN

References 1 A. R. Ubbelohde and F. A. Lewis, “Graphite and Its Crystalline Compounds”, Oxford University Press, Oxford, U.K., 1960 2 F. A. Lewis, ‘The Hydrides of Palladium and Palladium Alloys’, Platinum Metals Rev., 1960, 4, (4), 132 3 F. A. Lewis, ‘The Hydrides of Palladium and Palladium Alloys’, Platinum Metals Rev., 1961, 5, (1), 21 4 F. A. Lewis, ‘The Hydrided Palladium Electrode’, Platinum Metals Rev., 1962, 6, (1), 22 5 F. A. Lewis, ‘Novel Method of Electrical Storage’, Platinum Metals Rev., 1963, 7, (3), 88 6 F. A. Lewis, ‘Hydrogen in Palladium and its Alloys’, Platinum Metals Rev., 1968, 12, (4), 140

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7 F. A. Lewis, ‘Absorption of Hydrogen by Palladium Alloys’, Platinum Metals Rev., 1970, 14, (4), 131 8 R. Burch and F. A. Lewis, ‘The Form of the Interaction between Palladium and Hydrogen’, Platinum Metals Rev., 1971, 15, (1), 21 9 F. A. Lewis, ‘Current Research on the PalladiumHydrogen System’, Platinum Metals Rev., 1977, 21, (4), 134 10 F. A. Lewis, ‘Platinum Group Metal Hydrides’, Platinum Metals Rev., 1980, 24, (3), 102 11 F. A. Lewis, ‘The Palladium-Hydrogen System’, Platinum Metals Rev., 1982, 26, (1), 20 12 F. A. Lewis, ‘The Palladium-Hydrogen System’, Platinum Metals Rev., 1982, 26, (2), 70 13 F. A. Lewis, ‘The Palladium-Hydrogen System’, Platinum Metals Rev., 1982, 26, (3), 121 14 F. A. Lewis, ‘Hydrogen in Palladium and Its Alloys’, Platinum Metals Rev., 1984, 28, (1), 13 15 F. A. Lewis, K. Kandasamy and B. Baranowksi, ‘The “Uphill” Diffusion of Hydrogen’, Platinum Metals Rev., 1988, 32, (1), 22 16 F. A. Lewis, ‘Hydrogen in Amorphous Palladium Alloys’, Platinum Metals Rev., 1988, 32, (2), 83 17 R.-A. McNichol and F. A. Lewis, ‘The Hydride Phase Miscibility Gap in Palladium-Rare Earth Alloys’, Platinum Metals Rev., 1990, 34, (2), 81 18 F. A. Lewis, X. Q. Tong and R. V. Bucur, ‘Permeation of Hydrogen through Palladium-Silver Membranes’, Platinum Metals Rev., 1991, 35, (3), 138 19 F. A. Lewis, ‘Metal-Hydrogen Systems and the Hydrogen Economy’, Platinum Metals Rev., 1992, 36, (4), 196 20 F. A. Lewis, ‘Hydrogen Interstitial Structures in Palladium-Silver Membranes’, Platinum Metals Rev., 1993, 37, (4), 220 21 F. A. Lewis, ‘Hydrogen Material Science and Metal Hydride Chemistry’, Platinum Metals Rev., 1994, 38, (1), 20 22 F. A. Lewis, ‘Palladium-Hydrogen System’, Platinum Metals Rev., 1994, 38, (3), 112 23 F. A. Lewis, ‘International Conference on “Noble and Rare Metals”’, Platinum Metals Rev., 1995, 39, (1), 29 24 F. A. Lewis, ‘Fundamentals and Applications of Metal-Hydrogen Systems’, Platinum Metals Rev., 1995, 39, (2), 75 25 F. A. Lewis, ‘First International Workshop on Diffusion and Stresses’, Platinum Metals Rev., 1995, 39, (3), 127

26 F. A. Lewis, ‘Hydrogen Treatment of Materials’, Platinum Metals Rev., 1996, 40, (1), 36 27 F. A. Lewis, ‘Problems of the Palladium-Hydrogen System’, Platinum Metals Rev., 1996, 40, (4), 180 28 F. A. Lewis, ‘Rapid Hydrogen Permeation in Palladium and Palladium Alloys’, Platinum Metals Rev., 1997, 41, (1), 33 29 F. A. Lewis, ‘Platinum Metals Involvement in the Hydrogen Economy’, Platinum Metals Rev., 1997, 41, (4), 163 30 F. A. Lewis, ‘Hydrogen Treatment of Materials’, Platinum Metals Rev., 1998, 42, (3), 99 31 F. A. Lewis, ‘Advancements in Hydrogen Technology’, Platinum Metals Rev., 1999, 43, (4), 166 32 F. A. Lewis, ‘Hydrogen Treatment of Materials’, Platinum Metals Rev., 2001, 45, (3), 130 33 X. Q. Tong, F. A. Lewis, S. E. J. Bell and J. Cermák, ‘Uphill Effects on Hydrogen Diffusion Coefficients in Pd77Ag23 Alloy Membranes’, Platinum Metals Rev., 2003, 47, (1), 32 34 F. A. Lewis, ‘Hydrogen Economy Forum in Russia’, Platinum Metals Rev., 2003, 47, (4), 166 35 F. A. Lewis, J. P. Magennis, S. G. McKee and P. J. M. Ssebuwufu, Nature, 1983, 306, (5944), 673 36 V. A. Goltsov and T. N. Veziroglu, ‘A Step on the Road to Hydrogen Civilization, The “HTM-2001” review’, in The Third International Conference “Hydrogen Treatment of Materials”, Donetsk National Technical University, Ukraine, 14th–18th May, 2001: http://donntu.edu.ua/hydrogen/conf.html 37 F. A. Lewis, “The Palladium-Hydrogen System”, Academic Press, London, New York, 1967 38 F. A. Lewis, K. Kandasamy and X. Q. Tong, ‘Platinum and Palladium-Hydrogen’, in “Hydrogen in Metal Systems II”, eds. F. A. Lewis and A. Aladjem, Solid State Phenomena, Vols. 73–75, Scitec Publications, Zürich, Switzerland, 2000, Chapter 6, pp. 207–501 39 P. A. Sermon, ‘Symposium on Hydrogen in Metals’, Platinum Metals Rev., 1985, 29, (3), 115 40 F. A. Lewis and E. Wicke (Eds.), Z. Phys. Chem. Neue Folge, 1985, 143, 1–254; 1985, 145, 1–278; 1985, 146, 137–242; and 1986, 147, 1–290 41 “Hydrogen in Metals”, eds. F. A. Lewis and E. Wicke, R. Oldenbourg Verlag, Munchen, Germany, 1986, Proceedings of the International Symposium on Hydrogen in Metals, 26th–29th March, 1985, Queen’s University of Belfast, Northern Ireland

The Author Ted Flanagan was a postdoctoral worker with Fred Lewis at Queen’s University, Belfast, from 1957 to 1959; he had an enjoyable and productive time. The collaboration with Fred resulted in twelve papers in the area of H2 absorption by Pd and its alloys. After that, Flanagan spent two years at Brookhaven National Laboratory, Upton, New York, and then joined the University of Vermont, Burlington, Vermont, where he has remained. Flanagan’s main interests have been in the area of metal, alloy and intermetallic-H systems. Following in the footsteps of his collaboration with Fred Lewis, his special interests have been Pd- and Pd alloy-H systems. Flanagan received his B.S. from the University of California, Berkeley and his Ph.D. from the University of Washington, Seattle. He subsequently received a Ph.D. honoris causa from Uppsala University (1992) for his research on H in Pd-based compounds such as Pd6P. He is on the Editorial Advisory Board of the Journal of Alloys and Compounds and the Materials Science Forum. Although he is now an Emeritus Professor, he has continued his research on metal-H systems.


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DOI: 10.1595/147106708X292508

Fuel Cell Today Industry Review 2008 “FUEL CELLS: COMMERCIALISATION” The commercialisation of fuel cells started in 2007, according to Fuel Cell Today in its 2008 Industry Review, released on 30th January. The Review, titled “Fuel Cells: Commercialisation”, reports that in the last year the fuel cell industry has seen a growth of 75 per cent in new units delivered, with some 12,000 new fuel cell units shipped during 2007. Fuel Cell Today believes that the current global manufacturing capability for fuel cells is around 100,000 units per annum, with a quarter of this coming from companies whose business activity is exclusively the development of hydrogen and fuel cell technologies. The Fuel Cell Industry Review aims to provide a concise and accurate summary of worldwide fuel cell activity. Alongside information on legislation, finance, applications and key fuel cell companies, the Review publishes, for the first time, the Fuel Cell Today analysts’ forecasts of fuel cell shipments for the next two years. The forecasts include data by geographical region, fuel cell technology type and end use application. According to the Review, the last three years have seen the commercialisation of a number of fuel cell products at the luxury end of the market. Currently, fuel cells are relatively expensive and a number of issues are still outstanding in terms of research, development and demonstration (RD&D), codes and standards, and fuel infrastructure/distribution. However, price reductions are expected as manufacturing costs fall and subsidies for adoption become available.

The Review shows that worldwide government funding for RD&D topped £500 million (U.S.$1000 million) during 2007, with seven countries making up £400 million (U.S.$800 million) of this. Government funding has helped to support development of fuel cells for stationary and transport applications, while funding for portable fuel cells has come mainly from the private sector. Fuel Cell Today believes that the current commercial opportunities for fuel cells favour the low-temperature electrolytes, direct methanol fuel cells (DMFCs) and proton exchange membranes (PEMs), with over 98 per cent of manufacturing today being low-temperature units. The cost of PEM products currently varies from £1500 (U.S.$3000) per kW for a 5 kW unit up to £17,000 (U.S.$34,000) per kW for a micro 100 W fuel cell. Annual cost reductions of between 10 and 20 per cent are currently being reported. Dr Kerry-Ann Adamson, Principal Analyst at Fuel Cell Today, said: “Fuel cells are starting the process of becoming a mainstream market technology and although this will not be completed until well after the period under scrutiny in this report, commercialisation has finally begun”. The Fuel Cell Today Industry Review (ISBN: 978-0-9557963-0-2; ISSN: 1756-3186), priced at £500 (U.S.$1000), is available to order from http://www.fuelcelltoday.com/events/industryreview. For more information please contact Dr Kerry-Ann Adamson: [email protected].

Horizon Fuel Cell Technologies’ H-racer and refuelling set. Horizon came to the attention of the wider public in 2006 with the release of this toy-scale, six-inch long fuel cell car, which circumvents the need for an external refuelling infrastructure by coming complete with a miniature hydrogen production plant powered by a solar cell (Courtesy of Horizon Fuel Cell Technologies)

Platinum Metals Rev., 2008, 52, (2), 123

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DOI: 10.1595/147106708X300535

ABSTRACTS CATALYSIS – APPLIED AND PHYSICAL ASPECTS

CATALYSIS – REACTIONS

FT-IR Investigation of NOx Storage Properties of Pt–Mg(Al)O and Pt/Cu–Mg(Al)O Catalysts Obtained from Hydrotalcite Compounds S. MORANDI, F. PRINETTO, G. GHIOTTI, M. LIVI and A. VACCARI,

Microporous Mesoporous Mater., 2008, 107, (1–2), 31–38

The NOx storage capability upon admission of NO and NO2 with or without excess O2 at ≤ 623 K was investigated by in situ FT-IR for the title catalysts. Pure NO2 is adsorbed by a dismutation reaction with simultaneous formation of nitrates and nitrites that evolve to nitrates (“dismutation route”); nitrite evolution is promoted by the metal phase. When metal phase is present, the nitrites’ oxidation is further accelerated by O2. When O2 is present, NO is stored by: (a) oxidation to nitrites followed by their oxidation to nitrates; or (b) oxidation to NO2, followed by the “dismutation route”. (a) and (b) are promoted by the metal phases. Complete Oxidation of Methane over Palladium Supported on Alumina Modified with Calcium, Lanthanum, and Cerium Ions B. STASINSKA, W. GAC, T. IOANNIDES and A. MACHOCKI , J. Nat. Gas Chem., 2007, 16, (4), 342–348

The supports for Pd/Al2O3 and Pd/(Al2O3 + MOx) (M = Ca, La, Ce) were prepared by a sol-gel method. They were dried either conventionally or with sc-CO2, and then impregnated with Pd nitrate solution. The introduction of Ca, La or Ce oxide caused a decrease of the surface area, dependent on the support precursor drying method. These modifiers decreased the activity of the Pd catalysts for CH4 oxidation. Improvement of the Pd activity by La and Ce support modifier was observed only at low temperatures. Carbon Microsphere Supported Pd Catalysts for the Hydrogenation of Ethylene K. C. MONDAL, L. M. CELE, M. J. WITCOMB and N. J. COVILLE,

Catal. Commun., 2008, 9, (4), 494–498

C microspheres were prepared from acetylene at 800ºC. The microspheres were loaded with 2% Pd, both before and after H2SO4/HNO3 acid treatment. The acid-treated C microsphere-supported Pd catalyst exhibited better ethylene hydrogenation activity. Selective Oxidation of Styrene to Acetophenone over Supported Au–Pd Catalyst with Hydrogen Peroxide in Supercritical Carbon Dioxide X. WANG, N. S. VENKATARAMANAN, H. KAWANAMI IKUSHIMA, Green Chem., 2007, 9, (12), 1352–1355

and Y.

Selective oxidation of styrene to acetophenone was carried out over supported Pd-Au catalysts with H2O2 in sc-CO2. The Al2O3 support showed the best catalytic performance. The presence of the sc-CO2 improved the oxidation of styrene to acetophenone and inhibited the formation of byproducts.


Stille Cross-Coupling Reaction Using Pd/BaSO4 as Catalyst Reservoir A. V. COELHO, A. L. F. DE SOUZA, P. G. DE LIMA, J. L. WARDELL and O. A. C. ANTUNES, Appl. Organomet. Chem., 2008, 22, (1),

39–42

Stille cross-couplings of iodobenzene and tributylphenyltin were achieved in EtOH/H2O using different amounts of Pd/BaSO4 as catalyst reservoir in a ligandfree system. The catalyst was reused up to three times with some loss in activity. Filtration of the catalyst and product extraction gave a solution that kept its activity, indicating that Pd(0)/Pd(II) are the catalytic species. α-Arylation of Ketones Using Highly Active, AirStable (DtBPF)PdX2 (X = Cl, Br) Catalysts G. A. GRASA

and T. J. COLACOT, Org. Lett., 2007, 9, (26),

5489–5492

α-Arylation of ketones with aryl chlorides and bromides using (DtBPF)PdX2 (X = Cl, Br) catalysts gave 80–100% yield of the coupled products under relatively mild conditions at low catalyst loadings. The X-ray structure determination of (DtBPF)PdCl2 revealed the largest P–Pd–P bite angle (104.2º) for a ferrocenyl bisphosphine ligand. 31P NMR monitoring of the (DtBPF)PdCl2-catalysed reaction of 4-chlorotoluene with propiophenone indicated that the DtBPF remained coordinated in a bidentate mode. C-C Coupling Reaction of Triphenylbismuth(V) Derivatives and Olefins in the Presence of Palladium Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes Y. B. MALYSHEVA, A. V. GUSHCHIN, Y. MEI, Y. LU, M. BALLAUFF, S. PROCH and R. KEMPE, Eur. J. Inorg. Chem., 2008, (3),

379–383

C–C couplings were carried out at 50ºC using Ph3BiX2 (X = O2CH, O2CMe, O2CEt, O2CnPr, O2CnBu, O2CtBu, O2CPh, O2CCH2Cl, O2CCCl3, O2CCF3) and a range of olefins in the presence of Pd nanoparticles immobilised in spherical polyelectrolyte brushes (Pd@SPB). Ph3Bi(O2CCF3)2 was the most active. This route allows the formation of Heck-type products without the addition of base. A very low Pd loading was used. Ruthenium-Based NHC-Arene Systems as RingOpening Metathesis Polymerisation Catalysts N. LEDOUX, B. ALLAERT and F. VERPOORT, Eur. J. Inorg. Chem., 2007, (35), 5578–5583

The coordination of the standard NHC ligand H2IMes to [(p-cymene)RuCl2]2 was established to be unattainable, so bidentate NHC analogues were synthesised instead. These analogues are O-hydroxyaryl-substituted NHCs, capable of binding with the metal centre through the O atom as well as through the carbene C atom. Their chelating properties improved the stability of the corresponding Ru arene complexes.

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FUEL CELLS Synergistic Effect of CeO2 Modified Pt/C Electrocatalysts on the Performance of PEM Fuel Cells H. XU and X. HOU,

Int. J. Hydrogen Energy, 2007, 32, (17),

4397–4401

Pt/C electrocatalysts were modified by CeO2 with sol-gel and dipping processes. TEM and CV results showed that some Pt active surfaces were covered by CeO2, the electrochemical surface area of modified Pt/C was less than that of the unmodified one, and the sol-gel process covered less electrocatalyst surface area than the dipping process. Using modified Pt/C as a cathode electrocatalyst enhanced performance. Synthesis and Characterization of Electrodeposited Ni–Pd Alloy Electrodes for Methanol Oxidation K. S. KUMAR, P. HARIDOSS and S. K. SESHADRI, Surf. Coat. Technol., 2008, 202, (9), 1764–1770

A wide compositional range of Ni-Pd alloy electrocatalysts (1) were prepared by electrodeposition for use as anode materials for DMFCs in alkaline conditions. As-plated (1) were nanocrystalline, single phase, f.c.c. materials, indicating the formation of a complete solid solution in the alloy. Compositional analysis of the alloys indicated that the Pd composition increased with decrease in current density. (1) were active for MeOH oxidation in alkaline medium. Pd-Co Carbon-Nitride Electrocatalysts for Polymer Electrolyte Fuel Cells V. DI NOTO, E. NEGRO, S. LAVINA, S. GROSS

and G. PACE,

Electrochim. Acta, 2007, 53, (4), 1604–1617

Two groups of materials with the formula Kn[PdxCoyCzNlHm] were synthesised with: (a) molar ratio y:x > 1; and (b) molar ratio y:x < 1. Vibrational studies revealed that (a) and (b) systems consisted of two polymorphs of α- and graphitic-like CN nanomaterials. The electrochemical performance of the Pd-Co-CNs of (a) obtained at tf ≥ 700ºC was higher than that measured for a Pt-based commercial electrocatalyst in terms of both activity towards the O reduction and H oxidation reactions; also the resistance towards poisoning by MeOH.

Synthesis of Colloidal Particles of Poly(2vinylpyridine)-Coated Palladium and Platinum in Organic Solutions under the High Temperatures and High Pressures M. HARADA, M. UEJI and Y. KIMURA, Colloids Surf. A: Physicochem. Eng. Aspects, 2008, 315, (1–3), 304–310

Colloidal dispersions of PVP-coated Pd and Pt particles in toluene/1-propanol were synthesised by the decomposition and reduction of Pd(acac)2 and Pt(acac)2 respectively under high-temperature and high-pressure conditions. At 473 K and 25 MPa, colloidal dispersions ([Pd] = 7.5 mM) of Pd particles of average diameter 1.9 nm with narrow particle size distributions, were synthesised within seconds. Pt particles (average diameter of 2.1 nm) were also obtained. Synthesis of Palladium Nanowire Arrays with Controlled Diameter and Length G. KARTOPU, S. HABOUTI and M. ES-SOUNI, Mater. Chem. Phys., 2008, 107, (2–3), 226–230

Pd nanowire (NW) arrays were synthesised using porous alumina templates and direct current electrodeposition. The electrolyte was K2PdCl4 in H2SO4. Final pore sizes of the alumina templates were ~ 65 and 35 nm. A high filling rate (> 90%) was obtained using 65 μm thick templates. The NWs synthesised in 65 nm pores were polycrystalline and textured, but those in 35 nm pores were single crystalline. The alumina template was dissolved away, leaving self-standing NWs supported on a conductive thin film. Creep Deformation Mechanisms in Ru-Ni-Al Ternary B2 Alloys F. CAO and T. M. POLLOCK, Metall. Mater. Trans. A, 2008, 39,

(1), 39–49

The creep behaviour of five Ru-Ni-Al alloys with compositions across the ternary RuAl-NiAl B2 phase field was studied within the range 1223–1323 K. These alloys exhibited exceptional creep strength compared to other high melting temperature intermetallics. A continuous increase of the melting temperature and creep resistance with increasing Ru:Ni ratio was observed.

APPARATUS AND TECHNIQUE METALLURGY AND MATERIALS Infrared Spectroscopy of Physisorbed and Chemisorbed N2 in the Pt(111)(3×3)N2 Structure

Temperature-Independent Ceria- and Pt-Doped Nano-Size TiO2 Oxygen Lambda Sensor Using Pt/SiO2 Catalytic Filter and Y. MORTAZAVI, Sens. Actuators B: Chem., 2008, 129, (1), 47–52

K. GUSTAFSSON, G. S. KARLBERG and S. ANDERSSON, J. Chem. Phys., 2007, 127, (19), 194708 (6 pages)

F. HAGHIGHAT, A. KHODADADI

The adsorption of N2, at 30 K, on Pt(111) and Pt(111)(1×1)H surfaces was investigated using IR spectroscopy and LEED. The IR spectra revealed that N2 exclusively physisorbed on the Pt(111)(1×1)H surface, whereas both physisorbed and chemisorbed N2 were detected on the Pt(111) surface. Physisorbed N2 was the majority species in the latter case, and the two adsorption states showed an almost identical uptake behaviour, which indicates that they are intrinsic constituents of the growing (3×3) N2 islands.

The overlap of TiO2-based O2 sensor responses in the rich and lean regions was eliminated by using a 1.0 wt.% Pt/SiO2 catalytic filter (1) located in front of the sensors. Nanosized TiO2 was prepared by a microemulsion method and then doped with 1.0 wt.% Pt and 10.0 wt.% CeO2 by an impregnation method. The sensor was exposed to synthetic exhaust gases with λ values in the range 0.8–1.4. All of the sensors showed low-high transitions at about λ = 1.0. By using (1) only CO or O2 reaches the sensors.


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Electrochemical DNA Biosensors Based on Palladium Nanoparticles Combined with Carbon Nanotubes Z. CHANG, H. FAN, K. ZHAO, M. CHEN, P. HE

and Y. FANG,

Electroanalysis, 2008, 20, (2), 131–136

MWCNTs and Pd nanoparticles were dispersed in Nafion, and used to modify a GCE. Oligonucleotides with amino groups at the 5' end were covalently linked to carboxylic groups of MWCNTs on the electrode. The hybridisation events were monitored by differential pulse voltammetry using methylene blue as an indicator. The detection limit of the method for target DNA was 1.2 × 10–13 M. Hydrogen Permeation of Thin, Free-Standing Pd/Ag23% Membranes Before and After Heat Treatment in Air A. L. MEJDELL, H. KLETTE, A. RAMACHANDRAN, A. BORG and R. BREDESEN, J. Membrane Sci., 2008, 307, (1), 96–104

The title membranes (thicknesses ~ 1.3–5.0 μm) were produced by magnetron sputtering. Thermal treatment in air at 300ºC significantly enhanced their H2 flux. The permeability values became fairly similar after treatment, indicating bulk diffusion was the main rate-limiting step for H2 flux. The effect on permeation was found to depend on the membrane thickness, with less enhancement for the ~ 5.0 μm thick membranes. The treated samples had higher surface roughness, larger surface area and larger surface grains.

PHOTOCONVERSION Aggregation-Induced Phosphorescent Emission (AIPE) of Iridium(III) Complexes Q. ZHAO, L. LI, F. LI, M. YU, Z. LIU, T. YI and C. HUANG,

Chem.

Commun., 2008, (6), 685–687

An AIPE was observed for Ir(ppy)2(DBM) and Ir(ppy)2(SB) (DBM = 1,3-diphenyl-1,3-propanedione, SB = 2-(naphthalen-1-yliminomethyl)phenol). These Ir(III) complexes, in powder form, exhibited moderately intense emissions. Furthermore, addition of non-solvent H2O into dilute MeCN solutions can turn on their photoluminescent emission. Ultrafast Luminescence in Ir(ppy)3 G. J. HEDLEY, A. RUSECKAS and I. D. W. SAMUEL,

Chem. Phys.

Lett., 2008, 450, (4–6), 292–296

For Ir(ppy)3, an emission with a lifetime component of 230 fs in the spectral region 500–560 nm is assigned to the population equilibration between electronic substates of the lowest excited triplet state, with energy dissipation by intramolecular vibrational redistribution. At shorter wavelengths a strong emission with a faster decay was observed, which is attributed to a state with a higher admixture of singlet character. A slower decay on a 3 ps timescale is attributed to vibrational cooling.

SURFACE COATINGS Platinum OMCVD Processes and Precursor Chemistry Coord. Chem. Rev., 2008, 252,

BIOMEDICAL AND DENTAL

C. THURIER and P. DOPPELT,

On the Hydrolysis Mechanism of the SecondGeneration Anticancer Drug Carboplatin

Organometallic chemical vapour deposition (OMCVD) allows the formation of Pt thin films as a fine dispersion of Pt particles. Pt precursors having good volatility and a good thermal stability window are available. The best systems are MeCpPtMe3 and EtCpPtMe3, the latter being O2- and H2O-stable at ambient temperature. (cod)Pt(Me)2 is less volatile but it is easily synthesised in high yield. These precursors benefit from facile decomposition under the CVD conditions. Decomposition is rapid in the presence of O2(g) or H2(g). Films can be obtained with only traces of impurities, C being the most common. (97 Refs.)

M. PAVELKA, M. F. A. LUCAS and N. RUSSO, Chem. Eur. J., 2007,

13, (36), 10108–10116

The hydrolysis reaction mechanisms of carboplatin were investigated by combining DFT with the conductor-like dielectric continuum model (CPCM) approach. The theoretical calculations on carboplatin were used to obtain energy profiles and optimised structures for the rate-limiting process in its neutral hydrolysis. The results indicated that if carboplatin undergoes a hydration process, it should be doubly hydrated prior to reaction with DNA.

CHEMISTRY Double Complex Salts of Pt and Pd Ammines with Zn and Ni Oxalates – Promising Precursors of Nanosized Alloys A. V. ZADESENETS, E. YU. FILATOV, K. V. YUSENKO, YU. V. SHUBIN, S. V. KORENEV and I. A. BAIDINA, Inorg. Chim. Acta,

2008, 361, (1), 199–207

[M(NH3)4][M'(Ox)2(H2O)2]·2H2O (M = Pd, Pt; M' = Ni, Zn) were synthesised from solutions containing [M(NH3)4]2+ and [M'(Ox)2(H2O)2]2–. Thermal decomposition of the prepared salts in He or H2 atmosphere at 200–400ºC resulted in formation of nanosized bimetallic powders with crystallite sizes 50–250 Å.


(1–2), 155–169

Electrochemical Formation of Ir Oxide/Polyaniline Composite Films H. ELZANOWSKA, E. MIASEK and V. I. BIRSS, Electrochim. Acta,

2008, 53, (6), 2706–2715

IrOx/PANI composite films were made by forming an anodic IrOx film on bulk Ir and then depositing PANI into its pores. All of the PANI film that was electrochemically active was in direct electrical contact with the Ir surface at the base of the IrOx film pores. Thin films of Ir nanoparticles, subsequently converted to IrOx, were also used as a template for PANI formation within the porous structure. These hybrid films exhibited an enhanced internal porosity, high charge densities, unusual electrochromic behaviour, and very rapid charge transfer kinetics.

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NEW PATENTS CATALYSIS – APPLIED AND PHYSICAL ASPECTS Platinum-Ruthenium Catalyst for Methanol Oxidation JPN. ADV. INST. SCI. TECHNOL. HOKURIKU

Japanese Appl. 2007-190,454

A PtRu-based catalyst with increased uniformity of Pt particle size is claimed, where coagulation of Pt particles is prevented. Ru particles are dispersed on a carrier surface, followed by Pt particles with average diameter 0.5–15 nm. Standard deviation of the Pt particle diameter is 7–13. The supported particles are heat treated at < 300ºC in a non-oxidising atmosphere.

CATALYSIS – INDUSTRIAL PROCESS Hydroformylation Process with Rhodium Recovery DOW GLOBAL TECHNOL. INC

World Appl. 2007/133,379

A non-aqueous hydroformylation process with liquid catalyst recycle includes a hydroformylation step and one or more phase separation stages to recover high molecular weight aldehyde product and Rh catalyst. Hydroformylation is carried out at 250–450 psia (1724–3103 kPa). The product mix contains aldehydes, conjugated polyolefins, a Rh-organophosphorus ligand complex, free organophosphorus ligand and a polar organic solubilising agent. Phase separation stages use added H2O with CO(g), H2(g) or a mixture, and are carried out at 20–400 psia (138–2758 kPa). Sum of pressures in both steps is > 360 psia (2482 kPa). Preparation of 3-Methylbut-1-ene OXENO OLEFINCHEMIE GmbH

World Appl. 2008/006,633

The title compound is prepared from a hydrocarbon stream containing ≥ 70 wt.% isobutene with linear butenes or olefins containing 3–5 C atoms, by hydroformylation in the presence of a Rh catalyst with organophosphorus ligands, followed by hydrogenation of the resulting aldehyde to an alcohol. Elimination of H2O gives the final product.

CATALYSIS – REACTIONS Cross Metathesis of Cyclic Olefins MATERIA INC

World Appl. 2008/008,440

Ring-opening, ring insertion cross metathesis of cyclic olefins with internal olefins such as seed oils is carried out in the presence of a Ru alkylidene olefin metathesis catalyst. Olefinic substrates may include an unsaturated fatty acid or alcohol or an esterification product of an unsaturated fatty acid with a saturated or unsaturated alcohol. The Ru catalyst may be a Grubbs-Hoveyda complex and may contain an N-heterocyclic carbene ligand associated with the Ru centre, and is present in < 1000 ppm concentration relative to olefinic substrate.


Synthesis of 10-Hydroxycamptothecin Chinese Appl. 1,054,381

UNIV. FUDAN

The title compound is synthesised from 20(S)-camptothecin by catalytic hydrogenation using Pt/C or Rh/C, in the presence of a mitigator containing organic compound, followed by oxidation of the resulting tetrahydrocamptothecin to obtain the desired product. Yield is 70–75% and product purity is > 98.5%.

EMISSIONS CONTROL Removing Mercury from Gas Streams JOHNSON MATTHEY PLC

World Appl. 2007/141,577

Heavy metals such as Hg can be removed from high-temperature gases such as coal-derived syngas streams, using a sulfided Pd-containing absorbent, preferably Pd4S. Pd content is > 1.5 wt.%, preferably ~ 2 wt.%, loaded on a support, preferably γ-alumina. Hg forms a PdHg phase on contact with absorbent. Exhaust Gas Purifying Catalyst MAZDA MOTOR CORP

European Appl. 1,859,851

A catalyst for purifying exhaust gas containing HC, CO, NOx and H2O contains a catalyst layer on a honeycomb support. A first catalyst powder contains composite oxide RhZrCeNdO and a second contains RhZrXO, where X = a rare earth element other than Ce, and Rh is present on the surface. RhZrXO forms 1–50% of the total catalyst powder. Diesel Particulate Filter NISSAN MOTOR CO LTD


A DPF which can be partially regenerated at relatively low temperatures is claimed. A Pt catalyst is coated on the surface of a porous monolithic filter, with Pt concentration higher in the centre part to increase the probability of contact between the Pt catalyst and exhaust particulate.

FUEL CELLS Palladium-Ruthenium Electrocatalyst JOHNSON MATTHEY PLC

World Appl. 2008/012,572

An electrocatalyst for the anode of a DMFC is made from a PdRu alloy with a single crystalline phase, and contains (in at.%): 5–95 Pd, 5–95 Ru and < 10 other metals, but not 50 Pd and 50 Ru. Preferred compositions contain (in at.%): 5–49 Pd, 51–95 Ru and < 10 other metals on a support of high surface area. Water Management of PEMFC Stack GM GLOBAL TECHNOL. OPER. INC


A fuel cell system includes a means of humidifying the cathode inlet airflow and the H2(g) to the anode. A surface active agent such as EtOH is added to reduce surface tension and allow wicking of H2O to the flow field channels. The catalyst layers may include Ru as well as Pt to mitigate poisoning of Pt by CO formed by oxidation of EtOH on the cathode side.

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Platinum Catalyst on Carbon Nanotube Support Korean Patent 0,726,237

KOREA INST. ENERGY RES.

A highly dispersed C-nanotube supported Pt catalyst for a fuel cell is prepared by growing C nanotubes on graphite paper by CVD, pretreating to remove impurities and modify surface structure, then treating with chloroplatinic acid in aqueous H2SO4 solution to deposit Pt by an electrochemical process.

METALLURGY AND MATERIALS Palladium-Iridium Hydrogen Storage Alloy KYUSHU UNIV.


A H2 storage alloy is composed of PdIr nanoparticles which may form a core/shell structure with a core of Pd and a shell of Ir, or may be a solid solution with single crystal lattice. The alloy contains 40–90 at.% Pd and 10–60 at.% Ir. H2 storage content at 303 K and H2 pressure 0.1 MPa is ≥ 0.4 mol% and is claimed to exceed that of PdPt nanoparticles or bulk Pd. Platinum Alloy for Jewellery SEKI KK


A Pt alloy contains ≥ 99.7 wt.% Pt with 0.002–1.0 wt.% P, S or Be, preferably 0.005–0.3 wt.%. The alloy can be hallmarked Pt 1000, and Pt content is controlled in the range 98.90–99.94 wt.%, preferably 99.70–99.94 wt.%. Good wear and deformation resistance and low susceptibility to casting defects are claimed.

APPARATUS AND TECHNIQUE Temperature Measuring System European Appl. 1,860,414

WEBRESULTS SRL

A temperature measuring system includes a Pt resistance thermometer sensor, a managing circuit and a control circuit. The sensor incorporates at least two rheophores made from Pt or Ag (preferably 99.9999% Pt), with interconnecting wires made from the same material, sealed inside a metallic sheath with inert gas or dry air. Electrochemical Detection of DNA GENEOHM SCI.

BIOMEDICAL AND DENTAL Anticancer Drug Combinations BAYER PHARM. CORP

World Appl. 2007/139,930

Drug combinations and pharmaceutical compositions are claimed for treating cancer such as non-small cell lung carcinoma. The compositions contain at least one substituted-diaryl urea, at least one taxane and at least one Pt complex antineoplastic nucleic acid binding agent such as carboplatin, oxaplatin or cisplatin.

ELECTRICAL AND ELECTRONICS Rechargeable Battery with Ultracapacitor European Appl. 1,876,669

APOGEE POWER INC

A composite battery set for an electronic device includes a Li-ion, Li-polymer or Ni metal hydride battery and an ultracapacitor made from Pt, Au or preferably a metal-ceramic Ru oxide. The set may optionally include a protective circuit module. Pulse rise time provided to the electronic device is < 5 ms. Palladium-Plated Lead Finishing Structure SHINKO ELECTR. IND. CO LTD

U.S. Appl. 2007/0,272,441

A Pd-plated lead finishing structure for a semiconductor part includes Pd or Pd alloy plated at ≤ 0.3 μm thickness onto the surfaces of external connection terminals made from Cu, Cu alloy, Fe or a Fe-Ni alloy. No intermediate or underlying layer is required. Au or Au alloy may optionally be plated onto the Pd or Pd alloy to a thickness of ≤ 0.1 μm. Short circuits between terminals due to whiskers are prevented. Inkjet Printhead with Platinum Alloy SAMSUNG ELECTR. CO LTD

U.S. Appl. 2008/0,012,906

Thermal inkjet printheads include a heater to heat ink by direct contact, formed from Pt-Ru or Pt-Ir-X, where X = Ta, W, Cr, Al or O. Thickness of the heater is 500–3000 Å and the area of the heat generation part is ≤ 650 μm2. The Pt-Ru alloy contains 20–80% Ru; the Pt-Ir-X alloy may contain a proportion of Pt ≈ Ir, with 0–30% Ta or 0–40% O.

U.S. Appl. 2008/0,026,397

An assay for detecting a polynucleotide such as DNA includes the steps of immobilising the polynucleotide on an electrode, contacting with a Ru complex having a reduction potential which does not coincide with that of O2(g), such as Ru(III) pentaamine pyridine, and electrochemically detecting the Ru complex as an indicator of the presence of immobilised target polynucleotide. The process can be carried out in the presence of O2(g) and no deaeration step is required.

SURFACE COATINGS Platinum-Coated Refractory Oxide Ceramic Part JOHNSON MATTHEY PLC

World Appl. 2007/148,104

A refractory metallic oxide ceramic part for use in molten glass processing is surface treated to provide an array of slots or closed-end holes, and may then be spray coated with a Pt group metal or alloy of thickness 200–500 μm for erosion and corrosion protection. Dental Mirror with Ruthenium Coating U.S. Appl. 2007/0,268,603

Iridium Spark Plug Alloy

I. A. McCABE

TANAKA KIKINZOKU KOGYO KK

A dental mirror includes a glass substrate coated with a Ru film on either the front or rear surface. The coating thickness is 250–650 Å, preferably 350–550 Å. An adhesion enhancing layer may optionally be included between the glass substrate and the Ru coating, and an optical layer such as a reflection enhancing layer may be coated over the Ru.

World Appl. 2008/013,159

A spark plug chip is made from Ir with (in wt.%): 0.2–6.0 Cr plus 2.0–12.0 Fe and/or Ni. The surface may be oxidised by heating at 300–900ºC in an oxidising atmosphere, to give an oxide of Cr-Fe, Cr-Ni or Cr-Fe-Ni of thickness 5–100 μm.


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FINAL ANALYSIS

Crystallite Size Analysis of Supported Platinum Catalysts by XRD X-Ray diffraction (XRD) is often used in combination with transmission electron microscopy (TEM) and, for fuel cell electrocatalysts, electrochemical methods such as cyclic voltammetry, in the characterisation of supported platinum catalysts. Crystal/particle size information obtained from fresh or aged samples generally correlates with catalytic activity. Average Pt crystallite size is frequently calculated from XRD peak broadening using the Scherrer equation (1). As a bulk technique, XRD has certain advantages over TEM digital processing of particles (2). Direct comparison of the resulting XRD volume-weighted size of the crystallite with the TEM number-weighted size of the particle (often formed from several primary crystallite grains) is however often erroneously made in the literature. In order to obtain a realistic determination of Pt crystallite size by XRD, the effects of Pt particle shape, microstructure and very small, so-called ‘XRD amorphous’ Pt must be considered during X-ray analysis. The effect of interference from supports of high surface area and the presence of poorly crystalline oxidic Pt phases must also be considered. In addition, super-Lorentzian peak shapes are often encountered (Figure 1), which are

a result of either a broad lognormal or multimodal Pt crystallite size distributions (3). Issues outlined above have been successfully addressed for Pt/C catalysts by using the small angle X-ray scattering (SAXS) technique (4), as SAXS is well suited to size determination of very small particles (< 3 nm diameter). Although SAXS measurements can be made either in the laboratory or at a synchrotron facility, instrumentation is not commonplace, and data analysis of supported catalysts is often not routine. Alternative approaches more suited to the laboratory utilise XRD whole-pattern fitting based on the Rietveld method (5), using either Fourier (6) or non-Fourier transform methodology. One non-Fourier approach that allows for the determination of metric and microstructural parameters is given in the program FormFit (7). Similar to the Rietveld method, an analytical function is used to describe the measured X-ray pattern. Each line of the pattern is described by a split pseudo-Voigt function in terms of line width, Lorentzian fraction and an asymmetry term. After accounting for the instrument apparative intensity distribution function, the microstructure of the specimen can be determined.

Intensity, counts per second

1200 Aged 20 wt.% Pt/C Platinum

1000 800 600 400 200 0 30

40

50

60

70 80 90 2-Theta-Scale


100

110

120 130

Fig. 1 X-Ray powder diffraction data for an aged 20 wt.% platinum/carbon electrocatalyst

129

Fig. 2 Comparison of normalised, volume-weighted size distributions determined by XRD, TEM and SAXS for an aged 20 wt.% platinum/carbon electrocatalyst

0.25 XRD

Frequency, a.u.

0.20

SAXS TEM

0.15 0.10 0.05

0

10

20

30

40

50

D, nm

It is usual to assume a lognormal distribution gLN of diameters D of spherical crystallites with the median D0 and a variance parameter σ (see Equation (i)): (i)

Having an empirical relationship between this variance and the parameter η describing the Lorentzian fraction, FormFit can derive microstructural parameters for Pt such as anisotropic crystallite sizes and their distribution width, mean microstrain and stacking-fault densities calculated according to various models. A comparison of normalised, volumeweighted FormFit XRD, TEM and SAXS approaches is summarised in Figure 2 for an aged 20 wt.% Pt/C electrocatalyst sample. All approaches give on refinement a bimodal distribution of crystallites with a close match in XRD and SAXS distributions. Differences in the magnitude of this bimodal distribution are evident on comparing to TEM. In this example many of the particles measured by TEM are


TIM HYDE

References

g LN(D) = [D lnσ (2π)½]–1 exp[–(ln(D/D0))2/2(lnσ)2]

likely to contain several primary crystallite grains leading to a larger TEM particle size than that measured by XRD or SAXS.

1 2 3 4 5 6 7

P. Scherrer, Nachr. Ges. Wiss. Göttingen, 26 September, 1918, 98 D. Ozkaya, Platinum Metals Rev., 2008, 52, (1), 61 N. C. Popa and D. Balzar, J. Appl. Cryst., 2002, 35, (3), 338 D. A. Stevens, S. Zhang, Z. Chen and J. R. Dahn, Carbon, 2003, 41, (14), 2769 H. M. Rietveld, J. Appl. Cryst., 1969, 2, (2), 65 P. Scardi and P. L. Antonucci, J. Mater. Res., 1993, 8, (8), 1829 R. Haberkorn, ‘FormFit V5.5 – A Program for X-ray Powder Pattern Deconvolution and Determination of Microstructure’, Analytik und Datenverarbeitung, Dudweiler, Germany, 2005

The Author Dr Tim Hyde is a Principal Scientist in the Analytical Department at the Johnson Matthey Technology Centre, Sonning Common, U.K. Since joining Johnson Matthey in 1989 he has specialised in analytical characterisation of catalysts, primarily by laboratory X-ray powder diffraction and more recently synchrotron radiation techniques.

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Platinum Metals Review Johnson Matthey Plc, Precious Metals Marketing, Orchard Road, Royston, Hertfordshire SG8 5HE, U.K. E-mail: [email protected] http://www.platinummetalsreview.com/