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Cite this: J. Mater. Chem. A, 2014, 2, 12208

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When long bis(pyrazolates) meet late transition metals: structure, stability and adsorption of metal–organic frameworks featuring large parallel channels† S. Galli,*a A. Maspero,*a C. Giacobbe,a G. Palmisano,a L. Nardo,b A. Comotti,c I. Bassanetti,c P. Sozzanic and N. Masciocchia A family of bis(pyrazolato)-based metal-organic frameworks (MOFs) was isolated by reacting 1,4-bis(1Hpyrazol-4-ylethynyl)benzene (H2BPEB) with a number of transition metal ions. Special attention was dedicated to their structural features, their thermal and chemical stability, as well as their spectroscopic and adsorption properties. The rod-like ligands, connecting Zn(II), Ni(II) and Fe(III) nodes, fabricate 3-D networks containing 1-D pervious channels. The combination of thermal analysis and variabletemperature XRPD demonstrated the remarkable thermal robustness of the three materials, which are stable in air up to at least 410 C, and showed their structural response to increasing temperature. Specific experiments permitted us to test the chemical stability of the three species toward water as well as moderately acidic and basic solutions, the Ni(II) derivative being stable and hydrophobic in all the conditions assayed. The electronic transitions of both the ligand and the MOFs were investigated by solid-state UV-Vis absorption as well as by steady-state and time-resolved fluorescence analysis, which showed that the high fluorescence of the linker is perturbed in the three MOFs, suggesting high

Received 13th April 2014 Accepted 28th May 2014

sensitivity to environmental changes. N2 adsorption measurements at 77 K allowed to estimate promising Langmuir specific surface areas, peaking at 2378 m2 g1 in the case of the Ni(II) derivative. The best CO2 and CH4 uptake performances were achieved with the Fe(III)-based MOF. Indeed, adsorption

DOI: 10.1039/c4ta01798f

experiments with CO2 revealed that a considerable amount, up to 40% wt, is adsorbed by the Fe(III)

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derivative under the mild conditions of 298 K and 10 bar.

1. Introduction In the past two decades, metal–organic frameworks (MOFs)1 have been the subject of dedicated investigations in numerous disciplines, ranging from chemistry to physics and materials science. Their fame originated from the evidence that they

a

Dipartimento di Scienza e Alta Tecnologia, Universit` a dell'Insubria, via Valleggio 11, 22100 Como, Italy. E-mail: [email protected]; angelo.maspero@uninsubria. it

b

Dipartimento di Scienze della Salute, Universit` a di Milano Bicocca, via Cadore 48, 20900 Monza, Italy

c Dipartimento di Scienza dei Materiali, Universit` a di Milano Bicocca, via Cozzi 55, 20125 Milano, Italy

† Electronic supplementary information (ESI) available: Final Rietveld renement plots for species [Fe2(BPEB)3], [Ni(BPEB)] and a-[Zn(BPEB)]; TGA/DSC traces for [Fe2(BPEB)3], [Ni(BPEB)], a-[Zn(BPEB)] and b-[Zn(BPEB)]; XRPD monitoring of the thermal behaviour of species [Ni(BPEB)], a-[Zn(BPEB)] and b-[Zn(BPEB)]. XRPD monitoring of the chemical behaviour of species [Fe2(BPEB)3] and a-[Zn(BPEB)]; uorescence decay distributions and corresponding best-tting curves for H2BPEB, [Fe2(BPEB)3], [Ni(BPEB)] and a-[Zn(BPEB)]; CO2/N2 selectivity of [Fe2(BPEB)3], [Ni(BPEB)] and a-[Zn(BPEB)]; isosteric heat of CO2 adsorption of [Fe2(BPEB)3]. See DOI: 10.1039/c4ta01798f

12208 | J. Mater. Chem. A, 2014, 2, 12208–12221

represented an improvement in the eld of hydrogen storage2 with respect to the conventionally applied, natural and synthetic, inorganic compounds. Hydrogen as guest was soon followed by carbon dioxide and methane. Concerns about global warming continue to stimulate research for new solutions to sequester carbon dioxide and thus reduce CO2 concentrations in the atmosphere. Porous materials provide a feasible way to reduce CO2 concentration; in particular, MOFs can effectively store CO2 by physisorption mechanisms.3 Methane, the main component of natural gas, is a suitable alternative as on-board fuel, and interest in its storage has nurtured intense research activity.4 A vast number of MOFs with interesting structural features and functional properties5 have been constructed by juxtaposing selected metal ions with specic stereochemical requirements to organic linkers, the size, shape and function of which could be modulated on an ad hoc basis. Polyazolato-based ligands may be suitable alternatives to polycarboxylato-containing ones: many tetrazolato-, 1,2,3- and 1,2,4-triazolato-, imidazolato- and pyrazolato-based MOFs have been reported.6 In this respect, our work7 and that of others6,8

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Journal of Materials Chemistry A


Scheme 1

1,4-bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB).

have demonstrated that due to the higher pKa of pyrazole,9 pyrazolato-containing spacers provide stronger metal-to-ligand coordinative bonds, conferring to the corresponding MOFs higher thermal robustness, and, in specic instances, chemical resistance to acidic and basic conditions7c or to boiling solvents.10 Signicant examples along this line of research are the isoreticular families [M(BPZ)],7b [M(Me4BPZ)]11 and [M(BDP_X)] (M ¼ Ni, Zn; H2BPZ ¼ 4,40 -bis(pyrazole), H2Me4BPZ ¼ 3,30 ,5,50 tetramethyl-4,40 -bis(pyrazole), H2BDP ¼ 1,4-bis(1H-pyrazol-4-yl) benzene; X ¼ H,7f NO2, NH2, OH7a). All of them feature 3-D networks with square- (the Zn(II) derivatives) or rhombic-shaped (the Ni(II) derivatives) pervious channels, and couple a remarkable thermal robustness with interesting adsorption properties. As a representative example, [Ni(BDP_H)] and [Zn(BDP_H)], which possess 57% and 65% of potentially accessible void volume and 0.86 and 0.71 g cm3 bulk densities, respectively, selectively adsorb benzene from benzene–cyclohexane mixtures, and trap traces of thiophene from CH4/CO2 uxes—the Ni(II) derivative being active even in the presence of humidity.7f Our current interests include construction of 3-D networks isoreticular to [M(BPZ)], [M(Me4BPZ)] and [M(BDP_X)], but possessing a signicantly higher empty volume and lower crystal density, for potential applications as nano-reactors to build highly anisotropic metal nanoparticles. Extended spacers are suitable to achieve this goal. Two building units that can be mounted in series along a common symmetry axis are the pphenylene and ethynyl groups. Accordingly, the long, rigid, and rod-like spacer 1,4-bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB, Scheme 1) was engineered, prepared and coupled to the 3d transition metal ions Ni(II), Zn(II) and Fe(III) to fabricate microporous MOFs. As a result, in the following we report on the synthesis, structural aspects, and thermal and chemical behaviour of the metal–organic frameworks [Fe2(BPEB)3], [Ni(BPEB)], a[Zn(BPEB)] and b-[Zn(BPEB)]. Moreover, the electronic-transition spectroscopic features and adsorption properties toward gases of environmental and industrial interest have been investigated.

2.

the proper metal salt and H2BPEB either in pyridine (for Ni(II) and Zn(II)) or in DMF (for Fe(III)) at reux for 6 h. The use of a base (triethylamine) was deemed necessary only in the case of [Fe2(BPEB)3] and [Ni(BPEB)]. Unexpectedly, the application of a microwave-assisted route in the case of the Zn(II) derivative led to the isolation of a phase (b-[Zn(BPEB)] in the following) possessing the same formula unit and the same unit cell metrics as a-[Zn(BPEB)], but different relative intensities of the low-angle XRPD peaks (indicating a different structure; see the next section). Independently from the synthetic route, all the species precipitate in satisfactory yields in the form of air- and lightstable powders insoluble in water and in the most common solvents, thereby suggesting the polymeric nature of their crystal structures.

2.2. Structural analysis of [Fe2(BPEB)3], [Ni(BPEB)], a-[Zn(BPEB)] and b-[Zn(BPEB)] Structural analysis of [Fe2(BPEB)3]. The compound [Fe2(BPEB)3] crystallizes in the orthorhombic space group Fddd. The asymmetric unit contains one independent Fe(III) ion and two independent BPEB2 ligands, all lying on special positions. Remarkably, we achieved our goal of obtaining a material isostructural to [Fe2(BDP_H)3].12 Thus, in [Fe2(BPEB)3] the metal centres possess an octahedral stereochemistry dened by six nitrogen atoms of six BPEB2 ligands (Fig. 1a), and are arranged in parallel 1-D chains running along the crystallographic axis a ˚ apart by the (Fig. 1b). Along the chains, they are kept 3.699(5) A 2 BPEB spacers, bridging consecutive Fe(III) ions with one of their pyrazolato moieties. Along the [0,1,0] direction, each chain

Results and discussion

2.1. Synthesis of [Fe2(BPEB)3], [Ni(BPEB)], a-[Zn(BPEB)] and b-[Zn(BPEB)] In order to identify the most straightforward reaction path toward each target MOF, various synthetic strategies were adopted by changing parameters such as metal ion salt, solvent, base, and temperature. As a result of this preliminary screening, [Fe2(BPEB)3], [Ni(BPEB)] and a-[Zn(BPEB)] could be successfully isolated by following solution-phase one-pot reactions, reacting


Fig. 1 Representation of the crystal structure of species [Fe2(BPEB)3]: (a) the octahedral coordination sphere of the Fe(III) ions. (b) The 1-D chains of ligand-bridged, collinear metal ions (the coordination sphere of the metal centres is highlighted with dashed lines). (c) The 3-D network featuring triangular channels running along the crystallographic axis a. Horizontal axis, c; vertical axis, b. Iron, yellow; nitrogen, blue; carbon, gray; hydrogen, light gray. Significant bond distances (˚ A) and angles ( ): Fe–N 1.52(2), 1.755(3), 2.16(4); N–Fe–N 76(2)-170(1).

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is connected to six parallel ones by the BPEB2 linkers, generating a 3-D porous framework (Fig. 1c). The architecture is characterized by triangular 1-D channels ˚ long, are dened by running along a, the edges of which, 18.0 A the spacers, while the vertices are occupied by the FeN6 nodes. ˚ The channels can host an innite-length cylinder of about 3.0 A diameter.13 The estimated empty volume of [Fe2(BPEB)3] amounts to 64%,14 which results in a calculated bulk density of 0.69 g cm3. Notably, small size channels can exhibit intriguing functional properties: for example, [Fe2(BDP_H)3], possessing even smaller channels than [Fe2(BPEB)3], has been recently found to selectively trap the lower-octane components of fuel, separating them from the higher-octane species in a way that could prove less expensive than the current industrial method. As detailed below, we anticipate that [Fe2(BPEB)3] possesses a remarkable ability for adsorbing CO2. Structural analysis of [Ni(BPEB)]. As already found in the simpler analogues [Ni(BDP_X)],7a,f assembled with the shorter spacers H2BDP_X, [Ni(BPEB)] crystallizes in the orthorhombic space group Imma, the asymmetric unit featuring one Ni(II) ion and one BPEB2 ligand, both lying in special positions. More specically, [Ni(BPEB)] and [Ni(BDP_X)] possess the same structural motif, hence constituting, as we originally planned, an isoreticular family. Thus, in the case of [Ni(BPEB)] as well, each metal centre is coordinated in square-planar stereochemistry by four nitrogen atoms of four BPEB2 moieties, acting overall in the exo-tetradentate coordination mode (Fig. 2a). With one of their pyrazolato rings, the ligands bridge Ni/Ni ˚ (a/2) and generate parallel 1-D chains of vectors of 3.400(2) A metal ions (Fig. 2b). Along the [0,1,1] direction, each chain is further linked to four nearby chains by the spacers, with the consequent formation of a 3-D network possessing large rhombic channels running along a (Fig. 2c). The edges of the

Fig. 2 Representation of the crystal structure of species [Ni(BPEB)]: (a) the square-planar coordination sphere of the Ni(II) ions. (b) The 1-D chains of ligand-bridged, collinear metal ions (the coordination sphere of the metal centres is highlighted with dashed lines). (c) The 3-D network featuring rhombic channels running along the crystallographic axis a. Horizontal axis, b; vertical axis, c. Nickel, yellow; nitrogen, blue; carbon, gray; hydrogen, light gray. Significant bond distances (˚ A) and angles ( ): Ni–N 1.70(1); N–Ni–N 87.3(1), 92.7(1).

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˚ long, are dened by the ligands, while the channels, 18.1 A ˚ long, are dened by the crystallodiagonals, 31.9 and 18.3 A graphic axes b and c, respectively. Overall, the channels can host ˚ 2 wide.13 an ellipse 9.7 5.7 A As predicted, the different sizes of BDP2 and BPEB2 result in a signicantly higher empty volume in [Ni(BPEB)] than in [Ni(BDP_H)]7f (70% versus 57%, respectively),14 and into a lower estimated bulk density (0.53 versus 0.86 g cm3, respectively). Structural analysis of a-[Zn(BPEB)]. The compound a-[Zn(BPEB)] crystallizes in the orthorhombic space group Cccm. The asymmetric unit contains one independent Zn(II) ion and one independent BPEB2 ligand, both lying on special positions. Each metal centre is coordinated by four nitrogen atoms of four ligands and possesses a distorted tetrahedral stereochemistry (Fig. 3a). As expected, the linkers adopt the exotetradentate coordination mode: the nitrogen atoms of the ˚ same pyrazolato moiety bridge nearby metal ions 3.6537(6) A apart, building up 1-D chains of collinear metal centres running along the crystallographic axis c (Fig. 3b). Adjacent chains are further connected by the spacers in the [1,1,0] direction (and the crystallographically related ones) in such a way as to generate two mutually interpenetrated 3-D networks, reciprocally dis˚ along b (Fig. 3c and d). placed by about 7.75 A For the sake of comparison, it is worth noting that the analogous species [Zn(BDP_X)],7a,f crystallizing either in the tetragonal space group P42/mmc (a supergroup of Cccm; X ¼ H) or in the orthorhombic space group Cccm (X ¼ NO2, NH2, OH), invariably showed a non-interpenetrated 3-D network featuring 1-D channels. A similar consideration holds even for [Zn(BPZ)]7b

Fig. 3 Representation of the crystal structure of species a-[Zn(BPEB)]: (a) the tetrahedral coordination sphere of Zn(II) ions. (b) The 1-D chains of ligand-bridged, collinear metal ions (the coordination sphere of the metal centres is highlighted with dashed lines). (c) The 3-D network featuring rectangular-shaped channels running along the crystallographic axis a. Horizontal axis, b; vertical axis, c. (d) The two interpenetrated networks, depicted with different colours for the sake of clarity. Zinc, yellow; nitrogen, blue; carbon, gray; hydrogen, light gray. Significant bond distances (˚ A) and angles ( ): Zn–N 1.710(8), 1.870(8); N–Zn–N 99.21(7), 99.6(6), 108.4(6), 126.52(7).


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and [Zn(Me4BPZ)],11 built up with smaller bis(pyrazolato)-based linkers. In the case of a-[Zn(BPEB)], the failure of the isoreticular approach and the insurgence of interpenetration is reasonably promoted by the limited steric hindrance of the C^C triple bonds, the two interpenetrated networks actually crossing about them. Despite the presence of interpenetration, the crystal structure of a-[Zn(BPEB)] features two sets of rectangular-shaped 1-D channels running along c, the vertices of which are dened by the Zn(II) ions, while the walls are decorated by the ligands. The smaller channels, centred in [0,0,z], ˚ 13 while the wider ones, centred in have a diameter of only 2.1 A, ˚ 13 We expect that the [1/2,0,z], have a larger diameter of 13.7 A. major contribution to the gas adsorption properties is essentially due to the wider channels. On the whole, the structure possesses 42% of potentially accessible empty volume,14 which corresponds to an estimated bulk density of 0.87 g cm3, rather low for an interpenetrated network. Structural analysis of b-[Zn(BPEB)]. Compound b[Zn(BPEB)] crystallizes in the orthorhombic space group Cccm and possesses the same unit cell metrics of the a phase. In spite of this, their XRPD patterns are considerably different in terms of relative intensities of the peaks, especially the low-angle ones (Fig. S2†). The electronic density provided by the structural model of the a phase does not result in a satisfactory description of the XRPD pattern of b-[Zn(BPEB)]. On the basis of elemental analysis and infrared spectroscopy, the electronic density missing in the structural model cannot be ascribed to the presence of solvent within the 1-D channels. An alternative structural model, such as the non-interpenetrated one of the parent species [Zn(BDP_H)],7f equally fails. To shed some light on this challenging puzzle, an electronic density map was produced with the structure factors calculated with the interpenetrated network alone, and was compared with the map produced with the observed structure factors. The comparison between the two maps (Fig. S3†) highlights that the unexplained electronic density shows a non-serendipitous distribution, as if another [Zn(BPEB)] network could be present, possibly incommensurate with the “major” one. 2.3. Thermal behaviour of [Fe2(BPEB)3], [Ni(BPEB)], a[Zn(BPEB)] and b-[Zn(BPEB)] The thermal behaviour of [Fe2(BPEB)3], [Ni(BPEB)], a[Zn(BPEB)] and b-[Zn(BPEB)] was investigated by coupling thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), under a ux of nitrogen, to variabletemperature X-ray powder diffraction (VT-XRPD) measurements in air. Two sets of VT-XRPD experiments were performed: (a) XRPD patterns were collected during progressive heating from room temperature up to decomposition, in order to prove the thermal stability of the porous structures; (b) a series of heating–cooling cycles was carried out in the range 50–210 C, to assess the behaviour along consecutive thermal treatments. The acquired TGA/DSC traces are collectively gathered in Fig. S4 of the ESI.† As a representative example, Fig. 4 supplies the results of the VT-XRPD experiments and of the consequent data treatment for [Fe2(BPEB)3]. The results obtained with [Ni(BPEB)], aThis journal is © The Royal Society of Chemistry 2014


Fig. 4 (a) Plot of the X-ray powder diffraction patterns measured on [Fe2(BPEB)3] as a function of the temperature heating, with steps of 20 C, up to decomposition. The permanent porosity of species [Fe2(BPEB)3] can be appreciated. (b) Percentage variation of the unit cell parameters (pT) of [Fe2(BPEB)3] as a function of the temperature. The values at 30 C (p30) have been taken as the references. a, blue rhombi; b, red triangles; c, green circles; V, orange squares. (c) Plot of the X-ray powder diffraction patterns measured on [Fe2(BPEB)3] during heating–cooling cycles within the range 50–210 C. Heating step, red; cooling step, blue.

and b-[Zn(BPEB)] are reported in the ESI as Fig. S5–S7,† respectively. It is worthy of note that [Fe2(BPEB)3], [Ni(BPEB)] and a[Zn(BPEB)] possess remarkably high thermal stability: as demonstrated by their TGA/DSC proles, no transitions are observed up to 415, 422 and 410 C, respectively (Fig. S4†). This evidence corroborates the key role of poly(azolato)-based spacers to form strong metal-to-ligand coordinative bonds, conferring solidity to the entire material. The VT-XRPD measurements not only conrmed the high thermal stability of [Fe2(BPEB)3], [Ni(BPEB)] and a-[Zn(BPEB)] even in air, but also demonstrated that they possess permanent porosity: increasing the temperature does not inuence their structural features, and their network does not collapse (Fig. 4a and S5a–S7a†). Finally, the compounds are resistant to successive heating– cooling cycles (Fig. 4c, S5c and S6c†). Thermal behaviour of [Fe2(BPEB)3]. A Le Bail parametric treatment of the data acquired in the temperature range 30–270 C reveals that the framework of [Fe2(BPEB)3] is denitely rigid (Fig. 4b). The unit cell axis along which the Fe/Fe chains run, a, undergoes almost no variation, while b and c shrink less than 0.4%, with an overall volume shrinkage of less than 0.4%. Thermal behaviour of [Ni(BPEB)]. In the case of [Ni(BPEB)], a parametric data treatment in the range 30–290 C demonstrated that heating does not substantially affect the crystallographic axis a (Fig. S5b†). This evidence, already observed e.g., in the case of [Ni(BDP_H)],7f is somewhat expected, a being the axis along which the chains of pyrazolato-bridged metal ions run. In contrast, as a response to the thermal stimulus, b and c undergo moderate variations. In particular, up to 230 C they show an opposite trend (the former increases by less than 1.0%, while

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the latter decreases by about 1.5%, with an overall negative thermal expansion of about 0.8%). Thus, the rhombic channels behave like an accordion, as already observed for other species possessing the same structural motif, such as [Ni(BDP_H)],7f [Ni(BPZ)]7d and the renowned family of MIL-53(M) derivatives (M ¼ Al, Cr, Fe), which were investigated in detail by F´ erey and coworkers.15 Over 230 C, c inverts its trend and undergoes a moderate thermal expansion, with the overall volume shrinkage at 290 C being only 0.5%. Thermal behaviour of a- and b-[Zn(BPEB)]. In the case of a[Zn(BPEB)], a parametric data treatment in the range of 30– 350 C highlights opposite variations of a and b upon heating (Fig. S6b†): while the former decreases about 2.0%, the latter increases about 1.0%. Overall, the a/b ratio increases from 0.96 to 0.99, that is, as a response to the external stimulus, the rectangular channels (in Cccm) tend to transform into square ones (toward the P42/mmc symmetry). The unit cell axis along which the ligand-bridged Zn/Zn chains run, c, undergoes the lowest variation (shrinking less than 0.2%). Collectively, in the temperature range analyzed, the unit cell volume experiences a moderate negative thermal expansion of about 1.0%. It is noteworthy that the simultaneous thermal analysis conveys another piece of information (Fig. S4†): before decomposition, an exothermic transformation invariably takes place. Given the absence of weight loss and the enthalpy value involved (DH 75 kJ mol1), too high to invoke a phase transition, it is reasonable to propose that a thermally stimulated, chemical reaction takes place. In particular, thermally or photochemically promoted reactions between two adjacent acetylenic groups are not unknown in organic chemistry. When the two moieties are perpendicular to each other and bear sterically hindered groups, as is the case in a-[Zn(BPEB)], the formation of a tetrahedrane (Fig. S7†) is favoured,16 involving a DH of about 80 kJ mol1.17 Even more interesting, the DSC trace of the b-polymorph also invariably shows the same thermal event (Fig. S4†); this occurrence supports the structural hypothesis proposed above for this species, invoking an interpenetrated network crossing at the acetylenic moieties. In the case of a-[Zn(BPEB)], the reaction has never resulted in the appearance of a new phase during the VT-XRPD experiments: yet, by heating a sample in a more controlled manner with the STA apparatus, a novel phase was trapped (g-[Zn(BPEB)] in the following).18 The XRPD pattern of g-[Zn(BPEB)] can be described by a primitive tetragonal unit cell (P42/mmc, a supergroup of Cccm),19 suggesting that the reaction does not disrupt the entire crystal structure, but affects it only locally, as the formation of a tetrahedrane should do. At variance with the a-polymorph, in the case of b-[Zn(BPEB)] it was possible to trap g-[Zn(BPEB)] also by VT-XRPD starting from about 300 C (Fig. S8a†). We reasonably attribute the apparent discrepancies between the STA and the VT-XRPD experiments (lack of formation of the g-phase during VT-XRPD in the case of a, and difference in the temperature at which the reaction is observed with the two techniques in the case of b) to the markedly different conditions (atmosphere, heating rate) adopted for the two measurements.

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Although overnight data collections were carried out on g-[Zn(BPEB)], performing a structural determination was not feasible due to the less-than-ideal crystallinity and to the relevant anisotropic peak broadening of the sample. Finally, a parametric Le Bail renement of the data for b[Zn(BPEB)] acquired in the range 30–310 C revealed that while a decreases about 0.8%, b does not have a monotonic behaviour (Fig. S8b†). On the whole, the b/a ratio varies from 0.98 to 0.99, leading to the progressive transformation of the 1D channels' shape from rectangular to square. A small expansion of about 0.5% was observed for c, which is likely due to the presence, along this axis, of Zn/Zn chains, as already highlighted above for a-[Zn(BPEB)] and [Ni(BPEB)]. 2.4. Chemical stability of [Fe2(BPEB)3], [Ni(BPEB)] and a-[Zn(BPEB)] The chemical stability of [Fe2(BPEB)3], [Ni(BPEB)] and a-[Zn(BPEB)] was assayed toward water as well as toward aqueous acidic and basic solutions of varying pH. The effect of the chemical treatment was veried, in all cases, by means of XRPD. As a representative example, the XRPD traces acquired aer the chemical treatment on the Ni(II) derivative are collected in Fig. 5, while those for the Fe(III) and Zn(II) MOFs are reported in Fig. S9 and S10, respectively, of the ESI.† Chemical stability of [Fe2(BPEB)3]. [Fe2(BPEB)3] does not show pronounced stability and hydrophobicity: as a matter of fact, for all the solution-phase tests performed, Le Bail renements of the XRPD data acquired aer 1, 5 and 8 hours of treatment reveal that [Fe2(BPEB)3] survives for only 1 h; aer 5 h, the most intense peaks of the H2BPEB ligand start to appear (Fig. S9†), indicating a progressive hydrolysis of the MOF concomitant with the formation of a hydroxo-Fe(III) species (IR evidence20). The chemical treatments affect the relative intensities of the (0kl) peaks: this evidence supports the hypothesis that water could enter the 1-D channels (running along a), favouring disruption of the framework. Finally, hydrolysis takes place as well upon exposure to water vapours (Fig. S9†). Chemical stability of [Ni(BPEB)]. [Ni(BPEB)] is recovered intact aer exposition to water vapours at room temperature for 5 days, aer suspension in boiling water for 2 days, in acetone at room temperature for 5 days, in moderately acidic (pH 5) or

Chemical stability of [Ni(BPEB)] upon exposure to water vapours at room temperature, boiling water, aqueous acidic (pH 5) or basic (pH 9) solutions at room temperature, or acetone at room temperature.

Fig. 5


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basic (pH 9) aqueous solutions at room temperature for 8 hours (Fig. 5). As veried by Le Bail renements, none of these treatments modies its unit cell parameters, which remain almost unchanged, or the relative intensities of the XRPD peaks; at least in the assayed conditions, the material is hydrophobic and does not allow the solvent to enter the channels. Chemical stability of a-[Zn(BPEB)]. a-[Zn(BPEB)] progressively transforms into b-[Zn(BPEB)] (Fig. S10a†) not only when suspended in boiling water or in acidic or basic aqueous solutions at room temperature, but also when exposed to water vapours at room temperature. Visual inspection of the XRPD traces of the aliquots collected from boiling water aer 1, 5 and 8 hours demonstrated the progressive a to b transformation (Fig. S10b†). 2.5. Gas adsorption measurements

material shows a reduced specic surface area (1224 m2 g1), as a consequence of the interpenetrated structure. Remarkably, in our efforts to fabricate longer and longer rod-like, bis(pyrazolato)-based ligands, we succeeded in obtaining the highest specic surface area of the Ni-based series.7a,f The porosity of the three MOFs was further explored for the capture of the environmentally and industrially important gases CO2 and CH4. The carbon dioxide adsorption capacities of the three MOFs at various temperatures and pressures are shown in Fig. 7. The CO2 adsorption isotherms of [Fe2(BPEB)3] and a[Zn(BPEB)] at 195 K exhibited Langmuir proles that reached values of 300 and 250 cm3(STP) per g (13.4 and 11.2 mmol g1), respectively, at 1 bar (Table 2). The measured CO2 capacities correspond to an occupied volume of 0.77 and 0.63 cm3 g1 (dCO2(l) ¼ 0.77 g cm3 was used), which match the pore volumes of 0.79 and 0.60 cm3 g1 estimated by the Tarazona method

The permanent porosity of the title MOFs was demonstrated by N2 adsorption–desorption measurements performed at 77 K, as reported in Fig. 6. The main textural parameters of the three materials, as derived from the N2 isotherms, are collected in Table 1. The adsorption isotherms display Type I proles, which are characteristic of microporous solids: the Ni(II)- and Fe(III)-based compounds exhibit the highest Langmuir specic surface areas (2378 and 1598 m2 g1, respectively), while the Zn(II)-containing

Fig. 6 N2 adsorption isotherms at 77 K for [Fe2(BPEB)3] (dark grey diamonds), [Ni(BPEB)] (orange diamonds), and a-[Zn(BPEB)] (light grey diamonds). Desorption branches are depicted as white diamonds.

Table 1 Textural parameters of [Fe2(BPEB)3], [Ni(BPEB)] and a[Zn(BPEB)], as derived from the N2 adsorption isotherms at 77 K

[Fe2(BPEB)3] [Ni(BPEB)] a-[Zn(BPEB)]

SBET m2 g1

SLangmuir m2 g1

Vmicroa cm3 g1

1273 1900 985

1598 2378 1224

0.79 1.05 0.60

a

The pore volume Vmicro was calculated considering a cylindrical pore model and adopting the Tarazona method.


Fig. 7 CO2 (circles) and N2 (triangles) adsorption isotherms for [Fe2(BPEB)3] (dark grey), [Ni(BPEB)] (orange), and a-[Zn(BPEB)] (light grey) at 195 K (a), 273 K (b) and 298 K (c).

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Table 2 CO2 adsorption capacities of [Fe2(BPEB)3], [Ni(BPEB)] and a[Zn(BPEB)] at 298 K, 273 K and 195 K

[Fe2(BPEB)3]


[Ni(BPEB)] a-[Zn(BPEB)]

a

TK

P bar

Vads cm3 (STP) per g

mads mmol g1

wt%

298 273 195 298 273 195 298 273 195

10 10 1a 10 10 1a 10 10 1a

220 268 304 142 192 260 125 184 254

9.2 12.0 13.6 5.9 8.5 11.6 5.2 8.2 11.3

40.5 52.8 59.8 26.0 37.4 51.0 22.9 36.1 49.7

Qst kJ mol1 26

23

23

P/P0 is reported.

from the N2 adsorption isotherms, to be compared to 0.88 and 0.67 cm3 g1, as calculated by considering the crystal density ˚ and pore volume explored by a sphere having a radius of 1.7 A (equal to the kinetic radius of CO2). Interestingly, these results indicate the virtually complete lling of the empty space, which is easily accessible to this gas via diffusion. Remarkably, as regards CO2 capture under the mild conditions of room temperature, [Fe2(BPEB)3] adsorbs 220 cm3(STP) per g, corresponding to 40% of the host weight, already at 10 bar, overcoming many well-known, high-capacity materials such as HKUST-1, MIL-101, MIL-53, ZiF-8, Nott-140 and UiO-66.21 Notably, a high CO2 uptake of 105 cm3(STP) per g, equal to 20.6% of the host weight, is achieved at 273 K and a pressure as low as 1 bar. This value is higher or comparable to highperformance MOFs: for example, SNU-50 , Zn2(BTetB) and SNU-4 adsorb 19.2%, 19.7% and 20.3% of the host weight under the same conditions.22 To the best of our knowledge, a restricted number of MOFs, such as SNU-5, Dy(BTC), Cu-EBTC, CAU-1 and SNU-4, are superior to [Fe2(BPEB)3].3 The reason for the efficient uptake of [Fe2(BPEB)3] can be ascribed to the framework structure, featuring microporous 1-D channels and small interstices in close proximity of the channel edges that match the size of CO2, providing a suitable environment to efficiently adsorb this gas. In the case of [Ni(BPEB)], the CO2 adsorption isotherm at 195 K reaches the plateau value of 260 cm3(STP) per g at 1 bar, equivalent to 0.66 cm3 g1, to be compared to the pore volume of 1.0 cm3 g1, as derived from the N2 adsorption isotherm. This difference is tentatively ascribed to the detrimental inuence of the activation treatments on the porous structure of the Ni(II) derivative. In contrast, aer several cycles of gas absorption– desorption, the specic surface areas and pore volumes of [Fe2(BPEB)3] and a-[Zn(BPEB)] are preserved (Fig. S13 and S14†), making these materials appropriate for technological applications. As evident from Fig. 7, the porous MOFs preferably adsorb CO2 over N2 both at room temperature and at 273 K. The N2 adsorption isotherms show low uptake even at high pressures; the amount adsorbed is proportional to the pressure up to 10 bar, indicating an extremely low affinity of the three MOFs for

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nitrogen. This evidence can be favourably exploited for the selective adsorption of carbon dioxide in a mixture with nitrogen. The selectivity of the three MOFs toward CO2/N2 binary mixtures was determined from the single-component isotherms using the ideal adsorbed solution theory (IAST), which has been successfully applied to calculate gas mixtures separation by porous materials.23 At low pressure and 273 K, the title MOFs show the highest CO2/N2 selectivity of 25, 21 and 19 for [Fe2(BPEB)3], a-[Zn(BPEB)] and [Ni(BPEB)], respectively, in a CO2/N2 binary mixture of 15 : 85 (mol : mol), which is representative of industrial operative conditions (see ESI, Fig. S11†). The selectivity is associated with the isosteric heat of CO2 adsorption at low pressures, estimated to be 26 kJ mol1 for [Fe2(BPEB)3] and 23 kJ mol1 for a-[Zn(BPEB)] and [Ni(BPEB)] (Fig. S12†), in agreement with exposure of the pyrazolate and phenylene groups on the cavity walls. These values are consistent with those calculated, for instance, for the imidazolate framework ZIF-8 and for hydrophobic molecular zeolites.24

Fig. 8 CH4 adsorption isotherms for [Fe2(BPEB)3] (dark grey), [Ni(BPEB)] (orange), and a-[Zn(BPEB)] (light grey) at 195 K (a), 273 K (b) and 298 K (c).


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The porous MOFs have been tested also for methaneadsorption properties (Fig. 8). The CH4 isotherms, measured at the temperatures of 195 K, 273 K and 298 K, show that [Fe2(BPEB)3] possesses once again the highest capacity: for example, at 195 K it adsorbs 7.6 mmol g1 of CH4, equal to 12.1 wt%, while a-[Zn(BPEB)] and [Ni(BPEB)] reach values of 3.1 and 2.8 mmol g1 (4.9 wt% and 4.5 wt%), respectively (Table S1†). Also under the mild conditions of room temperature and in the medium-pressure range (up to 10 bar), [Fe2(BPEB)3] exhibits a higher gravimetric methane-uptake capacity (3.6 mmol g1 equal to 5.7 wt%) with respect to a-[Zn(BPEB)] and [Ni(BPEB)]. The volumetric uptake of methane, of importance for practical applications, was calculated considering the crystal density as the packing density. [Fe2(BPEB)3] displayed the highest capacity at 10 bar (59 v/v), followed by a-[Zn(BPEB)] (26 v/v) and [Ni(BPEB)] (21 v/v). At room temperature and at a moderate pressure, [Fe2(BPEB)3] showed remarkable gravimetric and volumetric CH4 uptake values owing to small pore size, which promotes overlapping of multiple CH4/framework energy potentials and, thus, efficient capture of this gas. 2.6. Electronic transitions spectroscopy A preliminary characterization, in the solid state, of the UV-Vis absorption as well as of the steady-state and time-resolved uorescence emission of the ligand and the metal–organic frameworks [Fe2(BPEB)3], [Ni(BPEB)] and a-[Zn(BPEB)] was undertaken. UV-Vis absorption spectroscopy. The UV-Vis absorption spectra of the ligand and of the three metal–organic frameworks are gathered and directly compared in Fig. 9. The absorption spectra of the ligand and of a-[Zn(BPEB)] are similar and exhibit one main UV band peaked at about 335 nm, which possesses residual vibronic structural features, denounced by a blueshied shoulder at about 315 nm, and two red-shied ones at 380 nm and 430 nm. This band (which is slightly sharper for a-[Zn(BPEB)], suggesting reduced vibrational freedom) represents a signature of the phenyl p / p* transition, and its red shi with respect to the free chromophore25 suggests high

Fig. 9 UV-Vis absorption spectra collected in solid state on the H2BPEB ligand (red curve), [Fe2(BPEB)3] (cyan curve), [Ni(BPEB)] (purple curve), and a-[Zn(BPEB)] (blue curve).



intramolecular, and likely also intermolecular, conjugation within the crystal structure. For both H2BPEB and a-[Zn(BPEB)], a secondary absorption band, peaked at 240 nm, might be attributed to the p / p* transition of the acetylenic cromophores,26 provided that, once again, a highly conjugated system is postulated. Finally, the band resolved at the UV edge of the acquisition interval may be attributed to the absorption of pyrazoles. The latter appear to be only marginally involved in the charge conjugation pattern as, in a-[Zn(BPEB)], the pertinent band is not red shied with respect to that of the isolated chromophore.27 The structure of the absorption spectrum changes signicantly for [Ni(BPEB)]: the main band is broader and much more intense, and its absorption maximum falls at about 300 nm. We attribute this hypsochromic shi and the concomitant hyperchromism to the superposition of absorption bands due to the chromophores with another band. The latter is likely due to a charge transfer from the lone pairs of the ligand p-orbitals to the metal unsaturated b1g d-orbitals.28,29 The acetylenic chromophore band, integrated in the main band, is detectable as a shoulder barycentered at approximately 250 nm. On the contrary, the band attributable to the pyrazole rings is not detectable. Finally, for [Fe2(BPEB)3], only one band is detected in the UV region, peaking at 278 nm. Similar to [Ni(BPEB)], this band might result from the superposition of a charge-transfer band with bands due to the chromophores. Indeed, for Fe(II) in similar geometry, an 1A1g / 1T1g transition in the near UV region (300 nm) was observed in the 1960s29 in simple [Fe(CN)6]4 aqueous solutions, and was shown to be conserved in hetero-metallic cubic lattices obtained from K2Ni[Fe(CN)6]. In addition to the UV-absorption features, a fainter, broadened band in the visible region (peaking at 480 nm) is detected for [Fe2(BPEB)3], and is ascribable to the envelope of the lowintensity, spin-forbidden metal d–d transitions. Similar transitions were previously evidenced at very similar wavelengths for several minerals.30 Two additional bands fall in the red region of the spectrum (with peaks at 640 nm and 720 nm). The latter bands might be generated by electron hopping between the d orbitals of neighbouring Fe(III) ions in the crystal structure, in the case of partial photochemically induced reduction of Fe(III) to Fe(II) occurring in correspondence with local distortions of the octahedral structure.30 The notably lower intensity of this band with respect to those reported in ref. 30 stems in support of minimal perturbation of the octahedral stereochemistry in [Fe2(BPEB)3]. The above described spectral features account for the black colour of the compound. Fluorescence emission spectroscopy. The uorescence emission spectra of the ligand and a-[Zn(BPEB)] were recorded by using the PTI spectrouorimeter. Their peak-normalized line-shapes are reported in Fig. 10. Complexation of the ligand by the Zn(II) ion induces a notable hypsochromic shi of the uorescence emission band (namely from 500 nm for the ligand to 460 nm for a-[Zn(BPEB)]). Moreover, a roughly twofold increase in the uorescence quantum yield, F, is observed. On the other hand, the time-resolved uorescence decay patterns of H2BPEB and a-[Zn(BPEB)] are almost

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Fig. 10 Fluorescence emission spectral line-shapes of H2BPEB (red), [Fe2(BPEB)3] (cyan), [Ni(BPEB)] (purple), and a-[Zn(BPEB)] (blue), either recorded by the PTI spectrofluorimeter (dotted lines, H2BPEB and a-[Zn(BPEB)] only), or reconstructed by acquisition of the fluorescence photo-count rate through interferential filters by means of the TCSPC setup (solid lines; the actual wavelengths in correspondence of which photo counts were acquired are highlighted with symbols—see Experimentals).

superimposable (Fig. S14a†), suggesting unmodied excitedstate dynamics and very similar microenvironments for the uorophore (likely the phenyl ring) within the two compounds. For this reason, and taking into account that the absorption spectra of H2BPEB and a-[Zn(BPEB)] are also very similar (Fig. 9), which entails minimal perturbation of the ligand's ground-state energy levels upon Zn(II) complexation, the observed differences in uorescence emission were traced back to a signicant perturbation of the ligand lower unoccupied molecular orbital (LUMO) charge distribution by the Zn(II) ion, resulting in a remarkable change in the LUMO vibronic potential well. The latter change would straightforwardly explain both the hypsochromic shi,31 and the F increase.32 Both the ligand and the a-[Zn(BPEB)] decay patterns are optimally tted by a tripleexponential decay model. The best-tting parameters are reported in Table S2.† The latter were used to calculate the average uorescence lifetime values, also reported in Table S2,† according to the formula: .X X sav ¼ si Ai Ai (1) i

i

Because the uorescence emission intensity was too faint to be measured by means of the spectrouorimeter for both the Ni(II)- and the Fe(III)-containing materials, their spectral lineshapes and spectrum-integrated F were reconstructed by exploiting the single-photon sensitivity of the time-correlated single-photon counting (TCSPC) setup. The same procedure was also applied to H2BPEB and a-[Zn(BPEB)]. In Table S1† the F values relative to a-[Zn(BPEB)] as obtained by using the TCSPC setup are reported for all compounds.33 The spectral line-shapes of the Ni(II) and Fe(III) MOFs are very similar to one

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another, and exhibit a main peak closely resembling that of a-[Zn(BPEB)], and a shoulder similar to the ligand emission band. Considering the much higher relative F value of the ligand with respect to both [Fe2(BPEB)3] and [Ni(BPEB)], one is strongly tempted to attribute the latter shoulder to traces of residual uncomplexed ligand. If this speculation holds true, the experimentally observed spectra of the Fe(III) and Ni(II) samples are recovered by assuming an actual spectral line-shape equal to that of the Zn(II) species and quantities of uncomplexed ligand of less than 1% and 5%, respectively (undetectable by XRPD). The substantial similarity among the emission line-shapes of the three investigated MOFs suggests that metal complexation induces perturbation of the ligand LUMO charge distribution, independent of the chosen metal and crystal structure. This entails a quite loose coupling of the ligand LUMO with the metal AOs. However, the dramatic drops in F upon complexation with either Fe(III) or Ni(II) ions with respect to the Zn(II) derivative demand at least a tentative explanation. The drops appear to be due to distinct phenomena. Indeed, the uorescence decay pattern measured for [Fe2(BPEB)3] is not noticeably different from that recorded for H2BPEB and a-[Zn(BPEB)], indicating substantially similar excited-state dynamics. The apparently lower F value could be thus primarily associated with the Fe(III) strong absorption along the entire ligand emission band. Indeed, if the measured count rate is corrected for the metal opacity, a relative F value of 0.44, much closer to that obtained for the highly uorescent H2BPEB, is obtained. Conversely, the uorescence decay distribution observed in the case of [Ni(BPEB)] is best tted by a four-exponential decay model (Table S2†), with a much shorter average uorescence lifetime value (according to eqn (1), Table S2†). This fact provides strong evidence in favour of an additional, very efficient excited-state depopulation pathway. The square planar stereochemistry of the Ni(II) ion in [Ni(BPEB)], in which the phenyl rings of neighbouring ligands are parallel and aligned, suggests that collective stacking effects involving several transition dipole moments on the same stack might play a primary role.

3.

Experimental section

3.1. Materials and methods All reactions requiring an anhydrous or oxygen-free environment were performed in ame- or oven-dried glassware under nitrogen pressure. All solvents were dried and distilled under nitrogen by standard procedures.34 Unless otherwise specied, the reagents were obtained from commercial suppliers and used as received. 4-Iodo-1-(ethoxyethyl)-1H-pyrazole was prepared according to an already published procedure.35 All manipulations and reactions were carried out under an argon atmosphere by using standard Schlenk techniques. Microwave heating was performed by means of a CEM Discover SP instrument with a single-mode microwave cavity producing continuous irradiation at 2.45 GHz and power up to 0.3 kW. All reactions carried out with microwave heating were performed in 10 mL vessels equipped with a Teon septum and a magnetic stir bar. The temperature, pressure and irradiation power were


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continuously monitored during the course of the reaction. Flash chromatography was carried out using Merck Kieselgel 60 F254 (230–400 mesh) silica gel. Thin liquid chromatography (TLC) was performed on Merck glass plates precoated with F254 silica gel, visualizing the deposited materials upon exposure to UV light (lmax ¼ 254 nm). IR spectra were acquired in nujol mulls by means of a Shimadzu FT-IR Prestige 21 instrument over the range 4000–600 cm1; in the following, signal intensities are denoted as br ¼ broad, vs ¼ very strong, s ¼ strong, m ¼ medium, and w ¼ weak. 1H and 13C(APT) NMR spectra were recorded at 400 and 100 MHz on a Bruker Avance 400 spectrometer in DMSO-d6. 1H and 13C data are reported as follows: chemical shis (in ppm and referenced to internal TMS), integration, multiplicity (s ¼ singlet, d ¼ doublet, t ¼ triplet, q ¼ quartet, m ¼ multiplet), and coupling constants (in Hz). Thermogravimetric analyses and differential scanning calorimetry were performed simultaneously with a Netzsch STA 409 instrument under N2, from 30 C up to 900 C, increasing the temperature at a rate of 10 C min1. Elemental analyses were obtained with a Perkin Elmer CHN Analyzer 2400 Series II.

3.2. Synthesis of 1,4-bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB) An oven-dried Schlenk tube was purged with Ar and charged with 4-iodo-1-(1-ethoxyethyl)-1H-pyrazole (5.27 g, 19.8 mmol), Pd(PPh3)2Cl2 (0.56 g, 0.8 mmol) and CuI (0.15 g, 0.8 mmol). The tube was sealed with a rubber septum and Ar-purged triethylamine (TEA, 50 mL) was added by means of a syringe. The obtained solution, purged by bubbling Ar at room temperature, was stirred for 20 min; aerwards, 1,4-diethynylbenzene (1.00 g, 7.9 mmol) was added and the resulting mixture was heated up to 70 C and kept at this temperature under stirring for 4 h, that is, until the TLC analysis (silica gel; EtOAc–hexane 1 : 1) revealed the complete consumption of 1,4-diethynylbenzene. Then, the brown mixture was cooled down to room temperature and EtOAc (100 mL) was added. The mixture was then stirred for 10 min and ltered through a Celite plug. The pad was rinsed with EtOAc (2 50 mL). The ltrates were combined and solvents removed by a lab-scale rotary evaporator, leaving a brownish oil. The latter was dissolved in EtOAc (100 mL) and sequentially washed with water (50 mL), aqueous ammonium hydroxide (28% NH3, 50 mL) and brine. Aer the organic layer was dried over anhydrous Na2SO4 and concentrated in vacuo, the residue was puried by silica-gel ash chromatography (EtOAc–hexane 1 : 1) to afford bis(ethoxylethyl)-1,4-bis(1H-pyrazol-4-ylethynyl)benzene as a pale yellow solid (2.10 g, yield 66%). IR (nujol, cm1): 3091(w), 2220(w), 1336(w), 1268(w), 1157(m), 1124(vs), 1069(m), 981(m), 944(m), 868(m), 819(s), 715(m), 641(m). 1H NMR (DMSO-d6, ppm): 8.32 (1H, s), 7.76 (1H, s), 7.48 (1H, s), 5.55 (1H, q, J ¼ 6.0), 3.43 (1H, dq; J ¼ 7.0, 7.1), 3.21 (1H, dq; J ¼ 7.0, 7.2), 1.60 (3H, d, J ¼ 6.0), 1.04 (3H, t, J ¼ 8.1). 13C NMR (DMSO-d6, ppm): 142.7 (CH); 132.0 (CH); 131.7 (CH); 123.0 (C); 102.5 (C); 89.9 (C); 87.1 (CH); 83.9 (C); 63.7 (CH2); 21.5 (CH3); 15.2 (CH3). Elem. Anal. calc. for C24H26N4O2 (FW ¼ 402.5 g mol1): C, 71.62; H, 6.51; N, 13.92%; found C, 70.22; H, 5.71; N, 13.02%. This intermediate (1.00 g, 2.5 mmol)



was dissolved in dioxane (50 mL) aer the addition of 12 N aqueous HCl (0.3 mL), and the resulting solution was kept at room temperature. Precipitation of a pale yellow solid occurred very quickly and the resulting slurry was stirred for 30 min. The white solid was then collected by vacuum ltration, washed twice with a mixture of acetone–EtOH (1 : 1, 50 mL) and kept under vacuum (80 C, 0.01 kPa) for 2 h, until the weight remained constant (0.58 mg, yield 89%). IR (nujol, cm1): 3169(br), 2221(m), 1141(s), 1101(w), 1050(vs), 1037(vs), 1002(vs), 992(vs), 950(s), 941(s), 845(w), 868(s), 861(s), 834(vs), 798(br), 654(vs), 620(vs). 1H NMR (DMSO-d6, ppm): 13.3 (1H, br, s), 8.14 (1H, s), 7.74 (1H, s), 7.47 (2H, s). 13C NMR (DMSO-d6, ppm): 133.2 (CH), 131.6 (CH), 123.0 (C), 101.3 (C), 89.7 (C), 84.5 (C). Elem. Anal. calc. for C16H10N4 (FW ¼ 258.3 g mol1): C, 74.40; H, 3.91; N, 21.69%; found C, 74.08; H, 3.21; N, 21.32%. 3.3. Synthesis of [Fe2(BPEB)3] To a warm solution (60 C) of H2BPEB (0.12 g, 0.5 mmol) and iron(III) chloride hexahydrate (0.11 g, 0.3 mmol) in DMF (5 mL), TEA (1 mL) was added dropwise. The resulting dark brown solution was reuxed for 6 h and subsequently cooled down to room temperature. Then, the black solid obtained was recovered by ltration, washed with MeOH (5 2 mL) and dried under vacuum (150 C, 0.01 kPa) to give the title compound as a black solid (0.09 g, yield 62%). IR (nujol, cm1): 2218(w), 1164(s), 1064(s), 1020(w), 844(s), 831(s), 770(w), 720(w), 635(s). Elem. Anal. calc. for C48H24Fe2N12 (FW ¼ 884.5 g mol1): C, 65.12; H, 2.71; N, 18.99%. Found: C, 64.86; H, 3.01; N, 18.39%. 3.4. Synthesis of [Ni(BPEB)] To a warm (60 C) solution of H2BPEB (0.12 g, 0.5 mmol) in pyridine (5.0 mL), anhydrous nickel(II) nitrate hexahydrate (0.13 g, 0.5 mmol) was added under stirring. The solution was allowed to slowly warm up to 80 C, changing from green to red as it was warmed. Aerwards, TEA (2.0 mL) was added dropwise. The solution was then reuxed for 8 h, which resulted in a lot of orange/brick red precipitate. The mixture was allowed to cool down to room temperature and the solid was collected by vacuum ltration. The lter cake was washed with MeOH (5 2 mL) and dried (130 C, 0.01 kPa) to give a red solid consisting of the title compound (0.11 g, yield 78%). IR (nujol, cm1): 2203(w), 1228(w), 1164(w), 1055(w), 1015(w), 1007(w), 840(w), 769(w), 719(w), 638(w). Elem. Anal. calc. for C16H8N4Ni (FW ¼ 315.0 g mol1): C, 60.96; H, 2.54; N, 17.78%. Found: C, 60.48; H, 2.87; N, 16.87%. 3.5. Synthesis of a-[Zn(BPEB)] H2BPEB (0.12 g, 0.5 mmol) was suspended in pyridine (5 mL), heated up to 60 C and maintained at this temperature for 10 min. To the resulting solution, zinc perchlorate hexahydrate (0.17 g, 0.5 mmol) was added and the mixture was then reuxed for 6 h. Subsequently, the mixture was allowed to cool down to room temperature and the resulting white precipitate was collected by ltration, washed with MeOH (5 2 mL) and dried under vacuum (130 C, 0.01 kPa), to give the title compound as a pale yellow solid (0.10 g, yield 68%). IR (nujol, cm1): 2228(w),

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1052(w), 1064(s), 1017(w), 855(s), 848(w), 835(s), 827(w), 768(w), 721(w), 640(s), 637(s). Elem. Anal. calc. for C16H8N4Zn (FW ¼ 321.7 g mol1): C, 59.69; H, 2.49; N, 17.41%. Found: C, 59.21; H, 2.72; N, 16.85%.


3.6. Synthesis of b-[Zn(BPEB)] by a microwave-assisted path In the attempt to obtain a-[Zn(BPEB)] through a faster and more efficient way, a microwave-assisted synthesis was carried out. Unexpectedly, a distinct phase, [Zn(BPEB)]$0.75H2O (b[Zn(BPEB)] in the following), was recovered. In detail, a mixture of H2BPEB (0.12 g, 0.5 mmol), zinc acetate dihydrate (0.10 g, 0.5 mmol), TEA (1 mL) and dimethylformamide (DMF, 6 mL) was irradiated for 25 min. The reaction tube was allowed to cool down to 50 C and the white solid obtained was collected by ltration, washed with MeOH (5 2 mL) and dried (130 C, 0.01 kPa) to give a pale yellow solid consisting of the title compound (0.12 g, yield 84%). Elem. Anal. calc. for C16H9.5N4O0.75Zn (FW ¼ 335.17 g mol1): C, 57.34; H, 2.86; N, 16.72%. Found: C, 57.31; H, 3.07; N, 16.03%. 3.7. X-ray powder diffraction crystal structures determination Powdered, polycrystalline batches of compounds [Fe2(BPEB)3], [Ni(BPEB)], a-[Zn(BPEB)] and b-[Zn(BPEB)] were ground in an agate mortar; then, they were deposited in the hollow of an aluminium-framed, zero-background silicon sample holder. Diffraction data were collected at room temperature on a Bruker AXS D8 Advance q:q diffractometer, equipped with Ni-ltered ˚ a Lynxeye linear positionCu-Ka radiation (l ¼ 1.5418 A), sensitive detector, and the following optics: primary beam Soller slits (2.3 ), xed divergence slit (0.5 ), receiving slit (8 mm). The generator was set at 40 kV and 40 mA. To carry out the structure determinations, overnight scans were performed in the 2q range of 3–105 , with steps of 0.02 . A standard peak search in the region between 3 and 30 (2q) was followed by indexing through the singular value decomposition method36 implemented in TOPAS-R,37 allowing the determination of the unit cell parameters. Systematic absences permitted individuation of the most probable space groups. Prior to the structure solution, Le Bail renements were carried out to conrm unit cells and space groups. Preliminary structural models were determined ab initio for [Fe2(BPEB)3], [Ni(BPEB)] and a[Zn(BPEB)] by the simulated annealing approach implemented in TOPAS-R. An idealized rigid model was used38 to describe the crystallographically independent portion of the ligand. Structure renements were carried out by means of the Rietveld method39 with TOPAS-R, maintaining the rigid bodies adopted during the structure solution stage. Benecial for the values of the gures of merit, rotational disorder with respect to the main axis of the ligand was introduced for the central aromatic ring in all the three species, allowing for two equiprobable orientations. The peak shapes were described with the fundamental parameters approach.40 In all the cases the anisotropic broadening of the peaks was modeled by means of spherical harmonics. The background was modeled by a Chebyshev polynomial function. The thermal effect was simulated by using

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a single isotropic parameter for the metal ions, augmented by ˚ 2 for lighter atoms. 2.0 A b-[Zn(BPEB)] deserves a special comment: complementary information such as elemental analysis and infrared spectroscopy address the same composition as that of the a-phase (except for, in some preparations, clathrated water molecules). In addition, its diffractogram can be described by the same metrics as that of a-[Zn(BPEB)]. In spite of all these similarities, the relative intensities of the XRPD peaks vary considerably, with particularly remarkable differences seen for the [110] and [200] ones: while the [110] Bragg reection is the most intense for a-[Zn(BPEB)], in the case of b-[Zn(BPEB)] its relative intensity decreases with a concomitant increase of the [200] peak. This phenomenon results in the impossibility of describing satisfactorily the diffractogram of b-[Zn(BPEB)] with the structural model retrieved for a-[Zn(BPEB)]. A non-interpenetrated model, as already observed for [Zn(BDP_H)],7f equally fails. Both models do not supply sufficient electronic density to grant a reasonable agreement between the observed and calculated diffractograms. Calculation of the Fourier difference map aer carrying out renement with the interpenetrated model alone allowed insight into the residual electronic density, as detailed in the section dedicated to crystal structure analysis. Table 3 contains the most relevant crystallographic data and structure renement details for [Fe2(BPEB)3], [Ni(BPEB)] and a[Zn(BPEB)]. The nal Rietveld renements plots are collectively supplied in Fig. S1 of the ESI.† Crystallographic data in CIF format have been deposited at the Cambridge Crystallographic Data Centre as supplementary publications 984473–984475. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK

Main crystallographic data and refinement details for species [Fe2(BPEB)3], [Ni(BPEB)] and a-[Zn(BPEB)]

Table 3

[Fe2(BPEB)3]

[Ni(BPEB)]

a-[Zn(BPEB)]

Empirical formula C48H24Fe2N12 C16H8N4Ni C16H8N4Zn Mr 880.5 314.9 321.7 Crystal system Orthorhombic Orthorhombic Orthorhombic SPGR, Z Fddd, 8 Imma, 4 Cccm, 8 ˚ a, A 7.397(5) 6.800(2) 25.303(4) ˚ b, A 36.357(3) 31.73(1) 26.464(3) ˚ c, A 63.44(1) 18.356(3) 7.3073(6) ˚3 V, A 1 7060(13) 3960(2) 4893(1) rcalcd, g cm3 0.69 0.53 0.87 F(000) 3584 640 1296 m(CuKa), cm1 29.27 6.96 13.77 T, K 298 298 298 Renement 2q range, deg 4–105 4–105 4–105 Data, parameters 5051, 56 5051, 31 5051, 34 Rp, Rwpa 0.020, 0.026 0.053, 0.074 0.051, 0.071 RBragga 0.008 0.076 0.016 P P P P a 2 2 1/2 PRp ¼ i|yi,o Pyi,c|/ i|yi,o|; Rwp ¼ [ iwi (yi,o yi,c) / iwi(yi,o) ] ; RB ¼ n|In,o In,c|/ nIn,o, where yi,o and yi,c are the observed and calculated prole intensities, respectively, while In,o and In,c are the observed and calculated intensities. The summations run over i data points or n independent reections. Statistical weights wi are normally taken as 1/yi,o.


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(fax: +44-1223-335033; e-mail: [email protected] or http://www.ccdc.cam.ac.uk).


3.8. Variable-temperature X-ray powder diffraction Variable-temperature X-ray powder diffraction (VT-XRPD) experiments were performed on the as-synthesized species [Fe2(BPEB)3], [Ni(BPEB)], a-[Zn(BPEB)] and b-[Zn(BPEB)] to highlight their structural response to temperature variations. For this purpose, two different sets of measurements were carried out: step-by-step heating up to decomposition allowed assessment of thermal robustness and permanent porosity of the materials, while consecutive heating–cooling cycles permitted evaluation of their stability during thermal activation series. Both sets of experiments were performed in air in a suitable low-angle 2q range, using a custom-made sample heater assembled by Officina Elettrotecnica di Tenno, Ponte Arche, Italy, and mounted on the Bruker Advance D8 diffractometer described in the previous section. Powdered polycrystalline batches of [Fe2(BPEB)3], [Ni(BPEB)], a-[Zn(BPEB)] and b-[Zn(BPEB)] were ground in an agate mortar and deposited in the hollow of an aluminium sample holder. The thermal behaviour was followed up to decomposition, heating the samples in situ with steps of 20 C, and acquiring the data in isothermal conditions. The parametric treatment41 of the VTXRPD data with the Le Bail method allowed us to depict the variation in unit cell parameters as a function of temperature. The temperature cycles were performed by varying the temperature in situ between 50 C (to prevent possible undesired contributions by moisture) and 210 C, in the same angular range as before. When rst reaching 210 C, a number of XRPD traces was acquired until an asymptote was reached, to ensure the removal of moisture potentially present. When comparing the results of thermal analysis and VT-XRPD, the reader must be aware that the thermocouple of the VT-XRPD setup is not in direct contact with the sample; thus, there is a slight difference in the temperature at which the same event is detected by the two techniques. The temperatures deriving from the thermal analysis must be considered more reliable. 3.9. Chemical stability tests To perform a typical test, a 90 mg sample of [Fe2(BPEB)3], [Ni(BPEB)] or a-[Zn(BPEB)] was suspended in water (6 mL) and stirred at reux. At different time intervals (1 hour, 5 and 8 hours), an aliquot of each sample was collected by ltration, washed with successive aliquots of water (5 3 mL), dried under vacuum, and analyzed by X-ray powder diffraction. The same procedure was applied to perform the stability tests in acidic (aqueous HCl, pH 5 and 6) and basic (aqueous NaOH, pH 8 and 9) media at room temperature. 3.10.

Gas adsorption measurements

Nitrogen adsorption–desorption isotherms were acquired at the liquid nitrogen temperature using a Micromeritics analyzer ASAP 2020 HD. The samples were previously outgassed at 120 C for 3 h. The Brunauer, Emmet, Teller (BET) and Langmuir models were used to evaluate the specic surface areas. The


pore volume was evaluated following non-local density functional theory (NLDFT) analysis for cylindrical pores and the Tarazona method. CO2 and CH4 adsorption isotherms at 195 K, 273 K and 298 K and up to 10 bar were collected using a Micromeritics analyzer ASAP 2050. The selectivity of CO2/N2 binary mixtures was determined from the single-component isotherms using the ideal adsorbed solution theory (IAST) and a CO2/N2 ratio of 15 : 85 (mol : mol). Isosteric heats of adsorption were calculated by applying the Clausius–Clapeyron equation. 3.11.

Electronic transitions spectroscopy

UV-Vis absorption spectra were recorded on an Agilent Technologies Cary 5000 UV-Vis-NIR spectrophotometer. Steady-state uorescence emission spectra were acquired by using a PTI Fluorescence Master System spectrouorimeter. The instrument was driven by dedicated soware (Felix 2000), performing automatic corrections for the excitation lamp spectral intensity and the detector spectral quantum efficiency. The samples were introduced in 1 mm-thick quartz cells (Hellma) that were positioned, with the help of a rotator, at 45 with respect to the excitation beam direction. The emission spectra were recorded upon excitation of the compounds at their main absorption bands and at 355 nm. Time-resolved uorescence analysis was performed by reconstructing the uorescence decay distributions by means of a time-correlated single-photon counting apparatus endowed with a temporal resolution lower than 30 ps, which is fully described elsewhere.42 The uorescence of the samples was excited at 355 nm by the third harmonic of a SESAM mode locked Nd:VAN laser (GE-100 SHG, Time Bandwidth Products, Zurich), delivering 5 ps pulses at a 113 MHz repetition rate. The uorescence was selected through a cut-off lter (LL-400, Corion, Holliston, MA). In order to reconstruct the emission spectra of the faintly uorescent Ni(II) and Fe(III) derivatives, the wavelength-resolved, uorescence-photons count rate was measured for all compounds by using a set of 4 nm band-pass interferential lters (CVI Laser Optics, Albuquerque, NM, USA). The spectrum-integrated quantum yield of the compounds upon excitation at 355 nm by the Nd:VAN laser was determined by comparison with that pertaining to a[Zn(BPEB)] (arbitrarily set equal to 1), aer normalization for the relative absorbance at 355 nm as measured by UV-Vis spectroscopy. The uorescence decay data were tted, without performing a deconvolution of the system pulse response, with either three- or four-exponential components above a constant background, by minimizing the c2 value through a Levenberg– Marquardt algorithm. Addition of further decay components did not improve the c2-value and the residual randomness around zero. Three decay curves were acquired for each sample: the uorescence lifetimes and relative initial amplitudes were calculated as averages of the values obtained from the ts, with errors given by standard deviations.

4. Conclusions Herein, we have reported the isolation and characterization of the novel bis(pyrazolato)-based MOFs [Fe2(BPEB)3], [Ni(BPEB)] J. Mater. Chem. A, 2014, 2, 12208–12221 | 12219

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and a-[Zn(BPEB)]. As assessed by XRPD structure determination methods, all possess porous 3-D networks featuring 1-D pervious channels. Stable in air at least up to 415 C, the three materials show remarkable thermal robustness. The Ni(II) derivative also possesses moderate chemical stability as veried toward water vapours, in boiling water and in moderately acidic and basic aqueous solutions at room temperature. The high uorescence of the ligand is perturbed by different mechanisms in the three MOFs, promising high sensitivity to environmental changes. The permanent porosity of the three MOFs was demonstrated by N2 adsorption isotherms at 77 K, which revealed high Langmuir surface areas, achieving 2300 m2 g1. Finally, the relevant capture of CO2 and CH4, noted especially for the Fe(III)-based compound, combined with high thermal stability, makes these materials good competitors for industrial applications.

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Acknowledgements Funding for this work was provided by the Fondazione Cariplo (nos 2011-0289 and 2012-0921) and the Italian Ministry of Education, Universities and Research (PRIN 2011). The authors are grateful to Dr G. Cernuto for acquiring the UV-Vis absorption spectra.

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Notes and references 1 See e.g. M. S. Silverstein, N. R. Cameron and M. A. Hillmyer, Porous Polymers, Wiley, 2011; S. R. Batten, S. M. Neville and D. R. Turner, Coordination Polymers: Design, Analysis and Application, Springer, New York, 2010. 2 See e.g. H. Li, M. Eddaoudi, M. O'Keeffe and O. M. Yaghi, Nature, 1999, 402, 276–279; N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O'Keeffe and O. M. Yaghi, Science, 2003, 300, 1127–1129; J. L. C. Rowsell and O. M. Yaghi, Angew. Chem., Int. Ed., 2005, 44, 4670– 4679; D. J. Collins and H.-C. Zhou, J. Mater. Chem., 2007, 17, 3154–3160. 3 K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724–781. 4 T. A. Makal, J.-R. Li, W. Lua and H.-C. Zhou, Chem. Soc. Rev., 2012, 41, 7761–7779. 5 See e.g. Chem. Rev., 2012, 112, issue 8; Chem. Soc. Rev., 2009, 38, issue 5. 6 See e.g. J.-P. Zhang, Y.-B. Zhang, J.-B. Lin and X.-M. Chen, Chem. Rev., 2012, 112, 1001–1033. 7 (a) V. Colombo, C. Montoro, A. Maspero, G. Palmisano, N. Masciocchi, S. Galli, E. Barea and J. A. R. Navarro, J. Am. Chem. Soc., 2012, 134, 12830–12843; (b) C. Pettinari, A. T˘ ab˘ acaru, I. Boldog, K. V. Domasevitch, S. Galli and N. Masciocchi, Inorg. Chem., 2012, 51, 5235–5245; (c) V. Colombo, S. Galli, H. J. Choi, G. D. Han, A. Maspero, G. Palmisano, N. Masciocchi and J. R. Long, Chem. Sci., 2011, 2, 1311–1319; (d) A. T˘ ab˘ acaru, C. Pettinari, N. Masciocchi, S. Galli, F. Marchetti and M. Angjellari, Inorg. Chem., 2011, 50, 11506–11513; (e) N. Masciocchi,

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S. Galli, V. Colombo, A. Maspero, G. Palmisano, B. Seyyedi, C. Lamberti and S. Bordiga, J. Am. Chem. Soc., 2010, 132, 7902–7904; (f) S. Galli, N. Masciocchi, V. Colombo, A. Maspero, G. Palmisano, F. J. L´ opez-Garz´ on, M. Domingo-Garc´ıa, I. Fern´ andez-Morales, E. Barea and J. A. R. Navarro, Chem. Mater., 2010, 22, 1664–1672. See e.g. (a) H. J. Choi, M. Dinc˘ a, A. Dailly and J. R. Long, Energy Environ. Sci., 2010, 3, 117–123; (b) H. J. Choi, M. Dinc˘ a and J. R. Long, J. Am. Chem. Soc., 2008, 130, 7848–7850; (c) J.-P. Zhang and S. Kitagawa, J. Am. Chem. Soc., 2008, 130, 907–917. F. G. Bordwell, Acc. Chem. Res., 1988, 21, 456–463. E. Quartapelle Procopio, S. Rojas, N. M. Padial, S. Galli, N. Masciocchi, F. Linares, D. Miguel, E. J. Oltra, J. A. R. Navarro and E. Barea, Chem. Commun., 2011, 47, 11751–11753. A. T˘ ab˘ acaru, C. Pettinari, I. Timokhin, F. Marchetti, F. Carrasco-Mar´ın, F. J. Maldonado-H´ odar, S. Galli and N. Masciocchi, Cryst. Growth Des., 2013, 13, 3087–3097. Z. R. Herm, B. M. Wiers, J. A. Mason, J. M. van Baten, M. R. Hudson, P. Zajdel, C. M. Brown, N. Masciocchi, R. Krishna and J. R. Long, Science, 2013, 340, 960–964. Calculated as the van der Waals corrected distance between the nearest atoms protruding into the channel. The percentage of void volume was estimated by PLATON, A. L. Spek, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1990, 46, C34. C. Serre, F. Millange, C. Thouvenot, M. Nogu` es, G. Marsolier, D. Lou¨ er and G. F´ erey, J. Am. Chem. Soc., 2002, 124, 13519– 13526. On the hypothetical formation of the unsubstituted tetrahedrane, see S. A. Kandil and R. E. Dessy, J. Am. Chem. Soc., 1966, 88, 3027–3034, On the formation of sterically hindered tetrahedranes, see e.g. M. Nakamoto, Y. Inagaki, T. Ochiai, M. Tanaka and A. Sekiguchi, Heteroat. Chem., 2011, 22, 412–416; G. Maier, J. Neudert, O. Wolf, D. Pappusch, A. Sekiguchi, M. Tanaka and T. Matsuo, J. Am. Chem. Soc., 2002, 124, 13819–13826. On the thermochemistry of tetrahedrane, see M. N. Glukhovtsev, S. Laiter and A. Pross, J. Phys. Chem., 1995, 99, 6828–6831, On the thermochemistry of acetylene, see M. W. Chase Jr, NIST-JANAF Themochemical Tables, Fourth Edition, J. Phys. Chem., Ref. Data, Monograph 9, 1998, 1–1951. The g-phase was isolated through the STA instrument by heating at 10 C min1 up to 320 C, then reducing the heating rate down to 4 C min1 up to 420 C. Aer the treatment, the brownish sample was analyzed by means of XRPD. ˚ c ¼ 7.768(3) A, ˚ V ¼ 6772(5) A ˚ 3. P42/mmc, a ¼ 29.53(1) A, In the IR spectrum of the material aer the treatment a band at 3620 cm1 appears, reasonably attributable to the presence of the hydroxo moiety. Z. Liang, M. Marshall and A. L. Chaffee, Energy Procedia, 2009, 1, 1265–1271; Z. Liang, M. Marshall and A. L. Chaffee, Energy Fuels, 2009, 23, 2785–2789; P. Chowdhury, C. Bikkina and S. Gumma, J. Phys. Chem. C,


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32 Associated with increased radiative decay rate. 33 The line-shape reconstruction procedure yields optimal results: for both the ligand and a-[Zn(BPEB)], a comparison of the reconstructed with the directly recorded emission spectra is available in Fig. 10. The ligand-to-a[Zn(BPEB)] relative F value obtained by spectra reconstruction with the TCSPC setup is equal, within a few percent, to that calculated from direct spectra measurements with the PTI uorimeter (reported in brackets in Table S1†). The spectral line-shapes obtained for the Fe(III) and Ni(II) derivatives, and reported as crossed and starred lines in Fig. 10, are thus denitely reliable. 34 D. D. Perrin and W. L. F. Armarego, Purication of Laboratory Chemicals, Pergamon Press, Oxford, 3rd edn, 1988. 35 Q. Lin, D. Meloni, Y. Pan, R. Xia, J. Rodgers, S. Shepard, M. Li, L. Galya, B. Metcalf, T.-N. Yue, P. Liu and J. Zhou, Org. Lett., 2009, 11, 1999–2002. 36 A. Coelho, J. Appl. Crystallogr., 2003, 36, 86–95. 37 TOPAS Version 3.0, Bruker AXS, 2005, Karlsruhe, Germany. 38 The z-matrix formalism was used to describe the BPEB2 moiety. Idealized bond distances and angles were adopted ˚ as follows: C–C, C–N, N–N of the penta-atomic ring 1.36 A; ˚ exocyclic C–C single C–C of the hexa-atomic ring 1.39 A; ˚ exocyclic C–C triple bond 1.24 A; ˚ C–H, N–H bond 1.50 A; ˚ penta-atomic ring internal bond angles 108 ; hexa0.95 A; 39 40 41 42

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