Functionalized o-Quinones - MDPI

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May 13, 2018 - Manipulation of the oxidation state of the o-quinone ligand can be realized in .... transformations are observed in acetonitrile at −0.45 V and −1.25 V vs. SCE, respectively ..... Reproduced from Dalton Trans. 2017, 46,. Scheme ...
Review

Functionalized o-Quinones: Concepts, Achievements inorganics and Prospects Review

Konstantin Martyanov * and Viacheslav Kuropatov *

Functionalized o-Quinones: Concepts, Achievements G.A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Box-445, and Prospects

Tropinina str. 49, Nizhny Novgorod 603950, Russia * Correspondence: [email protected] (K.M.); [email protected] (V.K.); Tel.: +7-831-462-7682 (V.K.) ID Konstantin Martyanov * and Viacheslav Kuropatov *

Received:G.A. 30 March 2018; Accepted: 10 May 2018; Published: date Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Box-445, Tropinina str. 49, 603950 Nizhny Novgorod, Russia * Correspondence: [email protected] (K.M.); [email protected] (V.K.); Tel.: +7-831-462-7682 (V.K.)

Abstract: o-Quinones are both well-studied and promising redox-active chelating ligands. There are  plenty of interesting effects to be found on o-quinone-derived metallocomplexes, such as photo Received: 30 March 2018; Accepted: 10 May 2018; Published: 13 May 2018 thermo mechanical effect in solid phase and in solution, luminescence, SMM properties and so on. Abstract: o-Quinones are both well-studied and promising redox-active chelating ligands. There are A combination o-quinone function with an additional coordination site or redox active fragment in plenty of interesting effects to be found on o-quinone-derived metallocomplexes, such as the samephoto-thermo molecule might sufficiently extend an assembling as well as functional capability of such mechanical effect in solid phase and in solution, luminescence, SMM properties ligand inand complexes. In this paper, the authors focusanon o-quinones decorated with additional nonso on. A combination o-quinone function with additional coordination site or redox active dioxolene chelating site or with electron donor redoxasactive fragments. fragment in thecoordination same molecule might sufficiently extend an assembling well asannelated functional capability of such ligand in complexes. In this paper, the authors focus on o-quinones decorated with additional non-dioxolene chelating coordination site orredox with electron donor redox active annelated fragments. Keywords: o-quinone; bifunctional ligands; amphoteric ligands; molecular devices Keywords: o-quinone; bifunctional ligands; redox amphoteric ligands; molecular devices

1. Introduction

1. Introduction

o-Quinones and their redox forms catecholates and semiquinonates have been continuously o-Quinones and their redox forms catecholates and semiquinonates have been continuously studied studied for many decades (Scheme 1).1).AArich chemistry of o-quinones provides for many decades (Scheme rich synthetic synthetic chemistry of o-quinones provides a nice a nice opportunity for preparation of o-quinone-based biologically active compounds for pharmaceutical opportunity for preparation of o-quinone-based biologically active compounds for pharmaceutical applications well for construction the constructionof ofnovel novel redox-active ligands for coordination chemistry. applications as wellasas forasthe redox-active ligands for coordination chemistry.

Scheme 1. Redox o-quinone. Scheme 1. Redoxtransformations transformations of of o-quinone.

The redox naturenature of o-quinone species thatdetermines determines activity in The redox of o-quinone speciesisisa akey key property property that theirtheir activity in biological o-Quinones are involved in oxygentransfer transfer chains organisms. biological systems. systems. o-Quinones are involved in oxygen chainsininlivelive organisms. In In coordination chemistry a redox activity of o-quinone ligands is led to relevant applications in material coordination chemistry a redox activity of o-quinone ligands is led to relevant applications in material sciences and catalysis. Redox isomerism of metallocomplexes in solid state and solution as sciences well andascatalysis. Redox isomerism of metallocomplexes in solid state and solution as well as reversible bending of crystals of Rh and Co semiquinonate complexes are also the result of reversible bending of crystals of o-quinone Rh and Co semiquinonate complexes are also the result of redox redox transformations within ligand [1–3]. transformations o-quinone ligandpoint [1–3]. Fromwithin a coordination chemistry of view o-quinones have some useful features: they form stable complexes with almost all metals; moreover they can change oxidation state, From a coordination chemistry point of view o-quinones have someformal useful features: they form directly being coordinated to the metal ion. Manipulation of the oxidation state of the o-quinone stable complexes with almost all metals; moreover they can change formal oxidation state, directly ligand can be realized in different ways, e.g., by optical or thermo-activation, by addition/ being coordinated to the metal ion. Manipulation of the oxidation state of the o-quinone ligand can displacement/removal of auxiliary ligand in the coordination sphere of the metal ion and so on. be realized in different ways, reservoir e.g., by optical or thermoaddition/displacement/removal A concept of an electron is typically attributedactivation, to o-quinoneby ligands: ligand-centered redox of auxiliary ligand in the coordination of complexes the metalwith ionnon-transition and so on. metals A concept processes provide catalytic activity forsphere o-quinone [4]. of an electron reservoir is typically attributed to o-quinone ligands: ligand-centered redox processes provide 2018, doi:10.3390/inorganics6020000 www.mdpi.com/journal/inorganics catalyticInorganics activity for6, 0;o-quinone complexes with non-transition metals [4]. Due to these peculiarities, o-quinones are promising building blocks for the construction of molecular devices and functional materials. From this point of view a topology of ligand that

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Inorganics 6, x FORpeculiarities, PEER REVIEW o-quinones are promising building blocks for the construction 2 of of 25 Due2018, to these molecular devices and functional materials. From this point of view a topology of ligand that provides an ordering of of thethe molecules in provides an opportunity opportunity for forsynthesis synthesisofofcoordination coordinationpolymers polymersororanan ordering molecules the crystal phase may be very useful. o-Quinone itself might be a bridging ligand in case of binding in the crystal phase may be very useful. o-Quinone itself might be a bridging ligand in case of of metal ions both by dioxolene and by π-complexation with diene system The chemistry binding of metal ions both by site dioxolene site and by π-complexation with [5–10]. diene system [5–10]. of o-quinones is immensely rich and diverse, so there are many ways to functionalize an o-quinone The chemistry of o-quinones is immensely rich and diverse, so there are many ways to functionalize molecule. An molecule. extension An of coordination functionaland capabilities an o-quinone molecule by an o-quinone extension of and coordination functionalofcapabilities of an o-quinone means of introduction an additional site (i.e., redox-active) a very promising molecule by means of of introduction of chelating an additional chelating site (i.e.,is redox-active) is a way. very There are several recent papers that describe the synthesis, coordination modes and important promising way. There are several recent papers that describe the synthesis, coordination modes properties of bridging di-o-quinones (OO~OO) and their isoelectronic di-o-iminoquinones and important properties of bridging di-o-quinones (OO~OO) and analogues their isoelectronic analogues (NO~NO) and di-o-diimines (NN~NN) [4,11–13]. In this review, we focus on the chemistry di-o-iminoquinones (NO~NO) and di-o-diimines (NN~NN) [4,11–13]. In this review, we focusand on coordination of bifunctional o-quinones decorated with additional alternative chelating the chemistrycapabilities and coordination capabilities of bifunctional o-quinones decorated with additional coordination sites (NN~OO, SS~OO systems) SS~OO as wellsystems) as on annulated di-o-quinone assemblies alternative chelating coordination sites (NN~OO, as well as on annulated di-o-quinone containing an additional redox-active unit. assemblies containing an additional redox-active unit.

2. OO~NN OO~NN Systems Systems

2.1. Coordination Properties of 1,10-Phenanthroline-5,6-dione One of the first reported examples of such bifunctional ligands is 1,10-phenanthroline-5,6-dione (pdon). compound was synthesized in 1947 as ainbyproduct of nitration of 1,10-phenanthroline (pdon). This This compound was synthesized 1947 as a byproduct of nitration [14]. of Now there is a simple, procedure the preparation of 1,10-phenanthroline-5,6-dione 1,10-phenanthroline [14].effective Now there is a for simple, effective procedure for the preparation of (Scheme 2) [15]. 1,10-phenanthroline-5,6-dione (Scheme 2) [15].

Scheme 2. Synthesis Scheme 2. Synthesis of of 1,10-phenanthroline-5,6-dione. 1,10-phenanthroline-5,6-dione.

Pdon combines two coordination centers. The dioxolene coordination site exhibits typical Pdon combines two coordination centers. The dioxolene coordination site exhibits typical o-quinone properties. A diimine chelating function at the backbone behaves as a 2,2′-bipyridine-like o-quinone properties. A diimine chelating function at the backbone behaves as a 2,20 -bipyridine-like unit. Numerous papers describe diimine coordination mode of pdon ligand only [16–18]. Concerning unit. Numerous papers describe diimine coordination mode of pdon ligand only [16–18]. Concerning to bifunctional ligands bearing two different coordination centers, the crucial problem is a separate to bifunctional ligands bearing two different coordination centers, the crucial problem is a separate manipulation by binding of the metal ions with each chelating site. manipulation by binding of the metal ions with each chelating site. The coordination bifunctionality of 1,10-phenanthroline-5,6-dione was first shown by Balch in The coordination bifunctionality of 1,10-phenanthroline-5,6-dione was first shown by Balch in 1975 [19]. Depending on the nature of attacking metal fragment, the resulting product will be NN- or 1975 [19]. Depending on the nature of attacking metal fragment, the resulting product will be NNOO-coordinated (Scheme 3). Oxidative addition of the low-valent Pt(PPh3)4, to the dioxolene site or OO-coordinated (Scheme 3). Oxidative addition of the low-valent Pt(PPh3 )4 , to the dioxolene site yields to formation of catecholate complex 2, while (MeCN)2PdCl2 selectively reacts towards the yields to formation of catecholate complex 2, while (MeCN)2 PdCl2 selectively reacts towards the N-donor center with generation of 3. N-donor center with generation of 3. Both mononuclear complexes 2 and 3 bear vacant coordination sites, and could be regarded as either “benzoquinone equivalent” (for complexes (N,N 0 -pdon) MLn ) or “bipyridine equivalent” (for complexes (O,O0 -pdon) MLn ), respectively. Binuclear complex 4 containing both occupied diimine and dioxolene coordination site was described later (Scheme 4) [20].

Scheme 3. Coordination bifunctionality of 1,10-phenanthroline-5,6-dione.

Both mononuclear complexes 2 and 3 bear vacant coordination sites, and could be regarded as either “benzoquinone equivalent” (for complexes (N,N′-pdon) MLn) or “bipyridine equivalent”

manipulation by binding of the metal ions with each chelating site. The coordination bifunctionality of 1,10-phenanthroline-5,6-dione was first shown by Balch in 1975 [19]. Depending on the nature of attacking metal fragment, the resulting product will be NN- or OO-coordinated (Scheme 3). Oxidative addition of the low-valent Pt(PPh3)4, to the dioxolene site Inorganics 6, 0 3 ofthe 28 yields to2018, formation of catecholate complex 2, while (MeCN)2PdCl2 selectively reacts towards N-donor center with generation of 3.

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(for complexes complexes (O,O′-pdon) (O,O′-pdon) ML MLnn),), respectively. respectively. Binuclear Binuclear complex complex 44 containing containing both both occupied occupied (for Scheme 3. Coordination bifunctionality of 1,10-phenanthroline-5,6-dione. Scheme 3. Coordination bifunctionality of 1,10-phenanthroline-5,6-dione. diimine and and dioxolene dioxolene coordination coordination site site was was described described later later (Scheme (Scheme 4) 4) [20]. [20]. diimine Both mononuclear complexes 2 and 3 bear vacant coordination sites, and could be regarded as either “benzoquinone equivalent” (for complexes (N,N′-pdon) MLn) or “bipyridine equivalent”

Scheme 4. Binuclear complex 4. Scheme4. 4.Binuclear Binuclearcomplex complex4. 4. Scheme

X-ray crystallography crystallography study showed showed that both both metal metal centers centers (Pd(II) (Pd(II) at at diimine site site and and Pt(II) Pt(II) at at X-ray X-ray crystallographystudy study showedthat that both metal centers (Pd(II) diimine at diimine site and Pt(II) dioxolene site, respectively) are in almost square-planar surrounding, which is typical for such dioxolene site,site, respectively) areare in in almost at dioxolene respectively) almostsquare-planar square-planarsurrounding, surrounding,which whichisistypical typicalfor for such such complexes. A A stacking stacking motif motif was was found found for for this this complex complex in in the the crystal crystal lattice: lattice: the the molecules molecules in in stack stack complexes. complexes. A stacking motif was found for this complex in the crystal lattice: the molecules in are parallel parallel to to each each other, other, and interplanar interplanar distance distance is is 3.29 3.29 ÅÅ .. Moreover, Moreover, the the fragments fragments are are rotated rotated are stack are parallel to each and other, and interplanar distance is 3.29 Å. Moreover, the fragments are relative to each other by approximately 90° in order to provide a short distance between donor and relative each other by approximately 90° in order a short distance between donor and rotated to relative to each other by approximately 90◦toinprovide order to provide a short distance between acceptor regions regions of of the the adjacent adjacent molecules molecules as as well well as as to to reduce reduce a steric contact contact between between PPh PPh 3 ligands ligands acceptor donor and acceptor regions of the adjacent molecules as well aassteric to reduce a steric contact3between (Figure 1). 1). The stacking stacking motif motif in in aa crystal crystal phase phase was was not not observed observed in in case of of (Figure PPh3 ligands The (Figure 1). The stacking motif in a crystal phase was not observed incase case of (PPh 3)2Pt(O,O′-pdon-N,N′)Ru(PPh 3 ) 2 Cl 2 binuclear complex where a bulkier metallofragment is 0 0 (PPh 3)23Cl complex (PPh33)2)Pt(O,O′-pdon-N,N′)Ru(PPh )22Clbinuclear complexwhere wherea abulkier bulkier metallofragment metallofragment isis 2 Pt(O,O -pdon-N,N )Ru(PPh 2 binuclear coordinated at at the diimine diimine site. coordinated coordinated at the the diimine site. site.

Figure 1. Intermolecular pairing of molecules 4 in solid state. Reprinted with permission from Inorg. Figure 1. Intermolecular pairing of molecules 4 in solid state. Reprinted with permission from Inorg. Figure1991, 1. Intermolecular pairing of (1991) molecules Chem. 30, 2895–2899. Copyright ACS.4 in solid state. Reprinted with permission from Chem. 1991, 30, 2895–2899. Copyright (1991) ACS. Inorg. Chem. 1991, 30, 2895–2899. Copyright (1991) ACS.

A reaction reaction of of “bipyridine “bipyridine equivalent” equivalent” (O,O′-pdon)Pt(PPh (O,O′-pdon)Pt(PPh33))22 (2) (2) with with Pt Pt22dba dba33 (dba— (dba— A dibenzylideneacetone) results results in in coordination coordination of of the the platinum platinum (0) (0) at at the the diimine diimine site site to to give give binuclear binuclear dibenzylideneacetone) complex 5 (Scheme 5). complex 5 (Scheme 5).

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A reaction of “bipyridine equivalent” (O,O0 -pdon)Pt(PPh3 )2 (2) with Pt2 dba3 (dba— dibenzylideneacetone) results in coordination of the platinum (0) at the diimine site to give binuclear complex 5 (Scheme Inorganics 2018, 6, x FOR5). PEER REVIEW 4 of 25 Inorganics 2018, 6, x FOR PEER REVIEW

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Scheme 5. 5. Platinum complex 5. 5. Scheme Platinum complex Scheme 5. Platinum complex 5.

In the reactions with o-quinones this binuclear adduct behaves similar to (bpy)Pt(dba) giving In the the reactions reactions with with o-quinones o-quinones this this binuclear adduct adduct behaves behaves similar similar to to (bpy)Pt(dba) giving In (bpy)Pt(dba) corresponding catecholate complexesbinuclear [20]. Thus, an interaction of 5 giving with corresponding catecholate complexes [20]. Thus, an interaction of 5 with 3,5-di-tert-butyl-ocorresponding catecholate complexes [20]. Thus, an interaction of 5 with 3,5-di-tert-butyl-o-benzoquinone yields to catecholate adduct 6 (Scheme 6). benzoquinone yields to catecholate adduct 6 (Scheme adduct 6). 3,5-di-tert-butyl-o-benzoquinone yields to catecholate 6 (Scheme 6).

Scheme 6. Binuclear complex 6. Scheme 6. Binuclear Binuclear complex complex 6.

Electrochemical study of 1,10-phenanthroline-5,6-dione and its complexes showed that Electrochemical 1,10-phenanthroline-5,6-dione complexes showed that introduction of the study diimineof made the o-quinone and unit its a more powerful oxidant. Electrochemical study of function 1,10-phenanthroline-5,6-dione and its complexes showed that introduction of made o-quinone aa more Two one-electron peaks function corresponding the unit o-quinone‒o-semiquinone and introduction of the the diimine diimine function made the the to o-quinone unit more powerful powerful oxidant. oxidant. Two one-electron peaks corresponding to theacetonitrile o-quinone‒o-semiquinone o-semiquinone‒catecholate transformations are observed in −0.45 V and −1.25 Vand vs. Two one-electron peaks corresponding to the o-quinone-o-semiquinone andato-semiquinone-catecholate o-semiquinone‒catecholate transformations are observed in acetonitrile at −0.45 V and −1.25 V vs. SCE, respectively [21]. At the same conditions 9,10-phenanthrenequinone reduces at −0.64 V and transformations are observed in acetonitrile at −0.45 V and −1.25 V vs. SCE, respectively [21]. At the SCE, [21]. At the same 9,10-phenanthrenequinone reduces at coordination −0.64 V and −1.22 respectively V. Moreover, coordination of conditions a metal ion atatthe diimine alsoV.significantly affects the same conditions 9,10-phenanthrenequinone reduces −0.64 V andsite −1.22 Moreover, −1.22 V. Moreover, coordination of a metal ion at the diimine site also significantly affects electrochemical of the unit, increasing theelectrochemical oxidation ability. In a range of of a metal ion atproperties the diimine site o-quinone also significantly affects the properties of the the electrochemical properties of the o-quinone unit, increasing the oxidation ability. In a range of 0 complexes unit, [M(N,N′-pdon) )2 (M = Co, Ru, Fe), values the first reduction potential o-quinone increasing3](PF the 6oxidation ability. In the a range of of complexes [M(N,N -pdon) 3 ](PFare 6 )2 complexes [M(N,N′-pdon) 3](PF6)2 (M = Co, Ru, Fe), the values of the first reduction potential are shifted from −0.45 V to −0.11 −0.13 and −0.18potential V, respectively. Very similar (M = Co, Ru, Fe), the values ofV,the firstVreduction are shifted from −0.45results V to −were 0.11 V,observed −0.13 V shifted from to −0.11 V,similar −0.13 Vresults and −0.18 V, respectively. Very site similar results were observed 0 -pdon)]Cl by Winpenny et al.Vfor [M(N,N′-pdon)]Cl 2 (M = Pd, Pt): theby dioxolene processes were2 and −0.18 V, −0.45 respectively. Very were observed Winpenny etreduction al. for [M(N,N by Winpenny et al. for [M(N,N′-pdon)]Cl 2 (M = Pd, Pt): the dioxolene site reduction processes were found to Pt): move more positive potentialsprocesses on coordination to a metal center to achieve −0.16 V and (M = Pd, thetodioxolene site reduction were found to move to more positive potentials found move to to more positive potentials on [22]. coordination to a−metal center to achieve and −0.12 Vtofor Pd and Ptaderivatives respectively The reduction process at the diimine site inVpdon on coordination metal center to achieve −0.16 V and 0.12 V for Pd and Pt −0.16 derivatives −0.12 V for Pd andThe Pt to derivatives respectively Thesite reduction the diimine was not observed up −2.0 V. process respectively [22]. reduction at the [22]. diimine in pdonprocess was notatobserved up site to −in 2.0pdon V. was not observed up to −2.0 V. Besides of catecholate coordination, there is a possibility of dicarbonyl binding of dioxolene site Besides of catecholate there is a possibility of dicarbonyl bindingform of dioxolene site metal ion. ion. coordination, In dioxolene site exists in an o-quinone of pdon at the metal In this case, the dioxolene as a neutral of pdon atligand. the metal ion. this basicity case, the dioxolene site exists anthan o-quinone form as a site, neutral Since the Lewis ofof the diimine sitesite is greater thatthat of dioxolene the chelating ligand. Since theIn Lewis basicity the diimine is in greater than of dioxolene site, chelating ligand. Since the Lewis basicity of the diimine site is greater than that of dioxolene site, the Lewis acids in ainratio 1:11:1 with pdon form mononuclear the Lewis acids a ratio with pdon form mononucleardiimine diimineadducts adductsatatthe thefirst first stage. stage. Dioxolene Dioxolene Lewis acids ininvolved a ratio 1:1 pdononly form mononuclear adducts at is thepresent first stage. Dioxolene site becomes inwith binding when an excessdiimine of the Lewis acid in the reaction when site becomes involved in binding only when an excess of the Lewis acid is present in the reaction mixture (Scheme 7) [23]. mixture (Scheme 7) [23].

Scheme 7. Synthesis of binuclear Ti complex of pdon.

Besides of catecholate coordination, there is a possibility of dicarbonyl binding of dioxolene site of pdon at the metal ion. In this case, the dioxolene site exists in an o-quinone form as a neutral chelating ligand. Since the Lewis basicity of the diimine site is greater than that of dioxolene site, the Lewis acids in a ratio 1:1 with pdon form mononuclear diimine adducts at the first stage. Dioxolene site becomes in binding only when an excess of the Lewis acid is present in the reaction Inorganics 2018, 6,involved 0 5 of 28 mixture (Scheme 7) [23].

Scheme 7. 7. Synthesis Synthesis of of binuclear binuclear Ti Ti complex complex of of pdon. pdon. Scheme

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It was was established established that that the thebasicity basicityof ofthe thediimine diimineunit unitininpdon pdonisissignificantly significantlyenlarged enlarged when It when a a metal is coordinated to the dioxolene site. This is due to the electron donation from the catecholate metal is coordinated to the dioxolene site. This is due to the electron donation from the catecholate unit being being occupied occupied with with aa metal Any modification modification of of the the surrounding unit metal ion ion [24]. [24]. Any surrounding at at one one coordination coordination site on the bifunctional ligand affects the properties of the remote coordination site. Sothe that the site on the bifunctional ligand affects the properties of the remote coordination site. So that order order of priority in the binding of different coordination sites at such bifunctional ligands is essential. of priority in the binding of different coordination sites at such bifunctional ligands is essential. When When employing the proper synthetic strategy, one can manipulate the selectivity for direction of the employing the proper synthetic strategy, one can manipulate the selectivity for direction of the metal metal ion coordination and therefore to avoid undesirable products. ion coordination and therefore to avoid undesirable products. Reduced form form of of pdon pdon 1,10-phenanthroline-5,6-diol 1,10-phenanthroline-5,6-diol (pdol, precursor for for metal metal Reduced (pdol, 9) 9) can can also also act act as as aa precursor complexes. The The reaction reaction of of 99 with with the the corresponding corresponding metal metal halide halide in in presence presence of of base base was was used used for for complexes. 0 -pdon) (10, tbbpy-di-tert-butylbipyridine) (Scheme 8) [25]. synthesis of (tbbpy)Pt(O,O synthesis of (tbbpy)Pt(O,O′-pdon) (10, tbbpy-di-tert-butylbipyridine) (Scheme 8) [25].

Scheme 8. Synthesis Scheme 8. Synthesis of of complex complex 10. 10.

Combination of two dissimilar coordination sites in one molecule provides an opportunity to Combination of two dissimilar coordination sites in one molecule provides an opportunity to use this ligand as a building block in the construction of chain-like or network structures. A series of use this ligand as a building block in the construction of chain-like or network structures. A series of polynuclear complexes of platinum, as well as heteronuclear Pt-Ru derivatives, were obtained using polynuclear complexes of platinum, as well as heteronuclear Pt-Ru derivatives, were obtained using the step-by-step assembly strategy starting from catecholate (tbbpy)Pt(O,O′-pdon) [25]. The initial the step-by-step assembly strategy starting from catecholate (tbbpy)Pt(O,O0 -pdon) [25]. The initial complex 10, being a “bipyridine equivalent”, interacts with Pt(PhCN)2Cl2 and then with pdol. The complex 10, being a “bipyridine equivalent”, interacts with Pt(PhCN) Cl and then with pdol. resulting adduct 12 is treated with Ru(tbbpy)2Cl2 to give trinuclear species213 2(Scheme 9). The resulting adduct 12 is treated with Ru(tbbpy)2 Cl2 to give trinuclear species 13 (Scheme 9). Pampaloni et al. also reported the design of multinuclear systems using the pdon ligand as a building block. A reactivity of the dioxolene and diimine coordination sites towards binding of transition metal ions was studied [23,26–28]. It was shown that, depending on the nature of the metallofragment Cp2 M(CO)2 , the reaction with pdon proceeds towards the dioxolene site to give semiquinonate (M = Ti) or catecholate (M = Zr, V) species. The oxidation state of the metal ion in Cp2 M(O,O0 -SQ-pdon) was described as +3 in case of Ti and +4 for Zr and V, respectively (Scheme 10) [23].

Combination of two dissimilar coordination sites in one molecule provides an opportunity to use this ligand as a building block in the construction of chain-like or network structures. A series of polynuclear complexes of platinum, as well as heteronuclear Pt-Ru derivatives, were obtained using the step-by-step assembly strategy starting from catecholate (tbbpy)Pt(O,O′-pdon) [25]. The initial complex2018, 10, 6,being a “bipyridine equivalent”, interacts with Pt(PhCN)2Cl2 and then with pdol.6 of The Inorganics 0 28 resulting adduct 12 is treated with Ru(tbbpy)2Cl2 to give trinuclear species 13 (Scheme 9).

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Pampaloni et al. also reported the design of multinuclear systems using the pdon ligand as a building block. A reactivity of the dioxolene and diimine coordination sites towards binding of transition metal ions was studied [23,26–28]. It was shown that, depending on the nature of the metallofragment Cp2M(CO)2, the reaction with pdon proceeds towards the dioxolene site to give semiquinonate (M = Ti) or catecholate (M = Zr, V) species. The oxidation state of the metal ion incomplex Cp 2M(O,O′-SQ-pdon) was described as +3 in case Scheme 9. 9. Generation 13 by by step-by-step step-by-step assembly. assembly. Scheme Generation of of trinuclear trinuclear complex 13 of Ti and +4 for Zr and V, respectively (Scheme 10) [23].

Scheme to o-quinone o-quinone site. site. Scheme 10. 10. Oxidative Oxidative addition addition of of Ti, Ti, Zr Zr and and V V metallofragments metallofragments to

Cr(CO)6 and Mo(CO)6 react with pdon towards the dioxolene site to give corresponding Cr(CO)6 and Mo(CO)6 react with pdon towards the dioxolene site to give corresponding tris(o-semiquinonate) complexes [27]. Reaction of pdon with bis(arene)titanium and vanadium tris(o-semiquinonate) complexes [27]. Reaction of pdon with bis(arene)titanium and vanadium complexes also proceeds as an oxidative addition to the dioxolene site. The mixed-valent complexes complexes also proceeds as an oxidative addition to the dioxolene site. The mixed-valent complexes MIV (O,O′-SQ-pdon)2(O,O’-pdon) (M = V (17), Ti (18)) were formed at stoichiometry 1:3 of the initial MIV (O,O0 -SQ-pdon)2 (O,O’-pdon) (M = V (17), Ti (18)) were formed at stoichiometry 1:3 of the initial reagents (Scheme 11) [26]. A comparison of these complexes with 9,10-phenanthrenequinone (phenQ) structure analogues is worth mentioning: the titanium derivatives indicate a mixed-valent redox state of the dioxolene ligands both in case of pdon and phen-Q, whereas vanadium adduct with phen-Q exhibits semiquinonate state for all three dioxolene ligands. These data correlate well with the electrochemical properties of pdon and phen-Q: the oxidation capacity of 1,10-phenanthroline5,6-dione is higher than that of 9,10-phenanthrenequinone.

Scheme 10. Oxidative addition of Ti, Zr and V metallofragments to o-quinone site.

Cr(CO)6 and Mo(CO)6 react with pdon towards the dioxolene site to give corresponding tris(o-semiquinonate) complexes [27]. Reaction of pdon with bis(arene)titanium and vanadium Inorganics 2018, 6, 0 7 of 28 complexes also proceeds as an oxidative addition to the dioxolene site. The mixed-valent complexes MIV(O,O′-SQ-pdon)2(O,O’-pdon) (M = V (17), Ti (18)) were formed at stoichiometry 1:3 of the initial (phen-Q) reagents (Scheme (Scheme 11) 11)[26]. [26]. A A comparison comparisonofofthese thesecomplexes complexeswith with9,10-phenanthrenequinone 9,10-phenanthrenequinone (phenstructure analogues is worth mentioning: the titanium derivatives indicate a mixed-valent Q) structure analogues is worth mentioning: the titanium derivatives indicate a mixed-valent redox state of the dioxolene ligands both in case of pdon and phen-Q, whereas vanadium adduct with phen-Q exhibits dioxolene ligands. These data correlate wellwell withwith the exhibits semiquinonate semiquinonatestate statefor forallallthree three dioxolene ligands. These data correlate electrochemical properties of pdon and phen-Q: the oxidation capacity of 1,10-phenanthroline-5,6-dione the electrochemical properties of pdon and phen-Q: the oxidation capacity of 1,10-phenanthrolineis higher than that of 9,10-phenanthrenequinone. 5,6-dione is higher than that of 9,10-phenanthrenequinone.

Scheme and titanium titanium (19). (19). Scheme 11. 11. Mixed-valent Mixed-valent complexes complexes of of vanadium vanadium (18) (18) and

Chromium tris-(pdon) semiquinonate derivative 19 was used as a starting reagent for the syntheses of heteropolynuclear metal complexes. Free diimine unit in 19 interacts with the Lewis acids (TiCl2 (THF)2 , TiCl4 , ZrCl4 , HfCl4 , FeCl2 (THF)1.5 , CoCl2 and NiBr2 (DME)) leading to the corresponding tetranuclear species [24,28]. Step-by-step assembling of dendrimer-like structure from Cr(O,O0 -SQ-pdon)3 consistently treated with FeCl2 (THF)1.5 and pdon were also reported (Scheme 12) [27]. It seems fascinating that the preparation of such heteronuclear species was realized, since it shows the coordination capabilities of the ligand. However, it should be noted that the evidence for the formation of these compounds is insufficient, since the authors rely only on the data of spectroscopic studies. There are also some examples of lanthanide ions coordination on pdon. It was shown that tris-(β-diketonate) lanthanide (III) complexes attack the diimine site to give corresponding eightcoordinated species. Some of them exhibit luminescent properties. The use of 1,10-phenanthroline-5, 6-dione as a neutral ligand in such systems is efficient for several reasons: (i) polynuclear aromatic structure is a chromophore unit, so it can be used as an organic antenna for the sensitization of specific luminescence on lanthanide ion; (ii) coordination of d-metals on the dioxolene center makes possible tuning of the parameters of sensitization. The complexes Pt(O,O0 -pdon-N,N 0 )Ln(tta)3 (tta—thenoyltrifluoroacetonate, Ln = La (21), Gd (22), Nd (23), Er (24), Yb (25)) were obtained (Scheme 13) [29,30]. Excitation of 23–25 at 520 nm leads to luminescence in the near-IR region both in solution and in the solid state. The platinum metal center at a ligand backbone was found to be photosensitive at this wavelength, so it acts as “antenna” [29,30].

Chromium tris-(pdon) semiquinonate derivative 19 was used as a starting reagent for the syntheses of heteropolynuclear metal complexes. Free diimine unit in 19 interacts with the Lewis acids (TiCl2(THF)2, TiCl4, ZrCl4, HfCl4, FeCl2(THF)1.5, CoCl2 and NiBr2(DME)) leading to the corresponding tetranuclear species [24,28]. Step-by-step assembling of dendrimer-like structure from Cr(O,O′-SQ-pdon) 3 consistently treated with FeCl 2(THF)1.5 and pdon were also reported (Scheme Inorganics 2018, 6, 0 8 of12) 28 [27].

Scheme blocks. Scheme 12. 12. Generation Generation of of dendrimer-like dendrimer-like structure structure with with pdon pdon as as building building blocks.

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It seems fascinating that the preparation of such heteronuclear species was realized, since it shows the coordination capabilities of the ligand. However, it should be noted that the evidence for the formation of these compounds is insufficient, since the authors rely only on the data of spectroscopic studies. There are also some examples of lanthanide ions coordination on pdon. It was shown that tris-(β-diketonate) lanthanide (III) complexes attack the diimine site to give corresponding eight-coordinated species. Some of them exhibit luminescent properties. The use of 1,10-phenanthroline-5,6-dione as a neutral ligand in such systems is efficient for several reasons: (i) polynuclear aromatic structure is a chromophore unit, so it can be used as an organic antenna for the sensitizationScheme of specific luminescence on lanthanide ion; (ii) coordination of d-metals on the 13. Tris-(β-diketonate) lanthanide (III) complexes complexes with pdon pdon ligand. ligand. Scheme 13. Tris-(β-diketonate) lanthanide (III) with dioxolene center makes possible tuning of the parameters of sensitization. The complexes Pt(O,O′-pdon-N,N′)Ln(tta) 3 (tta—thenoyltrifluoroacetonate, Ln in = La (21), Gd (22), Excitation of 23–25 at 520 nm leads to luminescence in the near-IR region both solution and in 2.2. Bifunctional Catechols, Containing Schiff-Bases Nd (23), Er (24), Yb (25)) were obtained (Scheme 13) [29,30]. the solid state. The platinum metal center at a ligand backbone was found to be photosensitive at this A seriesso ofitbifunctional redox-active wavelength, acts as “antenna” [29,30]. chelating ligands were derived from diaminocatechols. Bifunctional catechols, containing Schiff-bases, were synthesized via condensation of diaminocatechols with 2-pyridinecarboxaldehyde, salicylic aldehyde etc. 2.2. Bifunctional Catechols, Containing Schiff-Bases In 1993, Coucouvanis et al. proposed a concept of “metal complexes as ligands” [31]. A series of bifunctional redox-active chelating ligands were derived from diaminocatechols. Catechol H4 ETC, annelated with tetraazacyclotetradeca-7,12-dienyl macrocycle, comprises the Bifunctional catechols, containing Schiff-bases, were synthesized via condensation of redox-active catecholate site and N4 -tetradentate coordination center. Several synthetic approaches diaminocatechols with 2-pyridinecarboxaldehyde, salicylic aldehyde etc. In 1993, Coucouvanis et al. proposed a concept of “metal complexes as ligands” [31]. Catechol H4ETC, annelated with tetraazacyclotetradeca-7,12-dienyl macrocycle, comprises the redox-active catecholate site and N4-tetradentate coordination center. Several synthetic approaches resulting in metallocatecholates have been developed. The most versatile one is the condensation of 4,5-diaminoveratrol with 2,2′-(ethane-1,2-diyl-imino)bis(5-methyl-benzaldehyde) in presence of

2.2. Bifunctional Catechols, Containing Schiff-Bases A series of bifunctional redox-active chelating ligands were derived from diaminocatechols. A series ofcatechols, bifunctional redox-active chelating ligands derived from Bifunctional containing Schiff-bases, were were synthesized via diaminocatechols. condensation of Bifunctional catechols, containing Schiff-bases, were synthesized via condensation of diaminocatechols with 2-pyridinecarboxaldehyde, salicylic aldehyde etc. diaminocatechols with 2-pyridinecarboxaldehyde, salicylic aldehyde etc. as ligands” [31]. Catechol In 1993, et al. proposed a concept of “metal complexes Inorganics 2018, 6,Coucouvanis 0 9 of 28 In 1993, Coucouvanis et al. proposed a concept of “metal complexes as ligands” Catechol H4ETC, annelated with tetraazacyclotetradeca-7,12-dienyl macrocycle, comprises the[31]. redox-active H 4ETC, annelated with tetraazacyclotetradeca-7,12-dienyl macrocycle, comprises the redox-active catecholate site and N4-tetradentate coordination center. Several synthetic approaches resulting in catecholate site and Nhave 4-tetradentate coordination Several resulting in resulting in metallocatecholates been developed. The mostsynthetic versatile one is the condensation metallocatecholates been have developed. Thecenter. most versatile one isapproaches the condensation of 0 -(ethane-1,2-diyl-imino)bis(5-methyl-benzaldehyde) metallocatecholates have been developed. The most versatile one is the condensation of 4,5-diaminoveratrol with 2,2 in presence of of 4,5-diaminoveratrol with 2,2′-(ethane-1,2-diyl-imino)bis(5-methyl-benzaldehyde) in presence of 4,5-diaminoveratrol with 2,2′-(ethane-1,2-diyl-imino)bis(5-methyl-benzaldehyde) in 14) presence M(OAc) Ni (26), (26), Co (27)) (27)) followed by by deprotection deprotection of [31]. M(OAc)22 (M (M = = Ni Co followed of catechol catechol function function (Scheme (Scheme 14) [31]. of M(OAc)2 (M = Ni (26), Co (27)) followed by deprotection of catechol function (Scheme 14) [31].

Scheme 14. 14. Synthesis Synthesis of of complexes complexes 26 26 and and 27. 27. Scheme Scheme 14. Synthesis of complexes 26 and 27.

Dioxolene site of complex could be oxidized to 28 by the ferricinium cation [31]. Crystallography Dioxolene site site of of complex complex could could be be oxidized oxidized to to 28 28 by by the theferricinium ferricinium cation cation [31]. [31]. Crystallography Crystallography Dioxolene study of the resulting protonated semiquinonate species 28 shows a dimer structure due to hydrogen study of of the resulting resulting protonated semiquinonate species 28 shows a dimer structure due to hydrogen study bondingthe (Scheme 15).protonated semiquinonate species 28 shows a dimer structure due to hydrogen bonding (Scheme 15). bonding (Scheme 15).

Scheme 15. Dimeric structure of species 28. Scheme 15. Dimeric structure of species 28. Scheme 15. Dimeric structure of species 28.

Temperature dependence study of the magnetic susceptibility of this dimer reveals antiferromagnetic exchange interactions: the effective magnetic moment is 2.08 µB at 300 K. In contrast, solutions of 28 in DMF and DMSO are diamagnetic and display a narrow 1 H-NMR spectrum. It was explained by an electron-proton exchange dynamics in solution leading to formation of catechol-quinone pair. Treatment of 26 and 27 with tBu4 NOH or KOMe in presence of [(DMF)3 Mo2 O2 S2 –(S2 )] results in heterometallic compounds 29 and 30. Dimer species 31 and 32 containing Mo–Mo bridge were obtained in the reaction of 26 or 27 with [(DMF)3 Mo2 O2 S2 (DMF)3 ] (Scheme 16) [32]. Jonasdottir et al. reported the synthesis of trinuclear complex 33 where two NiETC moieties are bridged by copper (II) ion (Scheme 17) [33].

spectrum. It was explained by an electron-proton exchange dynamics in solution leading to formation of catechol-quinone pair. contrast, solutions of 28 in DMF and DMSO are diamagnetic and display a narrow 1H-NMR of catechol-quinone pair. Treatment 26 and 27bywith tBu4NOH or KOMe in presence of [(DMF) 3Mo2O2S2–(S2)] results in spectrum. It wasof explained an electron-proton exchange dynamics in solution leading to formation Treatment of 26 and 27 with tBu 4NOH or KOMe in presence of [(DMF) 3Mo2O2S2–(S2)] results in heterometallic compounds 29 and 30. Dimer species 31 and 32 containing Mo–Mo were of catechol-quinone pair. 29 and 30. Dimer species 31 and 32 containing Mo–Mo bridge heterometallic compounds bridge were obtained in the reaction of 26 or 27 with [(DMF) 3Mo2O2S2(DMF)3] (Scheme 16) [32]. Treatment of 26 andof2726with 4NOH or KOMe in presence of [(DMF) 3Mo2O2S2–(S2)] results in obtained in the reaction or 27tBu with [(DMF)3Mo2O2S2(DMF)3] (Scheme 16) [32]. Inorganics 2018, 6, 0compounds 29 and 30. Dimer species 31 and 32 containing Mo–Mo bridge10were of 28 heterometallic obtained in the reaction of 26 or 27 with [(DMF)3Mo2O2S2(DMF)3] (Scheme 16) [32].

Scheme 16. Heterometallic derivatives 29–32. Scheme 16. Heterometallic derivatives 29–32.

Jonasdottir et al. reported the synthesis of trinuclear complex 33 where two NiETC moieties are Scheme 16. Heterometallic derivatives 29–32. Jonasdottir et al. the synthesis of trinuclear complex 33 where two NiETC moieties are Scheme 16.[33]. Heterometallic derivatives 29–32. bridged by copper (II)reported ion (Scheme 17) bridged by copper (II) ion (Scheme 17) [33]. Jonasdottir et al. reported the synthesis of trinuclear complex 33 where two NiETC moieties are bridged by copper (II) ion (Scheme 17) [33].

Scheme 17. 17. Trinuclear Ni-Cu-Ni complex complex 33. 33. Scheme Trinuclear Ni-Cu-Ni Scheme 17. Trinuclear Ni-Cu-Ni complex 33.

A family of OO~NN ligands is also presented by bifunctional SALPHEN–catechols that are A of OO~NN ligands is also presented by bifunctional that are Scheme Trinuclear Ni-Cu-Ni complex 33.SALPHEN–catechols A family family ligands is 17. also presented bywith bifunctional SALPHEN–catechols are synthesized byof theOO~NN condensation of 4,5-diaminocatechol salicylic aldehydes (Scheme 18)that [34,35]. synthesized by the condensation of 4,5-diaminocatechol with salicylic aldehydes (Scheme 18) [34,35]. synthesized by the condensation of 4,5-diaminocatechol with salicylic aldehydes (Scheme 18) [34,35]. A family of OO~NN ligands is also presented by bifunctional SALPHEN–catechols that are synthesized by the condensation of 4,5-diaminocatechol with salicylic aldehydes (Scheme 18) [34,35].

Scheme 18. Bifunctional SALPHEN–catechols. Scheme 18. 18. Bifunctional Bifunctional SALPHEN–catechols. SALPHEN–catechols. Scheme Scheme 18. Bifunctional SALPHEN–catechols.

Malinak et al. developed the procedure for selective preparation of the dimer catecholate adduct (Bu4 N)2 [Mo2 O5 [H2 (EtO)2 SALPHEN–(O)2 ]2 34 (Scheme 19) [34]. Treatment of its vacant SALPHEN sites with M(OAc)2 yields to the corresponding heteronuclear complexes (M = Cu (35), Ni (36), Co (37), Fe (38), Mn (39)).

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Inorganics 2018, 6, x FOR PEER REVIEW 10 of 25 Inorganics 2018, 6,et x FOR PEER REVIEW 10 of 25 Malinak al. developed the procedure for selective preparation of the dimer catecholate adduct

Malinak al. developed the procedure selective preparation of the dimer catecholate adduct (Bu4N) 2[Mo2Oet 5[H 2(EtO) 2SALPHEN–(O) 2]2 34for (Scheme 19) [34]. Treatment of its vacant SALPHEN Malinak et al. developed the procedure for selective preparation of the (M dimer catecholate adduct (Bu 4N) 2[Mo 2O5[H2(EtO) 2SALPHEN–(O) 2]2 34 (Scheme 19) [34]. Treatment of its vacant SALPHEN sites with M(OAc) 2 yields to the corresponding heteronuclear complexes = Cu (35), Ni (36), Inorganics 2018, 6, 0 11 ofCo 28 (Bu 4N)2[Mo2O5[H2(EtO)2SALPHEN–(O)2]2 34 (Scheme 19) [34]. Treatment of its vacant SALPHEN sites Fe with M(OAc) 2 yields to the corresponding heteronuclear complexes (M = Cu (35), Ni (36), Co (37), (38), Mn (39)). sites M(OAc) 2 yields to the corresponding heteronuclear complexes (M = Cu (35), Ni (36), Co (37), with Fe (38), Mn (39)). (37), Fe (38), Mn (39)).

Scheme 19. M-Mo-Mo-M complexes. Scheme 19. M-Mo-Mo-M complexes. Scheme 19. M-Mo-Mo-M complexes. M-Mo-Mo-M complexes. Tetranuclear complexes 35 Scheme and 3619. were characterized structurally; their molecules have a

Tetranuclear complexes 35 and 36 were were characterized structurally; their molecules havea Tetranuclear complexes 35SALPHEN and 36 characterized structurally; their molecules pocket-like structure so that the ligand planes in dimer are almost parallel to eachhave other Tetranuclear complexes 35 and 36 were characterized structurally; their molecules have a a(Figure pocket-like structure so that the SALPHEN ligand planes in dimer are almost parallel to each pocket-like 2). structure so that the SALPHEN ligand planes in dimer are almost parallel to each other pocket-like structure so that the SALPHEN ligand planes in dimer are almost parallel to each other other (Figure 2). (Figure 2). (Figure 2).

Figure 2. ORTEP plot of complex 36 anion. Reprinted with permission from Inorg. Chem. 1996, 35, 4810–4811. Copyright American Chemical Society. Figure 2.2. ORTEP ORTEP plot(1996) ofcomplex complex 36anion. anion. Reprinted with permission permission from from Inorg. Inorg. Chem. Chem. 1996, 1996, 35, 35, Figure plot of 36 Reprinted with Figure 2. ORTEP plot of complex 36 anion. Reprinted with permission from Inorg. Chem. 1996, 35, 4810–4811. Copyright (1996) American Chemical Society. 4810–4811. Copyright (1996) American Chemical Society. 4810–4811. American Chemical Society. A curious Copyright feature of (1996) pocket-opening was found for complex 35. The Cu–Cu bond length in dimer

curious feature of pocket-opening was foracomplex Cu–Cu bond length incation dimer 35 is A 4.110 A. This distance can be changed byfound adding mixture35. of The pyridine and pyridinium A curious curious feature feature of of pocket-opening pocket-opening was found for complex 35. The Cu–Cu Cu–Cu bond bond length length in in dimer dimer A was found for complex 35. The 35 issolution: 4.110 A. the Thisneutral distance can be molecule changed by adding a on mixture of pyridine and cation into pyridine coordinates the molybdenum ion, pyridinium while pyridinium 35 is A. distance can changed byby adding a mixture of pyridine andand pyridinium cation into 35 is 4.110 4.110 A.This This distance canbebe changed adding a mixture of catecholate pyridine pyridinium cation into solution: the neutral pyridine molecule coordinates onpairs the molybdenum ion, while Due pyridinium cation forms hydrogen bonds with the vacant electron of oxygen. to the solution: the neutral pyridine molecule coordinates on the molybdenum ion, while pyridinium cation into solution: pyridine molecule coordinates onpairs the molybdenum while Mo–Mo pyridinium cation forms the hydrogen bonds with the vacant electron of catecholate oxygen. Due to and the rearrangement of neutral coordination spheres, valence angle Mo–O–Mo as well as theion, distances forms hydrogen bonds with the vacant electron pairs of catecholate oxygen. Due to the rearrangement cation forms hydrogen bonds with the vacant electron pairs of catecholate oxygen. Due20)to[34]. the rearrangement coordination angle Mo–O–Mo asof well the distances Mo–Mo and Cu–Cu increase,ofand the pocketspheres, betweenvalence the SALPHEN fragments 35 as is opened (Scheme of coordination of spheres, valencespheres, angle Mo–O–Mo as well as the distances Mo–Mo and Cu–Cu increase, rearrangement coordination valence angle Mo–O–Mo as well distances Mo–Mo and Cu–Cu increase, and the pocket between the SALPHEN fragments of 35asisthe opened (Scheme 20) [34]. and the pocket between the SALPHEN fragments of 35 is opened (Scheme 20) [34]. Cu–Cu increase, and the pocket between the SALPHEN fragments of 35 is opened (Scheme 20) [34].

Scheme 20. Activation of pocket-like Mo2O5-bridged system. Scheme 20. Activation of pocket-like Mo2O5-bridged system. Scheme 20. Activation of pocket-like Mo2O5-bridged system. Scheme 20. Activation of pocket-like Mo2 O5 -bridged system.

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Another interesting effect was found for iron complex 38. It was shown that the dimer Another effect was found for iron iron complex complex38. 38. It was was shownthat thatthe thedimer dimer Anotherinteresting interesting effect found for SALPHEN–complexes of iron (II) was interact with dioxygen or sulfur It to formshown inner Fe–X–Fe (X = O, S) SALPHEN–complexes ofofiron dioxygen or sulfur sulfur to to forminner innerFe–X–Fe Fe–X–Fe(X(X = O, S) SALPHEN–complexes iron(II) (II)interact interactwith with dioxygen or O, S) bonds, so that a SALPHEN pocket becomes locked. Complexes withform bridging oxygen (40)=and sulfur bonds, so that a SALPHEN pocket becomes locked. Complexes with bridging oxygen (40) and sulfur thatisolated a SALPHEN pocket becomes (41) bonds, atoms so were (Scheme 21) [35]. locked. Complexes with bridging oxygen (40) and sulfur (41) atoms were (41) atoms wereisolated isolated(Scheme (Scheme21) 21) [35]. [35].

Scheme 21. Intramolecular bridging atoms with sulfur and oxygen atoms. Scheme 21. Intramolecular of Fe Featoms atomswith with sulfur and oxygen atoms. Scheme 21. Intramolecularbridging bridging of of Fe sulfur and oxygen atoms.

Themore moresterically stericallyhindered hindered (tBu)44SALPHEN–(OH) SALPHEN–(OH)2 forms mixed-ligand catecholate complexes The formsmixed-ligand mixed-ligand catecholate complexes The more sterically hindered(tBu) (tBu)4 SALPHEN–(OH)2 2forms catecholate complexes 43–46. SALPHEN–catechol replaces only one catecholate fragment (3,5DBCat) in adduct 42 (Scheme 43–46. SALPHEN–catechol onecatecholate catecholate fragment (3,5DBCat) in adduct 42 (Scheme 43–46. SALPHEN–catecholreplaces replaces only only one fragment (3,5DBCat) in adduct 42 (Scheme 22). 22). The formation of a homoligand dimer adduct is unfavorable due to the steric reasons [35]. The formation of a homoligand dimer adduct is unfavorable due to the steric reasons [35]. 22). The formation of a homoligand dimer adduct is unfavorable due to the steric reasons [35].

Scheme 22. The formation of mixed-ligand molybdenium complexes.

Scheme 22.22.The molybdenium complexes. Scheme Theformation formation of of mixed-ligand mixed-ligand molybdenium complexes.

A series of Zr heterobinuclear complexes 51–53 were reported for SALPHEN ligands (Scheme 23). Some of them have been investigated as catalysts forwere polymerization ofSALPHEN olefins [36]. A series of Zr heterobinuclear reportedforfor ligands (Scheme A series of Zr heterobinuclearcomplexes complexes 51–53 51–53 were reported SALPHEN ligands (Scheme 23). 23). Some of them have been investigated forpolymerization polymerization of olefins Some of them have been investigatedas as catalysts catalysts for of olefins [36].[36].

Scheme 23. Mono- and binuclear complexes 47–53.

Scheme 22. The formation of mixed-ligand molybdenium complexes.

A series 23). Inorganics 2018, 6,of0 Zr heterobinuclear complexes 51–53 were reported for SALPHEN ligands (Scheme 13 of 28 Some of them have been investigated as catalysts for polymerization of olefins [36].

Scheme 23. 23. MonoMono- and and binuclear binuclear complexes complexes 47–53. 47–53. Scheme

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Weber et reported OO~NN OO~NN ligand ligand that that was was prepared prepared via via condensation condensation reaction of et al. al. reported 4,5-diaminocatechol keto-enol-ether [37–39]. A A series series of of Fe (II) complexes 54–60 bound at the 4,5-diaminocatechol with keto-enol-ether diimine site of this ligand were studied (Scheme 24). Some interesting effects including an influence of intramolecular intramolecularhydrogen hydrogenbonding bonding magnetic behavior of these complexes were found. onon thethe magnetic behavior of these complexes were found. There There arevery also very promising results concerning to construction polymericchains chainsusing using bidentate are also promising results concerning to construction of of 1D1Dpolymeric pyridine auxiliary ligands as a bridge. bridge. Some of these coordination polymers were found to exhibit spin-crossover phenomena. phenomena.

Scheme 24. A series of Fe (II) complexes with keto–enol–ether bifunctional ligand. Scheme 24. A series of Fe (II) complexes with keto–enol–ether bifunctional ligand.

Trinuclear complexes [Cu3L2](Bu4N) (61) and [V(O)Cu2L2](Bu4N) (62) have been studied in order Trinuclearthe complexes [Cu3of L2exchange ](Bu4 N) (61) and [V(O)Cu haveinbeen studied in 2 L2 ](Bu 4 N) (62) to understand mechanisms interactions between magnetic centers a planar linear order to understand the mechanisms of exchange magnetic in a planar coordination polymeric chain (Scheme 25). It wasinteractions shown thatbetween depending on thecenters symmetry of the linear coordination polymeric chain (Scheme 25). It was shown that depending on the symmetry of orbitals of the central metal ion exchange interactions between terminal Cu(II) centers may be the orbitals of the central metal ion exchange interactions between terminal Cu(II) centers may be antiferromagnetic or ferromagnetic as for 61 and 62, respectively [40]. antiferromagnetic or ferromagnetic as for 61 and 62, respectively [40].

Scheme 25. Trinuclear complexes with keto–enol–ether bifunctional ligand.

Trinuclear complexes [Cu3L2](Bu4N) (61) and [V(O)Cu2L2](Bu4N) (62) have been studied in order to understand the mechanisms of exchange interactions between magnetic centers in a planar linear coordination polymeric chain (Scheme 25). It was shown that depending on the symmetry of the orbitals 2018, of the Inorganics 6, 0 central metal ion exchange interactions between terminal Cu(II) centers may 14 ofbe 28 antiferromagnetic or ferromagnetic as for 61 and 62, respectively [40].

Scheme 25. Trinuclear complexes with with keto–enol–ether keto–enol–ether bifunctional bifunctional ligand. ligand. Scheme 25. Trinuclear complexes

Investigation of complexes containing NN~OO bifunctional chelating ligands indicates their Investigation of complexes containing NN~OO bifunctional chelating ligands indicates their potential for future technological applications. Two different coordination sites facilitate construction potential for future technological applications. Two different coordination sites facilitate construction of ordered structures. A redox activity of the dioxolene site is useful in managing the geometry or of ordered structures. A redox activity of the dioxolene site is useful in managing the geometry or other other parameters of complexes. At the same time, redox transformations at diimine site in such parameters of complexes. At the same time, redox transformations at diimine site in such bifunctional bifunctional ligands are not accessible at ordinary conditions. Introduction of dithiolene site instead ligands are not accessible at ordinary conditions. Introduction of dithiolene site instead of diimine one of diimine one provides an opportunity for the interplay between redox abilities of two sites. provides an opportunity for the interplay between redox abilities of two sites. 3. SS~OO Systems Inorganics 2018, 6, x FOR PEER REVIEW 3. SS~OO Systems Inorganics 2018, 6, x FOR PEER REVIEW

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There are number of redox-active benzenoid ligands ligands combining dioxolenedioxolene and dithiolene There area limited a limited number of redox-active benzenoid combining and There are a limited number of redox-active benzenoid ligands combining dioxolene and sites. A synthetic the preparation of the 4,5-dimercaptocatechol 63 was first reported by dithiolene sites. Aprocedure synthetic for procedure for the preparation of the 4,5-dimercaptocatechol 63 was first dithiolene sites. A synthetic procedure for the preparation of the 4,5-dimercaptocatechol 63 was first Coucouvanis et al. (Scheme 26) [41]. reported by Coucouvanis et al. (Scheme 26) [41]. reported by Coucouvanis et al. (Scheme 26) [41].

Scheme Scheme 26. 26. Synthesis Synthesis of of 4,5-dimercaptocatechol. 4,5-dimercaptocatechol. Scheme 26. Synthesis of 4,5-dimercaptocatechol.

All reported complexes containing this ligand reveal a dithiolene mode of coordination of the All reported complexes containing this ligand reveal a dithiolene mode of coordination of the metal ion. A series of bis(dithiolate) complexes were synthesized. According to the structural metal ion. A A series series of bis(dithiolate) complexes were synthesized. According to to the structural synthesized. According parameters the metal ion in [M(DtCat2)−−]−[Ph4P++] adopts M3+ state both in case of M = Cu (64) and + 3+ 3+ parameters the metal metal ion ion in in [M(DtCat [M(DtCat2)2 )] [Ph ] [Ph ] adopts in case Cu and (64) 4P4]Padopts M Mstatestate bothboth in case of Mof= M Cu=(64) M = Ni (65) (Scheme 27). and Ni (Scheme (65) (Scheme M=M Ni =(65) 27). 27).

Scheme 27. Bis(dithiolate) derivatives of 4,5-dimercaptocatechol [M(DtCat2)−−] [Ph4P++]. Scheme27. 27.Bis(dithiolate) Bis(dithiolate)derivatives derivativesofof4,5-dimercaptocatechol 4,5-dimercaptocatechol[M(DtCat [M(DtCat)2− ) ] [Ph4P +]. Scheme 2 ] [Ph4 P ].

Hydroxyl groups in 64 and 65 could be selectively oxidized by the molecular oxygen leading to Hydroxyl groups in 64 and 65 could be selectively oxidized by the molecular oxygen leading to the corresponding bis(dithiolates) and be 67,selectively those contain non-coordinated o-quinone sitesleading at the Hydroxyl groups in 64 and 6566 could oxidized by the molecular oxygen the corresponding bis(dithiolates) 66 and 67, those contain non-coordinated o-quinone sites at the termini (Scheme 28). bis(dithiolates) 66 and 67, those contain non-coordinated o-quinone sites at the to the corresponding termini (Scheme 28). termini (Scheme 28).

Scheme 27. Bis(dithiolate) derivatives of 4,5-dimercaptocatechol [M(DtCat2)−] [Ph4P+]. Scheme 27. Bis(dithiolate) derivatives of 4,5-dimercaptocatechol [M(DtCat2)−] [Ph4P+].

Hydroxyl groups in 64 and 65 could be selectively oxidized by the molecular oxygen leading to Hydroxyl groups in 64 and 65 66 could selectively oxidized by the molecular oxygensites leading to the corresponding bis(dithiolates) andbe67, those contain non-coordinated o-quinone atofthe Inorganics 2018, 6, 0 15 28 the corresponding bis(dithiolates) 66 and 67, those contain non-coordinated o-quinone sites at the termini (Scheme 28). termini (Scheme 28).

Scheme 28. 28. Bis(dithiolates) Bis(dithiolates) with with non-coordinated non-coordinated o-quinone o-quinone moiety. moiety. Scheme Scheme 28. Bis(dithiolates) with non-coordinated o-quinone moiety.

Unfortunately, the reactivity of metal ions towards binding at dioxolene site of the molecule has Unfortunately, the reactivity of metal ions towards binding at dioxolene site of the molecule has Unfortunately, not been studied yet.the reactivity of metal ions towards binding at dioxolene site of the molecule has not studied yet. not been been studied Recently, weyet. reported the substituted analogue of SS~OO bifunctional redox active chelating Recently, we reported the substituted analogue of SS~OO bifunctional redox active Recently, we reported substituted analogue of SS~OO bifunctional redox active chelating ligand. It was found that thethe decarbonylation of 4,7-di-tert-butyl-1,3-benzodithiole-2,5,6-trione yields chelating ligand. It was found that the decarbonylation of 4,7-di-tert-butyl-1,3-benzodithiole-2,5,6ligand. It was found that the decarbonylation of 4,7-di-tert-butyl-1,3-benzodithiole-2,5,6-trione yields the sterically-hindered o-quinone, annelated with dithiete (68) (Scheme 29) [42]. trione yields the sterically-hindered o-quinone, annelated dithiete (68) (Scheme 29) [42]. the sterically-hindered o-quinone, annelated with dithietewith (68) (Scheme 29) [42].

Scheme 29. Synthesis of o-quinone, annelated with dithiete cycle. Scheme 29. Synthesis with dithiete dithiete cycle. cycle. Scheme 29. Synthesis of of o-quinone, o-quinone, annelated annelated with

It is a unique bifunctional ligand, containing two different redox-active chelating chalcogen coordination centers. Since dithiete is an electronic analogue of 1,2-dithiocarbonyl function, the dithiolene and dioxolene sites in the ligand are in the same oxidation state. The different properties of sulfur and oxygen create an opportunity for manipulation of the direction of metal ion binding during the reaction of oxidative addition. This species presents a unique chance to compare an activity of dithiolene and dioxolene sites being in the same steric surrounding and under the same conditions in different reactions with metals. It was shown that the direction of an oxidative addition depends on the nature of metallofragment. Treatment of the ligand with one-electron reducing agents such as Li, Na, K, Tl, Cu (in presence of PPh3 ), Mn2 (CO)10 yields in o-semiquinonate species. Alkali metal o-semiquinonates can undergo further reduction to give corresponding catecholates. According to the EPR spectroscopy data vacant dithiolate site in the manganese semiquinonate adduct 69 can be also involved into the oxidative addition process. Interaction of 68 with an excess of Mn2 (CO)10 results in subsequent formation of semiquinonate 69 and then the trinuclear species 70 (Scheme 30) [42]. Mononuclear semiquinonate 69 has a sextet lines EPR spectrum in toluene due to splitting of the unpaired electron on the 55 Mn nucleus (Figure 3a). A multiplet spectrum that arises upon further reduction of 69 with Mn2 (CO)10 was attributed to the trinuclear species 70. The unpaired electron in 70 interacts with one manganese nucleus at the dioxolene site and with two equivalent manganese nuclei at the dithiolene unit (Figure 3c). The hyperfine splitting (HFS) constant values are 6.80 G and 1.39 G respectively.

Mononuclear semiquinonate 69 has at a sextet lines EPR spectrum in toluene due to splitting of the 70 interacts with one manganese nucleus the dioxolene site and with two equivalent manganese unpaired electron on the 55Mn nucleus (Figure 3a). A multiplet spectrum that arises upon further nuclei at the dithiolene unit (Figure 3c). The hyperfine splitting (HFS) constant values are 6.80 G and reduction of 69 with Mn2(CO)10 was attributed to the trinuclear species 70. The unpaired electron in 1.39 G respectively. 70 interacts with one manganese nucleus at the dioxolene site and with two equivalent manganese nuclei2018, at the Inorganics 6, 0dithiolene

unit (Figure 3c). The hyperfine splitting (HFS) constant values are 6.80 G28and 16 of

1.39 G respectively.

Scheme 30. The formation of trinuclear species 70.

Scheme 30. The formation of trinuclear species 70. Scheme 30. The formation of trinuclear species 70. [ * 10^ 6]

[ * 10^ 6] 3. 5

3. 0 3. 0

[ * 10^ 6] 3. 5

[ * 10^ 6]

2. 5

2. 5

3. 0

2. 0

2. 0

3. 0

1. 5

2. 5 1. 5

1. 0

2. 5 1. 0

2. 0

0. 5

1. 5

0. 5

2. 0

1. 5

0. 0

0. 0

- 0. 5

1. 0 - 0. 5

- 1. 0

1. 0

0. 5 - 1. 5

- 1. 0

0. 5

0. 0

- 1. 5

0. 0

- 0. 5

- 2. 0

- 2. 0

- 2. 5

- 3. 0 - 2. 5

- 0. 5

- 1. 0 3420

3425

3430

3435

3440

- 1. 0

3445 [G]

3450

3455

3460

3465

3420

3470

(a)

- 1. 5

3425

3430

3435

3440

- 1. 5

3445 [G]

3450

3455

3460

3465

3470

(b)

- 2. 0 [ * 10^ 6] 1. 8

- 2. 0

- 2. 5

1. 6 1. 4

- 2. 5

- 3. 0

1. 2

3420

3425

3430

3435

3440

3445 [G]

3450

3455

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3465

3420

3470

3425

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3440

1. 0

3445 [G]

3450

3455

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3470

0. 8

(a)

(b)

0. 6 0. 4 0. 2

[ * 10^ 6]

- 0. 0

1. 8

- 0. 2

1. 6

- 0. 4 - 0. 6

1. 4 - 0. 8

1. 2

- 1. 0

1. 0

- 1. 2 - 1. 4

0. 8

- 1. 6

0. 6

- 1. 8 3415

0. 4

3420

3425

3430

3435

0. 2

3440

3445 [G]

3450

3455

3460

3465

3470

(c)

- 0. 0 - 0. 2 - 0. 4

Figure 3. Evolution of the EPR spectrum of the mixture of o-quinone 68 and Mn2(CO)10 in toluene upon irradiation with halogen lamp. (a) 30 s later, the signal corresponds to the mononuclear adduct 69; (b) 10 min. later, the observed signal is a superposition of signals from paramagnetic species 69 and 70; (c) 20 min. later, the signal is attributed to the trinuclear adduct 70. - 0. 6

- 0. 8

- 1. 0

- 1. 2

- 1. 4

- 1. 6

- 1. 8

3415

3420

3425

3430

3435

3440

3445 [G]

3450

3455

3460

3465

3470

(c) Figure 3. Evolution of the spectrum EPR spectrum the mixture of o-quinone 68 and 68 Mnand in2toluene Figure 3. Evolution of the EPR ofofthe mixture of o-quinone Mn (CO)10 in toluene 2 (CO)10 upon irradiation with halogen lamp. (a) 30 s later, the signal corresponds to the mononuclear adduct 69; upon irradiation with halogen lamp. (a) 30 s later, the signal corresponds to the mononuclear adduct (b) 10 min later, the observed signal is a superposition of signals from paramagnetic species 69 and 70; 69; (b) 10 min. thethe observed signal to is the a superposition of signals from paramagnetic species 69 (c) 20later, min later, signal is attributed trinuclear adduct 70. and 70; (c) 20 min. later, the signal is attributed to the trinuclear adduct 70.

Reaction of 68 with two-electron reducing agents results in catecholate or dithiolate derivatives depending on the Lewis acidity of the metal ion. Binding at the dithiolate coordination site is preferable for the soft Lewis acids and vice versa. Group 10 zero-valent metal tetrakis (triphenylphosphine) complexes M(PPh3 )4 (M = Pd, Pt) selectively react towards dithiolene site of the ligand 68 with formation of 71 and 72 mononuclear adducts (Scheme 31) [43].

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Reaction of 68 with two-electron reducing agents results in catecholate or dithiolate derivatives Reactiononofthe 68 with two-electron results inatcatecholate or dithiolate derivatives depending Lewis acidity of reducing the metalagents ion. Binding the dithiolate coordination site is Reaction ofthe 68 with two-electron reducing agents results in catecholate or dithiolate derivatives depending on Lewis acidity of the metal ion. Binding at the dithiolate coordination site is preferable for the soft Lewis acids and vice versa. Group 10 zero-valent metal tetrakis depending on the Lewis acidity of the metal ion. Binding at the dithiolate coordination site is preferable for the soft Lewis M(PPh acids 3)and vice versa. Group 10 zero-valent metal tetrakis (triphenylphosphine) complexes 4 (M = Pd, Pt) selectively react towards dithiolene site17 ofofthe Inorganics 2018, 6, 0 28 preferable for the soft Lewis M(PPh acids 3)and vice versa. Group 10 zero-valent metal tetrakis (triphenylphosphine) complexes 4 (M = Pd, Pt) selectively react towards dithiolene site of the ligand 68 with formation of 71 and 72 mononuclear adducts (Scheme 31) [43]. (triphenylphosphine) complexes M(PPh 3)4 (M = Pd, Pt) selectively react towards dithiolene site of the ligand 68 with formation of 71 and 72 mononuclear adducts (Scheme 31) [43]. ligand 68 with formation of 71 and 72 mononuclear adducts (Scheme 31) [43].

Scheme 31. 31. Regioselective Regioselective oxidative oxidative addition addition of of M(PPh M(PPh33))44 (M (M = = Pd, Pd, Pt) Pt) towards towards dithiolene dithiolene site. site. Scheme Scheme 31. Regioselective oxidative addition of M(PPh 3)4 (M = Pd, Pt) towards dithiolene site. (Reproduced from Dalton Trans. 2016, 45, 7400–7405. Copyright (2006) Royal Society of Chemistry). (Reproduced from Dalton Trans. 2016, 45, 7400–7405. Copyright (2006) Royal Society of Chemistry). Scheme 31. Regioselective oxidative addition of M(PPh 3)4 (M = Pd, Pt) towards dithiolene site. (Reproduced from Dalton Trans. 2016, 45, 7400–7405. Copyright (2006) Royal Society of Chemistry). (Reproduced from Dalton Trans. 2016, 45, 7400–7405. Copyright (2006) Royal Society of Chemistry).

According to the cyclic voltammetry data, coordination of a metal fragment on the dithiolene According the voltammetry data, coordination of aa metal fragment on dithiolene According to todecreases the cyclic cyclicthe voltammetry data, coordination of metalsite fragment on the the dithiolene site significantly first reduction potential of free dioxolene from −0.42 V (68) to −0.90 According to the cyclic voltammetry data, coordination of a metal fragment on the dithiolene site significantly decreases the first reduction potential of free dioxolene site from − 0.42 Vto(68) to site significantly decreases the first reduction potential of free dioxolene site from −0.42 V (68) −0.90 V (71) and −0.80 V (72). Nevertheless, such mononuclear complex still might be regarded as an site significantly the first reduction of freecomplex dioxolene sitemight from Vbe (68) to as −0.90 − 0.90 Vand (71)−0.80 anddecreases − VNevertheless, (72). Nevertheless, such mononuclear complex still −0.42 might regarded Vo-quinone (71) V 0.80 (72). suchpotential mononuclear still be regarded an equivalent and can be reduced by many one-electron agents to give corresponding V (71) and −0.80 V (72). Nevertheless, such mononuclear complex still might be regarded as an as an o-quinone equivalent and can be reduced by many one-electron agents to give corresponding o-quinone equivalent and can be reduced by many one-electron agents to give corresponding heterobimetallic complexes (Scheme 32) [43]. o-quinone equivalent and (Scheme can be reduced heterobimetallic complexes 32) heterobimetallic complexes (Scheme 32) [43]. [43]. by many one-electron agents to give corresponding heterobimetallic complexes (Scheme 32) [43].

Scheme 32. Heterobimetallic semiquinonate adducts. Scheme 32. 32. Heterobimetallic Heterobimetallic semiquinonate semiquinonate adducts. adducts. Scheme Scheme 32. Heterobimetallic semiquinonate adducts.

A diversity in the oxidation addition of a metal to the ligand 68 was found in case of Pd(dppe) A diversity inBoth the oxidation of a metal to the ligandwere 68 was foundand in case of Pd(dppe) metallofragment. dithiolateaddition 73 and catecholate 74the isomers isolated characterized by A diversity diversity in in the the oxidation oxidation addition of aa metal metal to to ligand 68 68 was was found in in case case of Pd(dppe) Pd(dppe) A addition of the ligand found of metallofragment. Both dithiolate 73 and catecholate 74 isomers were isolated and characterized by NMR, IR-spectroscopy and X-ray analysis (Scheme 33)74 [44]. metallofragment. Bothdithiolate dithiolate 73and andcatecholate catecholate isomers were isolated characterized metallofragment. Both 74[44]. isomers were isolated andand characterized by NMR, IR-spectroscopy and X-ray 73 analysis (Scheme 33) by NMR, IR-spectroscopy and X-ray analysis (Scheme 33) [44]. NMR, IR-spectroscopy and X-ray analysis (Scheme 33) [44].

Scheme 33. The formation of two regioisomers 73 and 74. Reproduced from Dalton Trans. 2017, 46, Scheme 33. The formation of two regioisomers 73 and 74. Reproduced from Dalton Trans. 2017, 46, 16783–16786. Copyright (2017) Royal Society of Chemistry. Scheme 33. The formation of two regioisomers 73 and 74. Reproduced formation regioisomers Reproduced from from Dalton Dalton Trans. Trans. 2017, 2017, 46, 46, 16783–16786. Copyright (2017) Royal Society of Chemistry. Chemistry. 16783–16786. Copyright (2017) Royal Society of Chemistry.

According to DFT calculations the difference in energy between these isomers is rather small. According DFT calculations the difference in energy between theseof isomers rather small. This is the mainto reason for the diversity in the oxidative addition. The ratio isomerisproducts in the According to DFT calculations the difference in energy between these isomers is rather small. This is themixture main to reason the the oxidative addition. The ratio isomer products in According DFT for calculations the in difference in energy between theseofisomers is rather reaction depends on diversity the method of preparation, solvent polarity, temperature and small. so the on. This is the main reason for the diversity in the oxidative addition. The ratio oftemperature isomer products in the reaction mixture depends the method ofthe preparation, solvent polarity, andinto so on. This is the main foron the diversity oxidative addition. The ratio of isomer products in Nevertheless, thereason catecholate species 74 isin more stable, so that in solution it slowly isomerizes the reaction mixture depends onspecies the method of preparation, solvent polarity, temperature andinto so on. Nevertheless, the catecholate 74 is more stable, so that in solution it slowly isomerizes the the reaction mixture depends on the method of preparation, solvent polarity, temperature and so on. dithiolate one (Scheme 34). This is the first example of regioisomerism in coordination chemistry [44]. Nevertheless, catecholate species is more stable, so that in solution it slowly isomerizes into the dithiolate one the (Scheme 34). This is the74 first example of regioisomerism in coordination chemistry [44]. Nevertheless, the catecholate species 74 is more stable, so that in solution it slowly isomerizes into the dithiolate one (Scheme 34). This is the first example of regioisomerism in coordination chemistry [44]. dithiolate one (Scheme 34). This is the first example of regioisomerism in coordination chemistry [44].

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Scheme 34. Isomerization of 74 to 73. Reproduced from Dalton Trans. 2017, 46, 16783–16786. Copyright (2017) Royal Society of Chemistry. Scheme 34. Isomerization of 74 to 73. 73. Reproduced Reproduced from from Dalton Trans. Trans. 2017, 46, 16783–16786. Copyright (2017) Royal Society of Chemistry. (2017) Royal of Chemistry. Indeed, theSociety isomers 73 and 74 possess different chemical properties. Mononuclear dithiolate

complex 73 as an o-quinone equivalent is capable to form heterobinuclear semiquinonates being Indeed, the the isomers isomers 73 73 and and 74 74 possess different different chemical chemical properties. properties. Mononuclear Mononuclear dithiolate dithiolate Indeed, reduced with alkali metals or other possess one-electron reducing agents. Catecholate isomer 74 could be complex 73 as an o-quinone equivalent is capable to form heterobinuclear semiquinonates being complex an o-quinone equivalent is capable to form heterobinuclear beingspectrum reduced oxidized73 inas solution into the corresponding semiquinonate species. Thesemiquinonates parameters of EPR reduced with alkali metals or other one-electron reducing agents. Catecholate isomer 74 could with alkali adduct metals orare otherhighly one-electron reducing agents. Catecholate isomer could3)2be oxidized be in of this consistent with those reported for 74 (PPh Pd(3,6-t-Bu-ooxidized in solution into the corresponding semiquinonate species. The parameters of EPR spectrum solution into the corresponding semiquinonate species. The parameters of EPR spectrum of this adduct benzosemiquinonate) [45]. of highly this consistent adduct are highlyreported consistent those reported for (PPh 3)2[45]. Pd(3,6-t-Bu-oare with those (PPhwith Pd(3,6-t-Bu-o-benzosemiquinonate) 3 )2the Understanding the mechanisms thatforregulate direction of catecholate or dithiolate mode of benzosemiquinonate) [45]. Understanding theion mechanisms regulate the direction of from catecholate or dithiolate mode of binding of the metal in case of that ligand 68 could be useful the viewpoint of potential Understanding thein mechanisms that regulate the direction of viewpoint catecholateoforpotential dithiolate mode of binding of the ion case of ligand 68ordered could bestructures. useful from the application application in metal the stepwise assembling of Such binucleating constructions might binding of the metal ion in case of ligand 68 could be useful from the viewpoint of potential in the stepwise assembling of ordered structures. Such binucleating constructions might be applied as be applied as molecular devices. Moreover, the SS~OO system can be regarded as a junction between application in the stepwise assembling of ordered structures. Such binucleating constructions might molecular devices. Moreover, SS~OO be regarded as due a junction between the bulk the bulk metal electrode andthe a key partsystem of the can molecular device, to affinity of sulfur to metal noble be appliedand as molecular devices. Moreover, the SS~OO system can be regarded asmetals a junction between electrode a key part of the molecular device, due to affinity of sulfur to noble (Scheme 35). metals (Scheme 35). the bulk metal electrode and a key part of the molecular device, due to affinity of sulfur to noble metals (Scheme 35).

Scheme 35. Potential application of ligand 68 as a junction to a bulk metal. Scheme 35. Potential application of ligand 68 as a junction to a bulk metal. Scheme 35. Potential application of ligand 68 as a junction to a bulk metal. 4. Di-o-Quinones Containing Acceptor-Donor-Acceptor Systems 4. Di-o-Quinones Containing Acceptor-Donor-Acceptor Systems The energy gap between Acceptor-Donor-Acceptor the lowest unoccupied and highest occupied electronic level is an 4. Di-o-Quinones Containing Systems The energy gap between the lowest unoccupied and highest occupied electronic level is important parameter determining the redox, electronic, optical, and conductive properties of a an important parameter determining the redox, electronic, optical, and conductive properties The energy gap between the lowest unoccupied and highest occupied electronic level is an material. Multistage organic amphoteric redox systems with a small HOMO/LUMO gap (