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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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C−S Bond Cleavage, Redox Reactions, and Dioxygen Activation by Nonheme Dicobalt(II) Complexes Manish Jana and Amit Majumdar* Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India S Supporting Information *

ABSTRACT: Synthesis and reactivity of a series of thiolate/ thiocarboxylate bridged dicobalt(II) complexes were investigated in comparison with their carboxylate bridged analogues bearing free thiol/hydroxyl groups. Upon one-electron oxidation, complexes [Co2(N-Et-HPTB)(μ-SR1)](BF4)2 (R1 = Ph, 1a; Et, 1b; Py, 1c) and [Co2(N-Et-HPTB)(μ-SCOR2)](BF4)2 (R2 = Ph, 2a; Me, 2b) yielded [Co2(N-Et-HPTB)(DMF)2](BF4)3 (6) (DMF = dimethylformamide) along with the corresponding disulfides (where N-Et-HPTB is the anion of N,N,N′,N′-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1,3diaminopropane). Unlike the inertness of carboxylate bridged complexes [Co2(N-Et-HPTB)(μ-O2C-R3-SH)](BF4)2 (R3 = Ph, 3a; CH2CH2, 3b) and [Co2(N-Et-HPTB)(μ-O2CR4)](BF4)2 (R4 = Ph, 4a; Me, 4b; CH2CH2CH2OH, 5) toward O2, the bridging ethanethiolate in 1b was oxidized to yield a sulfinate bridged complex, [Co2(N-Et-HPTB)(μ-O2SEt)](BF4)2 (10). Detailed investigation of the synthetic aspects of 1a−1c led to the discovery of a C−S bond cleavage reaction and yielded the dicobalt(II) complexes [Co2(N-Et-HPTB)(SH)(H2O)](BF4)2 (8a), [Co2(N-CH2Py-HPTB)(SH)(H2O)](BF4)2 (8b) (where N-CH2Py-HPTB is the anion of N,N,N′,N′-tetrakis[2-(1-picolylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane)), and [Co2(N-Et-HPTB)(μ-S)](BF4) (9). Both 8a and 8b feature nonheme dinuclear Co(II) units containing a terminal hydrosulfide. The present study thus reports comparative redox reactions for a rare class of 16 dicobalt(II) complexes and introduces a selective synthetic strategy for the synthesis of unprecedented dicobalt(II) complexes featuring only one terminal hydrosulfide.

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INTRODUCTION Nonheme mono- and dinuclear cobalt(II) complexes have been studied since long ago due to their presence in the active sites of noncorrin cobalt-containing enzymes. To date, at least eight noncorrin cobalt-containing enzymes have been isolated,1 namely, methionine aminopeptidase, proline dipeptidase, nitrile hydratase, glucose isomerase, methylmalonyl-CoA carboxytransferase, aldehyde decarbonylase, lysine-2,3-aminomutase, and bromoperoxidase. These enzymes contain either mono or dinuclear Co(II) centers in a form that is quite different from that found in the corrin ring of vitamin B12. Two of the above-mentioned enzymes that contain dinuclear Co(II) units are methionine aminopeptidase from Escherichia coli2,3 and proline dipeptidase from Pyrococcus f uriosus.4 Methionine aminopeptidase cleaves the N-terminal methionine of the newly translated peptide chains in both eukaryotes and prokaryotes,3 while proline dipeptidase mediates the specific cleavage of proline-containing dipeptides at the C-terminus.4 Another interesting aspect of cobalt(II) complexes is their oxygen binding capability. Reversible oxygen binding for respiring organism is performed by hemoglobin, hemerythrin, and hemocyanin, of which the latter two contain dimetallic active sites. While cobalt has not been found in any naturally © XXXX American Chemical Society

occurring dioxygen carrier, cobalt-substituted hemoglobin (coboglobin) has been shown to bind oxygen reversibly.5 The oxygen binding affinity of coboglobin is, however, lower than the native iron proteins, and oxycoboglobin has been formulated as a Co(III) center with a coordinated superoxide.5 Bioinorganic chemistry of cobalt mainly involves synthesis of model mono- and dinuclear cobalt(II) complexes and investigation of their reactivity to mimic the functional aspects of the corresponding active sites. In model chemistry, a special emphasis has always been on the oxygen binding capability of the Co(II) complexes.6 Mononuclear cobalt complexes with salen7 and corrole8 ligands have been shown to achieve reversible O2 binding. Oxygenation study of dicobalt(II) complexes and characterization of the corresponding binuclear cobalt dioxygen complexes have been reported.9−12 A comparative thermodynamic study on the reactivity of diiron(II) and dicobalt(II) complexes containing different dinucleating ligands toward molecular oxygen is available in the literature.13 Recently it has also been reported that dicobalt(II) complexes can be tuned with respect to their Received: September 22, 2017

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DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Deprotonated Forms of Some Representative Dinucleating N,O-Donor Ligands

Chart 1. Abbreviationsa and Designations

a

HN-Et-HPTB, N,N,N′,N′-tetrakis(2-(l-ethylbenzimidazolyI))-2-hydroxy-1,3-diaminopropane,30,39 HN-CH2Py-HPTB, N,N,N′,N′-tetrakis(2-(1picolylbenzimidazolyl))-2-hydroxy-1,3-diaminipropane.

substituted benzimidazole-based ligands definitely offer a viable alternative option. For example, benzimidazole-based ligands have been successfully used in the oxygenation chemistry of diiron(II) complexes and related reactivity studies.27−29 Selected dinucleating ligands that have been used for the synthesis and reactivity study of dinculear Co(II) and/or Fe(II) complexes are collected in Scheme 1. Inspired by the successful use of a particular benzimidazole-based dinucleating ligand, HN-Et-HPTB (where N-Et-HPTB is the anion of N,N,N′,N′-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane; Scheme 1),30 in different aspects of various model diiron(II) complexes,28,29,31−36 we started using the ligand for the synthesis and reactivity study of Fe(II) and Co(II) complexes. While in general, a large number of Fe(II)/ Co(II)−thiolate complexes are available in the literature, nonheme diiron(II)/dicobalt(II) complexes with bridging or free thiolates/thiocarboxylates are relatively less explored37,38 and thus offer an opportunity to explore the synthetic aspects and reactivity of the later complexes in detail. In this line, we recently reported the reactivity study of carboxylate bridged

dioxygen affinity and reversible chemisorption by varying the electronic properties of the bridging carboxylate ligand.14 However, none of these studies included X-ray structural characterization of the dicobalt(II) complexes. Dicobalt(II) complexes using ligands other than typical dinucleating ligands used in the above-mentioned studies are also known.15−20 Synthesis, molecular structures, magnetic properties, and spectroscopic properties for a large number of dicobalt(II) and mixed-valence dicobalt(II, III) complexes with various types of dinucleating ligands have been reported recently.21−25 Examples include cobalt(II) centers containing mostly carboxylate bridges, although in some cases phosphodiester groups as auxiliary bridges are also reported.21 Dinuclear Co(II) complexes are mostly synthesized using N,O-donor dinucleating ligands along with carboxylates as ancillary coligands. Compartmental ligands of the “end-off” type containing phenolic or alcoholic oxygen as endogenous bridge have often been the choice of supporting dinucleating ligands.26 While the majority of the ligands used in the chemistry of dicobalt(II) complexes are pyridine-based, B

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Schematic Representation for the Synthesisa of 1a−10

a

Complex 8b was synthesized using procedure analogous to that shown for 8a but using HN-CH2Py-HPTB ligand.

Py) in 2:1:2:1 ratio in either DMF (1a, 1c) or MeCN (1b). Addition of RCOSH (R = Ph, Me) into a solution of 1b yielded 2a and 2b, respectively. Alternatively, 2a and 2b may be synthesized directly in a procedure analogous to the synthesis of 1a and 1b in 54−60% yield. Complexes 4a, 4b, and 5 could be obtained in 60−70% yield by following a procedure analogous to that described for 1a and 1b. Direct synthesis of 3a and 3b, however, may create a selectivity problem due to the possibility that both the thiolate and the carboxylate in 4-mercaptobenzoic acid and 3-mercaptopropionic acid, in principle, may bind to the Co(II) centers. This situation may create the possibility for the formation of three different species, namely, (i) 3a/3b, (ii) thiolate bridged dicobalt(II) complex bearing free carboxylic acid, and (iii) another species where two dicobalt(II) units may be bridged together by a 4-mercaptobenzoate/3-mercaptopropionate using both the thiolate and carboxylate functionality as bridging sites (Figure S1). It is therefore not surprising that attempted synthesis of 3a/3b following a procedure analogous to that described for 1a and 1b could not provide 3a/3b as pure crystalline samples. A similar situation was encountered by us during the synthesis of analogous diiron(II) complexes,39 and the synthetic problem was solved by introducing a selective proton exchange strategy. Successful synthesis of 3a and 3b (Scheme 2) thus relied on the possibility that during the reaction of the thiolate bridged dicobalt(II) complexes (1a−1c) with a mercaptocarboxylic acid reagent, the μ2-SR (R = Ph, Et, Py) group may preferentially accept one proton from the more acidic carboxylic acid groups rather than from comparatively less acidic aromatic/aliphatic thiol groups. Moreover, it is quite likely that a three atom bridge offered by a carboxylate group may be more stable than a single atom thiolate bridge. Thus, the proton exchange reaction selectively afforded 3a and 3b in 63−69% yields. Importance of the proton exchange reaction was also indicated by the reaction of the acetate bridged dicobalt(II) complex (4b) with PhCOSH, which yielded the thiobenzoate bridged dicobalt(II) complex (2a). In this reaction both CH3COO− and PhCOS− can potentially offer three atom bridges to the dicobalt(II) unit, and hence the driving force for the reaction may be related

diiron(II) complexes (using HN-Et-HPTB) in comparison with their thiolate bridged analogues.39 In a more recent work, a thioacetate bridged diiron(II) complex in the HN-Et-HPTB ligand platform was used by us to synthesize the first example of a mononitrosyl diiron(II) complex that could mediate the reduction of NO to N2O in relation with flavodiiron nitric oxide reductases.40 With these results, we planned to explore the synthetic aspects for various types of thiolate/thiocarboxylate bridged dicobalt(II) complexes using the HN-Et-HPTB ligand and study their reactivity in comparison with analogous carboxylate bridged dicobalt(II) complexes. Here we report the synthesis and characterization of five different types of dicobalt(II) complexes (Chart 1) that contain bridging thiolates (1a, 1b, 1c), bridging thiocarboxylates (2a, 2b), bridging carboxylates (4a, 4b), and bridging carboxylates with free hydroxy (5) and free thiol groups (3a, 3b). Solution stability and reactivity of these complexes with one-electron oxidants and O2 were explored, which yielded the complexes [Co2(N-Et-HPTB)(DMF)2](BF4)3 (6) (where DMF = dimethylformamide), [Co 2 (N-Et-HPTB)(μHNCOCH3)](BF4)2 (7), and [Co2(N-Et-HPTB)(μ-O2SEt)](BF4)2 (10). Most importantly, detailed investigation of the synthetic strategy for thiolate bridged dicobalt(II) complexes led to the discovery of a C−S bond cleavage reaction to yield two unprecedented dicobalt(II) complexes with only one terminal hydrosulfide, [Co2(L)(SH)(H2O)](BF4)2 (L = N-EtHPTB1−, 8a; N-CH2Py-HPTB1−, 8b), and a related sulfide bridged complex, [Co2(N-Et-HPTB)(μ-S)](BF4) (9). The C− S bond cleavage reaction was demonstrated by using different aliphatic thiolates (NaSR; R = tBu, Et) and dinucleating ligands (HN-Et-HPTB30 and HN-CH2Py-HPTB), and a plausible mechanism has been proposed. All of the 16 new complexes and a new ligand, HN-CH2Py-HPTB, were characterized by a combination of single-crystal X-ray structure determination and/or elemental analysis.

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RESULTS AND DISCUSSION General Synthesis. Complexes 1a−1c were synthesized directly in ∼60% yield (Scheme 2) by the addition of Co(BF4)2·6H2O into a mixture of the dinucleating ligand HNEt-HPTB,30,39 Et3N, and the corresponding NaSR (R = Ph, Et, C

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Molecular structures of selected complexes with 30% probability thermal ellipsoids and partial atom labeling schemes. Hydrogen atoms are omitted for clarity.

with the lower pKa of PhCOSH (pKa = 3.61) compared with that for CH3COOH (pKa = 4.76). Synthesis of a dicobalt(II) complex without any bridging ligand, [Co2(N-Et-HPTB)(DMF)n](BF4)3 (n = 1−4), was initially attempted by addition of Co(BF4)2·6H2O into a solution of HN-Et-HPTB and Et3N with 2:1:2 ratio in MeCN followed by recrystallization from DMF. However, other than

the desired complex [Co2(N-Et-HPTB)(DMF)2](BF4)3 (6) (minor product), a major product obtained in this synthetic procedure was found to be [Co 2 (N-Et-HPTB)(μHNCOCH3)](BF4)2 (7). Formation of 7 was also observed during the synthesis of 1a−1c in MeCN. This situation was, however, avoided by using a slight excess of NaSPh/NaSEt/ NaSPy. To confirm the source of the bridging acetamido D

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Molecular structures of 8a−10 with 30% probability thermal ellipsoids and partial atom labeling scheme. Except for hydrogen bonds in 8a−8b, hydrogen atoms are omitted for clarity.

discussed later in this manuscript. Molecular structures of selected complexes (1b, 2a, 3a, 4a, 5, and 6) are shown in Figure 1, while those for 1a, 1c, 2b, 3b, 4b, and 7 are provided in Figure S3. C−S Bond Cleavage of Aliphatic Thiolates by Dicobalt(II) Complexes. Synthesis and reactivity of thiolate (RS−) bridged diiron(II) complexes with R = Me, Et, tBu, Ph in HN-Et-HPTB ligand platform were recently reported by us.39 However, no account of analogous dicobalt(II) complexes is known in the literature. Synthesis of a series of dicobalt(II) complexes analogous to 1a was therefore attempted with various other bridging thiolates. In the present study, on the one hand, use of MeS− could not produce diffraction-quality single crystals. On the other hand, use of t BuS− in the synthesis of 1a−1c in MeCN resulted into the isolation of 7. To avoid the formation of 7, the latter reaction was therefore attempted in DMF. Interestingly enough, the latter reaction yielded an unexpected dicobalt(II) complex, [Co2(N-Et-HPTB)(SH)(H2O)](BF4)2 (8a), in 57% yield. Complex 8a was again isolated in 55% yield when the synthesis of 1b was attempted in DMF instead of in MeCN. Formation of 8a, which features a terminal hydrosulfide group coordinated to one Co(II) center of the dicobalt(II) unit (Figure 2), presumably indicates C−S bond cleavage of tBuS−/ EtS−. To reproduce this exciting synthetic procedure, a modified ligand system, HN-CH2Py-HPTB, was synthesized and characterized (Figure S4). Replacement of HN-Et-HPTB by HN-CH2Py-HPTB in the synthesis of 8a again yielded a dicobalt(II) complex containing one terminal hydrosulfide, [Co2(N-CH2Py-HPTB)(SH)(H2O)](BF4)2 (8b) (Figure 2).

group, isotope labeling experiments by using MeCN and CD3CN in two independent syntheses of 7 were performed followed by mass spectrometry of the reaction products in each case. The observed molecular ion peaks with m/z = 448.65 and 450.17 corresponding to [Co2(N-Et-HPTB)(μHNCOCH3)]2+ (expected m/z = 448.65) and [Co2(N-EtHPTB)(μ-HNCOCD3)]2+ (expected m/z = 450.16), respectively, confirmed that the bridging acetamide is indeed generated from MeCN (Figure S2). Base-catalyzed hydrolysis of MeCN in the presence of transition metals is welldocumented and has also been reported previously for iron.31,41 Hydrolysis of nitriles and related species by model cobalt and iron complexes in relevance to nitrile hydratase and mechanistic studies of such nitrile hydrolysis reaction are available in the literature.42−46 Moreover, reactions of pyrazolate bridged CoII-μ−OH-CoII dimers with acetonitrile to generate bridging acetamidato complexes have been studied extensively.47 These previous reports and the present observations thus strongly indicate that the bridging acetamido ligand in 7 might have been generated in situ by the basecatalyzed partial hydrolysis of MeCN in the presence of Co(II). Complex 7 was identified only by molecular structure determination, and no attempt was made to prepare this compound in pure form. Complex 6 may also be obtained, although not as an analytically pure sample, by addition of Co(BF4)2·6H2O into a solution of HN-Et-HPTB and Et3N in tetrahydrofuran (THF) (instead of MeCN) with 2:1:2 ratio, followed by recrystallization from DMF. Complex 6 was finally obtained as an analytically pure crystalline sample by a different procedure using (Cp2Fe)(BF4) and has been E

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 3. Set of Reactions Performed to Identify the Roles of a Preformed Dicobalt(II) Unit Identicala to 6, Et3N and H2O in the Formation of 8a via C−S Bond Cleavage of NaSEt

a

Or at least very similar.

The cleavage of C−S bonds mediated by cobalt48−53 and other transition metals49−51,53−57 has been reported in the literature, and in most of the cases the C−S bond of a coordinated ligand is involved. However, demonstration of C−S bond cleavage of simple aliphatic thiolates at room temperature (RT) and utilization of the reaction to synthesize unsymmetrical dinuclear transition metal complexes with only one terminal hydrosulfide have never been reported. Isolation of 8a was initially a fortunate stroke of serendipity during our attempt to extend the series of thiolate bridged dicobalt(II) complexes. However, the unique structural motif observed in 8a and 8b and its facile synthesis successfully established using two different thiolates (tBuS−, EtS−) in two different ligand systems (HN-Et-HPTB30 and HN-CH2Py-HPTB) prompted us to study the synthetic rationale for the formation of 8a and 8b in more detail. The formation of 8a may either have been triggered by Co(II) salt, such as Co(BF4)2·6H2O, in the presence of Et3N in DMF (without the ligand) or by a preformed dicobalt(II) unit (similar to 6). To examine these two possibilities, a set of reactions in different conditions was performed (Scheme 3), while keeping the crystallization procedures identical to that used for 8a. A mixture of Co(BF4)2·6H2O, Et3N, and NaStBu in 2:2:1 ratio could not produce any crystalline material that may be identified by single-crystal X-ray structure determination (Scheme 3, eqn (ii)). However, addition of a DMF solution of HN-Et-HPTB and Et3N into a stirred solution of Co(BF4)2·6H2O, NaStBu, and Et3N in DMF in 1:1:2:1:1 ratio yielded 8a in 60% yield (Scheme 3, eqn (iii)). While these two reactions indicate a major role of a preformed dicobalt(II) unit (similar to 6) in the reaction under discussion, it cannot rule out the possible role of Co(II) salts alone to cleave the C−S bond. Reaction of 6 with NaSEt in the presence of Et3N (not dried before reaction) in 1:1:1 ratio yielded 8a, although in ∼30% yield (Scheme 3, eqn (v)) compared with 57% yield in the original synthesis of 8a (Scheme 3, eqn (i)). Use of EtSH instead of NaSEt in the same reaction allowed the isolation of 8a in 35% yield. Upon addition of 5 equiv of H2O during the reaction with NaSEt (Scheme 3, eqn (vi)), 8a was again obtained, and the yield was increased to 55%, thus indicating

the importance of H2O in the reaction. Reaction of 6 with NaSEt in the presence of only 5 equiv of H2O (and no Et3N), however, could not produce any crystalline material that may be identified by a single-crystal X-ray structure determination, thus indicating the importance of Et3N in the reaction (Scheme 3, eqn (vii)). In the absence of both Et3N and H2O, 6 reacts with NaSEt (1:1) in DMF to yield 1b, which, upon further treatment with H2O, yielded uncharacterized products (Scheme 3, eqn (iv)). Therefore, considering all of the above-mentioned experiments (Scheme 3), it may be concluded that a dinuclear Co(II) complex, identical or at least very similar to 6, is surely capable of mediating the C−S bond cleavage of aliphatic thiolates to yield 8a and that both Et3N and H2O play major roles in triggering the reaction. Furthermore, it was also observed that 1b itself could not yield 8a upon treatment with H2O and Et3N, thus indicating that, once bridged, the thiolates may not undergo C−S bond cleavage reaction anymore. Attempted synthesis of 8a in anhydrous conditions using anhydrous CoCl2 (along with NaBF4) yielded [Co2(N-EtHPTB)(μ-S)](BF4) (9) (Scheme 2) in 52% yield. The striking difference between the structural motifs in 8a (one terminal −SH and one H2O) compared with 9 (one bridging sulfide) may be due to the absence of H2O in the synthesis of 9. Addition of H2O into a solution of 9 in DMF, however, could not yield 8a. This situation indicates that, once bridged, the sulfide cannot be protonated by H2O, and hence protonation of S2− may precede binding with dicobalt(II) unit. Addition of a 48% aqueous solution of HBF4 into a purple colored solution of 9 in DMF in 1.1:1 ratio resulted into a blue colored solution instead of the expected pink color of 8a. The absorption spectroscopic data (Figure S5) obtained for a mixture of 9 in DMF and 48% aqueous HBF4 in different ratios (1:0.8/1/1.2/ 1.4) also could not confirm the formation of 8a. However, pink colored crystals were obtained along with a blue colored oily mass upon crystallization of the reaction product over 2 d. Unit-cell determination of the pink crystals confirmed the formation of 8a. No attempt was made to characterize the blue oily mass. Therefore, we may conclude that, while 8a may be obtained upon treatment of 9 with aqueous solution of HBF4, F

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 4. A Plausible Working Mechanism for the Formation of 8a via C−S Bond Cleavage of Aliphatic Thiolatesa in the Presence of H2O and Et3N

Shown for tBuS−.

a

rather than occupying terminal positions at any or both of the metal centers. Reaction with One-Electron Oxidants and Interconversions of Selected Complexes. Redox properties of selected complexes were studied by cyclic voltammetry in MeCN, and the corresponding cyclic voltammetric traces are shown in Figures S7−S9. Investigation of the redox properties of 1a−9 indicates that the oxidations of the Co(II) centers in all the complexes are irreversible processes and in all the cases (except for 1c) take place at much higher potentials (>1 V vs Fc/Fc+). Oxidation of the bridging thiolates/thiocarboxylates, however, could not be observed. Similar situation was encountered in the electrochemical behavior of the analogous diiron(II) complexes,39 where the oxidation of the bridging thiolates could not be observed as well. However, the bridging thiolates in the analogous diiron(II) complexes could be oxidized by (Cp2Fe)(BF4) to yield a solvent-coordinated diiron(II) complex39 analogous to 6. In a similar way, the bridging thiolates in 1a−1c were oxidized by (Cp2Fe)(BF4) to the corresponding disulfides (Figures S10 and S11) along with the formation of presumably a MeCN-coordinated dicobalt(II) complex, [Co2(N-Et-HPTB)(MeCN)2]3+, which, when crystallized from DMF, yielded 6. In the same line, 2b was found to yield 6 and (CH3COS)2 (Figure S12) upon treatment with (Cp2Fe)(BF4). The bridging thiolates (R1S−, R1 = Ph, Et, Py) in 1a−1c may also react with (NO)(BF4) (E1/2 = 0.87 V vs Fc/Fc+) in 1:1 ratio to form R1SNO in solution, which will eventually form NO, corresponding disulfides, and 6. Complex 6 may then react with the in situ generated NO (available ratio = 1:1) to yield nitrosylated products. However, the reaction of 1a−1c with (NO)(BF4) yielded 6 (confirmed by unit-cell determination). Moreover, no reaction could be observed upon treatment of 6 with up to 4 equiv of Ph3CSNO in DMF. Direct reaction of 6 with excess NO gas was therefore investigated. In two separate experiments, NO gas was bubbled in solutions of 6 in MeCN and in CH2Cl2 for 5 min at RT. IR spectroscopic data for the reaction solutions in comparison with control experiments (Figure S13) indicated nitrosylation

it may not be the only product formed in this reaction. The molecular structures of 8a, 8b, and 9 are shown in Figure 2. To investigate the fate of the alkyl groups of the thiolates after C−S bond cleavage, diethyl ether was added to a reaction mixture containing HN-Et-HPTB, Et3N, Co(BF4)2·6H2O, and NaStBu in DMF to precipitate out the metal-containing species, and after filtration the filtrate was examined by gas chromatography (GC). Presence of tBuOH in the filtrate was identified by the GC experiments (see Figure S6). It was further observed that 8a may also be synthesized in comparable yields by using bases other than Et3N, such as t BuNH2 (yield of 8a = 58%). The fact that 8a could not be obtained from 6 in the absence of 1 equiv of a base (Et3N/tBuNH2) strongly indicates the important and necessary role of Et3N as a base in the synthesis of 8a−8b. Combining these results, a plausible working mechanism for the formation of 8a may be proposed (Scheme 4). A species identical (or at least very similar) to “A” may form initially, followed by attack of the coordinated H2O to the tertiary carbon atom (in case of StBu, shown in Scheme 4) of the thiolate coordinated to the adjacent Co(II) center to yield “B”, which subsequently may yield tBuOH and 8a. Presence of Et3N may promote activation of the coordinated H2O and thus may facilitate the attack on the thiolate carbon atom. While 8a−8b can only be synthesized in DMF and not in MeCN, the exact role of DMF in the above transformations cannot be defined at this stage. However, the failure of the reaction in MeCN to yield 8a may be due to the fast base-catalyzed partial hydrolysis of MeCN in the presence of Co(II) to form a potential bidentate ligand, acetamide, which eventually yields 7. Further treatment of 8a with NaSPh and NaSEt, however, could not afford the attempted monothiolato, monohydrosufido dicobalt(II) complex. The products obtained in these reactions were instead identified as 1a and 1b, respectively. Such results are not surprising, since, in the case of both diiron(II)39 and dicobalt(II) systems (this work), thiolates, even when used in excess, were always found to form bridges, G

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of 6 (νNO = 1712 cm−1). Upon crystallization under NO atmosphere, the product was isolated within 3 d as pink colored crystalline solid, which, however, upon IR spectroscopic measurements (both as KBR pellet and in CH2Cl2 solution) could not show the presence of any coordinated NO (Figure S14). Considering the synthesis and one-electron oxidation reactions of the complexes it becomes apparent that many of these complexes may undergo interconversions among themselves. The chemical interconversions of some selected dicobalt(II) complexes are summarized in Scheme 5. Results of

reports of dioxygen activity for other dicobalt(II) complexes are known since long ago.9,12,13 The dioxygen affinity of dicobalt(II) complexes is known to be influenced by the ligand nitrogen atoms and the chelate rings associated with the bridging oxygen.11,63 Nature of the dinucleating ligands is also known to influence the thermal stability of the μ-peroxo complexes against irreversible oxidation. An account of different dioxygen affinity of dicobalt(II) complexes with various “end-off” compartmental ligands is available in the literature.26 Representative examples of previously reported dicobalt(II) complexes with less or no dioxygen affinity includes [Co2(bpmp)(O2CCH3)2]2+, which does not react with O2 in MeCN, and [Co2(bpep)(O2CCH3)]2+ (see Scheme 1 for bpmp and bpep ligands), which shows no reactivity toward O2 even at −40 °C.26 The major difference between NEt-HPTB− and other ligands in Scheme 1 is the combination of benzimidazolyl N donors and bridging alkoxide O donor, which are present in N-Et-HPTB−, while the rest of the ligand systems contain different combinations of pyridyl N-donor and bridging phenolate/alkoxide O-donor sites. These differences in the ligand systems may therefore be responsible for the unusual stability of Co(II) centers in dicobalt(II) complexes (in the present study) toward molecular oxygen compared with the highly reactive nature of the dicobalt(II) complexes reported in the literature.9,12−14,26 Thiols are known to be prone toward oxidation by molecular oxygen to yield the corresponding disulfides, and very often this reaction can be catalyzed by transition metals. Representative examples include the oxidation of both aromatic and aliphatic thiols mediated by Co(II) phthalocyanines in ionic liquid,64 FeCl3/NaI in MeCN,65 [MoVIO2(O2CC(S)Ph2)2]2− in a Zn(II)−Al(III) layered double hydroxide host,66 RhH(PPh3)4/1,4-bis(diphenylphosphino)butane,67 VOCl3,68 [Fe4S4(SR)4]2−,69 (Et4N)[Co(PyPepS)2] and Na[Co(PyPepRS)2],70 [PPN][(NO)Fe(S,S−C6H4)2] (PPN = bis(triphenylphosphoranylidene)), 71 cis-Ru(bpy) 2 Cl 2 , 72 [PPN][Ru(DPPBT)3]73 [DPPBT = 2-diphenylphosphinobenzene thiolate], and a N 4 S(thiolate) iron(II) cysteine dioxygenase model complex.74 While some of the studies70−74 reported the formation of sulfinates, other studies64−69 did not involve the formation of sulfinic acids/sulfinates as a product. Moreover, a separate study on the oxidation of thiols by molecular oxygen reported that the ability of transition metals to catalyze oxidation of thiols to disulfide in aqueous solutions changes in the order Co ≪ Ni < Fe < Mn < Cu.75 Oxidation of thiols by hydrogen peroxide, however, leads to the formations of sulfenic, sulfinic, and sulfonic acids.76−78 Other methods for the conversion of thiols to sulfinic and sulfonic acids include the use of HOF·MeCN complex79 and Me2SO.80 Oxidation of n-octyl mercaptan and thiophenol in tert-butanol containing varying amounts of KOtBu to generate disulfide and/or acids (sulfinic and sulfonic) is also reported.81 The thiolate bridged complexes 1a and 1b are stable in solution under inert atmosphere, and no dissociation of the bridging thiolates could be observed within 2 d. Interestingly, attempted synthesis of 1b in open air at RT yielded a sulfinate bridged dicobalt(II) complex, [Co2(N-Et-HPTB)(EtSO2)](BF4)2 (10) (Scheme 2) in 33% yield. Cyclic voltammetric traces of 1b (Figure S7) show that the oxidation events for the Co(II) centers are irreversible events and take place at potentials higher than 1 V. Absorption spectroscopic measurements indicated that 1b did not react with oxygen (Figure S16) either at RT (in DMF) or at low temperatures (in DMF,

Scheme 5. Interconversions of Selected Dicobalt(II) Complexes Using Substitution and One-Electron Redox Reactions

the reactions in each case were confirmed by unit-cell determination of diffraction-quality single crystals obtained from the reaction products. These facile interconversions may indicate synthetic control over a wide variety of new dicobalt(II) complexes. Reaction with Molecular Oxygen. The oxidation events for the Co(II) centers in 1a−5 take place at much higher potentials (>1 V vs Fc/Fc+), which indicate their possible inertness toward reaction with molecular oxygen at RT. While all the complexes were initially synthesized under argon atmosphere, the carboxylate bridged dicobalt(II) complexes, 4a, 4b, and 5, as well as the DMF-coordinated dicobalt(II) complex 6 are perfectly stable in air at RT. No change in the absorption spectroscopic signatures of 6 in DMF could be observed upon purging of O2 at RT. However, there are literature reports of air-stable Co(II) complexes that rapidly react with O2 at low temperatures to form Co(III)-superoxo species.8,58−62 Therefore, possible reactions of 6 with O2 at low temperatures in different solvents were also investigated using absorption spectroscopic measurements (Figure S15). A DMF solution of 6 did not react with O2 at −50 °C upon bubbling of O2 for 15 min. No reaction could be observed either when a solution of 6 in MeCN was allowed to react with O2 at −35 °C. Similar situations prevailed when in two separate experiments solutions of 6 in CH2Cl2 and in acetone were allowed to react with O2 at −80 °C. On the contrary, a recent report on reversible O2 binding in dicobalt(II) complexes containing 2,6-bis(N,N-bis(2-pyridylmethyl)-aminomethyl)-4tert-butylphenolate ligand (bpdp in Scheme1) has described the dicobalt(II) complexes as “very air-sensitive pink powders” which “turn brown rapidly on exposure to air”.14 Similar H

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Selected Bond Distances (Å) and Angles (deg) for 1a−4a complexes parameters Co−OL

a

Co−S

1a

1b

1c

2a

2b

3a

3b

4a

1.923(2) 1.919(2) 2.466(1) 2.426(1)

1.930(2) 1.924(2) 2.410(1) 2.426(1)

1.909(2) 1.913(2) 2.452(1) 2.454(1)

1.962(4) 1.939(4) 2.371(1)

1.986(5) 1.956(5) 2.369(3)

1.950(3) 1.943(3)

1.964(4) 1.960(5)

1.931(3) 1.943(3)

1.989(4)

2.018(6)

2.059(5) 2.040(4) 2.274(5) 2.039(4) 2.021(5) 2.228(4) 3.496 127.3(1)

2.035(6) 2.036(8) 2.277(7) 2.048(7) 2.036(6) 2.245(7) 3.569 129.7(3)

2.012(4) 2.012(4) 2.051(4) 2.034(4) 2.264(4) 2.034(4) 2.030(4) 2.259(4) 3.406 122.1(1)

2.020(5) 2.004(5) 2.033(5) 2.035(6) 2.279(6) 2.024(6) 2.013(6) 2.298(6) 3.510 126.9(1)

2.018(4) 2.026(3) 2.027(5) 2.014(4) 2.250(4) 2.037(4) 2.009(4) 2.277(4) 3.405 123.1(1)

103.1(1)

101.0(1)

104.2(1)

102.7(2)

100.0(1) 101.5(1)

98.2(1) 98.1(1)

100.5(1) 100.0(1)

Co−Ob Co−NL

Co−Co Co−OLa−Co Co−S−Co S−Co−OLa

2.024(3) 2.012(3) 2.283(3) 2.023(3) 2.029(3) 2.276(3) 3.114 108.3(1) 79.1(1) 84.5(1) 85.7 (1)

2.017(2) 2.027(2) 2.305(2) 2.015(3) 2.030(2) 2.281(2) 3.131 108.6(1) 80.7(1) 83.5 (1) 84.0(1)

2.012(3) 2.012(3) 2.254(3) 2.013(3) 2.009(3) 2.253(3) 3.112 108.9(1) 78.7(1) 84.4(1) 84.5(1)

Ob−Co−OLa a

Deprotonated hydroxyl group from ligand. bO atom of thiocarboxylate (2a and 2b) or carboxylate (3a−4a).

−50 °C; in MeCN, −35 °C; in CH2Cl2, −80 °C; in acetone, −80 °C). Synthesis of 10 at −50 °C in open air (yield = 27%) or use of excess NaSEt (yield = 31%) in the synthesis did not have any significant effect on the yield of 10. Bubbling of O2 into the reaction mixture during the synthesis of 10 at RT also could not increase the yield significantly (35%). Addition of excess Et3N increased the yield from 33% to 40%, which may be considered only as a nominal increase. Considering the observations and the inertness of 6 toward dioxygen, the involvement of Co(II) centers in the oxidation of the bridged

ethanethiolate to the corresponding sulfinate via the formation of peroxo dicobalt(III) or related species may possibly be ruled out. Therefore, it may be concluded that the coordination of the thiolates with the Co(II) centers must have activated the bridged thiolates to undergo a direct reaction with O2 in the presence of Et3N to yield sulfinates at RT. Molecular Structures. All of the 16 dicobalt(II) complexes and HN-CH2Py-HPTB were characterized by single-crystal X-ray structure determinations. The molecular structures for selected dicobalt(II) complexes are shown in

Table 2. Selected Bond Distances (Å) and Angles (deg) for 4b−10 complexes parameters Co−OLa Co−Ob Co−μ-S HS−OH2 Co−SH Co−NL

Co−Co Co−OLa−Co Co−S−Co S−Co−OLa Ob−Co−OLa

4b

5

6

7

8a

8b

1.955(4) 1.949(4) 1.991(4) 2.013(5)

1.977(5) 1.978(5) 2.021(5) 2.032(5)

1.983 (4) 1.987(4) 2.033(5)c 2.043(5)c

1.977(4) 1.980(4) 2.007(5)

2.020(5) 2.000(5) 2.029(5)

2.000(4) 1.991(4) 2.024(5)

2.033(5) 2.044(5) 2.306(5) 2.016(6) 2.024(5) 2.286(6) 3.555 127.9(2)

2.843 2.318(3) 2.034(6) 2.061(7) 2.263(6) 2.032(7) 2.017(6) 2.248(6) 3.706 134.4(2)

2.800 2.232(3) 2.060(5) 2.059(5) 2.301(5) 2.031(5) 2.045(5) 2.248(5) 3.648 132.1(2)

103.9(1)

104.2(1)

97.6(2)

96.8(1)

9 1.908(5) 1.897(4)

10 1.986(4) 1.990(4) 2.035(5) 2.022(5)

2.456(1)2.455(2)

2.028(5) 2.038(5) 2.273(5) 2.016(5) 2.013(5) 2.304(5) 3.479 126.0(1)

99.0(1) 98.0(1)

2.022(6) 2.042(6) 2.321(6) 2.064(7) 2.044(6) 2.301(6) 3.531 126.4(2)

97.8(2) 98.5(2)

2.041(6) 2.056(6) 2.214(6) 2.012(6) 2.028(5) 2.300(6) 3.637 132.7(2)

108.4(1)c 99.8(1)c

101.1(1)

2.002(5) 2.005(6) 2.245(5) 1.999(6) 2.009(6) 2.228(6) 3.173 113.0(2) 80.6(1) 82.1(1) 82.4(1)

2.028(5) 2.049(6) 2.295(5) 2.058(5) 2.042(5) 2.275(5) 3.607 130.3(2)

98.1(1) 99.0(1)

a

Deprotonated hydroxyl group from ligand. bO atom of carboxylate (4b and 5), acetamide (7, Co1−N1 = 2.014(5)), H2O (8a, 8b), or sulfinate (10). cO atom of coordinated DMF. I

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

2.383(8) Å ([Co3(μ3-S)2(μ-SH)2(μ-PEt2)(PHEt2)6])84 reported for complexes containing bridging HS−. The CoII− OH2 distances of 2.029(5) and 2.024(5) Å in 8a and 8b are also comparable with previously reported CoII−OH2 bond distances of 2.073(2),85 2.072(1),86 2.081(2),87 and 2.095 (2) Å.88 The terminal −SH and H2O are engaged in hydrogen bonding (Figure 2) to make a H−S−H−O−H bridge similar to the few known H−O−H−O−H bridges in aquo-hydroxy bridged complexes,89−93 and the corresponding HS···HOH distances are 1.953(3) (8a) and 1.893(2) Å (8b). The SHS− OH2O distances in 8a and 8b are 2.843 and 2.800 Å, respectively, which are smaller than the corresponding van der Waals distance of 3.32 Å and much larger than the S−O distances in the thiocarboxylate bridged complexes 2a (2.623 Å) and 2b (2.590 Å). In 8b, the second hydrogen atom of the terminal water molecule is also engaged in hydrogen bonding with a DMF molecule, where the HOH···ODMF distance is 1.892(6) Å (Figure 2). It is the hydrogen bonding interaction between the −SH and H2O that presumably favors the nearly coplanar arrangement of these two terminal ligands in contrast to the bent arrangement of the coordinated DMF molecules in 6. The two Co(II) centers in 9 are bridged by a sulfide ligand with Co−μ-S distances of 2.456 (1) and 2.455(2) Å, while the Co−S−Co angle was found to be 80.6(1)°.

Figures 1 and 2, while those for the rest of the complexes are provided in Figure S3. Molecular structure of H-N-CH2PyHPTB is provided in Figure S3. Selected bond distances and angles for all the 16 complexes are collected in Tables 1 and 2. In all the dinuclear complexes, Co(II) centers are fivecoordinate. On the basis of the molecular structures, the dicobalt(II) complexes may be divided into seven different types: (i) thiolate bridged complexes (1a−1c), (ii) thiocarboxylate bridged complexes (2a, 2b), (iii) carboxylate bridged complexes with or without free hydroxyl/thiol groups (3a−5) and ethanesulfinate bridged complex (10), (iv) DMFcoordinated complex without any bridging ligand (6), (v) acetamido bridged complex (7), (vi) complexes featuring one terminal hydrosulfide and one terminal H2O (8a, 8b) and (vii) sulfide bridged complex (9). In general, two different sets of Co−NN‑Et‑HPTB distances are observed in all the complexes (Tables 1 and 2), of which the Co−benzimidazole nitrogen distances (2.009(3)−2.064(7) Å) are much smaller than the Co−tertiary amine nitrogen distances (2.214(6)−2.321(6) Å). The Co−Co distance varies a lot (3.112−3.706 Å) among the complexes (1a−10) due to the presence (or absence) of different bridging ligands in different types of dicobalt(II) complexes. As expected for comparatively more strained structures, the Co−Co distances in the thiolate bridged complexes 1a−1c range from 3.112 to 3.131 Å, while for the sulfide bridged complex 9, the distance is 3.173 Å. These distances are much smaller than any of the dicobalt(II) complexes with bidentate bridging ligands presented in this manuscript. The Co−Co distance increases upon going from 4a (benzoate bridged, 3.405 Å) to 4b (acetate bridged, 3.479 Å) to 5 (γ-hydroxy butyrate bridged, Co−Co 3.531 Å), while for 7 and 10 the distances are 3.555 and 3.607 Å, respectively. Complexes 2a, 2b, 3a, and 3b follow a similar trend, where 2a (thiobenzoate bridged, Co−Co = 3.496 Å) and 3a (4mercaptobenzoate bridged, Co−Co = 3.406 Å) exhibit smaller Co−Co distances compared with that observed in 2b (thioacetate bridged, Co−Co = 3.569 Å) and 3b (3mercaptopropionate bridged, Co−Co = 3.510 Å), respectively. Complexes 6, 8a, and 8b exhibit much larger Co−Co distances (3.637−3.706 Å) than the others due to the absence of any bridging ligand between the two Co(II) centers in 6, 8a, and 8b. These metric parameters are consistent with the Co−OL− Co angles (Table 1), which are smallest for 1a−1c (108.3(1)− 108.9(1)°) and largest for 8a (134.4(2)°). Unlike the symmetrical carboxylate bridges with nearly equal bond distances in 3a, 3b, 4a, 4b, and 5, the thiocarboxylate bridges in 2a and 2b show different C−O (1.262(9) and 1.212(10) Å), C−S (1.669(9) and 1.665(10) Å), Co−O (1.989(4) and 2.018(6) Å), and Co−S (2.371(1) and 2.369(3) Å) bond distances, where the bridging thiocarboxylates retain the C−O double bond character. The structural motifs in 8a and 8b exhibit unprecedented features, where one five-coordinate Co(II) center is coordinated by a terminal hydrosulfide, while the other five-coordinate Co(II) center is coordinated by a water molecule. Two Co(II) centers in 8a and 8b are separated by a distance of 3.706 and 3.648 Å, respectively, while the corresponding Co1−O1−Co2 angles are 134.4(2) and 132.1(2)°, respectively. The CoII−SH distances of 2.318(3) (8a) and 2.232 (3) Å (8b) are comparable with Co−SH distance of 2.2398(13) Å reported for a mononuclear Co(II) complex containing terminal SH, namely, [Co(C9H7N)2(SH)2],82 and Co−SH bond distances of 2.292− 2.303 Å ([{Co(μ-SH)(cyclam)}2][SH]283 and 2.343(8)−

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SUMMARY AND CONCLUSIONS Synthesis, molecular structures, and comparative reactivity for seven different types of nonheme dicobalt(II) complexes have been investigated using two benzimidazole-based dinucleating ligands. While analogous nonheme carboxylate bridged dicobalt(II) complexes are known in the literature, there are no precedents for the dicobalt(II) complexes that feature bridging carboxylates containing free thiol/hydroxyl functional groups. Unlike their diiron(II) analogues39 and other airsensitive dicobalt(II) complexes,9,12−14,26 all of the 16 dicobalt(II) complexes presented here are stable in air and show irreversible oxidation processes at potentials greater than 1 V. Depending upon the reaction conditions, selected fivecoordinate dinuclear cobalt(II) units may take part in (i) C−S bond cleavage of aliphatic thiolates, (ii) one-electron oxidation of bridging thiolates to the corresponding disulfides, and (iii) molecular oxygen mediated oxidation of thiolate to the corresponding Co(II) bound sulfinate. Most importantly, the C−S bond cleavage of aliphatic thiolates provides a unique synthetic strategy for the preparation of unprecedented dicobalt(II) complexes bearing only one terminal hydrosulfide. The synthetic strategy has been investigated in detail, and a plausible mechanism for the above transformation has been proposed. This new synthetic strategy has been established with two different thiolates using two different ligand systems and may be further investigated in future to synthesize similar complexes with other transition metals, especially iron.

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EXPERIMENTAL SECTION

Preparation of Compounds. All the reactions and manipulations described in this manuscript were performed under a pure argon atmosphere using either standard Schlenk techniques or an inert atmosphere glovebox, unless otherwise mentioned. Solvents were dried following literature procedures.94 All the filtrations were performed through diatomaceous earth, and solvent removal steps were performed in vacuo inside an inert atmosphere glovebox. Yields reported in each case are for recrystallized compounds and are average of individual yields obtained from multiple (at least three) batches of J

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as a pink crystalline solid (53 mg, 58%). Anal. Calcd for C45H54N10B2F8Co2S1O1 (1b): C, 50.30; H, 5.07; N, 13.04. Found: C, 50.25; H, 4.94; N, 13.26%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 529 (330 ± 10), 770 (60 ± 4). [Co2(N-Et-HPTB)(μ-SPy)](BF4)2 (1c). To a mixture of HN-Et-HPTB (0.08 mmol, 57.8 mg), Et3N (0.16 mmol, 16.2 mg), and 4mercaptopyridine (0.085 mmol, 9.45 mg) in 2 mL of DMF was added Co(BF4)2·6H2O (0.16 mmol, 54.5 mg) with stirring, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the dark red colored filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as a dark pink crystalline solid (50 mg, 55%). UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 349 (5650 ± 190), 520 (400 ± 20), 573 (170 ± 5), 780 (60 ± 4). Reasonable elemental analysis data could not be obtained for this compound. [Co2(N-Et-HPTB)(μ-SCOC6H5)](BF4)2 (2a). To a mixture of HN-EtHPTB (0.08 mmol, 57.8 mg) and triethyl amine (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Co(BF4)2·6H2O (0.16 mmol, 54.5 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of C6H5COSH (0.085 mmol, 11.75 mg) in 2 mL of MeCN was added, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the reddish-pink colored filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as a dark pink crystalline solid (50 mg, 54%). Anal. Calcd for C50H54B2F8Co2N10O2S1 (2a): C, 52.19; H, 4.73; N, 12.17. Found: C, 51.86; H, 4.52; N, 12.16%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 533 (380 ± 8). [Co2(N-Et-HPTB)(μ-SCOCH3)](BF4)2 (2b). To a mixture of HN-EtHPTB (0.08 mmol, 57.8 mg) and Et3N (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Co(BF4)2·6H2O (0.16 mmol, 54.5 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of CH3COSH (0.085 mmol, 6.46 mg) in 2 mL of MeCN was added, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the reddish-pink colored filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as a dark pink crystalline solid (53 mg, 61%). Anal. Calcd forC45H52B2F8Co2N10O2S1 (2b): C, 49.65; H, 4.81; N, 12.87. Found: C, 49.45; H, 5.01; N, 12.58%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 537 (450 ± 12), 758 (80 ± 2). [Co2(N-Et-HPTB)(μ-O2CC6H4SH)](BF4)2 (3a). To a pink colored solution of 1b (0.1 mmol, 107 mg) in 5 mL of MeCN was added a suspension of 4-mercaptobenzoic acid (0.1 mmol, 17.1 mg) in 2 mL of MeCN with stirring, and the reaction mixture was stirred for 4 h. The resulting pink solution was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as a pink colored crystalline solid (80 mg, 69%). Anal. Calcd for C50H54N10O3B2F8S1Co2 (3a·Et2O): C, 52.28; H, 5.20; N, 11.29. Found: C, 52.26; H, 5.39; N, 11.60%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 535 (375 ± 35), 775 (50 ± 4). [Co2(N-Et-HPTB)(μ-O2CC3H4SH)](BF4)2 (3b). To a pink colored solution of 1b (0.1 mmol, 107 mg) in 5 mL of MeCN was added a suspension of 3-mercaptopropionic acid (0.1 mmol, 10.61 mg) in 2 mL of MeCN with stirring, and the reaction mixture was stirred for 4 h. The resulting pink solution was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as a pink colored crystalline solid (71 mg, 63%). Anal. Calcd for C46H54N10O3B2F8S1Co2 (3b): C, 49.39; H, 4.87; N, 12.52. Found: C, 49.49; H, 4.85; N, 12.18%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 527 (380 ± 10), 760 (80 ± 5). [Co2(N-Et-HPTB)(μ-COOC6H5)](BF4)2 (4a). To a mixture of HN-EtHPTB (0.08 mmol, 57.8 mg) and triethylamine (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Co(BF4)2·6H2O (0.16

reactions. All the yields are calculated using corresponding molecular weights of the compounds shown in Tables S1−S3. HN-EtHPTB30,39 was synthesized following a reported procedure.30 HNCH2Py-HPTB was synthesized by a procedure analogous to that used for HN-Et-HPTB. Co(BF4)2·6H2O (99%), NaSEt (90%), NaStBu (90%), 4-mercaptobenzoic acid (90%), 3-mercaptopropionic acid (99%), thioacetic acid (96%), thiobenzoic acid (90%), sodium benzoate (99%), ferrocene (98%), ferrocenium tetrafluoroborate (technical grade), (NO)(BF4) (95%), and tetra-n-butylammonium hexafluorophosphate (98%) were obtained commercially and used without further purifications. Identity of all the 16 compounds in Chart 1 along with HN-CH2Py-HPTB was confirmed by molecular structure determinations, and bulk purity was confirmed (except for 1c and 9) by elemental analysis. N,N,N′,N′-Tetrakis(2-(1-picolylbenzimidazolyl))-2-hydroxy-1,3diaminipropane (HN-CH2Py-HPTB). 1,2-Diaminobenzene (5.25 g, 0.048 mol) was ground to a fine powder and thoroughly mixed with 2-hydroxy-1,3-diaminopropanetetraacetic acid (2.5 g, 0.008 mol). The solid mixture was heated at 170−180 °C for 1.5 h, at which stage effervescence had ceased. After the mixture was cooled to RT, the resulting red glass was taken up in 4 M HCl (total 150 mL) by using small portions each time to yield a bluish-gray precipitate. After the solution was filtered, the bluish-gray precipitate was washed by slurrying in acetone multiple times. The precipitate was then dissolved in water, and dilute ammonia was used to neutralize the solution. The off-white precipitate thus formed was collected, recrystallized from acetone, ground to a fine powder, and dried under vacuum to yield the precipitate as off-white fine dust (4.5 g, 94%). A portion of this off-white solid (1.73 g, 0.0029 mol) was suspended in dry tetrahydrofuran (30 mL) and stirred with NaOH (0.93 g) overnight. 2-(Chloromethyl) pyridine hydrochloride (1.9 g) was then added, and the resulting solution was stirred for 2 d. The solvents were then stripped to dryness, and the resulting solid powder was dissolved in chloroform. The chloroform solution was filtered to remove insoluble NaCl, and the filtrate was stripped to a very small volume. Acetone was added into the solution, and the resulting solution was kept standing for 30 min to yield white powder of HNCH2Py-HPTB (1.18 g, 42%). 1H NMR (300 MHz, CDCl3) δ 8.21− 8.20 (d, J = 3 Hz, 4H), 7.63−7.60 (dd, J = 3 Hz, 4H), 7.24−7.08 (m, 16H), 6.95−6.91 (q, J = 6 Hz, 4H), 6.57−6.54 (d, J = 9 Hz, 4H), 6.41 (s, 1H) 5.36−5.21 (q, J = 15 Hz, 8H), 4.18−4.13 (d, J = 15 Hz, 4H), 3.98−3.93 (d, J = 15 Hz, 4H), 2.84−2.77 (q, J = 6 Hz, 2H), 2.57− 2.52 (dd, J = 3 Hz, 2H). 13C NMR (75 MHz, CDCl3) 155.69, 151.96, 149.54, 142.33, 136.83, 135.68, 122.90, 122.54, 122.21, 120.86, 119.78, 110.05, 67.96, 59.87, 52.48, 48.54. Time-of-flight mass spectrometry (TOF-MS) (+) m/z calcd for C59H54N14O+ ([M + H]+) = 975.4678, ([M + Na]+) = 997.4498; found 975.1154, 997.1203. Identity of HN-CH2Py-HPTB was further confirmed by a single-crystal X-ray structure determination (Figure S3). Colorless block-shaped crystals, suitable for single-crystal X-ray diffraction study, were obtained by keeping a concentrated methanol solution of HN-CH2Py-HPTB at 0 °C for 2 d. [Co2(N-Et-HPTB)(μ-SPh)](BF4)2 (1a). To a mixture of HN-Et-HPTB (0.08 mmol, 57.8 mg), Et3N (0.16 mmol, 16.2 mg), and NaSPh (0.085 mmol, 11.2 mg) in 2 mL of DMF was added Co(BF4)2·6H2O (0.16 mmol, 54.5 mg) with stirring, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the dark pink colored filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as a dark pink crystalline solid (58 mg, 60%). Anal. Calcd for C49H54B2F8Co2N10O1S1 (1a·H2O): C, 51.60; H, 4.95; N, 12.28. Found: C, 51.49; H, 5.09; N, 12.78%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 345 (360 ± 30), 527 (460 ± 20), 780 (90 ± 4). [Co2(N-Et-HPTB)(μ-SEt)](BF4)2 (1b). To a mixture of HN-Et-HPTB (0.08 mmol, 57.8 mg) and Et3N (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Co(BF4)2·6H2O (0.16 mmol, 54.5 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of NaSEt (0.085 mmol, 7.9 mg) in 2 mL of MeCN was added, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the dark pink colored filtrate was evaporated to dryness. The K

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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C59H56B2F8N14S1O2Co2·2H2O (8b·2H2O): C, 52.38; H, 4.47; N, 14.5. Found: C, 52.47; H, 4.74; N, 14.28%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 527 (322 ± 15), 775 (90 ± 3). [Co2(N-Et-HPTB)(μ-S)](BF4) (9). To a mixture of HN-Et-HPTB (0.05 mmol, 36 mg), Et3N (0.075 mmol, 7.6 mg), NaBF4 (0.10 mmol, 10.93 mg), and NaStBu (0.075 mmol, 8.4 mg) in 2 mL of DMF was added anhydrous CoCl2 (0.10 mmol, 12.9 mg) with stirring, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the purple colored filtrate was allowed to diffuse overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as purple crystalline solid (30 mg, 52%). UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 537 (270 ± 15), 695 (46 ± 3), 820 (60 ± 2). Reasonable elemental analysis data could not be obtained for this compound. [Co2(N-Et-HPTB)(μ-O2SEt)](BF4)2 (10). To a mixture of HN-EtHPTB (0.08 mmol, 57.8 mg) and Et3N (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Co(BF4)2·6H2O (0.16 mmol, 54.5 mg) in 2 mL of MeCN with stirring in open air. After 15 min, a suspension of NaSEt (0.085 mmol, 7.9 mg) in 2 mL of MeCN was added, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the dark pink colored filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered. The filtrate was diffused overnight with Et2O at 0 °C and allowed to stand for 2 d at RT to afford the product as a pink crystalline solid (29 mg, 33%). Anal. Calcd for C45H54N10B2F8Co2S1O3 (10·H2O): C, 48.06; H, 5.02; N, 12.46. Found: C, 48.0; H, 4.97; N, 12.41%. UV− Vis (in MeCN) λ nm (ε M−1 cm−1): 530 (360 ± 20), 778 (60 ± 2). General Physical Methods. A PerkinElmer 2400 series II CHNS analyzer was used for the elemental analysis. Electrochemical studies of the dicobalt(II) complexes (1 × 10−3 M in MeCN) were performed using a CHI620E electrochemical analyzer (CH Instruments, USA). A three-electrode setup comprising a glassy carbon working electrode, a platinum wire auxiliary electrode, and a silver wire as the pseudo reference electrode was employed for the study. Tetra-n-butylammonium hexafluorophosphate (0.1 M) was the supporting electrolyte of choice. Electrochemical potentials for all the complexes are referenced to the ferrocene/ferrocenium couple at 0.0 V. Gas chromatography Agilent 7890B equipped with a packed column (Agilent J & W GC columns) and a 5977A MS detector was used for identification of the disulfides. X-ray Structure Determinations. The molecular structures of all the complexes 1a−10 in Chart 1 and HN-CH2Py-HPTB were determined. Diffraction-quality crystals were obtained as described in the respective syntheses. Single crystals coated with mineral oil were mounted under a 150 K nitrogen cold stream. Data were collected either at 150 K on a Bruker SMART APEX-II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) controlled by the APEX 2 (v. 2010.1−2) software package (1a−7, 10 and HN-CH2Py-HPTB) or at 100 K (8b and 9) on a Bruker D8VENTURE Microfocus diffractometer equipped with PHOTON II Detector, with Mo Kα radiation (λ = 0.710 73 Å), controlled by the APEX3 (v2017.3−0) software package. Raw data were integrated and corrected for Lorentz and polarization effects using the Bruker APEX II95/APEX III program suite. Absorption corrections were performed using SADABS. Space groups were assigned by analysis of metric symmetry and systematic absences (determined by XPREP) and were further checked by PLATON96,97 for additional symmetry. All the structures were solved by direct methods and were refined against all data in the reported 2θ ranges by full-matrix least-squares on F2 with the SHELXL program suite98 using the OLEX 299 interface. Hydrogen atoms at idealized positions were included during the final refinements of each structure. The OLEX 2 interface was used for structure visualization, analysis of bond distances and angles, and drawing ORTEP100,101 plots. Crystallographic data and final agreement factors for the 17 compounds (CCDC Nos. 1556873−1556886, 1573986, 1573990, and 1573991) are provided in Tables S1−S3. Refinement details and relevant explanations (wherever applicable) are included in the individual CIFs.

mmol, 54.5 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of C6H5COONa (0.085 mmol, 12.24 mg) in 2 mL of MeCN was added, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the pink colored filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as pale pink crystalline solid (55 mg, 60%). Anal. Calcd for C50H54B2Co2F8N10O3 (4a): C, 52.93; H, 4.80; N, 12.35. Found: C, 52.42; H, 4.47; N, 12.17%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 531 (385 ± 30), 770 (60 ± 2). [Co2(N-Et-HPTB)(μ-COOCH3)](BF4)2 (4b). To a mixture of HN-EtHPTB (0.08 mmol, 57.8 mg) and triethylamine (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Co(BF4)2·6H2O (0.16 mmol, 54.5 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of CH3COONa (0.085 mmol, 6.97 mg) in 2 mL of MeCN was added, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the pink colored filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as pale pink crystalline solid (60 mg, 70%). Anal. Calcd for C45H52B2Co2F8N10O3 (4b·0.5 C3H7NO): C, 50.36; H, 5.04; N, 13.26. Found: C, 50.26; H, 5.32; N, 12.98%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 525 (210 ± 10), 770 (36 ± 2). [Co2(N-Et-HPTB)(μ-O2CCH2CH2CH2OH)](BF4)2 (5). To a mixture of HN-Et-HPTB (0.08 mmol, 57.8 mg) and Et3N (0.16 mmol,16.2 mg) in 4 mL of MeCN was added a solution of Co(BF4)2·6H2O (0.16 mmol, 54.5 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of sodium γ-hydroxybutyrate (0.085 mmol, 10.7 mg) in 2 mL of MeCN was added, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the pink colored filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as pink colored crystalline solid (53 mg, 59%). Anal. Calcd for C47H56N10O4B2F8Co2 (5·H2O): C, 49.76; H, 5.15; N, 12.35. Found: C, 49.60; H, 5.34, N, 12.02%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 527 (365 ± 10), 775 (65 ± 2). [Co2(N-Et-HPTB)(DMF)2](BF4)3 (6). To a pink colored solution of 1a (0.04 mmol, 44.9 mg) in 5 mL of MeCN was added ferrocenium tetrafluoroborate (0.06 mmol, 16.4 mg) in 2 mL of MeCN with stirring, and the reaction mixture was stirred for 4 h. The resulting purple solution was evaporated to dryness. The residue was washed several times with THF and dried. The residue was then extracted with 2 mL of DMF and filtered, and the filtrate was allowed to diffuse overnight with Et2O at −35 °C followed by 1 d at RT to afford the product as pink colored crystalline solid (30.5 mg, 65%). Anal. Calcd for C49H63B3F12Co2N12O3 (6): C, 47.22; H, 5.09; N, 13.49. Found: C, 47.11; H, 4.96; N, 13.04%. UV Vis (in MeCN) λ nm (ε M−1 cm−1): 518 (300 ± 10), 742 (40 ± 2). [Co2(N-Et-HPTB)(SH)(H2O)](BF4)2 (8a). To a mixture of HN-EtHPTB (0.08 mmol, 57.8 mg), Et3N (0.16 mmol, 16.2 mg), and NaStBu (0.085 mmol, 9.5 mg) in 2 mL of DMF was added Co(BF4)2· 6H2O (0.16 mmol, 54.5 mg) with stirring, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the pink colored filtrate was allowed to diffuse overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as a pink crystalline solid (49 mg, 57%). Anal. Calcd for C43H52B2F8N10S1O2Co2 (8a·C3H7NO): C, 48.57; H, 5.23; N, 13.54. Found: C, 48.70; H, 5.26; N, 13.54%. UV−Vis (in MeCN) λ nm (ε M−1 cm−1): 531 (270 ± 2), 800 (45 ± 2). [Co2(N-CH2Py-HPTB)(SH)(H2O)](BF4)2 (8b). To a mixture of HNCH2Py-HPTB (0.06 mmol, 60 mg), Et3N (0.12 mmol, 12.5 mg), and NaStBu (0.085 mmol, 9.5 mg) in 2 mL of DMF was added Co(BF4)2· 6H2O (0.12 mmol, 40.8 mg) with stirring, and the resultant slurry was stirred for 4 h. The reaction mixture was filtered, and the pink colored filtrate was allowed to diffuse overnight with Et2O at −35 °C and allowed to stand for 1 d at RT to afford the product as pink colored crystalline solid (45 mg, 57%). Anal. Calcd for L

DOI: 10.1021/acs.inorgchem.7b02432 Inorg. Chem. XXXX, XXX, XXX−XXX

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Cobalt−Alkynyl Complexes. J. Am. Chem. Soc. 2004, 126, 2515− 2525. (9) Suzuki, M.; Kanatomi, H.; Murase, I. Synthesis and Properties of Binuclear Cobalt(II) Oxygen Adduct with 2,6-Bis Bis(2Pyridylmethyl)Aminomethyl −4-Methylphenol. Chem. Lett. 1981, 10, 1745−1748. (10) Suzuki, M.; Kanatomi, H.; Murase, I. Dinuclear Cobalt(II) Complexes with Various Dinucleating Ligands and Their Dioxygen Complexes. Bull. Chem. Soc. Jpn. 1984, 57, 36−42. (11) Suzuki, M.; Sugisawa, T.; Uehara, A. Dinuclear Cobalt(II) Complexes Containing 1,3-(or 1,5-)Bis[bis(2-pyridylmethyl)amino]2-propanolato (or −3-pentanolato): Preparation and Reaction with Molecular Oxygen. Bull. Chem. Soc. Jpn. 1990, 63, 1115−1120. (12) Suzuki, M.; Ueda, I.; Kanatomi, H.; Murase, I. Molecular Structure of a Binuclear Cobalt Dioxygen Complex, [Co2(2,6bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenolato)(benzoato)dioxygen](BF4)2·2H2O. Chem. Lett. 1983, 12, 185−188. (13) Sugimoto, H.; Nagayama, T.; Maruyama, S.; Fujinami, S.; Yasuda, Y.; Suzuki, M.; Uehara, A. Thermodynamic Study on Dioxygen Binding of Diiron(II) and Dicobalt(II) Complexes Containing Various Dinucleating Ligands. Bull. Chem. Soc. Jpn. 1998, 71, 2267−2279. (14) Vad, M. S.; Johansson, F. B.; Seidler-Egdal, R. K.; McGrady, J. E.; Novikov, S. M.; Bozhevolnyi, S. I.; Bond, A. D.; McKenzie, C. J. Tuning affinity and reversibility for O2 binding in dinuclear Co(II) complexes. Dalton Trans. 2013, 42, 9921−9929. (15) Chaudhuri, P.; Querbach, J.; Wieghardt, K.; Nuber, B.; Weiss, J. Synthesis, electrochemistry, and magnetic properties of binuclear cobalt complexes containing the Co2(μ-X)(μ-carboxylato)2n+ core (X = OH, Cl, or Br; n= 1−3). The crystal structures of [Co2II(μClH2CCO2)2(μ-Cl)L2]PF6 and [CoIICoIII(μ-MeCO2)2(μ-OH)L2][ClO4]2·0.5H2O (L = N,N′,N″-trimethyl-1,4,7-triazacyclononane). J. Chem. Soc., Dalton Trans. 1990, 271−278. (16) Drew, J.; Hursthouse, M. B.; Thornton, P.; Welch, A. J. Preparation, crystal structure, and magnetic properties of a binuclear cobalt(II) carboxylate. J. Chem. Soc., Chem. Commun. 1973, 52−53. (17) Catterick, J.; Hursthouse, M. B.; Thornton, P.; Welch, A. J. Crystal and molecular structure of tetra-[small micro]-benzoatobisquinolinedi-cobalt(II), a binuclear cobalt(II) carboxylate. J. Chem. Soc., Dalton Trans. 1977, 223−226. (18) Little, I. R.; Straughan, B. P.; Thornton, P. Magnetic and spectroscopic study of some binuclear and trinuclear cobalt(II) carboxylate complexes. J. Chem. Soc., Dalton Trans. 1986, 2211−2214. (19) Turpeinen, U.; Hämäläinen, R.; Reedijk, J. The dinuclear unit μ-aqua-bis(μ-carboxylato)dimetal. X-ray structure and magnetism of cobalt and nickel(II) compounds containing bridging carboxylato groups and a bridging water molecule. Polyhedron 1987, 6, 1603− 1610. (20) Davies, J. E.; Rivera, A. V.; Sheldrick, M. Tetra-[mu]-benzoatobis(4-methylquinoline)dicobalt(II). Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1977, 33, 156−158. (21) Siluvai, G. S.; Murthy, N. N. X-ray structure and spectroscopic characterization of divalent dinuclear cobalt complexes containing carboxylate- and phosphodiester- auxiliary bridges. Inorg. Chim. Acta 2009, 362, 3119−3126. (22) Johansson, F. B.; Bond, A. D.; Nielsen, U. G.; Moubaraki, B.; Murray, K. S.; Berry, K. J.; Larrabee, J. A.; McKenzie, C. J. Dicobalt II−II, II−III, and III−III Complexes as Spectroscopic Models for Dicobalt Enzyme Active Sites. Inorg. Chem. 2008, 47, 5079−5092. (23) Daumann, L. J.; Comba, P.; Larrabee, J. A.; Schenk, G.; Stranger, R.; Cavigliasso, G.; Gahan, L. R. Synthesis, Magnetic Properties, and Phosphoesterase Activity of Dinuclear Cobalt(II) Complexes. Inorg. Chem. 2013, 52, 2029−2043. (24) Zarei, S. A.; Piltan, M.; Hassanzadeh, K.; Akhtari, K.; Cinčić, D. Synthesis, characterization, crystal structure and predicting the second-order optical nonlinearity of a new dicobalt(III) complex with Schiff base ligand. J. Mol. Struct. 2015, 1083, 82−87. (25) Jacob, W.; Mishra, H.; Pandey, S.; Lloret, F.; Mukherjee, R. Sixcoordinate CoIII and four-coordinate MII (M = Co, Zn) mixed-valence

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02432. Cyclic voltammetric traces, GCMS data, molecular structures, spectroscopic data, X-ray crystallographic data, and final agreement factors for the compounds 1a−10 and HN-CH2Py-HPTB (PDF) Accession Codes

CCDC 1556873−1556886, 1573986, and 1573990−1573991 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Amit Majumdar: 0000-0003-0522-8533 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant Nos. SB/S1/IC-43/2013 (SERB, DST, India) and 01(2804)/14/EMR-II (CSIR, India). M.J. acknowledges CSIR, India, for a senior research fellowship. M.J. acknowledges Ms. M. Mukherjee and Md. E. Ahmed of Dr. A. Dey research group at IACS for help with the GC and solution IR experiments, respectively. The authors sincerely thank the anonymous reviewers for their valuable suggestions during the revision stage.

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