Redox Interconversion between Cobalt(III) Thiolate and Cobalt(II) Disulfide Compounds

The redox interconversion between Co­(III) thiolate and Co­(II) disulfide compounds has been investigated experimentally and computationally. Reactions of cobalt­(II) salts with disulfide ligand L¹SSL¹ (L¹SSL¹ = di-2-(bis­(2-pyridylmethyl)­amino)-ethyl disulfide) result in the formation of either the high-spin cobalt­(II) disulfide compound [Co^II₂(L¹SSL¹)­Cl₄] or a low-spin, octahedral cobalt­(III) thiolate compound, such as [Co^III(L¹S)­(MeCN)₂]­(BF₄)₂. Addition of thiocyanate anions to a solution containing the latter compound yielded crystals of [Co^III(L¹S)­(NCS)₂]. The addition of chloride ions to a solution of [Co^III(L¹S)­(MeCN)₂]­(BF₄)₂ in acetonitrile results in conversion of the cobalt­(III) thiolate compound to the cobalt­(II) disulfide compound [Co^II₂(L¹SSL¹)­Cl₄], as monitored with UV–vis spectroscopy; subsequent addition of AgBF₄ regenerates the Co­(III) compound. Computational studies show that exchange by a chloride anion of the coordinated acetonitrile molecule or thiocyanate anion in compounds [Co^III(L¹S)­(MeCN)₂]²⁺ and [Co^III(L¹S)­(NCS)₂] induces a change in the character of the highest occupied molecular orbitals, showing a decrease of the contribution of the p orbital on sulfur and an increase of the d orbital on cobalt. As a comparison, the synthesis of iron compounds was undertaken. X-ray crystallography revealed that structure of the dinuclear iron­(II) disulfide compound [Fe^II₂(L¹SSL¹)­Cl₄] is different from that of cobalt­(II) compound [Co^II₂(L¹SSL¹)­Cl₄]. In contrast to cobalt, reaction of ligand L¹SSL¹ with [Fe­(MeCN)₆]­(BF₄)₂ did not yield the expected Fe­(III) thiolate compound. This work is an unprecedented example of redox interconversion between a high-spin Co­(II) disulfide compound and a low-spin Co­(III) thiolate compound triggered by the nature of the anion