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.