Probing the redox-conversion of Co(II)-disulfide to Co(III)-thiolate complexes: the effect of ligand-field strength

The redox-conversion reaction of cobalt(II)-disulfide to cobalt(III)-thiolate complexes triggered by addition of the bidentate ligand 2,2'-bipyridine has been investigated. Reaction of the cobalt(II)-disulfide complex [Co-2((LSSL1)-S-1)(X)(4)] ((LSSL1)-S-1 = di-2-(bis(2-pyridylmethyl)amino)-ethyldisulfide; X = Cl or Br) [1(x)] with 2,2'-bipyridine (bpy) resulted in the formation of two different products, namely the cobalt(m)-thiolate complex [Co((LS)-S-1)(bpy)]X-2 and the unexpected side product (Co-2((LSSL1)-S-1)(bpy)(2)(X)(2)]X-2. Crystals of [Co-2((LSSL1)-S-1)(bpy)(2)(Cl)(2)](BPh4)(2) [2(Cl)](BPh4)(2) obtained after anion exchange showed the cobalt(II) ions to be in octahedral geometries with the nitrogen donors of the disulfide ligand arranged in a facial conformation and the chloride ion trans to the tertiary amine nitrogen. Remarkably, this side product cannot be converted to the cobalt(III)-thiolate compound [Co((LS)-S-1)(bpy)](SbF6)(2) [3](SbF6)(2) by removal of the chloride ion with use of a silver salt, as this causes scrambling of the ligands, resulting in the formation of [Co(bpy)(3)](n+). [Co((LS)-S-1)(bpy)](SbF6)(2) was obtained in a pure form by addition of bpy to a solution in acetonitrile of the compound [Co((LS)-S-1)(MeCN)(2)](2+) [4](2+). Addition of NEt4Cl to [3](SbF6)(2) regenerates the cobalt(II)-disulfide complex [1(Cl)] as confirmed spectroscopically. DFT studies revealed that the conversion from [1(Cl)] to [3](2+) most likely occurs via the hypothetical intermediate species [2(Cl)](mer)(2+), in which the nitrogen atoms of the disulfide ligand are arranged in a meridional conformation. Interestingly, the estimated d-orbital splitting energy of [3](2+) is lower than that of [4](2+), indicating that the ligand-field strength of bpy is lower than anticipated, which hampers clean conversion in the redox-conversion reaction. This study shows that the redox-conversion reaction between cobalt(n)-disulfide and cobalt(m)-thiolate complexes is intricate rather than straightforward.