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 [Co₂(L¹SSL¹)(X)₄] (L¹SSL¹ = di-2-(bis(2-pyridylmethyl)amino)-ethyldisulfide; X = Cl or Br) [1X] with 2,2'-bipyridine (bpy) resulted in the formation of two different products, namely the cobalt(III)-thiolate complex [Co(L¹S)(bpy)]X₂ and the unexpected side product [Co₂(L¹SSL¹)(bpy)₂(X)₂]X₂. Crystals of [Co₂(L¹SSL¹)(bpy)₂(Cl)₂](BPh₄)₂ [2Cl](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(L¹S)(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]. [Co(L¹S)(bpy)](SbF₆)₂ was obtained in a pure form by addition of bpy to a solution in acetonitrile of the compound [Co(L¹S)(MeCN)₂]²⁺ [4]²⁺. Addition of NEt₄Cl to [3](SbF₆)₂ regenerates the cobalt(II)-disulfide complex [1Cl] as confirmed spectroscopically. DFT studies revealed that the conversion from [1Cl] to [3]²⁺ most likely occurs via the hypothetical intermediate species [2Cl]²⁺_mer, 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(II)-disulfide and cobalt(III)-thiolate complexes is intricate rather than straightforward.