The chemical and electrochemical reduction of heteroligand o-semiquinonato-formazanato cobalt complexes

[Display omitted] •Reduction of heteroligand o-semiquinonato-formazanato cobalt complexes.•1D linear D-A-D-A motif in crystal of [Co(3,6-Cat)(Form)]−[Cp2Co]+ complex.•[Co(3,6-Cat)(Form)]−[Cp2Co]+ has a doublet ground state (S = 1/2).•[Co(3,6-Cat)(3,6-SQ)(Form)]−[Cp2Co]+ (S = 1/2) has a redox isomer with S = 3/2. The electrochemical investigation of heteroligand o-semiquinonato-formazanato cobalt complexes [CoII(3,6-SQ)(Form)] (1) and [CoIII(3,6-SQ)2(Form)] (2) (where 3,6-SQ is 3,6-di-tert-butyl-o-benzosemiquinonate radical anion; Form is 1,3,5-triphenylformazanate anion) shows that the reduction of complexes proceeds through some successive stages corresponding to the transitions of radical anionic o-semiquinonato ligands to dianionic catecholates and the subsequent reduction of formazanate ligand with a formation of stable metallaverdazyl radical. In the accordance with CV data, we have accomplished the chemical reduction of heteroligand o-semiquinonato-formazanato cobalt complexes by treatment with one eqv. of Cp2Co. The resulting complexes [Co(3,6-Cat)(Form)]−[Cp2Co]+ (1a) and [Co(3,6-SQ)(3,6-Cat)(Form)]−[Cp2Co]+ (2a) have been isolated and characterized in details. The molecular structure of 1a in crystal state has been determined by single-crystal X-ray analysis. The magnetic susceptibility measurements, EPR spectroscopic and DFT studies have shown that complex [CoIII(3,6-SQ)(3,6-Cat)(Form)]−[Cp2Co]+ (2a) exists as a mixture of two valence tautomeric forms: more stable low-spin cobalt(III) isomer [CoIII(3,6-SQ)(3,6-Cat)(Form)]−[Cp2Co]+ and high-spin cobalt(II) isomer [CoII(3,6-SQ)2(Form)]−[Cp2Co]+.