Electron-transfer oxidation of coenzyme B₁₂ model compounds and facile cleavage of the cobalt(IV)-carbon bond via charge-transfer complexes with bases. A negative temperature dependence of the rates

The electron-transfer oxidation and subsequent cobalt-carbon bond cleavage of vitamin B₁₂ model complexes were investigated using cobaloximes, (DH)₂Co^III(R)(L), where DH^- = the anion of dimethylglyoxime, R = Me, Et, Ph, PhCH₂, and PhCH(CH₃), and L = a substituted pyridine, as coenzyme B₁₂ model complexes and [Fe(bpy)₃](PF₆)₃ or [Ru(bpy) ₃](PF₆)₃ (bpy = 2,2′-bipyridine) as a one-electron oxidant. The rapid one-electron oxidation of (DH) ₂Co^III(Me)(py) (py = pyridine) with the oxidant gives the corresponding Co(IV) complexes, [(DH)₂Co^IV(Me)(py)] ⁺, which were well identified by the ESR spectra. The reorganization energy (λ) for the electron-transfer oxidation of (DH) ₂Co(Me)(py) was determined from the ESR line broadening of [(DH) ₂Co(Me)(py)]⁺ caused by the electron exchange with (DH)₂Co(Me)(py). The λ value is applied to evaluate the rate constants of photoinduced electron transfer from (DH)₂Co(Me)(py) to photosensitizers in light of the Marcus theory of electron transfer. The Co(IV)-C bond cleavage of [(DH)₂Co(Me)(py)]⁺ is accelerated significantly by the reaction with a base. The overall activation energy for the second-order rate constants of Co(IV)-C bond cleavage of [(DH)₂Co^IV(Me)(py)]⁺ in the presence of a base is decreased by charge-transfer complex formation with a base, which leads to a negative activation energy for the Co(IV)-C cleavage when either 2-methoxypyridine or 2,6-dimethoxypyridine is used as the base.