Cobalt complexes outer-sphere redox reactions

The stereoselective reduction of spinach plastocyanin with several cobalt cage complexes (Scheme 26) has been reported, too [60]. These cage complexes are very useful for investigation of outer-sphere electron transfer reactions because of their inertness to hydrolysis and to loss of ligands in the redox reaction. 

The biomethylation reaction between platinum and methylcobalamin involves both platinum(II) and platinum(IV) oxidation states. An outer-sphere complex is formed between the charged platinum(II) salts and the corrin macrocycle, which catalytically labilizes the Co—C o bond to electrophilic attack. A two-electron redox switch mechanism has been proposed between platinum(II) and platinum(IV). However, a mechanism consistent with the kinetic data is direct electrophilic attack by PtClg on the Co—C a bond in MeBu. Studies on [Pt(NH3)2(OH2)2] indicate that the bases on cobalt interact in the coordination sphere of platinum(II). Since both platinum(ll) and platinum(rV) are together required to effect methyl transfer from methylcobalamin to platinuni, Pt and C NMR spectroscopy have been used to show that the methyl group is transferred to the platinum of the platinum(n) reactant. The kinetics of demethylation by mixtures of platinum(II) and platinum(IV) complexes show a lack of dependence on the axial ligand. The authors conclude therefore that it is unlikely that the reaction involves direct attack by the bound platinum on the Co—C bond, and instead favor electron transfer from an orbital on the corrin ring to the boimd platinum group in the slow step, followed by rapid methyl transfer. ... 

Reaction 3. This reaction has been described as a "Redox-Switch" mechanism. The metal ion in its lower oxidation state first forms an "outer sphere" complex with the corrin macrocycle, followed by a two electron transfer of the complexed metal ion to a two electron acceptor. Oxidation of the complexed metal ion facilitates carbanion transfer from the cobalt to the complexed metal. 

Cobalt(in) oxidizes 2-mercaptoethylamine (HMea) in [Co(en)2(Mea)] to the corresponding co-ordinated disulphide complex by pathways involving Co + and [CoOH] +. An outer-sphere mechanism is suggested by the activation entropy (—3.1 cal K mol ) for reaction with Co and the reaction with [CoOH] + is substitution controlled. Redox proceeds by formation of a co-ordinated radical complex [Co(en)2(Mea)] +,... 

The second redox reaction involves one cobalt(m) centre, but the second cobalt(iii) site reacts only after the peroxo-bridge has been reduced. The Cr + reduction of (5) is too rapid for detection by flow techniques, but the [Cr(H20)e] + product is indicative of an outer-sphere route. The reduction of (6) is also fast and here the reaction product is not a mononuclear chromium(m) species but the first example of a jM-peroxo-complex involving different metal atoms,... 

---