Electron transfer. Part 148. Reactions of corrin-bound cobalt(III) with s2 metal-ion reducing centers

The cobalt(III)–corrin complex, aquacobalamin (vitamin B12a) is readily reduced to the corresponding Co(II) derivative (B12r) by the s2 metal-reducing centers, tin(II), germanium(II), and indium(I) in aqueous chloride solutions at low pH. Reactions with Sn(II) proceed through chlorocobalamin which has acquired an additional proton (presumably at an imidazole nitrogen), whereas reductions with Ge(II) involve both this protonated form and a species which has assumed still another H+. Kinetic [Cl−]-dependencies indicate the principal reducing species to be SnCl3 − and GeCl4 2−. Reductions by In(I) take competing paths involving the aqua and chloro forms of the oxidant. Excess Ge(II) forms a yellow 1:1 Co(II)–Ge(II) complex (λmax 447 nm). The corrin-free complexes, Cl(NH3)5Co2+ and its bromo analog, are readily reduced by In(I) but are inert to Ge(II) and Sn(II). Arguments are presented that these reductions of cob(III)alamin by Ge(II) and Sn(II) are initiated by a chloro-mediated 2e−transfer, forming cob(I)alamin (B12s), which is rapidly oxidized by Co(III), yielding 2Co(II). Reactions of In(I) more probably entail successive le− transactions, in which the initial transfer (In(I)→In(II)) is rate-determining.