Kinetics of electron transfer between macrocyclic organocobalt(III) and nickel(I) compounds

Abstract

Kinetic information about detailed mechanisms of reactions between macrocyclic complexes of organocobalt and nickel is still lacking, despite the fact that both compounds are relevant to important enzymic reactions such as vitamin B12-catalysed 1,2-rearrangements ~1) and the Factor 430-catalysed generation of CH4(2). Unlike classical transition metal complexes, where the metal may have two or more stable oxidation states, a one electron reduction of an organometallic compound, RM, yields a species, R M , which often decomposes easily, either homolytically ( R M ~ R " + M ) (3) or heterolytically ( R M + H + RH + M) ~4). Reductive migration of organic groups from a cobalt centre to nearby ligands has already been indirectly observed on a few occasions (s). Because reduction-induced C C o bond cleavage and subsequent methyl group transfer from one metal to another is known ~6), and given the ability of R,S,R,S-Ni(tmc) + [Ni(tmc) + hereafter; tmc = 1,4,8,11-tetramethyl-l,4,8,11-tetraazacyclotetradecane] to function both as a reducing reagent and as a radical scavenger to capture an alkyl radical forming a C N i bond (4), it is interesting to study the reaction between a pseudo-macrocyclic organocobalt(III) compound, such as MeCo(DH)2py (DH = dimethylglyoximato), and the macrocyclic nickel(I) complex Ni(tmc) +. The ligand set (DH)2 is structurally reminiscent of (dmgBF2)2, in which evidence of methyl migration from cobalt to the equatorial ligand was observed ~5).