Concertedness in proton-coupled electron transfer cleavages of carbon–metal bonds illustrated by the reduction of an alkyl cobalt porphyrin

Abstract

The cleavage of metal–oxygen or metal–carbon bonds is an essential step in the activation by transition metal complex catalysis of small molecules involved in the resolution of modern energy challenges. Deciphering the role of proton transfer as promoter of such reactions is an essential piece of knowledge in this connection, notably in establishing the degree of concertedness between the three associated events; electron transfer, proton transfer and bond breaking. The reductive cleavage of the cobalt–carbon bond of a tetraphenylporphyrin, reminiscent of vitamin B₁₂ derivatives, is taken as an example. A systematic cyclic voltammetric analysis of the catalytic currents resulting from the reduction of the cobalt(II) porphyrin upon addition of an alkyl halide and an acid shows that, among all possible mechanisms, proton transfer and bond breaking are concerted but are not concerted with electron transfer.