Metal Alkylation of Electrochemically Generated Low-Valent Metalloporphyrins. Is There Something Special with Rhodium?

Rhodium(III) porphyrins are converted into the corresponding σ-alkyl complexes by electrochemical reduction in the presence of an alkyl halide. The mechanism of this reaction is investigated by means of cyclic voltammetry and spectroelectrochemistry. On the basis of the kinetic data and on the recent finding that the rhodium(II) intermediates disproportionate rather than dimerize as believed earlier, it is shown that the σ-alkyl complex results from an S N 2 substitution by the rhodium(I) porphyrin rather than from a radical reaction involving the rhodium(II) intermediate. Rhodium(I) porphyrins, thus, exhibit an S N 2 reactivity similar to that of their cobalt and iron analogues.