Cobalt meso-Tetrakis(N-methyl-4-pyridiniumyl)porphyrin Becomes a Catalyst for the Electroreduction of O_2 by Four Electrons When [(NH_3)_5Os]^(n+) (n = 2, 3) Groups Are Coordinated to the Porphyrin Ring

Abstract

In a series of recent reports evidence has been offered to support the proposal that coordination of (NH_3)_5Ru^(2+) groups to ligand sites introduced on the meso positions of certain cobalt porphyrins enhances their abilities to act as electrocatalysts for the reduction of O_2 to H_2O. The enhancement is believed to be the result of back-bonding by the Ru(II) centers into the pendant ligands and the cobalt porphyrin to which they are linked. The catalytic mechanism that has been proposed leaves the coordinated Ru centers in their +2 oxidation state as the O_2 molecule coordinated to the Co(II) center of the porphyrin in the activated complex is reduced via electron-transfer from the electrode surface. Intramolecular electron-transfer from the Ru(II) centers to the coordinated O_2 is not consistent with the experimental data. To obtain additional information about the importance of back-bonding in enhancing the catalytic activity of these cobalt porphyrins, we sought to use Os(III) in place of Ru(II) as the source of the back-donation. Os(III), unlike Ru(III), exhibits substantial back-bonding capabilities, but Os(III) is a much weaker thermodynamic reductant than Ru(II). Thus, we reasoned that any increase in the catalytic activity of cobalt porphyrins that might result from the attachment of Os(III) centers to the porphyrin ring could be confidently attributed to back-bonding effects free of any contributions from intramolecular electron-transfer. The results of experiments designed to test this speculation are described in this report.