Electrocatalytic Oxygen Reduction by Iron Tetra-arylporphyrins Bearing Pendant Proton Relays

Iron(III) meso-tetra(2-carboxyphenyl)-porphine chloride (1) was investigated as a soluble electrocatalyst for the oxygen reduction reaction (ORR) in acetonitrile with [H(DMF)(+)]OTf-. Rotating ring-disk voltammetry, spectroelectrochemistry, and independent reactions with hydrogen peroxide indicate that 1 has very high selectivity for reduction of O-2 to H2O, without forming significant amounts of H2O2. Cyclic voltammetric measurements at high substrate/catalyst ratios (high oxygen pressure) allowed the estimation of a turnover frequency (TOE) of 200 s(-1) at -0.4 V vs Cp2Fe+/0. This is, to our knowledge, the first reported TOF for a soluble ORR electrocatalyst under kinetically controlled conditions. The 4-carboxyphenyl isomer of 1, in which the carboxylic acids point away from the iron center, is a much less selective catalyst. This comparison shows that carboxylate groups positioned to act as proton delivery relays can substantially enhance the selectivity of ORR catalysis.