Catalytic H2O2 Disproportionation and Electrocatalytic O2 Reduction by a Functional Mimic of Heme Catalase: Direct Observation of Compound 0 and Compound I in Situ

Abstract

A phenolate-bound Fe-porphyrin complex assembled in situ, atop self-assembled monolayer (SAM) covered Au surfaces, can disproportionate hydrogen peroxide chemically with a turnover frequency (TOF) of 25 s-1 and turnover number (TON) >105 in a pH 7 buffer solution at room temperature. The same catalase mimic is found to selectively reduce oxygen to water when the cathodic potential is applied to the same surface: i.e., electrochemically. In situ resonance Raman (rR) experiments reveal the accumulation of compound I during chemical H2O2 disproportionation and compound 0 during electrochemical O2 reduction, allowing spectroscopic detection of two key intermediates proposed to be involved in the mechanism of heme catalases that have, so far, remained elusive. The Fe=O vibration of a phenolate-bound compound I species is identified at 803 cm-1, which shifts to 796 cm-1 on hydrogen bonding. The Fe-O and O-O vibrations of the compound 0 species are identified at 587 and 830 cm-1, respectively. The observation of two different intermediates under two different reaction conditions implies the fundamental differences in the mechanisms of H2O2 disproportionation and O2 reduction.