Abstract
Heme enzymes catalyze several key reactions in nature. O(2) reduction and peroxidation are two such reactions crucial for respiration and oxidation of organic and inorganic substrates in nature, respectively. Over the last several decades, there has been a focused effort to generate small-molecule analogues which mimic these reactions. Small-molecule mimics of these metallo-enzymes may find potential use as an oxygen reduction reaction (ORR) catalyst in a fuel cell and as a catalyst for decontamination of wastewater in addition to providing deeper insights into the reactivity of the enzyme they mimic. An iron porphyrin with a pendant imidazole group appears to catalyze both these reactions quite efficiently. On the one hand, the pendant imidazole group provides a binding site for both H(2)O(2) and the substrate, which results in efficient peroxidase catalysis that shows an enzyme-like "ping-pong" mechanism which is extremely rare in a small molecule. On the other hand, the pendant imidazole aids stabilization of intermediates during the ORR and facilitates O-O bond cleavage, resulting in fast catalysis with a high selectivity for 4e(-)/4H(+) ORR under homogeneous as well as heterogeneous conditions. The reactive species produced during the ORR can oxidize organic substrates, acting like an oxygenase.