A Minimalist Iron Porphyrin Which Can Catalyze Both Peroxidation and Oxygen Reduction Reaction.

Abstract

Heme enzymes catalyze several key reactions in nature. O₂ 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 H₂O₂ 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.