Executive Summary : | Cytochrome c oxidase and multicopper oxidase are enzymes that catalyze the selective 4e-/4H+ reduction of O2 to H2O at ~400 mV overpotential. Oxygen reduction reaction (ORR) is crucial in fuel cell technology and air batteries, with H2/O2 fuel cells being environmentally friendly, lower emissions, and highly efficient. Platinum is an excellent catalyst for H2 oxidation and reduction, but ORR at the cathode is slower than at the Pt anode. Efforts have been made to develop efficient ORR catalysts based on earth-abundant metal ions, such as Fe and Co complexes supported by different macrocyclic ligand scaffolds. However, the selectivity of 4e-/4H+ vs. 2e-/2H+ reduction of O2 remains a major obstacle. The focus has been on modulating the secondary coordination sphere of macrocyclic ligand systems to push the cleavage of O‒O bond of a peroxide or hydroperoxide intermediate species. This research proposal aims to develop 3d transition metal complexes supported by nonheme ligands for chemical and electrocatalytic ORR. The proposal designs planar N4 donor sets of ligands consisting of reversible protonation sites poised over the redox-active cofactor. The aim is to synthesize and characterize the proposed ligands' 3d transition metal complexes and investigate electrocatalytic and chemical ORR with such complexes. Preliminary experimental results were obtained for the ORR of a Co(III) complex, showing electrocatalytic 4e-/4H+ reduction of O2 with over 98% selectivity. |