Coordination Tuning of Metal Porphyrins for Improved Oxygen Evolution Reaction

Abstract The nucleophilic attack of water or hydroxide on metal‐oxo units forms an O−O bond in the oxygen evolution reaction (OER). Coordination tuning to improve this attack is intriguing but has been rarely realized. We herein report on improved OER catalysis by metal porphyrin 1‐M (M=Co, Fe) with a coordinatively unsaturated metal ion. We designed and synthesized 1‐M by sterically blocking one porphyrin side with a tethered tetraazacyclododecane unit. With this protection, the metal‐oxo species generated in OER can maintain an unoccupied trans axial site. Importantly, 1‐M displays a higher OER activity in alkaline solutions than analogues lacking such an axial protection by decreasing up to 150‐mV overpotential to achieve 10 mA/cm 2 current density. Theoretical studies suggest that with an unoccupied trans axial site, the metal‐oxo unit becomes more positively charged and thus is more favoured for the hydroxide nucleophilic attack as compared to metal‐oxo units bearing trans axial ligands.