Achieving Reversible H2/H+ Interconversion at Room Temperature with Enzyme-Inspired Molecular Complexes: A Mechanistic Study

Abstract

Inspired by the contribution of the protein scaffold to the efficiency with which enzymes function, we used outer coordination sphere features to develop a molecular electrocatalyst for the reversible production/oxidation of H2 at 25 °C: [Ni(PCy2NPhe2)2]2+ (CyPhe; PR2NR′2 = 1,5-diaza-3,7-diphosphacyclooctane, Cy = cyclohexyl, Phe = phenylalanine). Electrocatalytic reversibility is observed in aqueous, acidic methanol. The aromatic rings in the peripheral phenylalanine groups appear to be essential to achieving reversibility based on the observation that reversibility for arginine (CyArg) or glycine (CyGly) complexes is only achieved with elevated temperature (>50 °C) in 100% water. A complex with a hydroxyl group in the para-position of the aromatic ring, R′ = tyrosine (CyTyr), shows similar reversible behavior. NMR spectroscopy and molecular dynamics studies suggest that interactions between the aromatic groups as well as between the carboxylic acid groups limit conformational flexibility, contributing t...