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

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 H<inf>2</inf> at 25 °C: [Ni(P^Cy<inf>2</inf>N^Phe<inf>2</inf>)<inf>2</inf>]²⁺ (CyPhe; P^R<inf>2</inf>NR′<inf>2</inf> = 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 to reversibility. NMR spectroscopy studies also show extremely fast proton exchange along a pathway from the Ni-H through the pendant amine to the carboxyl group. Further, a complex containing a side chain similar to tyrosine but without the carboxyl group (CyTym; Tym = tyramine) does not display reversible catalysis and has limited proton exchange from the pendant amine, demonstrating an essential role for the carboxylic acid and the proton pathway in achieving catalytic reversibility. This minimal pathway mimics proton pathways found in hydrogenases. The influence of multiple factors on lowering barriers and optimizing relative energies to achieve reversibility for this synthetic catalyst is a clear indication of the intricate interplay between the first, second, and outer coordination spheres that begins to mimic the complexity observed in metalloenzymes.