Amino acid modified Ni catalyst exhibits reversible H₂ oxidation/production over a broad pH range at elevated temperatures

Hydrogenases interconvert H₂ and protons at high rates and with high energy efficiencies, providing inspiration for the development of molecular catalysts. Studies designed to determine how the protein scaffold can influence a catalytically active site have led to the synthesis of amino acid derivatives of [Ni(P^R₂N^R'₂)₂]²⁺ complexes, [Ni(P^Cy₂ N₂^{Amino acid})₂]²⁺ (CyAA). It is shown that these CyAA derivatives can catalyze fully reversible H₂ production/oxidation at rates approaching those of hydrogenase enzymes. The reversibility is achieved in acidic aqueous solutions (pH = 0-6), 1 atm 25% H₂/Ar, and elevated temperatures (tested from 298 to 348 K) for the glycine (CyGly), arginine (CyArg), and arginine methyl ester (CyArgOMe) derivatives. As expected for a reversible process, the catalytic activity is dependent upon H₂ and proton concentrations. CyArg is significantly faster in both directions (∼300 s^-1 H₂ production and 20 s^-1 H₂ oxidation; pH = 1, 348 K, 1 atm 25% H₂/Ar) than the other two derivatives. The slower turnover frequencies for CyArgOMe (35 s^-1 production and 7 s^-1 oxidation under the same conditions) compared with CyArg suggests an important role for the COOH group during catalysis. That CyArg is faster than CyGly (3 s^-1 production and 4 s^-1 oxidation) suggests that the additional structural features imparted by the guanidinium groups facilitate fast and reversible H₂ addition/release. These observations demonstrate that outer coordination sphere amino acids work in synergy with the active site and can play an important role for synthetic molecular electrocatalysts, as has been observed for the protein scaffold of redox active enzymes.