Proton Channels on Molecular Electrocatlaysts for Hydrogenase Mimics

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^Cy₂N^{Amino acid}₂)₂]²⁺ (CyAA).  It is shown that these CyAA derivatives can electrochemically catalyze fully reversible H₂ production/oxidation, a feature reminiscent of enzymes.  The reversibility is achieved in acidic aqueous solutions, 1 atm 25% H₂/Ar, and elevated temperatures (tested up to 70 ¢ªC) for the glycine (CyGly), arginine (CyArg), and arginine methyl ester (CyArgOMe) derivatives. At pH=1 and 70 C, rates of up to ~300 s^-1 H₂ production and 20 s^-1 H₂ oxidation 348 K are observed.  The effect of higher pressure and the behavior of complexes without an outer coordination sphere will be discussed. These observations demonstrate that outer coordination sphere amino acids work in synergy with the active site in and can play an equally important role for synthetic molecular electrocatalysts as the protein scaffold does for redox active enzymes.