Amino acid modified Ni catalyst exhibits reversible H 2 oxidation/production over a broad pH range at elevated temperatures
Hydrogenases interconvert H 2 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 has led to the synthesis of amino acid derivatives, [Ni(P Cy 2 N Amino acid 2) 2 ] 2+ (CyAA), of [Ni(P R 2 N R' 2 ) 2 ] 2+ complexes. It is shown that these CyAA derivatives can catalyze fully reversible H 2 production/oxidation, a feature reminiscent of enzymes. The reversibility is achieved in acidic aqueous solutions, 0.25% H 2 /Ar, and elevated temperatures (tested up to 348 K) for the glycine (CyGly), arginine (CyArg), and arginine methyl ester (CyArgOMe) derivatives. As expected for a reversible process, the activity is dependent upon H 2 and proton concentration. CyArg is significantly faster in both directions than the other two derivatives (~300 s -1 H 2 production and 20 s -1 H 2 oxidation; pH = 1, 348 K). The significantly slower rates for CyArgOMe (35 s -1 production and 7 s -1 oxidation) compared to 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 under the same conditions) suggests that the additional structural features imparted by the guanidinium groups facilitate fast and reversible H 2 addition/release. Furthermore, these observations demonstrate that appended, outer coordination sphere amino acids work in synergy with the active site and can play an equally important role for synthetic molecular electrocatalysts as the protein scaffold does for redox active enzymes.