Arginine-Containing Ligands Enhance H2 Oxidation Catalyst Performance

Hydrogenase enzymes use Ni and Fe to oxidize H-2 at high turnover frequencies (TOF) (up to 10 000 s(-1)) and low overpotentials (< 100 mV). In comparison, the fastest reported synthetic electrocatalyst, [Ni-II(P-Cy N-2(2)tBu)(2)](2+), oxidizes H-2 at 60 s(-1) in MeCN under 1 atm H-2 with an unoptimized overpotential of ca. 500 mV using triethylamine as a base.[1] Here we show that a structured outer coordination sphere in a Ni electrocatalyst enhances H-2 oxidation activity: [Ni-II(P-Cy N-2(2)Arg)(2)]8(+) (Arg = arginine) has a TOF of 210 s(-1) in water with high energy efficiency (180 mVoverpotential) under 1 atm H-2, and 144 000 s(-1) (460 mV overpotential) under 133 atm H-2. The complex is active from pH 0-14 and is faster at low pH, the most relevant condition for fuel cells. The arginine substituents increase TOF and may engage in an intramolecular guanidinium interaction that assists in H-2 activation, while the COOH groups facilitate rapid proton movement. These results emphasize the critical role of features beyond the active site in achieving fast, efficient catalysis.