Arginine‐Containing Ligands Enhance H₂ Oxidation Catalyst Performance

Hydrogenase enzymes use Ni and Fe to oxidize H₂ 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^Cy₂N^tBu₂)₂]²⁺, oxidizes H₂ at 60 s⁻¹ in MeCN under 1 atm H₂ 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 H₂ oxidation activity: [Ni^II(P^Cy₂N^Arg₂)₂]⁸⁺ (Arg=arginine) has a TOF of 210 s⁻¹ in water with high energy efficiency (180 mV overpotential) under 1 atm H₂, and 144 000 s⁻¹ (460 mV overpotential) under 133 atm H₂. 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 H₂ 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.