Substituting Cyclohexyl with Phenyl at Phosphorus Makes a NiP ₂ N ₂ Molecular Catalyst Active for Hydrogen Evolution in Alkaline Conditions at Low Overvoltage

Among the large family of mononuclear bis-diphosphine nickel catalysts with pendant amines, the so-called NiCyArg complex, with a cyclohexyl group at the phosphine and an arginine substituted amine, has attracted attention for catalyzing H₂ oxidation and production reversibly in aqueous solutions, with a significant bias in favor of H₂ oxidation. Here, we analyze the voltammetric responses of this complex and an analogue where the cyclohexyl group was replaced with a phenyl group. Our analysis helps distinguish the molecular and kinetic distinct determinants of rate, directionality and reversibility of NiP₂N₂ redox molecular catalysts of H⁺/H₂ conversion. The NiPhArg complex described in this work is a bidirectional, reversible catalyst over a large range of pH. It is strongly biased in the direction of reductive catalysis, and still active for proton reduction at pH 9. It could become useful in applications where H₂ evolution should be catalyzed in aqueous solutions under mild conditions and at low overpotential.