Electrocatalytic Hydrogen Production by a Nickel Complex Containing a Tetradentate Phosphine Ligand

A nickel complex has been synthesized using the P₄N₂ ligand, a tetradentate phosphine ligand with two pendant amines in the ligand backbone. The rigidly square pyramidal [Ni(P₄N₂)(CH₃CN)]²⁺ complex is an electrocatalyst for the reduction of protons to hydrogen. Using N,N-dimethylformamidium ([DMF(H)⁺]) as the acid in an acetonitrile/water solution, [Ni(P₄N₂)(CH₃CN)]²⁺ displays a turnover frequency of 1.6 × 10⁶ s^-1, which is among the fastest rates reported for any molecular electrocatalyst. This high catalytic rate comes at the cost of a 1200 mV overpotential at the catalytic half-wave potential. The Ni(II) state of the catalyst was found to be stable under strongly acidic conditions, with only trace decomposition observed over 2 weeks in the presence of 0.1 M trifluoromethanesulfonic acid (HOTf). The catalyst was less stable in the Ni(0) state due to the inability of the rigid P₄N₂ ligand to adopt a tetrahedral geometry. Using variable scan rate cyclic voltammetry, a first-order rate constant of ∼1 s^-1 was measured for dissociation of a phosphine from Ni(0), which is proposed to be a key step for determining the lifetime of the catalyst during electrolysis.