Investigation of Immobilization Effects on Ni(P₂N₂)₂ Electrocatalysts

A new synthetic route to complexes of the type Ni(P₂N₂)₂²⁺ with highly functionalized phosphine substituents and the investigation of immobilization effects on these catalysts is reported. Ni(P₂N₂)₂²⁺ complexes have been extensively studied as homogeneous and surface-attached molecular electrocatalysts for the hydrogen evolution reaction (HER). A synthesis based on postsynthetic modification of P^ArBr₂N^Ph₂ was developed and is described here. Phosphonate-modified ligands and their corresponding nickel complexes were isolated and characterized. Subsequent deprotection of the phosphonic ester derivatives provided the first Ni(P₂N₂)₂²⁺ catalyst that can be covalently attached via pendent phosphonate groups to an electrode without involvement of the important pendent amine groups. Mesoporous TiO₂ electrodes were surface modified by attachment of the new phosphonate functionalized Ni(P₂N₂)₂²⁺ complexes, and these provided electrocatalytic materials that proved to be competent and stable for sustained HER in aqueous solution at mild pH and low overpotential. We directly compared the new ligand to a previously reported complex that utilized the amine moiety for surface attachment. Using HER as the benchmark reaction, the P-attached catalyst showed a marginally (9-14%) higher turnover number than its N-attached counterpart.