Electrocatalytic hydrogen production activity with a copper(ii)-dipyridylamine complex in acidic water

Abstract

A water-stable and structurally-characterized earth-abundant copper(II) complex, [Cu(dpa)₂(N₃)]Cl·4H₂O, containing an N,N-chelator (dpa = 2,2′-dipyridylamine) and an ancillary ligand (N₃⁻ = azide ion) was synthesized and evaluated for electrocatalytic H₂ production activity in water using acetic acid as a proton source. Crystal engineering approach revealed the presence of multiple lattice water molecules and a chloride ion in the secondary coordination zone of the distorted square pyramidal copper(II) complex forming an 8-membered tetrameric water cluster and, subsequently, an 11-membered hexameric water cluster in a repetitive mode in association with a water-chloride cluster, leading to a beautiful long-range supramolecular framework involving the chains of a copper(II) complex. The copper catalyst exhibited excellent hydrogen generation activity for the electrochemical proton reduction in acidic water with a rate of 49.81 s⁻¹. A Faradic efficiency of 71% was derived from controlled potential electrolysis (CPE) with [Cu(dpa)₂(N₃)]⁺ at −1.25 V vs. Ag/AgCl for 3 h using a glassy carbon electrode ensuring a high turnover number of 41.96 and the desired durability of the molecular electrocatalyst under electrochemical conditions. Further, electrochemical, spectroscopic, and computational studies propose the proton-coupled reduction mechanism for the hydrogen evolution where endo conformational H^δ+⋯H^δ− coupling effectively releases molecular hydrogen.