8-Aminoquinoline Pendant Ligand-Driven Proton Shuttling in Copper Electrocatalysis for Enhanced Proton Reduction into Hydrogen Gas.

Secondary interactions are a key feature of redox-active ligands that could affect (enhance or hinder) the reduction of protons into hydrogen gas during catalysis. The fine-tuning of the secondary interaction, which avoids a high-energy intermediate, follows selective and energetically favored steps for remarkably proton-reducing activity, is highly desirable. The current study describes the proton shuttling of synthesized tetra-coordinated copper amide molecular scaffolds via secondary interaction from the 8-aminoquinoline pendant N-donor atom for the evolution of hydrogen gas. The synthesized electrocatalysts demonstrate high Faradaic efficiency up to 92% with turnover frequency (kobs) up to 11,100 s-1 and moderate overpotential (640-690 mV) in the N,N-dimethylformamide (DMF) solvent using mild acetic acid as a proton source. In the mechanistic investigations, in situ sequentially generated intermediates were characterized by EPR, demonstrating the stepwise progression of the reaction pathway. Further, kinetic and DFT computational studies revealed that the coordinatively unsaturated copper, having the additional suitably positioned secondary N-donor atom, facilitates the Cu-hydride intermediate by proton shuttling from 8-aminoquinoline to copper under acidic conditions. Consequently, the 1-electron reduced protonated quinoline ligand upon intramolecular homolytic coupling with the Cu-hydride intermediate liberates H2 gas by involving the synergistic effect of the redox-active organic ligand with abundant copper metal.