Effect of Ligand Backbone on the Electrochemical Hydrogen Evolution Reaction and Hydrogen-Atom-Transfer Reactivity Using a Nickel Polypyridine Quinoline Complex

Redox-active quinoline-containing [Ni^II(2PyN2Q) (H₂O)]²⁺ complex (1) has been developed for the electrocatalytic (e) hydrogen evolution reaction (HER) in the presence of organic acids and water and for the hydrogen-atom-transfer (HAT) reaction with styrene in the presence of acids. Complex 1 shows promising e-HER performance in water up to pH 9. It exhibits a stepwise (E)ECEC mechanism with AcOH, while a potential-dependent bimolecular homolytic pathway and CEEC mechanism is operative with p-toluene sulfonic acid during the e-HER. The one- and two-electron-reduced species of 1 are characterized by spectro-electrochemistry, optical, and EPR studies. Moreover, the inverse kinetic isotope effect (KIE = 0.83) between AcOH and d₄-AcOH during the e-HER and e-HAT with styrene for the hydro-functionalization reaction using catalyst 1 possibly suggests the involvement of nickel hydride species. The e-HER and e-HAT reactivity of 1 have been compared with redox-inactive redox-inactive [Ni^II(N4Py)(H₂O)]²⁺ (2), demonstrating the prominent effect of quinoline in the e-HER and pyridine in the e-HAT. The proposed mechanism of the e-HER with AcOH is well supported by DFT studies.