Effect of Redox-Active Quinoline on the Reactivity and Mechanism of Hydrogen Evolution Reaction (HER) with Pentadentate Polypyridyl-Quinolyl Ligand-Coordinated Cobalt Complex

Herein, we have introduced a redox-active quinoline moiety in a pentadentate pyridine-containing N4Py ligand resulting in two quinolines and two pyridines containing 2PyN2Q ligand. The corresponding cobalt complex 1 has been synthesized and employed for electro- and photocatalytic hydrogen evolution reactions (HER) using different acids and water. The detailed mechanistic elucidation of the electrocatalytic HER steps is investigated by characterizing the plausible reaction intermediates using spectroelectrochemistry, optical, magnetic, and electron paramagnetic resonance (EPR) spectroscopy. The generation of one-electron- and two-electron-reduced species from complex 1 in acetonitrile under both chemical and electrochemical conditions and their in-depth characterization demonstrate the feasibility of the stepwise formation of reduced intermediates for the proposed ECEC (E = electron transfer step, C = proton addition step) mechanism during the catalysis. The two-electron-reduced species of 1 show an isotropic signal (at g = 2.003) in EPR analysis characteristic of a ligand-centered radical. Interestingly, in the native complex 1, the quinoline arm takes up the electron during the second electron transfer, whereas one of the protonated pyridine arms accepts that electron over quinoline during electrocatalytic HER. Herein, the weak basic character and better metal-to-ligand π-back-bonding ability trigger ligand decoordination followed by protonation at the pyridine arm, thereby participating in electron and subsequent proton transfer during HER. The in-depth computational studies (DFT) support the dichotomic redox behavior between quinoline and pyridine and corroborate with the experimentally proposed stepwise ECEC mechanism. Further, complex 1 has been found to show superior HER activity compared with all pyridine-containing [Co^III(N4Py)(H₂O)]³⁺ (2) in acids and water substantiating the crucial role of quinoline moiety of 1 in enhancing the HER efficiency.