Mechanisms of hydrogen evolution by six-coordinate cobalt complexes: a density functional study on the role of a redox-active pyridinyl-substituted diaminotriazine benzamidine ligand as a proton relay

Abstract

The hydrogen evolution reaction is an important process for energy storage. The six-coordinate cobalt complex [Co^III(L¹⁻)(LH)]²⁺ (LH = N-(4-amino-6-(pyridin-2-yl)-1,3,5-triazin-2-yl)benzamidine) was found to catalyze photocatalytic hydrogen evolution. In this work, we performed density functional calculations to obtain the reduction potentials and the proton-transfer free energy of possible intermediates to determine the preferred pathways for proton reduction. The mechanism involves the metal-based reduction of Co(III) to Co(II) before the protonation at the amidinate N on the pyridinyl-substituted diaminotriazine benzamidinate ligand L¹⁻ to form [Co^II(LH)(LH)]²⁺. Essentially, the subsequent electron transfer is not metal-based reduction, but rather ligand-based reduction to form [Co^II(LH)(LH˙¹⁻)]¹⁺. Through a proton-coupled electron transfer process, the cobalt hydride [Co^IIH(LH)(LH₂˙)]¹⁺ is formed as the key intermediate for hydrogen evolution. As the cobalt hydride complex is coordinatively saturated, a structural change is required when the hydride on Co is coupled with the proton on pyridine. Notably, the redox-active nature of the ligand results in the low acidity of the protonated pyridine moiety of LH₂˙, which impedes its function as a proton relay. Our findings suggest that separating the proton relay fragment from the electron reservoir fragment of the redox-active ligand is preferred for fully utilizing both features in catalytic H₂ evolution.