Carbonic Acid-Mediated Proton Transfer via the Hydrogen-Bond Network for Accelerating Photocatalytic H2 Evolution

Abstract Constructing a favorable catalytic environment adjacent to the active center is a promising strategy for achieving efficient photocatalytic performance with the aid of the solvent effect. We report in this work that the initial rate of H2 evolution catalyzed by Co(dmgH)2LCl (dmgH = dimethylglyoximate, L = 4-COOH-py, py = pyridine) can be significantly enhanced when CO2 is introduced into the reaction solution, giving an impressive turnover frequency of 0.77 min−1 with 35 times enhancement in sharp contrast to the N2 atmosphere. 1H NMR, IR spectroscopy, and DFT calculation were conducted to explore the mechanistic impact of CO2 derived H2CO3 in the system. This dramatic improvement originates from the H2CO3-mediated acceleration of the protonation of the CoI intermediate with decreased activation energy, a critical rate-determining step in the electron-transfer reactions. The protonation of the side-chain oxime group in the Co-cat is completed in advance due to CO2 acidity, while favorable hydrogen-bond interactions ensure the optimal geometric alignment for proton transfer from H2CO3 to CoI. This study unveils mechanistic insights into hydrogen-bond network-mediated proton transfer (PT) in H2 evolution and provides a paradigm for enhancing PT efficiency through the action of carbonic acid as a dynamic proton-transfer agent. Furthermore, it challenges the traditional view of competitive photocatalytic CO2 reduction and H2 production via water splitting, proposing a cooperative mechanism where carbonic acid functions as a nondepleting cosubstrate to accelerate H2 evolution.