Carbonic Acid-Mediated Proton Transfer via the Hydrogen-Bond Network for Accelerating Photocatalytic H$_2$ Evolution

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 H$_2$ evolution catalyzed by Co(dmgH)$_2$LCl (dmgH = dimethylglyoximate, L = 4-COOH-py, py = pyridine) can be significantly enhanced when CO$_2$ 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 N$_2$ atmosphere. $^1$H NMR, IR spectroscopy, and DFT calculation were conducted to explore the mechanistic impact of CO$_2$ derived H$_2$CO$_3$ in the system. This dramatic improvement originates from the H$_2$CO$_3$-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 CO$_2$ acidity, while favorable hydrogen-bond interactions ensure the optimal geometric alignment for proton transfer from H$_2$CO$_3$ to Co$^I$. ... mehrThis study unveils mechanistic insights into hydrogen-bond network-mediated proton transfer (PT) in H$_2$ 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 CO$_2$ reduction and H$_2$ production via water splitting, proposing a cooperative mechanism where carbonic acid functions as a nondepleting cosubstrate to accelerate H$_2$ evolution.