A secondary-sphere proton channel accelerating metal-hydride formation in Mn(i) catalysts for selective...

The selective formation of metal-hydride intermediates represents a key mechanistic step in Mn-based CO₂ reduction catalysis, yet remains kinetically challenging. Herein, we report the discovery of a secondary-sphere proton channel that markedly accelerates Mn-H formation in visible-light-driven CO₂-to-formate conversion. Mn(i) bipyridyl complexes bearing ethylene-bridged Brønsted acidic and basic pendants at the 6,6'-positions of the ligand establish a dynamic hydrogen-bond network that relays protons from protonated triethanolamine (TEOA(H)) directly to the metal center. Operando FTIR and DFT analyses reveal that this bio-inspired secondary coordination sphere (SCS) mimics the proton-transfer architecture of formate dehydrogenase (FDH), lowering the activation barrier for hydride formation while suppressing Mn-Mn dimerization. The optimized Mn-bpy^diOMe complex achieves a turnover number (TON) of approximately 300 and an exceptional formate selectivity exceeding 94%, representing the highest reported among Mn-based molecular systems to date. These findings demonstrate that strategic SCS engineering can emulate enzymatic proton channels, enabling precise control over hydride chemistry and guiding Mn-catalyzed CO₂ reduction exclusively toward formate formation.