A Secondary-Sphere Proton Channel Accelerating Metal–Hydride Formation in Mn(I) Catalysts for Selective CO2-to-Formate Conversion

Abstract

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 delivers a turnover number of ~300 with >94% formate selectivity—performance that ranks among the best for Mn-based molecular systems—and, notably, achieves a solar-to-fuel quantum yield of 25.9% for the reducing half reaction in the presence of sacrificial electron donors, highlighting the remarkable efficiency gained from SCS-assisted proton delivery. 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.