Hydroxide Ligands Cooperate with Catalytic Centers in Metal-Organic Frameworks for Efficient Photocatalytic CO Reduction.

Converting CO into fuels via photochemical reactions relies on highly efficient and selective catalysts. We demonstrate that the catalytic active metal center can cooperate with neighboring hydroxide ligands to boost the photocatalytic CO reduction. Six cobalt-based metal-organic frameworks (MOFs) with different coordination environments are studied at the same reaction condition (photosensitizer, electron donor, water/organic mixed solvent, and visible light). In pure CO at 1.0 atm, the MOFs bearing μ-OH ligands neighboring the open Co centers showed CO selectivities and turnover frequencies (TOFs) up to 98.2% and 0.059 s, respectively. More importantly, their TOFs reduced only ca. 20% when the CO partial pressure was reduced to 0.1 atm, while other MOFs reduced by at least 90%. Periodic density functional theory calculations and isotope tracing experiments showed that the μ-OH ligands serve not only as strong hydrogen-bonding donors to stabilize the initial Co-CO adduct but also local proton sources to facilitate the C-O bond breaking.