Achieving High-Efficient Carbon Radical-Mediated Cross-Coupling Reaction by Confined Radical Microenvironment in Metal-Organic Framework

Abstract

It has been well-demonstrated that the combination of photosensitive (PS), hydrogen atom transfer (HAT) and single electron transfer (SET) processes can achieve efficient radical-mediated organic synthesis, but such reaction systems are usually homogeneous reactions, require additional HAT agents and can only activate one substrate. Herein, we constructed two crystalline porous materials, Zr/Hf-NDI, which possess excellent light absorbing capacity and confined radical microenvironment, making them able to integrate PS, HAT and SET processes to activate two substrates simultaneously. Thus, as heterogeneous photocatalysts, they exhibited excellent catalytic performance for the carbon radical-mediated cross-coupling reaction between alcohol and o-phenylenediamine (OPD) to synthesize benzimidazoles (yield＞99%). More importantly, they displayed very good substrate compatibility, especially for the OPD substrates with electron-withdrawing groups, which even surpasses those of noble metal catalysts. In situ characterizations combined with theoretical calculations showed that the high activity of these catalysts aroused from: i) the metal-oxo clusters and radical NDI•− ligands can form hydrogen bonding traction activation for the alcohol substrate, and thus facilitate it to generate key intermediate α-carbon radical through HAT process; ii) the OPD substrate, acting as an electron donor, forms strong D-A interaction with the NDI ligand and activates the NDI and itself into radical NDI•− and OPD•+ respectively via SET process, further promoting the reaction. To the best of our knowledge, this is the best performing crystalline porous catalysts for photocatalytic carbon radical-induced benzimidazole synthesis.