Achieving highly efficient carbon radical-mediated cross-coupling reaction in a confined radical microenvironment within a 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, requiring additional HAT agents and can only activate one substrate. Here, we constructed two crystalline porous materials, Zr/Hf-NDI, which possess excellent light absorbing capacity and a confined radical microenvironment, making them able to integrate PS, HAT, and SET processes to simultaneously activate two substrates. Thus, as heterogeneous photocatalysts, they exhibited excellent catalytic performance for the carbon radical-mediated cross-coupling reaction between alcohols and o-phenylenediamine (OPD) to synthesize benzimidazoles (yield > 99%). More importantly, they displayed very good substrate compatibility, especially for OPD substrates with electron-withdrawing groups, even surpassing those of noble metal catalysts. In situ characterizations combined with theoretical calculations showed that the high activity of these catalysts arose 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 a 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 radicals NDI˙⁻ and OPD˙⁺, respectively, via an SET process, further promoting the reaction. To the best of our knowledge, this is the best performing crystalline porous catalyst for photocatalytic carbon radical-induced benzimidazole synthesis.