Cooperative Interplay of Brønsted Acid and Lewis Acid Sites in MIL-101(Cr) for Cross-Dehydrogenative Coupling of C-H Bonds.

Abstract

Cross-dehydrogenative coupling (CDC) is an effective tool for carbon-carbon bond formation in chemical synthesis. Herein, we report a metal-organic framework (MOF) possessing dual Lewis acidic Cr sites and sulfonic acid sites (MIL-101(Cr)-SO3H) as an efficient catalytic material for direct cross-coupling of xanthene and different nucleophiles using O2 as the oxidant. The highly porous structure of MIL-101(Cr)-SO3H enables the free access of reactants to the catalytic active sites inside MOF pores. Kinetic studies indicated that the Cr sites of MOF accelerate the rate-limiting autoxidation reaction of xanthene, which synergistically work with the sulfonic acid group on MOF ligands in promoting the CDC reactions. Besides, the catalytic system shows excellent functional group compatibility, and a variety of valuable xanthene derivatives were synthesized with satisfactory yields. Furthermore, MIL-101(Cr)-SO3H can be reused and its catalytic activity and crystal structure remain after six consecutive runs.