UiO-67 MOF-Encapsulated NHC-Based Single-Site Copper Catalyst and Its Application in Regioselective Borylation of Terminal Alkynes

Abstract

N-Heterocyclic carbenes (NHCs) act as versatile ligand backbones due to their strong σ-donation and π-acceptance properties. However, the encapsulation of NHC–coinage metal complexes in a metal–organic framework (MOF) to utilize them in organic catalysis is rare. In this work, an NHC-coordinated CuBr (NHC = Bn₂Im; 1,3-dibenzyl-imidazol-2-ylidene) complex was encapsulated in UiO-67 MOF ((Bn₂Im)₂CuBr@UiO-67) and further utilized toward the regioselective protoboration of terminal alkynes. (Bn₂Im)₂CuBr@UiO-67 was found to show superior catalytic performance in aiding the protoboration of terminal alkynes, with a very high turnover frequency (TOF) of 14333.3 h^–1, much higher than those of many other reported copper-based heterogeneous catalysts. Our catalyst also retained excellent catalytic efficiency for up to five cycles for the above-mentioned process. The newly synthesized (Bn₂Im)₂CuBr@UiO-67 and the recovered catalyst post-catalysis were characterized using various analytical techniques, including powder X-ray diffraction (PXRD), IR spectroscopy, field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS).