Conversion of CO2 to epoxides or oxazolidinones enabled by a CuI/CuII-organic framework bearing a tri-functional linker

Abstract

Metal–Organic Frameworks (MOFs) as effective catalysts can contribute to the effective green conversion of CO₂. However, more studies have focused on the conversion of epoxides to cyclic carbonate systems, but the equally important conversion of oxazolidinones has been rarely addressed. In general, the catalysts required for such reactions have to combine multiple different types of catalytic sites and synergistic effects at the same time. MOFs, however, have the advantage of structural tailorability and can be chemically designed to endow the material with a uniform distribution of multiple functional sites very easily and to exhibit catalytic conversion performance simultaneously. In this work, a Cu–MOF with mixed valence states, [Cu^II₂Cu^I₄I₄L], was synthesized by using CuI and a pre-designed linker consisting of triazole and dicarboxylic groups, which exhibits many advantages such as stable backbone architecture, abundant active sites, and modified amino pore channels. The dual coordination groups of the ligand can stabilize the copper moiety with different valence states, and the modified amino group can be used as a functional site to adsorb carbon dioxide and promote the conversion of the substrate. For the conversion of carbon dioxide, two different systems were tried by choosing a Cu–MOF as the catalyst. Firstly, a Cu–MOF/TBABr synergistic catalytic system was used to perform the cycloaddition reaction of CO₂ with a series of epoxides at atmospheric pressure, and was able to produce cyclic carbonate compounds in high yields. Mechanistically, it is speculated that the Lewis acid site on the Cu–MOF and the nucleophilic Br⁻ acts in synergy to lower the energy barrier required for the reaction, allowing the reaction to proceed more quickly. Secondly, under atmospheric pressure and co-catalyst-free conditions, the carboxycyclization reaction of CO₂ with a series of terminal alkynylamine compounds was performed using a Cu–MOF as a single catalyst to obtain different oxazolidinone products with higher yields. Mechanistically, it was analyzed that divalent copper, monovalent copper and amino groups produced a subtle synergistic effect, one without the other, to jointly complete the conversion of terminal alkynylamine compounds to oxazolidinones with CO₂. Therefore, the catalysts prepared by taking advantage of the structural tailorability of MOFs can be endowed with abundant active sites of Lewis acid and base at the same time, exhibiting excellent catalytic effects for CO₂ conversion requiring different catalytic sites.