Tolerant Bimetallic Macrocyclic [OSSO]-Type Zinc Complexes for Efficient CO2 Fixation into Cyclic Carbonates

Abstract

A new type of sustainable bimetallic zinc complexes bearing macrocyclic thioetherphenolate [OSSO]-type ligands has been developed and employed toward the chemical fixation of carbon dioxide into cyclic carbonates. The effects of reaction variables such as temperature, time, pressure, and molar ratio of epoxide to catalyst on the catalytic performance were systematically investigated. Based on the observations from structural and spectroscopic analyses, a plausible catalytic mechanism for the present reaction system was proposed, in which a Lewis acidic zinc center activates the epoxide and a nucleophilic halide counterion from the cocatalyst to ring open the activated epoxide. The two zinc centers are situated sufficiently close to each other to rationally allow a synergistic effect in the cycloaddition. The macrocyclic thioetherphenolate [OSSO]-type catalyst showed excellent conversions and selectivity for CO2 fixation into cyclic carbonates from a broad range of terminal epoxides, including bis-epoxides and biomass-derived epoxides substrates. Additionally, these catalysts demonstrate high stability to moisture and oxygen, resistance to many kinds of impurity, and good recyclability with negligible losses in catalytic activity.