Development of pyridine based o-aminophenolate zinc complexes as structurally tunable catalysts for CO2 fixation into cyclic carbonates

Abstract

Zinc complexes of ZnL^APIPX (L = 2,4-di-tert-butyl-6-(((E)-2-(((E)-pyridin-2-lmethylene)amino)benzylidene)amino)phenol) bearing different axial ligands (X = OAc, Cl, Br, I) have been successfully synthesized and characterized. It is evident from X-ray crystallography analysis that ZnL^APIPX exists as a distorted square-pyramid in which Zn^II is coordinated by an o-aminophenolate-based ligand and an X moiety in the axial position. These easily synthesized complexes have been shown to perform as effective catalysts for the coupling of epoxides and carbon dioxide to generate cyclic carbonates. Their axial ligand was found to play an important role in enhancing their catalytic activity. The ZnL^APIPI complex represents a versatile bifunctional catalyst due to the synergistic effect of both an electrophilic zinc ion and a nucleophilic iodide in one molecule to achieve a high-yield of cyclic carbonates, while ZnL^APIPOAc in cooperation with a co-catalyst affords a productive system for this coupling reaction. The effects of reaction variables such as temperature, time and pressure were systematically investigated. This solvent free route is relevant to the context of green chemistry as it provides an atom-efficient protocol for the conversion of CO₂ into valuable cyclic carbonates.