Bimetallic nickel complexes containing imidazole-based phenolate ligands as efficient catalysts for the copolymerization of carbon dioxide with epoxides

Abstract

The utilization of hexadentate imidazole-derived diamine-bisphenolate ligands to construct structurally well-defined bimetallic nickel catalysts that enable the mediation of the copolymerization of carbon dioxide with alicyclic epoxides was reported for the first time. A series of dinickel carboxylate/nitrophenolate complexes were facilely prepared through a one-pot procedure and their structures were fully determined by single crystal X-ray structural analysis. Dinickel complexes 1–10 were used as single-component catalysts, and were evaluated for the copolymerization of CO₂ and cyclohexene oxide (CHO), for which acetato-incorporated complex 1 was proved to exhibit the best activity. Not only has the controllability of binickel catalyst 1 for CO₂/CHO copolymerization been demonstrated, but also an “immortal” character for the same polymerization has been realized. Furthermore, detailed kinetic studies of polymerization catalysis of this type were undertaken, and the kinetics results revealed a first-order dependence on both Ni complex 1 and CHO concentrations. This is a successful example of the introduction of the easily accessible nitrogen-heterocycle group, the imidazole moiety, into phenolate ligands for the development of high-performance homogeneous catalysts towards the bimetallic complex-catalyzed copolymerization of CO₂ and epoxides.