Multinuclear Zinc-Magnesium Hydroxide Carboxylates: A Predesigned Model System for Copolymerization of CO2 with Epoxides

Abstract

Among numerous catalysts in the ring-opening copolymerization of epoxides with carbon dioxide (CO2), zinc dicarboxylate complexes are the most common type, and in the family of metal-based homogeneous catalysts, zinc and magnesium complexes have attracted widespread attention. We report on the synthesis and structural characterization of a zinc-magnesium benzoate framework templated by the central hydroxide anion with μ3-κ2:κ2:κ2 coordination mode, [ZnMg2(μ3-OH)(O2CPh)5]n (n = 1 or 2). The resulting heterometallic system forms stable Lewis acid-base adducts with tetrahydrofuran (THF) and cyclohexene oxide (CHO), which crystallize as the hexanuclear zinc-magnesium hydroxide carboxylate cluster [ZnMg2(μ3-OH)(O2CPh)5(L)2]2 (L = THF or CHO). Their X-ray crystal structure analysis revealed that the Zn center prefers 4-fold coordination and the Mg centers demonstrated the ability to accommodate higher coordination numbers, and as a result, the heterocyclic molecules are exclusively bonded to 6-fold Mg atoms. The heteronuclear carboxylate aggregates appeared active in the copolymerization reaction at elevated temperatures to produce an alternating poly(cyclohexene carbonate).