Multinuclear Zinc-Magnesium Hydroxide Carboxylates: A Predesigned Model System for Copolymerization of CO 2 with Epoxides.

Among numerous catalysts in the ring-opening copolymerization of epoxides with carbon dioxide (CO 2 ), 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 μ 3 -κ 2 :κ 2 :κ 2 coordination mode, [ZnMg 2 (μ 3 -OH)(O 2 CPh) 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 [ZnMg 2 (μ 3 -OH)(O 2 CPh) 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).