Di-nuclear zinc complexes containing tridentate imino-benzotriazole phenolate derivatives as efficient catalysts for ring-opening polymerization of cyclic esters and copolymerization of phthalic anhydride with cyclohexene oxide

Zinc catalysts incorporated by imino-benzotriazole phenolate (IBTP) ligands were synthesized and characterized by single-crystal X-ray structure determinations. The reaction of the ligand precursor (^C1DMeIBTP-H or ^C1DIPIBTP-H) with diethyl zinc (ZnEt₂) in a stoichiometric proportion in toluene furnished the di-nuclear ethyl zinc complexes [(μ-^C1DMeIBTP)ZnEt]₂ (1) and [(μ-^C1DIPIBTP)ZnEt]₂ (2). The tetra-coordinated monomeric zinc complex [(^C1PhIBTP)₂Zn] (3) or [(^C1BnIBTP)₂Zn] (4) resulted from treatment of ^C1PhIBTP-H or ^C1BnIBTP-H as the pro-ligand under the similar synthetic method with ligand to metal precursor ratio of 2:1. Single-crystal X-ray diffraction of bimetallic complexes 1 and 2 indicates that the ^C1DMeIBTP or ^C1DIPIBTP fragment behaves a NON-tridentate ligand to coordinate two metal atoms. Catalysis for ring-opening polymerization (ROP) of ε-caprolactone (ε-CL), β-butyrolactone (β-BL), and lactide (LA) of complexes 1 and 2 was systematic studied. In combination with 9-anthracenemethanol (9-AnOH), Zn complex 1 was found to polymerize ε-CL, β-BL, and L-LA with efficient catalytic activities in a controlled character. This study also compared the reactivity of these ROP monomers with different ring strains by Zn catalyst 1 in the presence of 9-AnOH. Additionally, Zn complex 1 combining with benzoic acid was demonstrated to be an active catalytic system to copolymerize phthalic anhydride and cyclohexene oxide.