Bimetallic bis(benzotriazole iminophenolate) cobalt, nickel and zinc complexes as versatile catalysts for coupling of carbon dioxide with epoxides and copolymerization of phthalic anhydride with cyclohexene oxide?

Catalysis Science & Technology Pub Date: 2015-10-13 DOI: 10.1039/C5CY01290B

Abstract

A family of cobalt, nickel and zinc complexes containing the 5CBiIBTP or 2CBiIBTP derivative (BiIBTP = bis(benzotriazole iminophenolate)) were synthesized and structurally characterized. The reactions of 5CBiIBTP-H2 pro-ligand with 2.0 molar equivalents of Mt(OAc)2·4H2O (Mt = Co or Ni) or Zn(OAc)2 furnished di-nuclear cobalt, nickel and zinc complexes (1: [(5CBiIBTP)Co2(OAc)2], 2: [(5CBiIBTP)Ni2(OAc)2], and 3: [(5CBiIBTP)Zn2(OAc)2]), but treatment of 2CBiIBTP-H2 as the ligand precursor under a similar synthetic method gave the acetate-free species [(2CBiIBTP)Ni] (5) or [(2CBiIBTP)Zn(H2O)] (6). Interestingly, 2CBiIBTP-H2 reacted with 2.0 molar equiv. of cobalt(II) acetate tetrahydrate to generate a water-solvated di-cobalt complex, [(2CBiIBTP)Co2(OAc)2(H2O)] (4). The solid-state structure of 1–4 displays a bimetallic metal(II) acetate (metal = Co, Ni, Zn) with a hexadentate BiIBTP ligand chelating two metal atoms, whereas complexes 5 and 6 reveal a monomeric feature with a tetra- or penta-coordinated metal centre chelated by one ONNO-tetradentate 2CBiIBTP ligand. Versatile catalysis towards coupling of CO2 with epoxides and ring-opening copolymerization of phthalic anhydride (PA) with cyclohexene oxide (CHO) by these bimetallic cobalt and nickel acetate complexes was systematically examined. BiIBTP-featuring di-nickel 2 was found to efficiently copolymerize CO2 and CHO with a high TOF (>200 h?1) in a controlled character, giving a highly alternating poly(cyclohexene carbonate) with high molecular weight (Mn > 30?000 g mol?1). Di-cobalt catalyst 1 in combination with n-Bu4NBr co-catalysts was able to catalyse CO2/CHO coupling to produce >99% cis-selectivity cyclohexene carbonate with good activity under optimal conditions. Not only was the efficient catalytic performance of di-Co complex 1 for CO2/epoxides coupling enabled, but 1 was also able to catalyse the copolymerization of PA and CHO to afford the molecular weight controllable poly(PA-alt-CHO)s with high ester linkage content. This is the first example of utilizing versatile BiIBTP-ligated cobalt(II) acetate catalysts for the production of cyclic carbonate, biodegradable polycarbonate or polyester.

Graphical abstract: Bimetallic bis(benzotriazole iminophenolate) cobalt, nickel and zinc complexes as versatile catalysts for coupling of carbon dioxide with epoxides and copolymerization of phthalic anhydride with cyclohexene oxide
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