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

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 M_t(OAc)₂·4H₂O (M_t = 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 (M_n > 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.