Dinuclear Nickel and Cobalt Complexes Containing Biocompatible Carboxylate Derivatives as Effective Catalysts for Coupling of Carbon Dioxide with Epoxides: Synthesis, Characterization, and Catalysis

We reported a series of well-characterized bimetallic bis(benzotriazole iminophenolate) (BiIBTP) nickel and cobalt complexes containing biocompatible mandelate or tert-butoxycarbonyl (Boc)-protected glycinate coligands, which were synthesized through a one-pot procedure. X-ray structural determinations of dinickel and dicobalt complexes 1-8 indicated that the BiIBTP ligand behaves as a N4O2 skeleton to bond two metal (Ni or Co) ions and that the hydroxyl or amide groups on the coligands do not react with the corresponding nickel or cobalt ions. Dinuclear nickel complexes 1-4 were all found to effectively mediate alternating CO2 copolymerization of cyclohexene oxide (CHO) to generate CO2-based poly(cyclohexene carbonates) (PCHCs), and Ni complex 2 incorporating two Boc-glycinate moieties was demonstrated to be the most active single-component catalyst among them. Note that complex 2 not only displayed a controllable character for CO2/CHO copolymerization but also exhibited the capability of catalyzing such a copolymerization in an "immortal" manner. Additionally, detailed kinetic investigations for this kind of copolymerization by 2 were carried out. Apart from CO2/CHO copolymerization by binickel catalysts, CO, cycloaddition of terminal or internal epoxides catalyzed by dicobalt complexes 5-8 in combination with quaternary ammonium salts as cocatalysts was also investigated. The catalysis results suggest that Boc-glycinate-supported Co complex 6 is an efficient catalyst to couple CO2 and epoxides with a broad substrate scope in the presence of n-Bu4NI under the optimized conditions, producing the associated cyclic organic carbonates from terminal epoxides with good activities and high selectivities.