Nickel-catalyzed copolymerization of carbon dioxide with internal epoxides by di-nuclear bis(benzotriazole iminophenolate) complexes

We report the synthesis of structurally well-characterized di-nuclear nickel bis(benzotriazole iminophenolate) (BiIBTP) complexes containing diverse mono- or bi-dentate monoanionic coligands for the utilization of copolymerization of carbon dioxide (CO2) with internal epoxides. The molecular structures of complexes 1-3 can be ascribed to bimetallic nickel(II) species containing a hexadentate BiIBTP main ligand and two homo-coligands of pentafluorobenzoate, pentafluorophenolate or chloride, whereas 4 and 5 are BiIBTP-ligated di-nickel chloride analogues mixed with carboxylate coligands. Di-nickel complexes 1 and 3 supported by O2CC6F5 and Cl coli-gands were demonstrated to be efficient catalysts for alternating copolymerization of CO2 with cyclohexene oxide (CHO) to afford CO2 -based poly(cyclohexene carbonate)s (PCHCs) with > 99% carbonate repeated units. Particularly, di-Ni di-chloride complex 3 was capable of giving high activity with a turnover frequency of 688 h (-1) for such copolymerization under the optimal conditions. Interestingly, di-Ni catalyst 1 enables us to produce the low molecular weight di-hydroxyl end-capped PCHCs with unimodal molecular weight distributions while commercially available CHO without further purification was used as the epoxide monomer. In addition to CO2 -CHO copolymerization, bimetallic complex 1 was also found to effectively copolymerize CO2 with cyclopentene oxide, generating highly alternating poly(cyclopentene carbonate)s with molecular weights in a broad range. Kinetic studies of CO2/CHO copolymerization catalyzed by 1 were investigated using H-1 NMR spectroscopy and revealed the first-order dependence for both catalyst 1 and CHO concentrations.