Nickel-Catalyzed Coupling of Carbon Dioxide with Cyclohexene Oxide by Well-Characterized Bis(N-Heterocyclic Carbene) Carbazolide Complexes

The facile synthesis, structural characterization, and catalytic studies for CO₂/epoxide coupling of nickel acetates based on carbazolide-bis(NHC) (NHC = N-heterocyclic carbene) were reported. Treatment of 3,6-di-tert-butyl-1,8-bis(3-alkylimidazolium-1-yl)carbazole salt proligands (bis(R-Im-X)Cz, R = benzyl (Bn), n-butyl (ⁿBu), methyl (Me), X = Br or I) with nickel acetate tetrahydrate in the presence of excess triethylamine generated monomeric four-coordinate nickel complexes [(bis(R-Im)Cz)Ni(OAc)] (R = Bn (3), R = ⁿBu (4), and R = Me (5)). Single-crystal X-ray diffraction of Ni complexes 4 and 5 indicates that the bis(NHC)-carbazolide fragment behaves as a CNC-tridentate pincer ligand to coordinate the metal center, and the ancillary acetate group assumes a terminal acetate bonding mode. Catalysis for coupling of carbon dioxide with cyclohexene oxide (CHO) by these carbazolide-bis(NHC)-ligated Ni complexes was systematically examined. Experimental results displayed that cycloaddition of CHO and CO₂ catalyzed with complex 4 could give cyclohexene carbonate (CHC) with >99% cis-isomer selectivity on using low catalyst concentrations and high reaction temperature, whereas catalyst 3 was able to copolymerize CHO and CO₂ to afford a narrowly dispersed and perfectly alternating poly(cyclohexene carbonate) (PCHC) as the major product at the higher catalyst loadings and lower copolymerization temperature. This is the first time that the air-stable bis(NHC)-carbazolide nickel(II) acetate is an effective and versatile catalyst for the formation of either biodegradable PCHCs or cis-CHCs.