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.