Thermal stability of CO2 adducts of N-heterocyclic carbenes (NHCs) was studied by means of in situ FTIR method with monitoring of the nu(CO2) region of the infrared spectra under various conditions. 1,3-Bis(2,6-diisopropylphenyl)imidazolinium-2-carboxylate (SIPr-CO2) shows higher thermal stability compared with 1,3-bis(2,6-diisopropylphenyl)imidazolium-2-carboxylate (IPr-CO2). The presence of free CO2 can significantly inhibit the decomposition of NHC-CO2 adducts, while the addition of an epoxide such as propylene oxide has a negative effect on stabilizing these adducts. As zwitterionic compounds, NHC-CO2 adducts were also proved to be effective organic catalysts for the coupling reaction of CO2 and epoxides to afford cyclic carbonates, for which a possible mechanism was proposed. Among these NHC-CO2 adducts, the relatively unstable IPr-CO2 exhibits the highest catalytic activity. The presence of an electrophile such as SalenAlEt could greatly improve the catalytic activity of IPr-CO2 due to intermolecular cooperative catalysis of the binary components.

N-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO2 sequestration, affording a CO2 adduct (NHO-CO2). X-ray single crystal analysis revealed the bent geometry of the binding CO2 in the NHO-CO2 adducts with an O-C-O angle of 127.7-129.9°, dependent on the substitute groups of N-heterocyclic ring. The length of the C(carboxylate)-C(NHO) bond is in the range of 1.55-1.57 Å, significantly longer than that of the C(carboxylate)-C(NHC) bond (1.52-1.53 Å) of the previously reported NHC-CO2 adducts. The FTIR study by monitoring the ν(CO2) region of transmittance change demonstrated that the decarboxylation of NHO-CO2 adducts is easier than that of the corresponding NHC-CO2 adducts. Notably, the NHO-CO2 adducts were found to be highly active in catalyzing the carboxylative cyclization of CO2 and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO2 pressure), selectively giving α-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10-200 times that of the corresponding NHC-CO2 adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO-CO2 adduct was tentatively ascribed to its low stability for easily releasing the CO2 moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle.

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