Trapping CO by Adduct Formation with Nitrogen Heterocyclic Carbenes (NHCs): A Theoretical Study.

Carbon dioxide can form compounds with nitrogen heterocyclic carbenes (NHCs) based on azoles through noncovalent interactions or by covalent bonding. A narrow dependence on the carbene structure has been observed for the preference for one or the other type of bonding, as revealed by a series of physicochemical descriptors. In our survey, a set of NHCs based on the azole family (three classical, three abnormal, and one remote) was shown to bind CO at the accurate G4MP2 computational level. In most cases, exothermic reaction profiles towards the covalently bound form were found, which reached stabilization enthalpies of up to -77 kJ mol for the remote carbene case. Both noncovalent and covalent minima and the corresponding transition state that connects them have been identified as stationary points along the reaction coordinate.