J. Phys. Chem. A, 121, 8136–8146 (2017)

DOI: 10.1021/acs.jpca.7b08393 (open access)

Carbon–Carbon Bonding between Nitrogen Heterocyclic Carbenes and CO2

Ab initio MP2/aug′-cc-pVTZ calculations were performed to identify equilibrium complexes and molecules and the transition structures that interconvert them, on the potential energy surfaces of a series of seven binary systems that have nitrogen heterocyclic carbenes (NHCs) as the electron-pair donors to CO2. Seven of the NHCs form complexes stabilized by C···C tetrel bonds, and six of these seven are also stabilized by a secondary interaction between an O of CO2 and the adjacent N–H group of the carbene. Six of the seven NHCs also form stable molecules with C–C covalent bonds, and with one exception, these molecules have binding energies that are significantly greater than the binding energies of the complexes. Charge-transfer stabilizes all of the NHC:CO2 complexes and occurs from the C lone pair of the carbene to the CO2 molecule. The six complexes that have secondary stabilizing interactions are also stabilized by back-donation of charge from the O to the adjacent N–H group of the carbene. Transition structures present barriers to the interconversion of complexes and molecules. With one exception, the barrier for converting a molecule to a complex is much greater than the barrier for the reverse reaction. Atoms in Molecules bonding parameters, shifts of IR C–O stretching and O–C–O bending frequencies, changes in NMR 13C chemical shieldings, and changes in C–C and C–O coupling constants as 1tJ(C–C) and J(C–O) for complexes and transition structures become 1J(C–C) and 2J(C–O) for molecules, are all consistent with the changing nature of the C···C tetrel bond in the complex through the transition state to a covalent C–C bond in the molecule.