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.