Carbenes as Electron-Pair Donors To CO2 for C···C Tetrel Bonds and C-C Covalent Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to identify stable complexes and molecules and the transition structures that interconvert them on the potential surfaces of ten singlet carbene bases acting as electron-pair donors to CO2. The carbene bases include cyclic C(NHCH)2 or NHC, C(NH2)2, an oxygen heterocyclic carbene C(OCH)2 or OHC, C(OH)2, C(CH3)2, cyclic C3H2, CCCH2, CCl2, CCH2, and CF2. Carbene:CO2 complexes stabilized by C···C tetrel bonds have been found on all potential surfaces, whereas carbene-CO2 molecules stabilized by C-C covalent bonds have been found on eight surfaces. Three of these molecules have open structures with C2v symmetry, whereas the remaining have cyclic three membered C-O-C rings with Cs symmetry. The transition structures which connect the complex and the molecule are bound on three of the potential surfaces. Whether the transition structure is bound or unbound relative to the carbene and CO2 depends on the relationship among C-C distances at the three stationary points on the surface. Charge-transfer interactions stabilize carbene:CO2 complexes. The primary charge transfer in complexes arises from electron donation from the carbene lone-pair to the CO2 molecule. There is also back-donation of charge from CO2 to the carbene in three complexes. Systematic changes in bonding properties occur as complexes go through transition structures and become molecules. EOM-CCSD inter- and intramolecular C-C and C-O spin-spin coupling constants have been computed and compared for complexes and molecules. A search of the CSD database found the (NH2)2C-CO2 structure and 17 NHC-CO2 derivatives. Computed bond distances and angles have been compared with experimental data.