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 C_2v symmetry, whereas the remaining have cyclic three membered C-O-C rings with C_s 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.