Azines as Electron-Pair Donors to CO2 for N•••C Tetrel Bonds

Ab initio MP2/aug'-cc-pVTZ calculations were performed to investigate tetrel-bonded complexes formed between CO2 and the aromatic bases pyridine, the diazines, triazines, tetrazines, and pentazine. Of the 23 unique equilibrium azine:CO2 complexes, 14 have planar structures in which a single nitrogen atom is an electron-pair donor to the carbon of the CO2 molecule, and 9 have perpendicular structures in which two adjacent nitrogen atoms donate electrons to CO2, with bond formation occurring along an N-N bond. The binding energies of these complexes vary from 13 to 20 kJ mol(-1) and decrease as the number of nitrogen atoms in the ring increases. For a given base, planar structures have larger binding energies than perpendicular structures. The binding energies of the planar complexes also tend to increase as the distance across the tetrel bond decreases. Charge transfer in the planar pyridine:CO2 complex occurs from the N lone pair to a virtual nonbonding orbital of the CO, carbon atom. In the remaining planar complexes, charge transfer occurs from an N lone pair to the remote in-plane pi*C-O orbital. In perpendicular complexes, charge transfer occurs from an N-N bond to the adjacent pi*O-C-O orbital of CO2. Decreases in the bending frequency of the CO2 molecule and in the C-13 chemical shielding of the C atom of CO2 upon complex formation are larger in planar structures compared to perpendicular structures. EOM-CCSD spin-spin coupling constants (1t)J(N-C) for complexes with planar structures are very small but still correlate with the N-C distance across the tetrel bond.