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 CO₂ carbon atom. In the remaining planar complexes, charge transfer occurs from an N lone pair to the remote in-plane π*C-O orbital. In perpendicular complexes, charge transfer occurs from an N-N bond to the adjacent π*O-C-O orbital of CO₂. Decreases in the bending frequency of the CO₂ molecule and in the ¹³C 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 ^1tJ(N-C) for complexes with planar structures are very small but still correlate with the N-C distance across the tetrel bond.