Azines as Electron-Pair Donors to CO₂for N···C Tetrel Bonds

Abstract

Ab initio MP2/aug'-cc-pVTZ calculations were performed to investigate tetrel-bonded complexes formed between CO and the aromatic bases pyridine, the diazines, triazines, tetrazines, and pentazine. Of the 23 unique equilibrium azine:CO complexes, 14 have planar structures in which a single nitrogen atom is an electron-pair donor to the carbon of the CO molecule, and 9 have perpendicular structures in which two adjacent nitrogen atoms donate electrons to CO, with bond formation occurring along an N-N bond. The binding energies of these complexes vary from 13 to 20 kJ mol 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:CO 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 π*C-O orbital. In perpendicular complexes, charge transfer occurs from an N-N bond to the adjacent π*O-C-O orbital of CO. Decreases in the bending frequency of the CO molecule and in the C chemical shielding of the C atom of CO upon complex formation are larger in planar structures compared to perpendicular structures. EOM-CCSD spin-spin coupling constants J(N-C) for complexes with planar structures are very small but still correlate with the N-C distance across the tetrel bond.

Solvent and Substituent Effects on the Phosphine + CO2 Reaction

“…Also, the CO2 molecule has been the subject of investigation from the theoretical point of view [29,34,35]. Our previous studies concerning A•••B complexes with B being CO2 are reported in references [36][37][38][39][40][41]. Simulation of phosphorous/boron frustrated lewis pair complexes (FLP) with CO2 in explicit solvent proposed a two-step mechanism [42].…”

Steric Crowding in Tetrel Bonds

“…Analogous to the venerable and thoroughly studied H-bond [1][2][3] , it is now well accepted that the bridging proton can be replaced by any of a litany of larger atoms, without compromising the strength of the interaction [4][5][6][7] . These substitute atoms are typically electronegative ones, drawn from the right side of the periodic table.…”

Complexes of CO2 with the Azoles: Tetrel Bonds, Hydrogen Bonds and Other Secondary Interactions

“…It is of interest to explore interactions that are important in biochemical reactions, such as those between azines and azoles with CO 2 . In a previous paper, we presented the structures, binding energies and other properties of CO 2 : azine complexes stabilized by N···C tetrel bonds [ 24 ]. The azoles have pyridine-like N atoms in positions 2, 3, 4 and 5 that may act as electron-pair donors as do the azines.…”