Experimental Demonstration of Simultaneous σ- and π-Hole-Driven and Highly Directional Dual Pnicogen Bonding in...

Phosphorus and nitrogen, as fundamental elements in biological systems, continue to attract interest due to their versatile bonding capabilities. Pnicogen bonding, an intricate noncovalent interaction, is particularly significant among these lighter pnicogen elements. The electron deficiency of the phosphorus atom in POCl₃ enables it to attract electron-rich species, such as nucleophiles or Lewis bases, which can donate electrons to stabilize the phosphorus center. In N₂O, the terminal nitrogen is electron-rich and thus acts as an electron donor, whereas the central nitrogen, bonded to both oxygen and nitrogen, is electron-deficient and serves as an electron acceptor. The positive electrostatic potential along the extension of the three P-Cl bonds in POCl₃ creates a σ-hole, thereby promoting σ-hole-driven pnicogen bonding. As a result, the approaching nucleophile adopts a nearly linear geometry (∼180°) relative to that of the phosphorus atom. In contrast, N₂O features a π-hole located on the central nitrogen due to its distinct molecular geometry, which facilitates π-hole-driven pnicogen bonding. In this case, the nucleophile approaches the central nitrogen in a nearly perpendicular orientation (∼90°). The present study provides spectroscopic evidence for the simultaneous occurrence of σ- and π-hole-driven, highly directional dual pnicogen bonding in prototypical POCl₃-N₂O dimers, using matrix-isolation infrared spectroscopy and ab initio computations. The experimentally observed dimers preserve the uncompromised geometries of both σ- and π-hole-driven bonding interactions. The dual-directional nature of this pnicogen bonding is further substantiated by molecular topographic analyses. The strength of these interactions is rationalized by the pnicogen affinity, defined as the energy released upon H^-'s attachment to the donor. POCl₃ exhibits a markedly higher affinity than N₂O, consistent with the dominant role of phosphorus in stabilizing dual pnicogen bonding.