Pentavalent P…N phosphorus bonding in the heterodimers of POCl3…nitrogen bases: Evidence from matrix isolation infrared spectroscopy and Ab initio computations

Abstract

The present work explores the P…N phosphorus bonding with phosphoryl chloride (POCl3) prototype against a series of nitrogen bases such as ammonia (NH3), aniline (C6H5NH2), and pyridine (C5H5N), arranged in increasing order of basicity. Experiments were performed in N2 and Ar matrixes at low temperatures and the structure of dimers was elucidated by infrared spectroscopy. Though N-H…O hydrogen bonding is expected to dominate in POCl3-NH3dimer due to the presence of phosphoryl oxygen, the phosphorus bonded P…N interaction plays a non-trivial role in determining the structure of the dimer with a distortion in the hydrogen bonding angle. With a severe distortion in the hydrogen bonding angle, the P…N phosphorus bonding completely influences the structure of POCl3-C6H5NH2dimer. A strong P…N interaction was noticed in the case of POCl3-C5H5N dimer with a co-operative C-H…O hydrogen bonding stabilization. The scanning of complete potential energy surface of POCl3 with nitrogen bases divulged competing structures,which are stabilized through Cl…N halogen bonding interaction in the case of POCl3-NH3 dimer and P…π phosphorus bonding for POCl3-C6H5NH2 and POCl3-C5H5N dimers.The distinctive features of the interaction prevailing in the heterodimers involving POCl3-nitrogen bases were examined using electrostatic potential mapping along with natural orbital, energy decomposition and non-covalent interaction analyses. In addition to P…N interaction, a faint P…σ interaction in which bonding σ(N-H) orbital interacts with phosphorus of POCl3 also was discerned in POCl3-NH3 and POCl3-C6H5NH2 dimers. Though the nitrogen atom of these bases (NH3,C6H5NH2, and C5H5N) cannot qualify to be acknowledged as pnicogens, due to the donation of the lone pair of electrons on nitrogen in these systems towards hydrogen bonding make it electron deficient. As a result, a surprising pnicogen character, though feeble, is witnessed on these nitrogen atoms, the observation of which is reported with caution.