Elusive Hypervalent Phosphorus… Interactions: Evidence for Paradigm Transformation from Hydrogen to Phosphorus Bonding at Low Temperatures
The electron systems are the conventional electron donors to the hydrogen acceptors in hydrogen bonding. Apart from the hydrogen atom, halogens, chalcogens and pnicogens can also be envisaged as electron acceptors for such bonding motifs involving clouds. Triel/tetrel bonding involving boron/carbon atoms too gains much attention in recent times. Although, experimental reports on halogen… and chalcogen… interactions are sizeable, the amount of reports on pnicogen… is sparse and boron/carbon… interaction has not yet opened its account. Markedly, in pnicogen… interactions, the focus thus far is towards pnicogen atom (or most commonly phosphorus atom), which is by and large exist in its trivalent state. The expansion from trivalent to hypervalent (predominantly pentavalent) state of phosphorus atom with localized and delocalized electrons as electron donors is elusive and can be thought as an intuitive extension to trivalent phosphorus… interactions. It is intriguing to investigate this novel and unique class of hypervalent phosphorus… interactions at low temperatures with a strong competing hydrogen and halogen bonding partners. In this work, on one side, POCl3 was taken as a prototypical molecule to explore these pentavalent phosphorus interactions and on the other side, it is composed of the electron donors such as acetylene (C2H2), ethylene (C2H4) and benzene (C6H6), in which phosphorus… bonding can be expected to compete with hydrogen and halogen bonding interactions. Though, hydrogen bonding dominates in POCl3-C2H2 and POCl3-C2H4 heterodimers, phosphorus bonding plays a definite and non-trivial role in their overall stabilization. An interesting paradigm transformation was noticed in POCl3-C6H6 system, where pentavalent phosphorus… bonding was observed to completely influence the hydrogen bonding interaction. All three POCl3-C2H2, POCl3-C2H4 and POCl3-C6H6 heterodimers were experimentally generated at low temperatures in Ar and N2 matrixes and were characterized both by infrared spectroscopy and the state-of-the-art quantum chemical computations. To further shed the light upon these P… systems, the interaction characteristics were analyzed with the help of electrostatic potential mapping, natural bond orbital and energy decomposition analyses.