Elusive hypervalent phosphorusπ interactions: evidence for paradigm transformation from hydrogen to phosphorus bonding at low temperatures.

The π electron systems are the conventional electron donors to the hydrogen acceptors in hydrogen bonding. Apart from the hydrogen atom, halogens, chalcogens, pnicogens and triel/tetrel atoms can also be envisaged as electron acceptors involving π clouds. Markedly, in pnicogenπ interactions, the bonding of the hypervalent (predominantly pentavalent) state of the phosphorus atom with π electron donors is elusive and can be thought of as an intuitive extension to trivalent phosphorusπ interactions. In this work, on the one hand, POCl ₃ was taken as a prototypical molecule to explore these pentavalent phosphorus interactions and on the other hand, acetylene (C ₂ H ₂ ), ethylene (C ₂ H ₄ ) and benzene (C ₆ H ₆ ), in which phosphorusπ bonding can be expected to compete with hydrogen and halogen bonding interactions, were taken as π electron donors. All three POCl ₃ -C ₂ H ₂ , POCl ₃ -C ₂ H ₄ and POCl ₃ -C ₆ H ₆ heterodimers were experimentally generated at low temperatures in Ar and N ₂ matrices and were characterized by both infrared spectroscopy and state-of-the-art quantum chemical computations. Though hydrogen bonding dominates in POCl ₃ -C ₂ H ₂ and POCl ₃ -C ₂ H ₄ heterodimers, phosphorus bonding plays a definite and non-trivial role in their overall stabilization. An interesting paradigm transformation was noticed in the POCl ₃ -C ₆ H ₆ system, where pentavalent phosphorusπ bonding was observed to completely influence the hydrogen bonding interaction. To further shed light on these Pπ systems, the interaction characteristics were analyzed with the help of electrostatic potential mapping, natural bond orbital and energy decomposition analyses.