Phosphorus as a simultaneous electron-pair acceptor in intermolecular P⋯N pnicogen bonds and electron-pair donor to lewis acids

Abstract

Ab initio MP2/aug'-cc-pVTZ calculations have been performed to investigate the structures and energies of binary complexes LA:PH2F and LA:PH3 and of ternary complexes LA:H2FP:NFH2 and LA:H3P:NH3 in which the pnicogen-bonded P atom also acts as an electron-pair donor to a Lewis acid (LA), for LA = BH3, NCH, ClH, FH, FCl, and HLi. Hydrogen bonds, halogen bonds, and dative covalent bonds are found at P in some cases, depending on the nature of the Lewis acid. HLi forms a lithium bond with P only in the binary complex HLi:PH3. The binding energies of ternary complexes exhibit a classical synergistic effect, although the computed cooperativity may be overestimated due to neglect of the interaction of the Lewis acid with NH2F or NH3 in some cases. The hydrogen-bonding Lewis acids appear to have little effect on the strength of the P⋯N bond, while the remaining Lewis acids strengthen the pnicogen bond. 31P absolute chemical shieldings increase in LA:H2FP:NFH2 complexes relative to the corresponding LA:PH2F complexes as the positive charge on P decreases, while chemical shieldings decrease in LA:H3P:NH3 relative to the corresponding LA:PH3 complexes as the positive charge increases. Absolute values of 1pJ(P-N) spin-spin coupling constants in complexes LA:H2FP:NFH2 decrease as the P-N distance decreases. It appears that this behavior is associated with the presence of a second intermolecular interaction, whether electron-donation by P or hydrogen bond formation at P-F.