Can HNNH, FNNH, or HNCHOH bridge the σ-hole and the lone pair at P in binary complexes with H2XP, for X = F, Cl, NC, OH, CN, CCH, CH3, and H?

Abstract

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the properties of complexes formed between H₂XP, for X = F, Cl, NC, OH, CN, CCH, CH₃, and H, and the possible bridging molecules HNNH, FNNH, and HNCHOH. H₂XP:HNNH and H₂XP:FNNH complexes are stabilized by P⋯N pnicogen bonds, except for H₂(CH₃)P:FNNH and H₃P:FNNH which are stabilized by N–H⋯P hydrogen bonds. H₂XP:HNCHOH complexes are stabilized by P⋯N pnicogen bonds and nonlinear O–H⋯P hydrogen bonds. For a fixed H₂XP molecule, binding energies decrease in the order HNCHOH > HNNH > FNNH, except for the binding energies of H₂(CH₃)P and H₃P with HNNH and FNNH. Binding energies of complexes with HNCHOH and HNNH increase as the P–N₁ distance decreases, but binding energies of complexes with FNNH show little dependence on this distance. The large binding energies of H₂XP:HNCHOH complexes arise from a cooperative effect involving electron-pair acceptance by P to form a pnicogen bond, and electron-pair donation by P to form a hydrogen bond. The dominant charge-transfer interaction in these complexes involves electron-pair donation by N across the pnicogen bond, except for complexes in which X is one of the more electropositive substituents, CCH, CH₃, and H. For these, lone-pair donation by P across the hydrogen bond dominates. AIM and NBO data for these complexes are consistent with their bonding characteristics, showing molecular graphs with bond critical points and charge-transfer interactions associated with hydrogen and pnicogen bonds. EOM-CCSD spin–spin coupling constants ^1pJ(P–N) across the pnicogen bond for each series of complexes correlate with the P–N distance. In contrast, ^2hJ(O–P) values for complexes H₂XP:HNCHOH do not correlate with the O–P distance, a consequence of the nonlinearity of these hydrogen bonds.