Interplay of F-H•••F Hydrogen Bonds and P•••N Pnicogen Bonds

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the influence of F-H center dot center dot center dot F hydrogen bonds on the P center dot center dot center dot N pnicogen bond in complexes nFH:(H2FP:NFH2) for n = 1-2, and a selected complex with n = 3. The NBO analysis indicates that the N(lp) -> P-F sigma* charge-transfer transition has a much greater stabilizing effect than the P(lp) -> N-F sigma* transition. When hydrogen bonding occurs at P-F, charge transfer associated with the pnicogen bond and the hydrogen bond are in the same direction but are in opposite directions when hydrogen bonding occurs at N-F. As a result, the formation of F-H center dot center dot center dot F hydrogen bonds at P-F leads to shorter P center dot center dot center dot N distances, increased strength of P center dot center dot center dot N bonds, and synergistic energetic effects; hydrogen bonding at N-F has opposite effects. P-31 and N-15 chemical shieldings do not correlate with charges on P and N, respectively, but P-31 shieldings correlate quadratically with the P-N distance. (lp)J(P-N) coupling constants do not correlate with the intermolecular P-N distance. However, when hydrogen bonding occurs only at P-F, (lp)J(P-N) decreases in absolute value as the P-N distance decreases, thereby approaching (1)J(P-N) for H2P-NH2. However, the P center dot center dot center dot N bond in 3FH:(H2FP:NFH2) has little covalent character, unlike the P center dot center dot center dot P bond in the corresponding complex 3FH:(PH2F)(2).