Influence of Hydrogen Bonds on the P···P Pnicogen Bond.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the influence of F-H···F hydrogen bonds on the P···P pnicogen bond in complexes nFH:(PH2F)2 for n = 1-3. The formation of F-H···F hydrogen bonds leads to a shortening of the P-P distance, a lengthening of the P-F distance involved in the hydrogen bond, a strengthening of the P···P interaction, and changes in atomic populations, NMR (31)P chemical shieldings, and (1p)J(P-P) coupling constants. The magnitude of these changes depends on the number of FH molecules and their positions in the complex and are relatively modest except for complexes 2FH:(PH2F)2 and 3FH:(PH2F)2 that have all FH molecules hydrogen bonded to the same F-atom. For these two complexes, (1p)J(P-P) decreases as the P-P distance decreases and approaches the value of (1)J(P-P) for P2H4. The dramatic changes in these two complexes reflect the changing nature of the hydrogen bonds and the pnicogen bond. Thus, the complex 3FH:(PH2F)2 acquires ion-pair character represented as [3(FH)F(-):(H2P-PH2F)(+)], and the P···P pnicogen bond acquires significant covalent character. These changes are observed to a lesser extent in 2FH:(PH2F)2.