Pnicogen-Bonded Complexes H_<i>n</i>F_5–<i>n</i>P:N-Base, for <i>n</i> = 0–5

Abstract

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out on the pnicogen-bonded complexes H_<i>n</i>F_5–<i>n</i>P:N-base, for <i>n</i> = 0–5 and nitrogen bases NC^–, NCLi, NP, NCH, and NCF. The structures of these complexes have either <i>C</i>_4<i>v</i> or <i>C</i>_2<i>v</i> symmetry with one exception. P–N distances and interaction energies vary dramatically in these complexes, while F_ax–P–F_eq angles in complexes with PF₅ vary from 91° at short P–N distances to 100° at long distances. The value of this angle approaches the F_ax–P–F_eq angle of 102° computed for the Berry pseudorotation transition structure which interconverts axial and equatorial F atoms of PF₅. The computed distances and F_ax–P–F_eq angles in complexes F₅P:N-base are consistent with experimental CSD data. For a fixed acid, interaction energies decrease in the order NC^– > NCLi > NP > NCH > NCF. In contrast, for a fixed base, there is no single pattern for the variations in distances and interaction energies as a function of the acid. This suggests that there are multiple factors that influence these properties. The dominant factor appears to be the number of F atoms in equatorial positions, and then a linear F_ax–P···N rather than H_ax–P···N alignment. The acids may be grouped into pairs (PF₅, PHF₄) with four equatorial F atoms, then (PH₄F, PH₂F₃) with F_ax–P···N linear, and then (PH₃F₂ and PH₅) with H_ax–P···N linear. The electron-donating ability of the base is also a factor in determining the structures and interaction energies of these complexes. Charge transfer from the N lone pair to the σ* P–A_ax orbital stabilizes H_<i>n</i>F_5–<i>n</i>P:N-base complexes, with A_ax either F_ax or H_ax. The total charge-transfer energies correlate with the interaction energies of these complexes. Spin–spin coupling constants ^1p<i>J</i>(P–N) for (PF₅, PHF₄) complexes with nitrogen bases are negative with the strongest bases NC^– and NCLi but positive for the remaining bases. Complexes of (PH₄F, PH₂F₃) with these same two strong bases and H₄FP:NP have positive ^1p<i>J</i>(P–N) values but negative values for the remaining bases. (PH₅, PH₃F₂) have negative values of ^1p<i>J</i>(P–N) only for complexes with NC^–. Values of ¹<i>J</i>(P–F_ax) and ¹<i>J</i>(P–H_ax) correlate with the P–F_ax and P–H_ax distances, respectively