P···N Pnicogen Bonds in Cationic Complexes of F₄P⁺ and F₃HP⁺ with Nitrogen Bases

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out on cationic pnicogen-bonded complexes F₄P⁺:N-base and F₃HP⁺:N-base, with linear F_ax–P···N and H_ax-P···N, respectively. The bases include the sp³-hybridized nitrogen bases NH₃, NClH₂, NFH₂, NCl₂H, NCl₃, NFCl₂, NF₂H, NF₂Cl, and NF₃, and the sp bases NCNH₂, NCCH₃, NP, NCOH, NCCl, NCH, NCF, NCCN, and N₂. The binding energies of these complexes span a wide range, from −15 to −180 kJ mol^–1, as do the P–N distances, which vary from 1.89 to 3.11 Å. There is a gap in the P–N distances between 2.25 and 2.53 Å in which no complexes are found. Thus, the equilibrium complexes may be classified as inner or outer complexes based on the value of the P–N distance. Inner complexes have P···N bonds with varying degrees of covalent character, whereas outer complexes are stabilized by intermolecular P···N bonds with little or no covalency. Charge-transfer stabilizes these pnicogen-bonded complexes. For complexes F₄P⁺:N-base, the dominant charge-transfer interaction is from the lone pair on N to the σ*P–F_ax orbital. In addition, there are three other charge-transfer interactions from the lone pair on N to the σ*P–F_eq orbitals, which taken together, are more stabilizing than the interaction involving σ*P–F_ax. In contrast, the dominant charge-transfer interaction for complexes F₃HP⁺:N-base is from the lone pair on N to the σ*P–F_eq orbitals. Computed EOM-CCSD Fermi-contact terms are excellent approximations to the total spin–spin coupling constants ^1p<i>J</i>(P–N) and ¹<i>J</i>(P–H_ax), but are poor approximations to ¹<i>J</i>(P–F_ax). ^1p<i>J</i>(P–N) values increase with decreasing P–N distance, approach a maximum, and then decrease and change sign as the P–N distance further decreases and the pnicogen bond acquires increased covalency. ¹<i>J</i>(P–F_ax) values for F₄P⁺:N-base complexes increase with decreasing distance. Although the P–H_ax distance changes very little in complexes F₃HP⁺:N-base, patterns exist which suggest that changes in ¹<i>J</i>(P–H_ax) reflect the hybridization of the nitrogen base and whether the complex is an inner or outer complex.