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 center dot center dot center dot N and H-ax-P center dot center dot center dot N, respectively. The bases include the sp(3)-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 N-2. 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 angstrom. There is a gap in the P-N distances between 2.25 and 2.53 angstrom 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 center dot center dot center dot N bonds with varying degrees of covalent character, whereas outer complexes are stabilized by intermolecular P center dot center dot center dot 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 sigma*P-F-ax orbital. In addition, there are three other charge-transfer interactions from the lone pair on N to the sigma*P-F-eq orbitals, which taken together, are more stabilizing than the interaction involving sigma*P-F-ax. In contrast, the dominant charge-transfer interaction for complexes F3HP+:N-base is from the lone pair on N to the sigma*P-F-eq orbitals. Computed EOM-CCSD Fermi-contact terms are excellent approximations to the total spin-spin coupling constants (1pJ)(P-N) and (1)J(P-H-ax), but are poor approximations to (1)J(P-F-ax). (1p)J(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. (1J)(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 (1J)(P-H-ax) reflect the hybridization of the nitrogen base and whether the complex is an inner or outer complex.