Properties of Cationic Pnicogen-Bonded Complexes F_4–<i>n</i>H_<i>n</i>P⁺:N-Base with F–P···N Linear and <i>n</i> = 0–3

Abstract

Ab initio MP2/aug′-cc-pVTZ calculations were performed to investigate the pnicogen-bonded complexes F_4–<i>n</i>H_<i>n</i>P⁺:N-base, for <i>n</i> = 0–3, each with a linear or nearly linear F–P···N alignment. The nitrogen bases include the sp³ 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 vary between −20 and −180 kJ·mol^–1, while the P–N distances vary from 1.89 to 3.01 Å. In each series of complexes, binding energies decrease exponentially as the P–N distance increases, provided that complexes with sp³ and sp hybridized bases are treated separately. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid–base pair is a factor in determining the binding energies of these complexes. Three different charge-transfer interactions stabilize these complexes. These arise from the nitrogen lone pair to the σ*P–F_ax, σ*P–F_eq, and σ*P–H_eq orbitals. The dominant single charge-transfer energy in all complexes is N_lp → σ*P–F_ax. However, since there are three N_lp → σ*P–F_eq charge-transfer interactions in complexes with F₄P⁺ and two in complexes with F₃HP⁺, the sum of the N_lp → σ*P–F_eq charge-transfer energies is greater than the N_lp → σ*P–F_ax charge-transfer energies in the former complexes, and similar to the N_lp → σ*P–F_ax energies in the latter. The total charge-transfer energies of all complexes decrease exponentially as the P–N distance increases. Coupling constants ^1p<i>J</i>(P–N) across the pnicogen bond vary with the P–N distance, but different patterns are observed for complexes with F₄P⁺ and complexes of the sp³ bases with F₃HP⁺. These initially increase as the P–N distance decreases, reach a maximum, and then decrease with decreasing P–N distance as the P···N bond acquires increased covalent character. For the remaining complexes, ^1p<i>J</i>(P–N) increases with decreasing P–N distance. Complexation increases the P–F_ax distance and ¹<i>J</i>(P–F_ax) relative to the corresponding isolated ion. ¹<i>J</i>(P–F_ax) correlates quadratically with the P–N distance