Properties of cationic pnicogen-bonded complexes F4-nHnP+:N-base with H–P···N linear and n = 1–4
Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes F4-nHnP+:N-base, for n = 1–4, each with a linear or nearly linear Hax–P···N alignment. The sp3-hybridised nitrogen bases include NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3, and the sp bases are NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. Binding energies increase as the P–N distance decreases, with an exponential curve showing this relationship when complexes with sp3 and sp hybridised bases are treated separately. However, the correlations are not as good as they are for the complexes F4-nHnP+:N-base for n = 0–3 with F–P···N linear. 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 stabilise these complexes, namely Nlp→σ*P–Hax, Nlp→σ*P–Feq, and Nlp→σ*P–Heq. Unlike the corresponding complexes with F–P···N linear, Nlp→σ*P–Hax is not always the dominant charge-transfer interaction, since Nlp→σ*P–Feq is greater in some complexes. Nlp→σ*P–Heq makes the smallest contribution to the total charge-transfer energy. The total charge-transfer energies of all complexes increase exponentially as the P–N distance decreases in a manner very similar to that observed for the series of complexes with F–P···N linear.
Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin–spin coupling constants 1pJ(P–N) across the pnicogen bond vary with the P–N distance, but different patterns are observed which depend on the nature of the acid, and for some acids, on the hybridisation of the nitrogen base. 1pJ(P–N) values for complexes of F3HP+ 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. 1pJ(P–N) for complexes with H–P···N linear and those with F–P···N linear exhibit similar distance dependencies depending on the number of F atoms in equatorial positions and the hybridisation of the base. Complexation may increase, decrease, or leave the P–Hax distance unchanged, but1J(P–Hax) always decreases relative to the corresponding isolated ion. Decreasing 1J(P–Hax) can be related to decreasing intermolecular P–N distance.