Using one halogen bond to change the nature of a second bond in ternary complexes with P⋯Cl and F⋯Cl halogen bonds

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to determine the effect of the presence of one halogen bond on the nature of the other in ternary complexes HXP:ClF:ClH and HXP:ClF:ClF, for X = F, Cl, H, NC, and CN. The P⋯Cl bonds remain chlorine-shared halogen bonds in the ternary complexes HXP:ClF:ClH, although the degree of chlorine sharing increases relative to the corresponding binary complexes. The F⋯Cl bonds in the ternary complexes remain traditional halogen bonds. The binding energies of the complexes HXP:ClF:ClH increase relative to the corresponding binary complexes, and nonadditivities of binding energies are synergistic. In contrast, the presence of two halogen bonds in the ternary complexes HXP:ClF:ClF has a dramatic effect on the nature of these bonds in the four most strongly bound complexes. In these, chlorine transfer occurs across the P⋯Cl halogen bond to produce complexes represented as (HXP-Cl):(F:ClF). In the ion-pair, the cation is also halogen bonded to the anion by a Cl⋯F halogen bond, while the anion is stabilized by an F⋯Cl halogen bond. The central ClF molecule no longer exists as a molecule. The binding energies of the ternary HXP:ClF:ClF complexes are significantly greater than the binding energies of the HXP:ClF:ClH complexes, and nonadditivities exhibit large synergistic effects. The Wiberg bond indexes for the complexes HXP:ClF, HXP:ClF:ClH, and HXP:ClF:ClF, and the cations (HXP-Cl) reflect the changes in the P-Cl and Cl-F bonds. Similarly, EOM-CCSD spin-spin coupling constants are also consistent with the changes in these same bonds. In particular, J(P-Cl) in HXP:ClF complexes becomes J(P-Cl) in the ternary complexes with chlorine-transferred halogen bonds. A plot of these coupling constants shows a change in the curvature of the trendline as chlorine-shared halogen bonds in HXP:ClF:ClH become chlorine-transferred halogen bonds in HXP:ClF:ClF. J(F-Cl) coupling constants also reflect changes in the nature of F⋯Cl halogen bonds.
This work was carried out with financial support from the Ministerio de Economía y Competitividad (Project No. CTQ2015-63997-C2-2-P) and Comunidad Autónoma de Madrid (S2013/MIT2841, Fotocarbon). Thanks are also given to the Ohio Supercomputer Center and CTI (CSIC) for their continued computational support.