Using one halogen bond to change the nature of a second bond in ternary complexes with PCl and FCl 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 H₂XP:ClF:ClH and H₂XP:ClF:ClF, for X = F, Cl, H, NC, and CN. The PCl bonds remain chlorine-shared halogen bonds in the ternary complexes H₂XP:ClF:ClH, although the degree of chlorine sharing increases relative to the corresponding binary complexes. The FCl bonds in the ternary complexes remain traditional halogen bonds. The binding energies of the complexes H₂XP: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 H₂XP: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 PCl halogen bond to produce complexes represented as (H₂XP-Cl)⁺:^-(F:ClF). In the ion-pair, the cation is also halogen bonded to the anion by a ClF^- halogen bond, while the anion is stabilized by an ^-FCl halogen bond. The central ClF molecule no longer exists as a molecule. The binding energies of the ternary H₂XP:ClF:ClF complexes are significantly greater than the binding energies of the H₂XP:ClF:ClH complexes, and nonadditivities exhibit large synergistic effects. The Wiberg bond indexes for the complexes H₂XP:ClF, H₂XP:ClF:ClH, and H₂XP:ClF:ClF, and the cations (H₂XP-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, ^1xJ(P-Cl) in H₂XP:ClF complexes becomes ¹J(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 H₂XP:ClF:ClH become chlorine-transferred halogen bonds in H₂XP:ClF:ClF. ^1xJ(F-Cl) coupling constants also reflect changes in the nature of FCl halogen bonds.