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 H2 XP:ClF:ClH and H2 XP:ClF:ClF, for X = F, Cl, H, NC, and CN. The PCl bonds remain chlorine-shared halogen bonds in the ternary complexes H2 XP: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 H2 XP: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 H2 XP: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 (H2 XP-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 H2 XP:ClF:ClF complexes are significantly greater than the binding energies of the H2 XP:ClF:ClH complexes, and nonadditivities exhibit large synergistic effects. The Wiberg bond indexes for the complexes H2 XP:ClF, H2 XP:ClF:ClH, and H2 XP:ClF:ClF, and the cations (H2 XP-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, 1x J(P-Cl) in H2 XP:ClF complexes becomes 1 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 H2 XP:ClF:ClH become chlorine-transferred halogen bonds in H2 XP:ClF:ClF. 1x J(F-Cl) coupling constants also reflect changes in the nature of FCl halogen bonds.