Abnormal synergistic effects between Lewis acid–base interaction and halogen bond in F₃B···NCX···NCM

An abnormal synergistic effect was found between the Lewis acid–base interaction and halogen bond in triads F₃B···NCX···NCM (X and M are halogen atoms), where the strong Lewis acid–base interaction between F₃B and NCX has a larger enhancement than the weak halogen bond between NCX and NCM. This is in contrast with the traditional cooperative effect. It is interesting that the alkali-metal substituent as well as the heavier halogen atom play a more remarkable role in the enhancement of the interaction F₃B···NCX than that of NCX···NCM, particularly, the alkali-metal substituent makes the abnormal synergistic effect be the traditional cooperative one.