Catalytic activation via pi-backbonding in halogen bonds

The role of halogen bonding (XB) in chemical catalysis has largely involved using XB donors as Lewis acid activators to modulate the reactivity of partner Lewis bases. We explore a more uncommon scenario, where a Lewis base modulates reactivity via a spectator halogen bond interaction. Our computational studies reveal that spectator halogen bonds may play an important role in modulating the rate of S(N)2 reactions. Most notably, pi acceptors such as PF3 significantly decrease the barrier to substitution by decreasing electron density in the very electron rich transition state. Such pi-backbonding represents an example of a heretofore unexplored situation in halogen bonding: the combination of both sigma-donation and pi-backdonation in this "non-covalent" interaction. The broader implications of this observation are discussed.