Putting Anion-π Interactions Into Perspective

Supramolecular chemistry is a field of scientific exploration that probes the relationship between molecular structure and function. It is the chemistry of the noncovalent bond, which forms the basis of highly specific recognition, transport, and regulation events that actuate biological processes. The classic design principles of supramolecular chemistry include strong, directional interactions like hydrogen bonding, halogen bonding, and cation-π complexation, as well as less directional forces like ion pairing, π-π, solvophobic, and van der Waals potentials. In recent years, the anion-π interaction (an attractive force between an electron-deficient aromatic π system and an anion) has been recognized as a hitherto unexplored noncovalent bond, the nature of which has been interpreted through both experimental and theoretical investigations. The design of selective anion receptors and channels based on this interaction represent important advances in the field of supramolecular chemistry. The objectives of this Review are 1) to discuss current thinking on the nature of this interaction, 2) to survey key experimental work in which anion-π bonding is demonstrated, and 3) to provide insights into the directional nature of anion-π contact in X-ray crystal structures. As easy as π systems found it to bond with cations, they concealed their attraction to anions until they were spotted together in too many crystal structures for it to be mere coincidence. Today, the anion-π relationship has an established place in the repertory of noncovalent interactions. Extensive theoretical and experimental studies illuminate the physical nature of the interaction and its utility in the design of hosts, and the engineering of solid-state structures. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.