A perspective on the nature of cation-π interactions

Recent years have witnessed a remarkable surge in the study of noncovalent interactions and their role in diverse areas of chemistry, biology, material science and allied fields. Among all, hydrogen bonding is quite extensively studied. Several other noncovalent interactions continue to attract wide attention. Notably, cation-π interactions play a crucial role in the function of several binding sites, reaction mechanisms, self-assemblies, catalytic mechanisms, adhesion and cohesion properties in many biological and chemical systems. The current review focuses on the character of cation-π interactions, its range and its occurrence from a conceptual point of view. The range and nature of cation-π interactions depend on the type of π-system and cation involved, besides solvent and environment. The size and polarizability of the π-systems and the effective nuclear charge on cation and its multiplicity and spin dictate the structural and energetic aspects of cation-π interaction. Further, factors affecting the modulation of strength and nature are brought out in the review. The interplay between the cation-π interaction and the other noncovalent interactions, solvent and counter-ion effects are analysed, and the cooperativity of these forces in organizing supramolecular architectures are discussed. How strong is cation-π interaction depends on various factors: It may be weak, medium or strong depending on the nature of cation, π system and environment. The strength of the interaction was shown to alternate a few orders of magnitude depending on the environment.