Hydrogen-bridged chelate ring-assisted pi-stacking interactions

Abstract

A salicylideneaniline (SA) derivative, (6Z)-6-({[2-(hydroxymethyl)phenyl]amino}methylidene)-3,5-dimethoxycyclohexa-2,4-dien-1-one monohydrate, has an increased aromaticity within its hydrogen-bridged chelate ring owing to its NH character. In the reported crystal structure, nonconventional pi-stacking interactions, which are referred to as hybrid pi-stacking interactions, are observed between a quasiaromatic chelate ring, formed as a result of the resonance-assisted intramolecular hydrogen bond and ordinary aromatic rings. Besides, pi-stacking interactions are also seen between two hydrogen-bridged quasiaromatic chelate rings, which are referred to as pure pi-stacking interactions. A CSD search has revealed that both kinds of interactions are frequently observed in molecular crystals of SA derivatives in fully or partially NH tautomeric form, and aromaticity levels of certain fragments of SA derivatives have dramatic effects on their stacking arrangements. These interactions are distinguished from the usual pi center dot center dot center dot pi interactions by their formation character, i.e. both sigma- and pi-deficient and sigma-deficient character of pure interactions is more pronounced than that of the hybrid ones.