Intramolecular hydrogen bonding in N-salicylideneanilines. X-ray diffraction and solid-state NMR studies

The crystal structures of four Schiff bases: N-(3,5-dichlorosalicylidene)-4-dimethylaminoaniline, N-(3,5-dichlorosalicylidene)-4-diethylaminoaniline, N-(3-nitrosalicylidene)-4-diethylaminoaniline and N-(2- hydroxynaphthylidene)-4-dimethylaminoaniline have been determined by single-crystal X-ray diffraction and solid-state NMR. We discuss the relationship between the variation of the hydrogen bonding on the one hand, and the geometry and electron density distribution in the N-salicylideneaniline fragment on the other. Mesomerism involving ionic structures is the dominant effect in changing the geometric parameters of the fragment. The concentration of ionic structures correlates very well with the C—O bond length and other structural parameters.