Through-resonance assisted ionic hydrogen bonding in 5-nitro-N-salicylideneethylamine

Structural and spectroscopic properties of hydrogen bonding in solid 5-nitro-N-salicylideneethylamine have been investigated. This is the first example of ionic [O-... H-N+] intramolecular hydrogen bonding in a structure of Schiff base. Single-crystal X-ray diffraction and C-13 magic-angle spinning NMR show that the title molecule is dominated by ionic canonical structures favored by through molecule conjugation between the nitro group and both groups of salicylidene fragment. Being involved in strong intramolecular [O-... H-N+] hydrogen bonding, 5-nitro-N-salicylideneethylamine forms its crystal lattice by means of different types of weak intermolecular [O-... H-N+], N-H ... O and C-H ... O hydrogen bonds. On the basis of the solid-state NMR results, it has been suggested that the acidic proton can also stay at the oxygen atom, and this is qualitatively supported by X-ray diffraction.