Nitroaniline Derivatives of 2-Oxo-1-naphthylideneamines—Molecular Self-Assembling via C—H⋯O Intermolecular Hydrogen Bonds and Stabilization of O—H⋯N and N—H⋯O Tautomers in Solution and Solid State

Abstract

Three Schiff-bases of the Ar—CH—N—Ar type were synthesized from 2-hydroxy-1-naphthaldehyde and o- (1), m- (2), and p-nitroaniline (3) in order to investigate the shift of the keto-amine/enol-imine tautomeric equilibrium that arises by the introduction of the electron-withdrawal nitro group. The compounds have been investigated experimentally, in the solid state and in the solution, by IR, NMR, UV spectroscopy, and X-ray single crystal analysis, and theoretically by using density functional theory (DFT). The tautomeric equilibrium is strongly shifted towards the keto-amine form despite of the nitro group position at N-phenyl ring in the solid state of molecular structures of 1, 2, and 3 (especially indicated by C—O and C—N bond distances). The keto-amine tautomeric form of the compounds is characterized by a strong, resonance assisted hydrogen bond (RAHB) of the N—H⋯ O type (the N⋅ ⋅ ⋅ O distances are 2.530(8), 2.554(2) and 2.555(5) A in 1, 2, and 3, respectively). The molecules are assembled in the crystalline state into a 3D HB network (1), centrosymmetrical dimers (2) or infinite chains (3) by the C—H⋯ O intermolecular hydrogen bonds. In contrast, UV/VIS spectra of the chloroform solution of the title compounds reveal predominance of the enol-imine form. The molecular geometries and relative stabilities of the compounds determined by using DFT find good agreement with the experimental data.