Imine-based Zwitterion: Synthesis, single-crystal characterization, and computational investigation

Abstract

The 2‑hydroxy-1-naphthaldehyde based Schiff base zwitterions: (E)-1-(((4-methoxyphenyl)-iminio)methyl)naphthalen-2-olate (3a) and (E)-1-(((2‑methoxy-4-nitrophenyl)iminio)methyl)-naphthalen-2-olate (3b) have been obtained from the condensation reaction of 2‑hydroxy-1-naphthaldehyde and substituted aniline. The structures of these Schiff base zwitterions have been verified unambiguously by the single-crystal X-ray diffraction analysis. The SC-XRD (single crystal X-ray diffraction) exploration revealed that both compounds have same crystal system, i.e., monoclinic, and same space group, i.e., P21/c. Also, there are intermolecular attractive forces that are responsible for the stabilization of these organic frameworks. Additionally, these structures have been thoroughly explored by the density functional theory (DFT) calculations. The optimized geometries were found to be close to the experimental structures. The analysis of frontier molecular orbitals (FMOs) suggested that 3a would be more stable towards redox reactions than 3b. Therefore, modifying the imine-zwitterion structure with the nitro group and changing the position of methoxy group would increase the reactivity of the resulting species. This is further supported by the computed global reactivity parameters (GRPs): both compounds should be considered as noticeably stable, both from thermodynamic and kinetic points of view, with 3a showing even higher stability. The compound 3b was shown to be more reactive in redox processes, being more prone to be reduced rather than oxidized. The higher stability of 3a was further supported by the NBO analysis results.