The crystal structure and characterization of a co-product in the synthesis of a Schiff base give evidence of its zwitterionic nature in the solid state

Abstract

During the synthesis of the Schiff base 2-methoxy-6-{[(tiophen-2-methyl)imino]methyl}phenol (o-HVATPNH2), a co-product was isolated and characterized. The crystal structure of this molecule, 4,10-dimethoxy-13-(thiophen-2-ylmethyl)-6,12-dihydro-6,12-epiminodibenzo[b,f][1,5]dioxocane, namely (o-VA)₂TPNH2 for short, was determined by X-ray diffraction methods. The chiral compound crystallizes as a racemate in the triclinic space group P with 2 molecules per unit cell. The (o-VA)₂ molecular fragment in (o-VA)₂TPNH2 shows a non-crystallographic two-fold axis that enables the two molecular stereogenic centers to have the same chirality. The formation reaction mechanism for (o-VA)₂TPNH2 is proposed and the structural properties of the Schiff base o-HVATPNH2 were revised. Evidence of the presence of the zwitterionic form of a Schiff base in the solid state was found.