Schiff Bases from TRIS and ortho-hydroxyarenecarbaldehydes: structures and tautomeric equilibria in the solid state and in solution

Three Schiff bases generated by conventional condensation of a simple aminopolyol (TRIS) and 5-bromo- and 5-iodosalicylaldehyde, and 2-hydroxy-1-naphthaldehyde, have been fully characterized, both in solution and in the solid state. This study provides a complete analysis of imine–enamine equilibria, and sheds light onto the mechanism of hydrogen transfer, which has been controversial in the literature. Low-temperature X-ray diffraction accompanied by electron-density maps have further been complemented by theoretical studies at the B3LYP/6-31G* and M06-2X/6-311++G** levels. To ascertain the influence of crystal packing on tautomeric stability, the lattice has also been simulated by computation. This protocol involves the assessment of a supramolecular cluster around a core tautomer possessing either imine or enamine structures. Such an analysis, in full agreement with solid-state data, reveals the greater stability of zwitterionic structures for the salicyl derivatives. In contrast, these substances show preferential imine forms in solution, whereas the naphthyl-based compound exhibits a prevalent keto-enamine structure in all case