Thermochromism of Salicylideneanilines in Solution:  Aggregation-controlled Proton Tautomerization

Abstract

Many salicylideneanilines have been known to exhibit thermochromism in the solid state. In contrast, in the solution, the thermochromism has rarely been observed. This paper reveals that salicylideneanilines are generally thermochromic in the solution of hydrocarbon solvents and that the proton tautomerization, which is the origin of the thermochromism, is controlled by the aggregation of molecules. This is evidenced by the electronic absorption spectra of salicylideneaniline ( 1), N-(5-chloro-2-hydroxybenzylidene)aniline (2), N-(2-hydroxy-4-methoxybenzylidene)aniline (3), N-(2-hydroxy-5-nitrobenzylidene)aniline (4), N-(2-hydroxy-5-nitrobenzylidene)-2,6-dimethylaniline (5), and N-(3,5-di-tert-butyl-2-hydroxybenzylidene)aniline (6) in isopentane or the mixture of isopentane and methylcyclohexane (volume ratio ) 3:1) at different temperatures between 297 and 77 K. They exhibited a remarkable spectral change with lowering temperature, except for 6. At 297 K the OH form is exclusively populated. When the temperature is lowered, the absorption bands of the OH form decrease in intensity and new bands that are assigned to the NH form emerge. At 77 K, the OH form disappears and the NH form is exclusively populated. The spectra of 6 do not change with lowering of the temperature.

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

“…Similar trends are found in other quinoid NH tautomers of naphthylideneamines or salicylideneamines [17,20,[44][45][46].…”

“…Moreover, Ogawa and co-workers indicate that the NH form can also be stabilized in solution (of isopentane or an isopentane/methylcyclohexane mixture) by similar electrostatic interactions as in the solid state [17]. Apparently the tautomeric equilibrium in Schiff-bases is not an exclusive crystal structure property, since aggreagtion-controlled proton tautomerization can be found in solution, too.…”

First-principles study of salicylideneaniline molecular crystals: Tautomerization reaction involving intermolecular hydrogen bonds

“…1͑b͒ and 1͑c͒, respectively͔, so that the NH form in crystalline phase can be considerably stabilized by electrostatic intermolecular interactions in the polar form of the zwitterion; Ogawa et al first claimed that the NH form is predominantly zwitterionic rather than quinoidal in some salicylideneaniline crystals from observation of the bond length determined by the x-ray analysis. [9][10][11] Thus the similar mechanism may be supposed in the present system. As will be discussed later, however, the information on the bond lengths only cannot always explain all the chemical properties of this system; in fact, the experimental NH form appeared neither a quinoidal nor zwitterionic form.…”

Bis[4-(salicylideneamino)phenyl]methane

“…The factors determining whether a given molecule will occur in the keto±amine or enol±imino form are manifold. It has been shown to depend on the substitution on the benzene rings (Filarowski et al, 2002;Popovic  et al, 2002) and intermolecular hydrogen bonding (Ogawa et al, 1998;Ogawa & Harada, 2003), and aggregation (packing) of the molecules plays an important role for the equilibrium in solution (Ogawa et al, 2001;Ogawa & Harada, 2003 The molecular structure of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.…”