engleski

Desmotropy, Polymorphism, and Solid-State Proton Transfer : Four Solid Forms of an Aromatic o-Hydroxy Schiff Base

The Schiff base derived from salicylaldehyde and 2-amino-3-hydroxypyridine affords diversity of solid forms, two polymorphic pairs of the enol-imino (D1a and D1b) and keto-amino (D2a and D2b) desmotropes. Isolated phases, identified by IR spectroscopy, X-ray crystallography and 13CP MAS NMR, display essentially planar molecular conformations characterized by strong intramolecular hydrogen bond of the O-H∙∙∙N (D1) or N-H∙∙∙O (D2) type. A change in the proton position within this O∙∙∙H∙∙∙N system is accompanied by substantially different molecular conformations, and subsequently with divergent supramolecular architectures. The appearance and interconversion conditions for each of the four phases have been established on the basis of a number of solution and solvent-free experiments, and evaluated against the results of computational studies. Solid phases readily convert to the most stable form (D1a) upon exposure to methanol vapor, heating or by mechanical treatment, and these transformations are accompanied by the change in the sample color. The course of thermally induced transformations has been monitored in detail by means of temperature-resolved X-ray powder diffraction and infrared spectroscopy. Upon dissolution, all forms equilibrate immediately, as confirmed by NMR and UV-Vis spectroscopies in several solvents, with the equilibrium shifted far towards the enol tautomer. This study reveals the significance of peripheral groups in stabilization of metastable tautomers in the solid state

desmotropy ; polymorphism ; proton transfer ; Schiff bases ; tautomerism

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