title: Desmotropy, polymorphism, and solid-state proton transfer: four solid forms of an aromatic o-hydroxy Schiff base.
abstract: The Schiff base derived from salicylaldehyde and 2-amino-3-hydroxypyridine affords a diversity of solid forms, two polymorphic pairs of the enol-imino (D1 a and D1 b) and keto-amino (D2 a and D2 b) desmotropes. The isolated phases, identified by IR spectroscopy, X-ray crystallography, and (13)C cross-polarization/magnetic angle spinning (CP/MAS) NMR spectroscopy, display essentially planar molecular conformations characterized by strong intramolecular hydrogen bonds of the O-H⋅⋅⋅N (D1) or N-H⋅⋅⋅O (D2) type. A change in the position of the proton within this O⋅⋅⋅H⋅⋅⋅N system is accompanied by substantially different molecular conformations and, subsequently, by 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 into the most stable form (D1 a) upon exposure to methanol vapor, heating, or by mechanical treatment, and these transformations are accompanied by a change in the color of the sample. The course of thermally induced transformations has been monitored in detail by means of temperature-resolved powder X-ray diffraction and infrared spectroscopy. Upon dissolution, all forms equilibrate immediately, as confirmed by NMR and UV/Vis spectroscopy in several solvents, with the equilibrium shifted far towards the enol tautomer. This study reveals the significance of peripheral groups in the stabilization of metastable tautomers in the solid state.
authors.affiliation: Department of Chemistry, Faculty of Science, University of Zagreb, Croatia. mirta@chem.pmf.hr
authors.forename: Mirta
authors.initials: M
authors.lastname: Rubčić
authors.forename: Krunoslav
authors.initials: K
authors.lastname: Užarević
authors.forename: Ivan
authors.initials: I
authors.lastname: Halasz
authors.forename: Nikola
authors.initials: N
authors.lastname: Bregović
authors.forename: Momir
authors.initials: M
authors.lastname: Mališ
authors.forename: Ivica
authors.initials: I
authors.lastname: Dilović
authors.forename: Zoran
authors.initials: Z
authors.lastname: Kokan
authors.forename: Robin S
authors.initials: RS
authors.lastname: Stein
authors.forename: Robert E
authors.initials: RE
authors.lastname: Dinnebier
authors.forename: Vladislav
authors.initials: V
authors.lastname: Tomišić
mesh_terms: D000447:Aldehydes / Q000138:chemical synthesis / Q000737:chemistry*; D000631:Aminopyridines / Q000138:chemical synthesis / Q000737:chemistry*; D003460:Crystallization; D018360:Crystallography, X-Ray; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D011522:Protons; D012545:Schiff Bases / Q000138:chemical synthesis / Q000737:chemistry*; D013055:Spectrophotometry, Infrared
chemical_list: D000447:Aldehydes; D000631:Aminopyridines; D011522:Protons; D012545:Schiff Bases; C013243:salicylaldehyde; C531569:2-amino-3-hydroxypyridine