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Polymorphs of 1‑(5-Methylthiazol-2-yl)-3-phenylthiourea and Various Anion-Assisted Assemblies of Two Positional Isomers

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posted on 2014-05-07, 00:00 authored by Nithi Phukan, Jubaraj B. Baruah
Orientations of the phenyl group and the intramolecular hydrogen bond play prime roles in the packing patterns of three conformational polymorphs of an unsymmetrical thiourea derivative, 1-(5-methylthiazol-2-yl)-3-phenylthiourea (PTH1). Self-assembly of each polymorph is composed of hydrogen-bonded dimeric motifs held together in head-to-tail arrangement but packed in different manners. Each has an intramolecular N–H···N hydrogen bond between an amide N–H and the nitrogen atom of the 5-methylthiazole unit. The packing pattern of 1-(4-methylthiazol-2-yl)-3-phenylthiourea (PTH2) is composed of dimeric assemblies of PTH2 in head-to-tail fashion. PTH2 is monomorphic as there are intermolecular C–H···S interactions between a C–H bond of the phenyl ring of each molecule and the sulfur atom of the thiocarbonyl group of a neighboring molecule. Such interactions lock the orientation of phenyl group in the solid state. The syn–anti conformation across the thiourea group, originally present in the positional isomers PTH1 and PTH2, is invariably transformed to syn–syn conformation in their salts. The extent of hydration of anions in the salts of PTH1 or PTH2 is dependent on the cation as well as the anion. The chloride salt of PTH1 has a large difference in packing patterns in comparison with the corresponding chloride salt of PTH2; they also differ in the numbers of symmetry-nonequivalent molecules in their respective unit cells. The anhydrous salt of PTH1 with hydrogen bromide having a 1:1 ratio of cation and anion is formed, whereas the bromide salt of PTH2 is a hydrate of composition (HPTH2)2(Br)2·6H2O. This salt has bromide–water clusters in its crystal lattice. Nitric acid reacts with PTH1 under different conditions to form hydrated or anhydrous salts. The hydrated salt (HPTH1)2(NO3)2·H2O has anions bridged by water molecules. The anhydrous nitrate salts of PTH1 and PTH2 are structurally similar in having nitrate···nitrate interactions. The deprotonation of polyacids by PTH1 and PTH2 is selective. The ability to abstract a proton from sulfuric acid to form crystalline salts by PTH1 and PTH2 differs. The sulfate salt (HPTH1)2(SO4) is formed by reaction of sulfuric acid with PTH1, but PTH2 forms the bisulfate salt (HPTH2)­HSO4·H2O. PTH1 forms the corresponding dihydrogen phosphate salt upon reaction with orthophosphoric acid; the dihydrogen phosphate anions are held together in the form of cyclic hydrogen-bonded hexameric assemblies in the lattice. Water loss from the assemblies of hydrated salt was determined by thermogravimetry and differential scanning calorimetry and showed that dehydration from anion-assisted assemblies was guided by the cationic host and the type of assembly.

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