Nithi Phukan

Imine-tautomers of aminothiazole derivatives: intriguing aspects of chemical reactivities

Intriguing reactivities of positional isomers 1-(5-methylthiazol-2-yl)-3-(4-nitrophenyl)thiourea (L1) and 1-(4-methylthiazol-2-yl)-3-(4-nitrophenyl)thiourea (L2) towards environment polluting ions such as mercuric, fluoride, bromide and solvents provide scope to study various self-assemblies formed under different conditions and allied applications. The solid state structures of L1 and L2 revealed that both adopt the imine form. However, the number of symmetry independent molecules in asymmetric units as well as the packing patterns of these two compounds have distinguishable features. The asymmetric unit of L1 has three symmetry non-equivalent molecules. The packing patterns of these two positional isomers differ as L1 has two arrays of dimeric self-assemblies in contrast to the single array found in the packing of L2. The reactivity of L1 and L2 with solvents was explored through crystal structure elucidation. The compound L1 forms solvates with N,N′-dimethylacetamide or dimethylsulphoxide. The structural analysis of solvates of L1 provided avenues to study conformational adjustments. An unusual example of a metastable solvate of L1 with N,N′-dimethylacetamide was isolated. A ring-opening reaction of 1,8-diazabicyclo [5.4.0] undec-7-ene with L1 has been observed. The compound L2 underwent hydrolysis cum bromination with hydrobromic acid in dimethylsulphoxide. The reactivities of L1 and L2 towards mercuric and fluoride ions contributed to chemodosimetric signal transductions. The study establishes the impact of subtle changes in structure, reaction and crystallization conditions, which can influence pre-designed crystal engineering.

Conformational adjustments over synthons of urea and thiourea based assemblies

Conformational adjustments in 1-(n-methylthiazol-2-yl)-3-naphthalen-1-yl-thiourea (n = 4, 5) through carbon–nitrogen bond rotation over an intramolecular hydrogen bonded synthon were established. Isosteric homodimeric hydrogen bonded synthons were analysed in their respective self-assemblies. The corresponding urea derivatives, namely, 1-(n-methylthiazol-2-yl)-3-naphthalen-1-yl-urea (n = 4, 5) also had similar homodimeric sub-assemblies in each of their respective assemblies. However, these two positional isomers of a urea derivative did not show polymorphism, which was observed in the corresponding thiourea counterpart. On the other hand, 1-(5-methylthiazol-2-yl)-3-naphthalen-1-yl-thiourea underwent a facile cyclisation reaction, whereas 1-(5-methylthiazol-2-yl)-3-naphthalen-1-yl-urea formed hydrated salts upon reaction with different inorganic acids. Self-assembly of chloride salt had a bifurcated hydrogen bond between two N–H of the urea moiety with chloride ion, whereas perchlorate salt had a similar bond with the oxygen atom of a water of crystallization, making a difference in the type of heterosynthon. Thus, heterosynthons of these salts were found to be anion dependent. Each salt has a cation differing in conformational adjustments guided by anions. Multi-component hydrated and anhydrous crystals of 1-(5-methylthiazol-2-yl)-3-naphthalen-1-yl-urea with tetrabutylammonium chloride were formed concomitantly. These are examples of anion-guided assemblies of thiazole based compounds containing a neutral host. The different naphthyl group orientations of the heterosynthon resulted in two polymorphic hydrated cocrystals.

A supramolecular assembly and complexes of zinc 2-hydoxy-3-naphthoate

Synthesis and characterisation of a cyclic binuclear five-coordinated complex [Zn(HA)2(22′bpy)]2 (HA = 2-hydroxy-3-naphthoate; 22′bpy = 22′-bipyridine) and a supramolecular assembly of two neutral zinc complexes namely [Zn(HA)2(phen)]·[Zn(HA)2(phen)(H2O)]·H2O (phen = 1,10-phenanthroline) are presented. Unsymmetrical cleavage of a cyclic dinuclear zinc intermediate complex is proposed as a plausible intermediate to form the supramolecular assembly. A four-coordinated helical zinc coordination polymer with a composition [Zn(HA)2(pep)]n which is derived from 4-(2-(pyridine-4-yl)propyl)pyridine (pep) as connecting ligands between zinc ions is presented.

Solvent and anion facilitated conformational changes in benzylamine substituted thiazolamine

Conformational adjustments in the solid state structures of the syn–anti–syn form of N,N′-(1,4-phenylene bis(methylene))-bis(5-methylthiazol-2-amine) (L) are studied. Changes are attributed to the rotation of the C–N bonds facilitated by different solvents and anions. Two conformational polymorphs of L, one having molecules with S-like geometry and the other with l-like geometry, were crystallized from solutions of L in different solvents. The crystalline salts of the compound L have anti orientation of the thiazole units across the phenylene ring but the extent of conformational adjustments depends on the anions. Conformational adjustments of the organic [H2L]2+ cation in each salt, [H2L]Br2·2H2O (1), [H2L](NO3)2 (2), and [H2L](H2PO4)2·2H3PO4 (3), provide different orientations to methylaminothiazole units. The organocation of the bromide and biphosphate salts has protonated L in S-like geometries, whereas the nitrate salt has cations in l-like geometry. Exceptionally, nitrate ions cause ∼175° twist with respect to the parent compound L across the C–N bond of the C(thiazole)–NH unit to adopt the anti–anti–anti form. Robust hydrogen bonded cyclic motifs are found in the self-assemblies of the salts.

Tetranuclear zinc(II)-oxy (benzothiazole)-2-thiolate aggregate and copper(I) phenylthiolate aggregate

Abstract A tetranuclear zinc-oxy (benzothiazole)-2-thiolate aggregate whose structure has a C3-axis passing through ZnO unit relating three other zinc ions and a tetranuclear copper(I) phenylthiolate aggregate having each thiphenolate ligand bridging three copper ions are reported. These aggregates were prepared by hydrothermal reactions of 2,2′-dithiobis-(benzothiazole) with zinc nitrate or copper(I) iodide, respectively. The reaction of zinc nitrate passed through in situ abstraction of a oxy ligand from moisture to form a Zn4O core holding six 2-benzothiazolethiolate ligands, and during the formation of the aggregate, cleavage of S–S bond of 2,2′-dithiobis-(benzothiazole) took place. Whereas, an aggregate formed by self-assembling of copper(I) phenylthiolate was formed after extensive degradation of 2,2′-dithiobis-(benzothiazole) during solvothermal reaction.