Arup Tarai

Oxime synthons in the salts and cocrystals of quinoline-4-carbaldoxime for non-covalent synthesis

The oxime–quinoline R22(8)-type heterosynthons in self-assemblies of quinoline-4-carbaldoxime change to form R22(14)-type oxime–oxime homosynthons and quinoline–carboxylic acid heterosynthons in cocrystals upon interactions with dicarboxylic acids such as adipic, succinic and fumaric acids. The general trend observed in the formation similar oxime–oxime homosynthons of these cocrystals is useful to demarcate these structures to have obtained through predesigned non-covalent synthesis. On the other hand, the maleate salt of quinoline-4-carbaldoxime has two self-interacting maleate anions flanked by two quinolinium-4-carbaldoxime cations, whereas the oxalate salt has the dianions flanked by two cations. Among the salts of acids like nitric acid and hydrochloric acid, the nitrate salt has nitrate ions bridging two quinolinium-4-carbaldoxime cations to form cyclic motifs, whereas the self-assembly of the chloride salt has chloride–water chains formed through hydrogen bonds. The self-assembly of the chloride salt has conventional R22(8)-type oxime–oxime homosynthons.

Quaternary and senary sub-assemblies in cocrystals and salts of quinoline-4-carbaldoxime with aromatic carboxylic acids

Quaternary sub-assemblies in cocrystals and salts of quinoline-4-carbaldoxime with aromatic carboxylic acids are analysed. Quinoline-4-carbaldoxime formed a 1:1 cocrystal with 2-methylbenzoic acid, whereas with 2-hydroxybenzoic acid, 3-nitrobenzoic acid, 4-nitrobenzoic acid, 2,3- or 2,4-dihydroxybenzoic acid, it formed 1:1 salts. A ternary cocrystal with 2-nitrobenzoic acid composed of a neutral 2-nitrobenzoic acid and 2-nitrobenzoate with protonated quinoline-4-carbaldoxime is observed. Accommodation of additional neutral 2-nitrobenzoic acid is due to the extension of quaternary sub-assemblies to senary sub-assemblies to achieve an appropriate packing pattern. Whereas, assemblies of hydrated salts can be considered to comprise senary sub-assemblies formed between water molecules and quaternary sub-assemblies. In such senary sub-assemblies, water molecules are held in the hydrophobic junctions formed by oximes and carboxylic acids.

A study on fluoride detection and assembly of hydroxyaromatic aldoximes caused by tetrabutylammonium fluoride

Tetrabutylammonium fluoride (TBAF) forms cocrystal H2NAP·TBAF with 2-hydroxynaphthaldoxime (H2NAP) or cocrystal 2(H3OHPA)·TBAF with 2,3-dihydroxyphenylaldoxime (H3OHPA), whereas a similar reaction with 2,4-dihydroxyphenylaldoxime (H3PHPA) forms tetrabutylammonium (TBA) salt TBA(H2.5PHPA)2. Formation of these cocrystals or salts is accompanied by a color change which enables the detection of fluoride ions. Cocrystal H2NAP·TBAF has a layered structure formed by hydrogen bonds between the fluoride ions and parent oxime molecules; the tetrabutylammonium cations are held in between layers of anionic assemblies. On the other hand, cocrystal 2(H3OHPA)·TBAF has a grid-like architecture constructed from hydrogen bonds between the parent oxime molecules and fluoride ions. TBA cations are encapsulated within the grids. It is shown that salt TBA(H2.5PHPA)2 forms anionic assemblies to encapsulate tetrabutylammonium cations which are devoid of fluoride ions. By interactions of the tetrabutylammonium fluoride ions with H3PHPA molecules, an anionic assembly is formed by the sharing of protons, which possesses a grid-like structure. The formation of such an assembly causes color change which enables one to detect fluoride ions by visual means.

Competing phenol–imidazole and phenol–phenol interactions in the flexible supramolecular environment of N,N′-bis(3-imidazol-1-ylpropyl)naphthalenediimide causing domain expansion

The relevance of a fundamentally important imidazole⋯phenol interaction in the self-assembly of cocrystals and a salt of a bis-imidazole hinged through a flexible tether to a naphthalenediimide, namely N,N′-bis(3-imidazol-1-ylpropyl)naphthalenediimide (L), with a series of phenolic compounds is studied. Cocrystals of L with phenolic compounds, 1,2-dihydroxybenzene (cat), 1,3-dihydroxybenzene (res), and 1,3,5-trihydroxybenzene (phi), namely L·cat, L·(res)2, L·phi·H2O, and a salt with 2,4-dinitrophenol (dnp), L2+·(dnp−)2, are structurally characterized. Hierarchical effects of oxime-imidazole synthons in the cocrystal of L with 2,3-dihydroxyphenylaldoxime, L·(dhpa)2, are also investigated. The cocrystals of L with cat and phi are formed in a 1 : 1 molar ratio, but the latter is a hydrated cocrystal. Compound L adopts a C-like shape in cocrystals L·cat and L·phi·H2O, whereas L adopts an S-like shape in the L·(res)2 cocrystal. It adopts an I-like shape in the salt L2+·(dnp−)2 or in the cocrystal L·(dhpa)2. Analyses of cocrystals of L with these phenolic compounds show a significant presence of charge-transfer interactions between naphthalenediimide only with phi. Phenol⋯imidazole interactions together with C–H⋯N(imidazole) and N⋯π interactions cause domain expansion in L·cat by forming dimeric sub-assemblies providing C-like geometry to L. In this example, one of the flexible arms bearing imidazole is crystallographically disordered, and a low temperature 1H-NMR study reveals conformational changes. In contrast, in the case of L·phi·H2O, domain expansion that takes place due to the interplay of charge-transfer, imidazole⋯phenol and phenol⋯water interactions provides a C-like shape to L. In these cases, phenol⋯phenol interactions have less relevance. In the case of the 1 : 2 salt L2+·(dnp−)2, imidazolium⋯phenolate interactions dominate and provide a stretched geometry to L, whereas competition to form homodimers between oximes and oxime⋯imidazole interactions predominates in the cocrystal L·(dhpa)2. This cocrystal has a stretched structure of L. In the lattice of this cocrystal, phenol⋯phenol interactions are not relevant. Cocrystal L·res is identified as one exception among the series where competition between phenol⋯phenol with phenol⋯imidazole interactions results in domain expansion facilitating an interpenetrated structure.

Anion assisted conformationally guided self-assemblies of multi-component cocrystals of dioxime

Multi-component cocrystals of 1,3-benzenedialdoxime (H2BNAD) with tetrabutylammonium salts provide examples of dendrimer-like anion assisted assemblies. The assembly of the multi-component cocrystal comprised of H2BNAD and tetrabutylammonium fluoride has dendrimer-like packing. It possesses dimeric cyclic sub-assemblies of dioximes and monomeric dioxime molecules to which fluoride ions act as connectors. Chloride ions act as connectors to three dioxime molecules in a dendrimer-like assembly of a multi-component cocrystal of H2BNAD with tetrabutylammonium chloride. In this assembly water molecules are held in the interstices by hydrogen bonding interactions with host oximes. The assembly forms cyclic ensembles in such a way as to accommodate the tetrabutylammonium cations. The cocrystal of H2BNAD comprised of tetrabutylammonium cations and HBNAD− possesses dimeric hydrogen bonded cyclic sub-assemblies of neutral H2BNAD and HBNAD− connected to each other in a chain-like structure. In general, two conformers of H2BNAD originating from different orientations of the oxime functional groups across the phenylene ring are key factors in providing directionality to the self-assemblies. Single crystal X-ray structure determination and Raman spectroscopy were used to ascertain signatures of the H2BNAD conformers present in these assemblies.

Study on divalent copper, nickel and zinc model complexes for fluoride ion detection

A series of new model complexes of 2,4-dihydroxybenzaldoxime (H3L) with divalent copper and nickel ions having a general formula [M(H2L)2]·sol (sol = solvent or nitrogen containing heterocycle) and a new zinc complex [Zn(H2L)2(pep)2] (where pep = 4-(2-(pyridine-4-yl)propyl)pyridine) were synthesized for the detection of fluoride ions. All complexes were structurally characterized by X-ray single crystal diffraction, powder X-ray diffraction and conventional spectroscopic tools. All these complexes show selective interactions with tetrabutylammonium fluoride. The position of the new UV-Vis absorption in each of these complexes in the presence of fluoride ions is guided by the electronic configuration of the metal ion. The complex [Zn(H2L)2(pep)2] has a distinct advantage over the other model complexes, as it can detect fluoride ions through both emission and absorption spectroscopy.

Different self-assemblies and absorption–emission properties of the picrate salts of aromatic amine or heterocycle linked oximes

The supramolecular oxime synthons in the self-assemblies of picrate salts of 4-(N,N-dimethylaminophenyl)aldoxime, quinoline-4-carbaldoxime and pyridine-4-carbaldoxime are different. In these salts, protonation in each case occurs at a nitrogen atom located at a remote site of the oxime. These results suggest that synthon analysis provides descriptors rather than predicting self-assemblies. The self-assembly of the picrate salt of 4-(N,N-dimethylaminophenyl)aldoxime lacks an oxime homodimer. The R22(8) type homodimeric sub-assemblies of oximes are observed in the picrate salt of quinoline oxime, whereas a similar picrate salt of pyridine-oxime has R22(4) type oxime homodimers. Water molecules in this salt act as filler molecules for a tight-packed structure and have a role in deciding the nature of oxime synthons. Protonation at a nitrogen atom of the oxime was observed in the picrate salt of indole-3-carbaldoxime. This salt undergoes charge-transfer between the aromatic rings. The differential scanning calorimetry data establish the higher melting points of salts over the parent components and exothermic decompositions just above or near the respective melting temperature. The charge-transfer interactions of the picrate salt of indole-3-carbaldoxime cause a color difference from other salts. The emission spectrum of each salt is invariably quenched in solution or in the solid state. The relative ability of different nitro-aromatic compounds to quench emissions of the oxime derivatives enables them to be differentiated in solution. It is shown that the modification of the ground state by charge-transfer interaction is not the sole factor to cause fluorescence quenching in these salts, but excited state proton transfer plays a decisive role.

Four-coordinated see-saw N-(aryl)-2-(propan-2-ylidene)hydrazinecarbothioamide complexes of nickel(II), copper(II) and zinc(II) and their propensity for catalytic cyclisation

A series of mononuclear complexes of divalent nickel and zinc with N-(4-methoxyphenyl)-2-(propan-2-ylidene)hydrazine carbothioamide (H2Lmethoxy) as well as with N-(4-nitrophenyl)-2-(propan-2-ylidene)hydrazine carbothioamide (H2Lnitro) have been structurally characterised. Among these two ligands, H2Lnitro formed an analogous copper(ii) complex to that of nickel and zinc, whereas H2Lmethoxy undergoes a catalytic cyclisation reaction in the presence of copper(ii) nitrate trihydrate. Control experiments based on ESR have revealed that copper(ii) is reduced to copper(i) and reverts back to copper(ii) as the cyclisation reaction proceeds, generating a catalytic reaction. The crystal structures of H2Lnitro and H2Lmethoxy show that the plane of the phenyl ring with respect to the plane of the hydrazine-containing unit in H2Lnitro is more or less coplanar, whereas the H2Lmethoxy molecule is non-planar with a 67.05° angle between these planes. DFT calculations have shown a large difference in the localisation of electrons in the HOMO of the two ligands; the HOMO of H2Lmethoxy is spread over the aromatic ring, which facilitates involvement of the ring in the formation of a C-N bond through a single electron transfer to a copper(ii) ion. This cyclic product has a distinguishable absorption maximum at 299 nm, which makes it possible to detect copper ions over other first row transition metal ions and alkali metal ions. On the other hand, the copper(ii) complex of H2Lnitro shows a characteristic absorption at 395 nm in the presence of fluoride ions, whereas the free ligand with fluoride shows absorption at 418 nm, which shows that the interactions of the ligand with fluoride and with the corresponding copper complex widely differ.