Arghya Basu

Encapsulation of a discrete cyclic halide water tetramer [X2(H2O)2]2−, X = Cl−/Br− within a dimeric capsular assembly of a tripodal amide receptor

A conformationally flexible C3v symmetric N-bridged tripodal amide receptor encapsulates a tetrameric halide water cluster [X2(H2O)2](2-) (X = Cl(-)/Br(-)) within its dimeric capsular assembly and forms a non-capsular 1D polymeric assembly with higher homologous iodide anions upon protonation.

A C3v-Symmetric Tripodal Urea Receptor for Anions and Ion Pairs: Formation of Dimeric Capsular Assemblies of the Receptor during Anion and Ion Pair Coordination

A new C(3v)-symmetric urea-based heteroditopic tripodal receptor capable of recognizing both anions and ion pairs was designed, synthesized, and characterized. The protonated receptor forms a sulfate complex which encapsulates a single DMF in the tripodal cavity of the receptor. However, the SO4(2-) anion is located outside the tripodal cavity and is stabilized by N-H···O hydrogen bonds from the urea functions of four receptor cations. With TBAHSO4 the receptor forms a contact ion pair complex, where both the TBA(+) and SO4(2-) groups are pseudoencapsulated in the tripodal cavity of the protonated receptor. Significantly, the receptor forms a charge-separated polymeric ion pair complex with K(+) and HPO4(2-) via formation of a dimeric capsular assembly of the receptor, in which three K(+) encapsulated dimeric capsular assemblies interdigitate to form a precise cavity that further encapsulates HPO4(2-). The receptor also forms an anion complex with CO3(2-) via formation of dimeric capsular self-assembly of the receptor. Solution-state binding studies of the receptor with oxyanions have also been carried out by (1)H NMR titration experiments, which show the oxyanion binding trend HCO3(-) > H2PO4(-) > HSO4(-), whereas no binding with NO3(-) and ClO4(-) anions is observed.

Synthesis, crystal structure and bio-macromolecular interaction studies of pyridine-based thiosemicarbazone and its Ni(II) and Cu(II) complexes

A new pyridine-based heterocyclic thiosemicarbazone ligand and its Ni(II) and Cu(II) complexes have been synthesized and characterized by structural, analytical and spectral methods. The mono-deprotonated anionic form of the ligand coordinates via NNS donor atoms to yield an octahedral Ni(II) complex and distorted square planar Cu(II) complex. UV-visible and fluorescence-based spectroscopic techniques revealed that both metal complexes interact with double stranded DNA via intercalation. A comparative assessment indicated that the Ni(II) complex displayed superior DNA binding. The interaction of these compounds with bovine serum albumin (BSA) suggested that the ligand and its Cu(II) complex quenched the intrinsic fluorescence of BSA in a static quenching process, whereas for the Ni(II) complex, fluorescence quenching of BSA was a combination of both static and collision/dynamic quenching processes. The quenching of the fluorescence of BSA is owing to energy transfer from the tryptophan residues of BSA to the compounds bound to BSA. Cytotoxicity tests based on the standard MTT assay revealed that the Cu(II) complex displayed prominent anti-proliferative activity against HeLa cells.

Neutral Acyclic Anion Receptor with Thiadiazole Spacer: Halide Binding Study and Halide-Directed Self-Assembly in the Solid State

A halide binding study of a newly synthesized neutral acyclic receptor LH(2) with a thiadiazole spacer has been methodically performed both in solution and in the solid state. Crystal structure analysis of the halide complexes elucidate the fact that fluoride forms an unusual 1:1 hyrogen-bonded complex with monodeprotonated receptor, whereas in the case of other congeners, such as chloride and bromide, the receptor binds two halide anions along with formation of a halide-bridged 1D polymeric chain network by participation of N-H···X(-) and aromatic C-H···X(-) hydrogen-bonding (where X = Cl and Br) interactions. The presence of a rigid thiadiazole spacer presumably opens up enough space for capturing two halide anions by a single receptor molecule, where the coordinated -NH protons are pointed in the same direction with respect to the spacer and eventually favor formation of halide (Cl(-) and Br(-)) induced polymeric architecture, although no obvious chloride- or bromide-directed polymeric assembly is found in solution. A significant red shift of 243 nm in the absorption spectra of LH(2) was solely observed in the presence of excess fluoride anion, which enables LH(2) as an efficient colorimetric sensor for optical detection of fluoride anion (yellow to blue). Furthermore, spectroscopic titration experiments with increasing equivalents of fluoride anion suggest formation of a H-bonded complex with subsequent stepwise deprotonation of two N-H groups, which can be visually monitored by a change in color from yellow to blue via pink.

Amidothiourea based colorimetric receptors for basic anions: evidence of anion induced deprotonation of amide –NH proton and hydroxide induced anion⋯π interaction with the deprotonated receptors

Two amidothiourea based receptors (L1 and L2) containing a π-acidic 3,5-dinitrophenyl chromophore have been synthesized in good yields and their anion recognition properties were evaluated both in organic and aqueous organic environment by spectroscopic techniques. Anions such as F−, AcO− and H2PO4− were examined to be suitable analytes for the receptor molecules, displaying optical signaling from colorless to orange/red, whereas anions of lower basicity such as Cl−, Br−, I−, NO3− and HSO4− did not cause any discernable spectral changes. The detailed 1H NMR titration experiments and single crystal X-ray structural analyses revealed that the receptor–anion(s) interaction encourages deprotonation of the amide –NH proton of the amidothiourea function. Interestingly, the highly basic OH− ion showed stepwise color changes with increasing equivalents, from colorless to red to green. The step wise color changes were found to be the outcome of OH− (1 equiv.) induced mono-deprotonation of the individual receptors (colorless to red), followed by anion⋯π interaction (red to green) with the π-acidic 3,5-dinitrophenyl ring of the receptors beyond one equiv. of OH− addition. This anion⋯π interaction between the OH− ion and the synthesized receptors has also been confirmed by monitoring the OH− induced absorption spectral changes of a control receptor (LC).

Anion coordinated capsules and pseudocapsules of tripodal amide, urea and thiourea scaffolds

This review aims to deliver a detailed and comparative account of the reported examples of anion (halide and oxyanions) coordinated capsules and pseudocapsules of tripodal receptors that employ hydrogen bonds and/or electrostatic hydrogen bonds offered by specific binding sites from amide, urea and thiourea functionalities. The review discusses both the structural aspects of anion binding and solution-state anion binding affinities of N-bridged and aryl-bridged tripodal receptors. Discussions relating to selective anion recognition and separation, carbondioxide uptake, and transmembrane anion transport, as demonstrated by some of these tripodal receptors have also been included in this review.

Structural insight into the anion–water cluster: stabilised by alcohol and carboxylic acid containing tripodal ligand

A new flexible N-bridged, unsymmetrical, water-soluble tripodal ligand (L) bearing alcohol and carboxylic acid groups has been synthesised and its solid-state interaction with anions has been investigated. The fully deprotonated ligand encapsulates a sodium cation in a half cryptand bowl-shaped cavity (1). The chloride complex 2, contains a cyclohexane-like water cluster incorporating ligand OH groups. However, complexes with bromide and nitrate (3 and 4) are dimeric. Tetrahedral clusters containing two water molecules and two anions were found in complexes 3 and 4. Perchlorate complex 5 forms perchlorate–methanol adducts. Complex 1 forms a hydrophilic cation–water channel and complexes 2–4 form anion–water channels between the hydrophobic layers of the naphthalene moieties. Complexes 2, 5 and 3, 4 are isostructural in nature having similar packing structures. Structurally diverse anion–water clusters and encapsulation of sodium in an alcohol–water medium are reported for a new flexible tripodal ligand.

Amidothiourea as a potential receptor for organic bases by resonance assisted low barrier hydrogen bond formation: Structure and Hirshfeld surface analysis

The complexes (1–3) of an amidothiourea receptor L with three different types of organic bases have been synthesized, and their crystal structures were solved using single crystal X-ray diffraction data. A detailed structural analysis of the complexes reveals that the complexes are primarily stabilized by a strong hydrogen bonding interaction between the amide oxygen of the deprotonated receptor and N–H proton of the protonated bases. Close inspection of the hydrogen bond parameters of the complexes shows that the receptor significantly forms a low barrier hydrogen bond with imidazole and hexamine, whereas no obvious low barrier hydrogen bond has been found in case of triethylamine. The Hirshfeld surface and fingerprint plot analysis of the complexes have been carried out, and reveal that the complexes are stabilized O–H⋯O, C–H⋯O, N–H⋯S and N–H⋯O, hydrogen bonds. Additionally, the nitro groups of the receptor are involved in different types of charge transfer or electron donor acceptor interactions, which also have a prominent effect in the solid state stabilization of the complexes (1–3).

Dual modes of binding on the hexafluorosilicate anion by a C3v symmetric flexible tripodal amide ligand in solid state

A para-nitrophenyl functionalized C3v symmetric flexible tripodal amide ligand, L, shows remarkable solvent dependent dual binding behaviour towards the octahedral hexafluorosilicate anion in solid state. In DMSO solvent the ligand encapsulates the hexafluorosilicate anion within its dimeric capsular assembly, whereas in the case of DMF solvent the ligand stabilizes the hexafluorosilicate anion via side cleft binding. The binding dissimilarities of the hexafluorosilicate anion in the complexes are also confirmed by Hirshfeld surface analysis.