Md. Najbul Hoque

Hydrated anion glued capsular and non-capsular assembly of a tripodal host: Solid state recognition of bromide–water [Br5–(H2O)6]5− and iodide–water [I2–(H2O)4]2− clusters in cationic tripodal receptor

Herein, we describe a flexible polyammonium tripodal (N4 unit) receptor for hydrated anions. Multiple protonation sites increased the degree of protonation and allowed to study binding of multiple anions towards the receptor in aqueous medium. All three arms are projected in the same direction in the protonated receptor and formed bowl shaped conformation. Whereas the free receptor L occupied an orientation in between open and bowl shaped conformation. We observe anion or anion–water assisted capsular and non-capsular assembled supramolecular structures stabilized by several H-bonding interactions. We show fluoride–water and chloride ion belt induced bimolecular capsular assemblies in complexes 1 and 3. On the other hand, we establish chloride–water, bromide–water and iodide–water templated non-capsular aggregations of the protonated receptor in complexes 2, 4 and 5. Most interestingly, a long chain fluoride–water cluster [F7–(H2O)8]7− in capsular complex 1, unique extended bromide–water [Br5–(H2O)6]5− and discrete iodide–water [I2–(H2O)4]2− clusters in non-capsular complexes 4 and 5 are also examined structurally. All supramolecular complexes are characterized by FTIR, NMR, TGA-DSC and X-ray analysis.

Influence of the cavity dimension on encapsulation of halides within the capsular assembly and side-cleft recognition of a sulfate–water cluster assisted by polyammonium tripodal receptors

The p-nitrophenyl and p-bromophenyl functionalized tris-polyamine receptors, L1 and L2, have formed capsular assembly with halide ions in an encapsulated fashion through efficient hydrogen-bonding. On the other hand, the positional isomer of L1, the m-nitrophenyl functionalized tripodal amine receptor L3, displays a rather flat-open conformation and is unable to bind halide anions in an encapsulated form. The presence of a smaller cavity in these receptors hinders the binding of larger oxyanions like sulfate. As a result, the protonated tripodal scaffold encapsulates small solvent molecules and helps in side-cleft binding of the larger sulfate anion. Herein, we report the design, synthesis and characterization of tren-based polyammonium receptors L1, L2 and L3 and their complexation as well as binding discrepancy with several anions in the presence of acid. The solid state crystal structure of the anion complexes with L1, L2 and L3 reveal that the anions are recognized via stable N–H⋯A, C–H⋯A, anion–π interactions with the protonated receptor molecule in a unimolecular fashion either inside or outside the cavity. The sulfate–water complexes of receptors L1, L2 and L3 are stabilized by (NH)+⋯O type H-bonding and electrostatic interactions among sulfate, water and ammonium groups. The polyammonium based tripodal scaffold with positional variation of the functional group shows significant difference in anion binding fashion through either capsular or non-capsular complex formation.

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.

Overview of the strategic approaches for the solid-state recognition of hydrated anions

The recognition of hydrated anions is a rapidly expanding area in the domain of supramolecular chemistry. Since the beginning of the research field of the supramolecular chemistry of anions, ample literature studies on the recognition of anions have become available. However, there is less literature on the solid-state recognition of hydrated anions in comparison. This highlight covers recent advances in the solid-state recognition of hydrated anions. Structural variation of the anion–water cluster is primarily governed by the shape and charge density of the anions. Given the interest in anion–water interactions, various structural assemblies of anion–water clusters are discussed in this highlight. The review includes both purely organic and metal–organic systems that are potentially suitable for stabilizing anion–water clusters. The recognition of hydrated anions by artificial ligands has attracted huge research interest for potential application in waste remediation, catalysis and transport. Due to their indigenous nature of a high propensity of hydration, inorganic anions can be crystallized with various degrees of hydration; however, their structures are very poorly understood. This review thus gathers the articles published concerning the recently developed field of hydrated anions. This short review might be helpful for researchers to gain an insight into some design features of ligands and anion–water interactions.