Entrapment of Cyclic Fluoride–Water and Sulfate–Water–Sulfate Cluster Within the Self-Assembled Structure of Linear meta-Phenylenediamine Based Bis-Urea Receptors: Positional Isomeric Effect

Abstract

meta-Phenylenediamine based two linear bis-urea anion receptors, L 1 (meta-nitro) and L 2 (para-nitro), have been synthesized which showed strong binding affinities toward halides as well as planar and tetrahedral oxyanions. In the presence of excess fluoride anion, L 1 self-assembles to entrap a cyclic fluoride–water tetrameric cluster within its tetrameric pillar-like assembly of each urea arm (complex 1a). In the presence of sulfate anion, L 1 self-assembles in an identical fashion to entrap the bigger tetrahedral anion (complex 1b), whereas two urea moieties of two different receptor molecules of L 1 have been structurally authenticated to be sufficient to stabilize the simple Y-shaped acetate anion in complex 1c. On the other hand, receptor L 2 , with a bis­(para-nitro urea) functional group, especially in the presence of sulfate anion, self-assembles like long straight pillars entrapping sulfate–water–sulfate cluster {SO4 2–-(H2O)3-SO4 2–} (complex 2a). Additionally, L 2 forms a host–guest assembly with the cation and solvent (DMSO) coordinated bromide anion (complex 2b) when treated with excess bromide anion as n-tetrabutylammonium salt. All the complexes have been characterized by powder X-ray diffraction and 1H NMR and 13C NMR spectroscopy, which are in full support and consistent with solid-state single crystal X-ray diffraction results.

Supramolecular Chemistry of Anionic Dimers, Trimers, Tetramers, and Clusters

“…Pores formed via the assembly of two or more functional building units are commonly used to capture specific anions. 35–37 Example include a fluoride anion dimer and a sulfate anion dimer stabilized by the isomeric meta -phenylene based bis-urea receptors 19 ( meta -nitro) and 20 ( para -nitro), respectively, as reported by Manna et al 38 Although both receptors favor a linear conformation, they proved capable of interacting with halide anions as well as planar and tetrahedral oxyanions. In the case of receptor 19 , a single crystal X-ray crystallographic analysis revealed that the four urea arms derived from four different molecules of 19 assemble into a barrel-like structure that acts to entrap a fluoride dimer bridged by two water molecules.…”

Anion Binding Studies of Urea and Thiourea Functionalized Molecular Clefts

“…It is now documented that a urea or thiourea group is capable to chelate an anion via two directional H-bonds (Jia et al, 2016). Therefore, the molecules containing (thio)urea moieties with flexible (Nishizawa et al, 1995;Nishizawa et al, 1998;Emgenbroich et al, 2008;Duke et al, 2012;Caltagirone et al, 2013;Olivari et al, 2015;Gillen et al, 2018) or rigid (Manna et al, 2016;Khansari et al, 2017b;Gale and Gunnlaugsson, 2010) linkers have been successfully applied as a promising class of receptors for anion recognition purpose in recent past years. Nishizawa et al (1995) reported bis(thio)urea receptors containing n-butyl terminals with a flexible metaxylylenediamine-based linker, exhibiting preferable binding for H 2 PO 4 − and SO 4 2− over other anions as evaluated by NMR studies.…”

Solution-State Anion Recognition, and Structural Studies, of a Series of Electron-Rich meta-Phenylene Bis(phenylurea) Receptors and Their Self-Assembled Structures

“…We previously reported allosteric effects in the binding of several anions to a series of acetamido-substituted diphenylureas and thioureas, due to the presence of the additional hydrogen bonding group adjacent to the urea. , Similar effects in symmetric phenylene bisureas have been noted: ortho -phenylene bisureas have been established by Gale and co-workers as excellent hydrogen bonding receptors for carboxylate anions and tetrahedral oxoanions due to the four convergent hydrogen bonds, while the bis­(urea)–carboxylate interaction displayed by these has been utilized as a supramolecular synthon . Wu and co-workers have used bis­(biurea) derivatives to form both PO 4 3– templated helicates and cages, − while Das and co-workers have developed several electron-poor meta -phenylene bis­(phenyl­urea) receptors, and studied their interactions with anions, with an emphasis on their crystalline adducts. − Others have extended the aromatic moieties of this framework: Ghosh et al developed a series of coumarin-containing receptors and studied their interactions with mono- and dicarboxylate anions, while the group of Caltagirone found that pyrophosphate binding in phenylene bis­(aryl­urea) systems is promoted in asymmetric hosts bearing a naphthalene and an indole ring …”