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A set of three positional isomeric nitro-phenyl functionalized linear bis-urea scaffolds has been purposefully synthesized from p-phenylenediamine, basically for investigating the extensive anion-coordination behavior in their neutral form. The para-isomer (L 1 ) and meta-isomer (L 2 ) can readily form bicarbonate dimer (HCO3)2 entrapped neutral self-assemblies in the solid state by either fluoride or hydroxide induced atmospheric CO2 fixation. In the bulk scale and based on the isolated yield of (HCO3)2 entrapped complexes of L 1 and L 2 , it has been found that 6.7, 7.3, and 6.2 mmol of CO2 can be fixed respectively by 1 mmol of linear bis-urea derivatives. The other planar acetate anions in their bare and hydrated form [(OCOCH3)2­(H2O)] are coordinated by hydrogen-bonding interactions in noncooperative fashion with L 1 and L 2 respectively. Interestingly, the para-isomer L 1 entraps bisulfate dimer (HSO4)2, while the meta-isomer L 2 has been found to self-assemble with divalent sulfate (SO4 2–) and polymeric dihydrogen-phosphate (H2PO4) n by noncooperative hydrogen-bonding interactions also. However, successful crystallization of any anion complexes with ortho-isomer (L 3 ) was not fruitful; instead the free L 3 ligand structure has been attained in most of the cases. This phenomena occurs owing to the steric hindrance provided by the nitro group at the ortho-position, which probably hampers the facile inclusion via coordination of an anion due to electrostatic factors, as observed and confirmed by the presence of intramolecular hydrogen-bonding interactions in the crystal structures of free ligand L 3 .

No abstract

The impact of the substituted moiety on the binding nature of the anion with a neutral receptor through non-cooperative interaction.

The naphthalimide-based linear monourea receptors L 1 and L 2 exhibit anion (oxyanions and halides) binding in the solid and solution states. The receptor L 1 could render fluoride-/hydroxide-induced aerial CO2 fixation efficiently, leading to air-stable assembling of dianions enclosed by n-TBA cations via the formation of an R2 2(8)-type cylindrical dimeric arrangement. The assembly of the anions is stabilized via short-range hydrogen-bonding interactions. A cyclic dimeric assembly of bisulfate anions sealed by the linear tetrameric assembly of the host molecule was also evidenced from the solid-state architecture. Upon addition of excess amounts of chloride and bromide anion, linearly assembled L 1 captured a cyclic chloride–water and bromide–water tetrameric cluster. The ligands were capable of discriminating five relevant anions with 100% classification accuracy in aqueous media on the basis of multivariate pattern analysis. Interestingly, the ligands L 1 and L 2 displayed notable bactericidal activity against the Gram-negative pathogen E. coli MTCC 433 and were nontoxic to cultured human embryonic kidney (HEK 293) cells.

Two bis-urea receptors bearing aromatic meta-substituted electron-withdrawing groups demonstrated  halide-coordination-triggered self-assemblies with contrasting coordination numbers and variation in morphology.