Terminal Substituent Induced Differential Anion Coordination and Self-Assembly: Case Study of Flexible Linear Bis-Urea Receptors

For a comprehensive analysis of host–guest binding propensity in their neutral form, three linear flexible bis-urea receptors (L1–L3) with different terminal substituents have been synthesized. It has been established that, with the existence of electron-withdrawing or π-acidic phenyl substituents, they act as a possible system that can proficiently coordinate with anions of diverse dimensions constantly initiated by the size of the countercations. The 3,5-bis(trifluoromethyl)phenyl-derived isomer (L1) can readily form cooperative neutral self-assemblies irrespective of the size the monovalent halides (viz. chloride, bromide, and iodide anions) and noncooperative neutral self-assemblies with planar divalent carbonate anion. The meta isomer L2 captures spherical halides, i.e. chloride and bromide, in an isostructural way, forming a 1:2 host–guest assembly, whereas the isomeric para receptor L3 shows cooperative binding with chloride anions, having coordination number 3. However, due to the greater flexibility and lesser hydrophobicity of receptor L2 and L3 in comparison to receptor L1, successful crystallization of any oxyanion complexes through the meta and para isomers was not successful.