Binding Consistency of Anion by Effect of Aromatic Meta-Substitution of Bis-urea Receptors: Entrapment of Hexafluorosilicate Clusters
With cyano group as a terminal substituent, three positional isomeric bis-urea receptors (L1-L3) have been synthesized for extensive analysis of host-anion binding propensity in their neutral form. The existence of electron-withdrawing or π-acidic phenyl substituents in the receptors L1-L3 supports efficient binding of the octahedral hexafluorosilicate anion within their dimeric cavity. L1 and its isomeric cyano-substituted para isomer receptor L3 have been found to entrap water-free naked SiF62-, while meta receptor found to trap hydrated SiF62- within its cavity. In the presence of excess fluorides L1 self-assembles to form dodeca-coordinated hexafluorosilicate complex 1a and L3 self-assembles to form octa-coordinated hexafluorosilicate complex 3a in the solid-state via cooperative and non-cooperative H-bonding interactions of urea moieties respectively. Whereas the meta receptor L2 self-assembles to build 3:2 host-guest assembly, where the two SiF62-- unit shows different coordination environments. FESEM imaging studies corroborated the result obtained from in the solid-state.