Binding consistency of anions by the effect of aromatic meta-substitution of bis-urea receptors: entrapment of hexafluorosilicate clusters

Abstract

With the 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 cyano-substituted para isomer receptor L₃ have been found to entrap water-free naked SiF₆²⁻, while the meta receptor has been 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. However, the meta receptor L₂ self-assembles to build a 3 : 2 host–guest assembly, where the two SiF₆²⁻ units show different coordination environments. FESEM imaging studies corroborated the result obtained in the solid-state.