Discrepancy in anion coordination directed by isomeric pyridine–urea receptors: Solid state recognition of hydrated anions

Abstract

Three isomeric planar pyridine–urea based receptors namely 2L (ortho), 3L (meta) and 4L (para) have been synthesized by Schiff base condensation reaction. All the receptors are decorated with suitable anion binding urea moieties. Pyridine acts as a protonation site, also serves for electrostatic stabilization for anions. Three isomeric receptors on treatment with inorganic acid generate isostructural/non-isostructural anion complexes. As an example, phosphate complexes of three isomers are non-isostructural, hence we observed three different types phosphate/phosphate-water clusters like [(H2PO4)2-H2O]∞2−, [HPO42−]∞ and [(HPO4)2-(H2O)3]∞4− by 2L (previously reported), 3L and 4L respectively. Perchlorate complexes of 2L (non-isostructural), 3L and 4L (isostructural) demonstrate several types of anion-water assemblies depending on the pyridine moiety. Position of N-atom in pyridine of particular receptor mainly controls the formation of diverse anion-water assemblies. Along with this we also report some additional hydrated anion cluster like iodide-water [I4-(H2O)6]4−, silicon hexafluoride-water [(SiF6)2-(H2O)2]∞4− and nitrate-water [(NO3)2-(H2O)6]∞2− clusters. The receptors are enriched with π electron cloud originating from protonated pyridine moiety, imine and carbonyl group. Various roles of these π clouds in supramolecular assembly of the receptor have also been investigated by π⋯π, π⋯π+ and π+⋯π+ interactions. Conformational variation of the isomeric receptors in their anion complexes mainly resulted different kinds of π interactions.