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

Three isomeric planar pyridineurea based receptors namely L-2 (ortho), L-3 (meta) and L-4 (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](infinity)(2-), [HPO42-](infinity) and [(HPO4)(2)-(H2O)(3)](infinity)(4-) by L-2 (previously reported), L-3 and L-4 respectively. Perchlorate complexes of L-2 (non-isostructural), L-3 and L-4 (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 [I-4-(H2O)(6)](4-), silicon hexafluoride-water [(SiF6)(2)-(H2O)(2)](infinity)(4-) and nitrate-water [(NO3)(2-)(H2O)(6)](infinity)(2-) clusters. The receptors are enriched with p electron cloud originating from protonated pyridine moiety, imine and carbonyl group. Various roles of these p clouds in supramolecular assembly of the receptor have also been investigated by pi...pi, pi...pi(+) and pi(+)...pi(+) interactions. Conformational variation of the isomeric receptors in their anion complexes mainly resulted different kinds of pi interactions. (C) 2016 Elsevier Ltd. All rights reserved.