Conformational adaptations of acyclic receptor templated by weakly coordinating anions

Abstract

Crystallization from methanol solutions resulted in new supramolecular complexes of a flexible polyamine receptor N,N′-3-azapentane-1,5-bis[3-(1-aminoethylidene)-6-methyl-3H-pyran-2,4-dione] (L) with spherical (Br⁻ and I⁻), linear (SCN⁻) or pyramidal (ClO₃⁻) anions. The recognition and self-assembly processes yielded five solids: solvated (1a) and non-solvated (1b) forms of HL : SCN⁻ complex with quite different receptor conformations, a supramolecular complex with ClO₃⁻ anion (3), which is isoskeletal to 1b, and HL : Br⁻ (2) and HL : I⁻ complexes which are isoskeletal to 1a. It is shown that the rich coordinating potential of the anions investigated arises not only from their geometry but also from their ambivalent accepting functionalities. The divergence in the way binding occurs is reflected on supramolecular synthons stabilizing crystal structures as well as on the physicochemical properties of the obtained solids. In these systems, anion binding could be interpreted as a competition between two “solvation spheres”, one provided by the solvent molecules and the other dominated by high-order supramolecular receptors built from HL molecules. This behaviour is related to previously reported complexes of HL with trigonal, tetrahedral and octahedral anions, emphasizing the receptor's conformational adaptability for providing the most favourable surrounding for guest inclusion in the rigid environment of a crystal.