meso-Bis(ethynyl) Versus meso-Bis(aryl) Calix[4]pyrroles: Dimensionally Well-Modulated Receptors That Can Regulate the Anion Binding Domains

meso-Substituted calix[4]pyrroles 2-6 containing a direct meso-ethynyl linker displayed high binding affinities and unique conformational features on halide anion binding. A general conformational bias for the equatorial alignments of the meso-(aryl)ethynyl groups was observed in the host-halide complexes which was attributed to the repulsive anion-alkyne interactions and released steric strain. Such conformational features of host-halide complexes persisted even in the case of calix[4]pyrrole 6 bearing cationic meso components, which displayed the highest binding affinity for chloride anions among known meso-aryl calix[4]pyrroles. Synthetic details, conformational features, and comparative halide anion binding properties of this series of calix[4]pyrroles are described.