Calix[4]arene lower rim ionic interaction and the formation of intra-molecular complexes

O. E. PIRO; E. E. CASTELLANO; A. F. DANIL DE NAMOR

Ginebra, Suiza

Congreso; XIXth Congress and General Assembly of the International Union of Crystallography; 2002

Allosteric effects are common in large organic systems: conformational modifications in a molecular region promote changes in other molecular parts. These synergetic effects are present in calix[4]arene compounds, where four phenyl rings enclose a hydrophobic pocket capable to interact and host small neutral organic molecules. By replacing the phenolic H-atoms by appropriate residues, a hydrophilic cavity can be created in the lower molecular rim where ionic and/or H-bonding interactions could occur. As a consequence, these calixarene derivatives act as bi-functional receptors of a small solvent molecule in the calyx and charged species in or around the hydrophilic cavity. The molecular structure of {tetrakis[4-(ethyl)morpholine]oxyl} p-tertbutylcalix[ 4]arene (1) and of the acetonitrile complex of the protonated calixarene trifluoromethanesulfonate salt (MeCN)1H(CF3SO3)4 (2) were determined by X-ray diffraction methods from 13876 (1) and 10478 (2) reflections with I>2 (I) and refined to R1-values of 0.084 (1) and 0.081 (2). The calyx of both independent molecules of 1 presents a squashed cone conformation where a pair of opposite phenyl rings are close to mutual perpendicularity while the other pair are nearly parallel to each other, a conformation exhibited by a whole variety of p-tert-butylcalix(4)arene derivatives which does not favor accommodation of a solvent molecule. In 2, ionic interactions mediated by H-bonding of the -O(CH2)2NH+(CH2)4O pendant arms with CF3SO3 - ions, promote the symmetric opening of the calyx and its subsequent filling with an acetonitrile solvent molecule to form an intramolecular complex (positive allostery).