Combined Crystallographic and Molecular Dynamics study on complexes of calixarene derivatives with contaminant metal ions

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

San Luís, Argentina

Congreso; III Reunión de la Asociación Argentina de Cristalografía (AACr); 2007

AACr

Abstract. X-ray diffraction crystallography is the most important tool to study structure-properties relationships of materials at atomic resolution. However, many important systems are functional in solution and the question arises about the certainly with which structural features found in the solid state can be extended to solutions. Calix[4]arene derivatives are versatile bi-functional macro-cyclic molecules consisting in an upper hydrophobic calix, enclosed by four phenyl rings, that can include small organic molecules, and a lower hydrophilic cavity, conformed by a variety of pendant arms, which is able to encapsulate metal ions. In this presentation we shall discuss the crystal vs solution structural relationship related to the ability of ester and ketone p-tert-butyl calix[4]arene derivatives to act as sequestering agents of the environmental contaminants Cd(II) and Pb(II) ions. To this purpose we shall use combined solid state X-ray diffraction methods and Molecular Dynamics (MD) simulations in vacuum and in acetonitrile (MeCN) solution. The in vacuum MD calculations dealing with the equilibrium molecular conformations of ester and ketone calix[4]arene derivatives and their complexes with Cd2+ and Pb2+ ions and acetonitrile solvent show a striking agreement with the experimental crystallographic structures. The MD simulations in MeCN solution uncover the following major behaviors: i) The strong metal ion-hydrophilic cavity interaction pre-organizes the calix hence promoting the inclusion of a stabilizing MeCN solvent molecule (allosteric effect), an association that survives crystallization by solvent evaporation. ii) The calculated equilibrium conformation of the ester calix[4]arene complexes closely agrees with the “endo” form observed in the solid state. iii) The theoretical structural results for the ketone calix[4]arene, however, are at variance with the crystallographic ones and show stronger solvent effects in the lower bore where the competing MeCN molecules pull the metal ion to the outer rim of the hydrophilic cavity to conform an “exo” complex.