Pre-organization and allosterism in complexes of Cd(II) and Pb(II) with calix[4]arene derivatives

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

Ginebra, Suiza

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

Internation Union of Crystallography (IUCr)

The bite ability of ethylester and methylketone pendant arms of butylcalix[4]arene derivatives (L and M) towards binding metal ions and the pre-organization shown by these macrocycles, prompted structural studies of their complexes with the environmentally relevant Cd2+ and Pb2+ ions. We determined the structures of (MeCN)Cd(L)(ClO4)2.3MeCN (1), Pb(L)(MeCN) (ClO4)2.5MeCN. 2H2O (2), (MeCN)Cd(M)(ClO4) .5MeCN (3) and Pb(M)(MeCN)(ClO4)2.2MeCN (4) at 120 K by X-ray diffraction methods from 10517 (1), 10969 (2), 14305 (3) and 10909 (4) reflections with I>2 (I) and refined them to R1-values of 0.047 (1), 0.077 (2), 0.0361 (3) and 0.0335 (4). All macrocycles adopt a slightly distorted cone conformation with a metal ion in the hydrophilic cavity and an acetonitrile solvent molecule in the calyx. In 1 and 3, at variance with most other calix[4]arene compounds, the MeCN molecule enters the calyx with the cyanide end pointing inward to form a Cd-N bond. There, the Cd2+ ion is further coordinated to all eight calixarene oxygen atoms in 1 and all but one carbonyl oxygen in 3. In 1 the ligand-to-cadmium atoms conform the corners of a capped Archimedean square antiprism with the N-atom at the capping site, while in (3) Cd2+ is at the center of a distorted triangular dodecahedral The lead complexes (2) and (4) show a quite similar environment around the metal. The Pb2+ ion is coordinated to the eight oxygen atoms, which are at the corners of a tetragonally squashed cube. These results strongly suggest the predominance of ionic interaction in the lower rim during metal complexation in promoting the inclusion of a solvent molecule within the calyx to form a molecular complex (positive allosteric effect).