Cation Selectivity of a Folded Ditopic Crown Receptor

Abstract

The synthesis and cation binding properties of a folded ditopic crown receptor, 3, are described. Intramolecular hydrogen-bonding interactions preorganize ditopic crown 3 into a helical conformation that positions the crown moieties in a manner that favors the formation of an intramolecular sandwich cation complex. Alkali-metal picrate extraction studies demonstrate that ditopic crown 3 exhibits a significantly different cation selectivity than mono-crown congener 2. A crystal structure of 3•2KClO⁴ indicates that two molecules of the complex are assembled into a molecular square in the solid state.

Keywords:

• Dendrimer
• Cation binding
• Crown macrocycle
• Hydrogen bonding

Acknowledgements

This work was supported by the National Science Foundation NSE program (CHE-0103133).

Notes

^†Crystallographic information: C⁴⁵H⁶⁰Cl¹¹N⁵O¹⁴•2KClO⁴•H²O, T = 200 (2) K; P ¹, a = 11.0714 (1) Å, b = 15.4689 \hspace{0.167em} (2) \hspace{0.167em} \AA , c = 18.0162 (2) Å. \mgreek{a} = 68.413 (1)°, \mgreek{b} = 77.512 (1)°, \mgreek{g} = 88.199 (1)°; V = 2797.26 (5) ų; Z = 2; 9822 unique data; R1(F) = 0.0516 , wR2(F^{2}) = 0.1391; {\rm GOF = 1.051} for I > 2sgr(I). The CIF file has been deposited with the Cambridge Crystallographic Data Centre and given the reference number CCDC 201180.

The Future of Supramolecular ChemistryProteins derive function through their highly organized, three-dimensional structure, and the reversible interconversion between different conformational states is often the means by which function is modulated or changed. In these molecules, conformational cooperativity that occurs over long distances causes small energetic differences relating conformational states to be dramatically magnified leading to highly stable folded materials. This phenomenon is commonly observed in biomacromolecules; however, there are relatively few synthetic materials that exhibit this cooperativity. Nevertheless, the remarkable efficacy exhibited by functional proteins suggests that synthetic macromolecules capable of adopting high levels of structural self-organization provide tremendous potential to develop materials with novel capabilities and properties not present in natural biomacromolecules. We have recently developed the first example of a well-defined monomolecular dendrimeric system with conformational properties consistent with such cooperativity. However, several important questions remain to be answered by further research in the area of dendrimer structure and stereochemistry. Can this cooperative conformational equilibrium be exploited in synthetic polymers and dendrimers to enhance the selectivity of a molecular recognition event or catalytic process? Can the folded dendron structures be induced to reversibly interconvert between different conformations through the action of an external stimulus in a manner that alters function? Such capabilities may provide the basis for development of catalysts, molecular switches, optical storage devices and drug-delivery vehicles among other functional devices. The work described in this manuscript represents a first step in trying to develop an understanding of how the conformational cooperativity present in folded dendrimer systems can control selectivity in a molecular recognition event. Progress in the development of functional materials will certainly be driven by a better understanding of how conformational equilibria can be controlled over relatively long distances in a manner reminiscent of biological macromolecules.Jon R. Parquette (born 1966) received a BS degree in chemistry from the University of California, Berkeley in 1988, and his PhD in 1994 from Stanford University working with Barry M. Trost. At Stanford, he was an American Chemical Society, Division of Organic Chemistry Fellow. After completing postdoctoral work at Caltech in 1996, working with Peter Dervan as an American Cancer Society Postdoctoral Fellow, he became a faculty member at The Ohio State University and was promoted to the rank of Associate Professor in 2002. His research interests revolve around the theme of synthetic macromolecular and supramolecular organic chemistry.