Capsule-like Assemblies in Polar Solvents.
Calixarene derivs. with four anionic groups at their upper rim form discrete 1:1 complexes with complementary calixarene derivs. bearing four cationic groups at their upper rim. Each cation is bound by two anions, and vice versa, in a mutual chelate arrangement, reinforced by a network of ionic hydrogen bonds. These multiple electrostatic interactions lead to the formation of highly stable capsule-like assemblies even in polar protic solvents such as methanol and water. In the capsule interior a cavity is formed that is in principle large enough for the encapsulation of small aliph. and arom. guests (170-230 .ANG.3). Monte Carlo simulations in water reproducibly lead to the same regular optimized structures. These differ mainly by their inner vol. and flexibility, as demonstrated by mol. dynamics calcns. Most half-spheres can be synthesized by way of the tetrakis(chloromethyl) or the tetrabromocalixarene intermediate. Oppositely charged calixarenes also form strong complexes, but no indication was found for a lock in the cone conformation. The formation of the ball-shaped complexes from calixarene building blocks was studied with Job plots, NMR titrns., NOESY, and variable-temp. expts., as well as ESI-MS measurements. Investigations aimed at the inclusion of various guest mols. were carried out with alcs., sulfoxides, benzene derivs., and ammonium, as well as pyrazinium guests. Although binding isotherms were generated with cationic guests, these must be considered to be loosely assocd. around the seam rather than included inside the capsule.
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