Calix[4]arene-Based Rotaxane Host Systems for Anion Recognition

The synthesis, structure and anion binding properties of the first calix[4]arene-based [2]rotaxane anion host systems are described. Rotaxanes 9 center dot Cl and 12 center dot Cl, consisting of a calix[4]arene functionalised macrocycle wheel and different pyridinium axle components, are prepared via adaption of an anion templated synthetic strategy to investigate the effect of preorganisation of the interlocked host's binding cavity on anion binding. Rotaxane 12 center dot Cl contains a conformationally flexible pyridinium axle, whereas rotaxane 9 center dot Cl incorporates a more preorganised pyridiniurn axle component. The X-ray crystal structure of 9 center dot Cl and solution phase H-1 NMR spectroscopy demonstrate the successful interlocking of the calix[4]arene macrocycle and pyridinium axle components in the rotaxane structures. Following removal of the chloride anion template, anion binding studies on the resulting rotaxanes 9 center dot PF6 and 12 center dot PF6 reveal the importance of preorganisation of the host binding cavity on anion binding. The more preorganised rotaxane 9 center dot PF6 is the superior anion host system. The interlocked host cavity is selective for chloride in 1:1 CDCl3/CD3OD and remains selective for chloride and bromide in 10% aqueous media over the more basic oxoanions. Rotaxane 12 center dot PF6 with a relatively conformationally flexible binding cavity is a less effective and discriminating anion host system although the rotaxane still binds halide anions in preference to oxoanions.