Monotopic and heteroditopic calix[4]arene receptors as hosts for pyridinium and viologen ion pairs: a solution and solid-state study

The binding efficiency of a series of monotopic (1–2) and heteroditopic (3–4) calix[4]arene-based receptors has been evaluated in chloroform solution toward N-methylpyridinium ion pairs using NMR and UV/vis spectroscopic techniques. These experiments provided evidence that, due to a positive cooperative effect, the presence of a phenylurea H-bond donor group on the upper rim of the calix[4]arene macrocycle increases the recognition abilities of the heteroditopic receptors by up to two orders of magnitude with respect to the monotopic receptors. The heteroditopic receptors are also able to form 2:1 host–guest inclusion complexes with dimethylviologen salts both in low polarity solvents and in the solid state. These complexes are stabilized through the cooperation of weak (CH–π and cation–π) and strong (hydrogen bonding) intramolecular interactions between the binding domains of the calix[4]arene host and the two ions of the guest ion pair