New guests for the cucurbit[8]uril host. Formation of G ₂H ternary complexes

On the basis of the highly stable G ₂H (2:1) ternary complex formed by two methyl viologen cation radicals inside the cavity of cucurbit[8]uril, we prepared three monocationic 4-phenylpyridinium derivatives: 1-(hydroxyethyl)-4-phenyl-pyridinium (1 ⁺) bromide, 1-(octaethyleneglycol)-4-phenyl-pyridinium (2 ⁺) chloride, and 4-[4-(methoxymethoxy)phenyl]pyridinium (3 ⁺) iodide, as possible guests for 2:1 complexation inside cucurbit[8]uril. We also investigated a fourth monocationic guest (4 ⁺), in which a central vinylidene group is inserted to elongate the 4-phenyl-pyridinium residue. Using ¹H NMR and UV-Vis spectroscopic data and mass spectrometric data, we obtained unequivocal evidence for the formation of G ₂H (2:1) ternary complexes in all cases. The stoichiometry of the complexes was further verified by continuous variation (Job) plots, and in some cases, high resolution ESI-MS spectrometric data. Diffusion coefficient measurements, using ¹H NMR pulse gradient spin echo techniques, yielded values consistent with the formation and expected structures of the ternary complexes.