Transfer of cationic cucurbit[7]uril inclusion complexes from water to non-aqueous solvents

Abstract

The redox-active guests (ferrocenylmethyl)trimethylammonium (2 ⁺) and methylviologen (5 ²⁺) form stable complexes with the cucurbit[7]uril host (CB7) in aqueous solution. In this work, we report a method to transfer these cationic inclusion complexes – as their hexafluorophosphate salts – to less polar, non-aqueous solvents, such as CH₃CN, dimethylformamide and dimethylsulphoxide (DMSO). Both inclusion complexes were found to be stable in these solvents, although the determination of accurate equilibrium association constants was hampered by the low solubility of the free host. Voltammetric experiments show that the one-electron oxidation of the ferrocenyl residue in the CB7·2 ⁺ complex increases its stability in CH₃CN and DMSO solutions, while the opposite effect was observed in aqueous media. These results provide valuable insight into the various roles played by hydrophobic interactions, solvation effects and ion–dipole interactions at stabilising CB7 inclusion complexes of cationic guests.

Keywords:

• host–guest complexes
• cucurbiturils
• electrochemistry
• non-aqueous solvents

Acknowledgements

The authors thank the National Science Foundation for the generous support of this work (to A.E.K., CHE-0848637). W.W. gratefully acknowledges a Maytag graduate fellowship from the University of Miami.

Notes

^†Dedicated to the memory of Prof Dmitry Rudkevic, a good friend and an excellent scientist.