Journal
EUROPEAN JOURNAL OF ORGANIC CHEMISTRY
Volume 2021, Issue 10, Pages 1547-1552Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/ejoc.202100014
Keywords
Complex stoichiometry; Crystal structure; Cucurbit[8]uril; Host-guest complexation; Viologens
Categories
Funding
- 'Departments of Excellence' program of the Italian Ministry for Education, University and Research (MIUR, 2018-2022)
- MIUR through the PRIN project [20179BJNA2]
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The study shows that as the alkyl chain length increases from 12 carbon atoms to 16 carbon atoms, the binding mode of viologens with cucurbit[8]uril in water switches from a 1:1 to 2:1 host/guest stoichiometry. This is due to the folding of the major portion of the longer alkyl chain in one CB[8] cavity and the inclusion of the full viologen unit by another host molecule.
Viologens are among the most studied guests for cucurbit[8]uril (CB[8]) and their complexation is usually driven by bipyridyl core inclusion inside the cavity to maximize both hydrophobic and cation-dipole interactions. The presence of alkyl substituents on the guest alters this complexation mode, switching to aliphatic chain inclusion in U-folded conformation. Herein, we report a thorough study of the influence of the alkyl chain length on the binding mode of methyl alkyl viologens. The chain length of the studied guests was increased by two methylene groups starting from methyl dodecyl viologen (MVC12) to the octadecyl analogue (MVC18). Complexation in water, investigated by NMR spectroscopy and ITC, revealed a clear switch from 1 : 1 to 2 : 1 host/guest stoichiometry moving from 12 to 16 carbon atoms, as a consequence of the chain folding of the major portion of the longer alkyl chain in one CB[8] cavity and the inclusion of the full viologen unit by another host molecule. The CB[8](2)(.)MVC18 complex crystal structure evidences the unprecedented 2 : 1 stoichiometry and quantified in 12 the number of carbon atoms necessary to fill the CB[8] cavity in U-shaped conformation.
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