Cucurbit[7]uril (CB7), a member of the cucurbit[n]uril host family, is large enough to form highly stable inclusion complexes with a wide range of simple three-dimensional guests (log K&gt;12), and exhibits a great water solubility of 30 mM. The properties of CB7 have demonstrated that it is possible to bind with synthetic receptors in aqueous solvent at picomolar magnitude that was previously thought as the exclusive terrain of biomolecules. Previously, we investigated the binding interactions of the CB7 host with a series of viologen derivatives and discovered that the encapsulation of the viologen moiety by the host only takes place when the N-substituents are either rather small or hydrophilic. In the current work, a new zwitterionic axle compounds containing a central viologen unit and two sulfonate terminated aliphatic N-substituents was prepared. This guest was chosen in order to probe the kinetic rate of association and dissociation of CB7 due to the electrostatic barriers created by the terminal sulfonate negative charges. We investigated this point, using a cationic ferocene derivative as the competitive guest to force the dissociation of the complex. The kinetic experiments were performed by 1H NMR to measure the dissociation rate constant of the 1:1 adduct: kout=2.09×10-4 s-1. The data of UV-Vis spectroscopic titrations indicate that the association constant for the formation of the 1:1 adduct is on the order of 6.02×104 M-1, which is lower relative to literature data of CB7 encircling the bipyridinium unit (1.5-5×105 M-1), suggesting that the thermodynamics of the association process is influenced by the negative charges at the terminal points of the axle. Furthermore, the association rate constant kin was calculated as K×kout =12.6 M-1s-1. Clearly, the negatively charged sulfonate units lead to a pronounced slow down on the kinetics of threading and unthreading by the CB7 host. ESI-MS data also helped us to support the conclusions derived from the spectroscopic measurements. We anticipate that the kinetics study of passage of CB7 over negatively charged arm of the zwitterionic viologen guest will help inform the design of switchable pseudorotaxanes, shuttle rotaxanes or related nanomolecular devices.

Kinetics of formation of the host-guest complex of zwitterionic viologen with cucurbit[7]uril

255th National Meeting (2018)

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