Inclusion of methylviologen in symmetrical α,α′,δ,δ′-tetramethyl-cucurbit[6]uril

Abstract

The host–guest interaction between symmetrical α,α′,δ,δ′-tetramethyl-cucurbit[6]uril (TMeQ[6]) and methylviologen (N,N′-dimethyl-4,4′-bipyridinium, MV) dication was investigated both in aqueous solution and in the solid state using NMR spectroscopic methods and single-crystal X-ray diffraction analysis. In the aqueous solution, TMeQ[6] forms a 1 : 1 inclusion complex with methylviologen MV²⁺, and the chemical exchange of the MV²⁺ guest in and out of the cavity of the TMeQ[6] host was fast on the NMR time scale. In the solid state, however, the MV²⁺ guest was partially encapsulated into the TMeQ[6] host. We found that the chemical environment of the TMeQ[6] host underwent a severe change during the encapsulation process. Interestingly, electrochemical studies revealed that, unlike other cucurbiturils, TMeQ[6] has the same level of binding affinity to the charged forms (MV²⁺ and MV⁺˙) and the fully reduced form (MV⁰). These studies contribute to the fundamental understanding of the interdependence of electron-transfer processes and molecular recognition.