Abstract
In aqueous media the deep‐cavity cavitand octaacid 1 forms stable dimeric molecular capsules 12, which are stabilized by hydrophobic effects. In this work we investigate the binding interactions in aqueous solution between these capsules and the redox active guests, ferrocene (Fc) and three 4,4′‐bipyridinium (viologen) dications: methyl viologen (MV2+), ethyl viologen (EV2+), and butyl viologen (BV2+). Using NMR spectroscopic and electrochemical techniques we clearly show that the hydrophobic Fc guest is encapsulated inside 12. An interesting effect of this encapsulation is that the reversible voltammetric response of Fc is completely eliminated when it resides inside the 12 capsular assembly, a finding that is attributed to very slow electrochemical kinetics for the oxidation of Fc@12. Diffusion coefficient measurements (PGSE NMR spectroscopy) reveal that all three viologen guests are strongly bound to the dimeric capsules. However, the 1H NMR spectroscopic data are not consistent with encapsulation and the measured diffusion coefficients indicate that two viologen guests can strongly associate with a single dimeric capsule. Furthermore, the (V2+)2⋅ 12 complex is capable of encapsulating ferrocene, clearly suggesting that the viologen guests are bound externally, via coulombic interactions, to the anionic polar ends of the capsule. The electrochemical kinetic rate constants for the reduction of the viologen residue in the V2+⋅12 complexes were measured and found to be substantially lower than those for the free viologen guests.
Captivated audience! Ferrocene and simple viologen derivatives interact strongly, although in very different ways, with dimeric molecular capsules formed by an octaacid cavitand. Ferrocene is encapsulated and its anodic voltammetric response is eliminated, whereas the viologens are bound to the periphery of the capsule and the electrochemical kinetics for their reduction is attenuated (see figure).
