Kinetics of the electron self-exchange and electron-transfer reactions of the (trimethylammonio)methylferrocene host-guest complex with cucurbit[7]uril in aqueous solution

The electron self-exchange rate constants for the (trimethylammonio)methylferrocene(+/2+) couple (FcTMA+/2+) have been measured in the absence and presence of the cucurbit7]uril (CB7]) host molecule in aqueous solution, using 1H NMR line-broadening experiments. The very strong binding of the ferrocene to CB7] results in slow exchange of the guest on the NMR time scale, such that resonances for both the free and bound forms of the reduced ferrocene can be observed. The extents of line broadening in the resonances of the two forms of the guest in the presence of the FcTMA2+ species can be monitored independently, allowing for the determination of the rate constants for the possible self-exchange pathways involving the bound and free forms of both the oxidized and reduced members of the redox couple. The encapsulation of both the reduced and oxidized forms of the ferrocene increases the rate constant (25 degrees C) from (2.1+/-0.1)x10(6) M-1 s-1 (for FcTMA+/2+) to (6.7+/-0.7)x10(6) M(-1) s(-1) (for {FcTMA.CB7]}+/2+), whereas inclusion of the reduced form only decreases the rate constant to (6+/-1)x10(5) M(-1) s(-1). The changes in the exchange rate constants upon inclusion of the reactants are related to the effects of CB7] acting as an outer, second-coordination sphere and are compared to those observed previously for the electron-exchange process in the presence of beta-cyclodextrin and p-sulfonated calix6]arene hosts. The binding of FcTMA+ and hydroxymethylferrocene to CB7] significantly reduces the rate constants for their oxidations by the bis(2,6-pyridinedicarboxylato)cobaltate(III) ion (which does not bind to CB7]) as a result of reduced thermodynamic driving forces and steric hindrance to close approach of the oxidant to the encapsulated ferrocenes.