Electron Self-Exchange Kinetics for a Water-Soluble Ferrocenium/Ferrocene Couple: Rate Modulation via Charge Dependent Calix[6]arene-p-sulfonate Encapsulation.

In several recent studies we have observed that stepwise encapsulation of simple redox partners by weakly interacting molecular hosts can exert substantial kinetic and/or energetic effects upon thermal (self exchange) and/or optical (intervalence) electron transfer (ET) reactions.1-3 The previous studies featured encapsulation via either hydrogen bonding (ammine ruthenium guests with crown ether hosts1,2) or hydrophobic interactions (trimethylaminomethyl(ferrocenium/ferrocene) (TMAFc2+/+) guests with a â-cyclodextrin host3,4). With either mode of encapsulation, substantial self-exchange rate decreases (i.e., 20to 300-fold) are induced when only one redox partner serves as a guest. Under these conditions electron transfer effectively becomes coupled to host transfer and an energetically unfavorable redox asymmetry effect arises in the isolated ET step.2,3 We reasoned that related effects could exist based on combined electrostatic/hydrophobic association of host and guests. Described below are TMAFc2+/+ self-exchange studies in water with calix[6]arene-p-sulfonate,5 a negatively charged molecular host. This system was chosen because strong host/ guest association had previously been established electrochemically and because the association strength was reported to depend upon guest oxidation state.6 Experimental Section