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Cross-Electron-Transfer Reactions of the [CuII/I(bite)]2+/+ Redox Couple

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journal contribution
posted on 08.06.2001, 00:00 by Boping Xie, Lon J. Wilson, David M. Stanbury
A series of outer-sphere cross-electron-transfer reactions involving the [Cu(bite)]2+/+ redox couple has been investigated in acetonitrile at 25 °C. In this complex, the bite ligand is a macrocyclic N2S2 ligand with a 2,2‘-biphenyl moiety as its backbone. The reaction of [CuII(bite)]2+ with [Ru(hfac)3]- produces [CuI(bite)]+ and [Ru(hfac)3] reversibly with K = 1.9. The rate law is second order in both directions, with a rate constant of (8.22 ± 0.27) × 102 M-1 s-1 in the forward direction. Rate constants were also obtained for the irreversible reactions of three Co(II) clathrochelates with [Cu(bite)]2+. The oxidation of [Cu(bite)]+ by [Fe(bpy)3]3+ was studied in order to obtain a rate constant for oxidation as well as reduction. Application of the Marcus cross relationship to these rate constants gives apparent self-exchange rate constants that are reasonably consistent yet unusually low, with an average value of 1.0 × 10-2 M-1 s-1. The self-consistence of the apparent self-exchange rate constants implies that all of the cross reactions proceed through the same intermediate, and hence, the outer-sphere self-exchange reaction should have a second-order rate law with k = 1.0 × 10-2 M-1 s-1. The much faster first-order self-exchange process reported previously for the [Cu(bite)]2+/+ couple in acetone implies a more efficient mechanism for the self-exchange reaction than for the cross reactions, such as an inner-sphere mechanism. Cyclic voltammograms of [Cu(bite)]n+ are strongly sensitive to the nature of the working electrode, thus precluding the use of these data in interpreting the homogeneous redox kinetics.