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

A series of outer-sphere cross-electron-transfer reactions involving the [Cu(bite)](2+/+) redox couple has been investigated in acetonitrile at 25 degrees C. In this complex, the bite ligand is a macrocyclic N(2)S(2) ligand with a 2,2'-biphenyl moiety as its backbone. The reaction of [Cu(II)(bite)](2+) with [Ru(hfac)(3)](-) produces [Cu(I)(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) x 10(2) 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 x 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 x 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.