ELECTROCHEMICAL KINETIC DISCRIMINATION OF THE SINGLE-ELECTRON-TRANSFER EVENTS OF A 2-ELECTRON-TRANSFER REACTION - CYCLIC VOLTAMMETRY OF THE REDUCTION OF THE BIS(HEXAMETHYLBENZENE)RUTHENIUM DICATION

The electrochemical reduction of [(eta-6-C6Me6)2Ru] [BF4]2 has been shown to occur in two one-electron steps, each manifesting solvent-dependent formal potentials. The heterogeneous charge-transfer kinetics varied with electrode material. Cyclic voltammetry (CV) was found to be sensitive to homogeneous reactions which occurred both within the electrode reaction layer and in the bulk of solution. Whereas the reduction waves Of (C6Me6)2RU2+/+/0 were resolved in CH2Cl2 and separated by -0.14 V (E(o)2 - E(o)1, E(o)2 = -1.45 V vs Fc/Fc+), only a single two-electron wave was observed in CH3CN because of a negative shift of E(o)1 with respect to E(o)2 (E(o)2 - E(o)1, = +0.03 V, E(o)2 = -1.40 V). Both reductions displayed Nernstian behavior at Hg electrodes. However, the Ru(I/0) couple showed quasireversible charge-transfer kinetics at Pt disk electrodes. At Pt, the two-electron wave was found to split into its one-electron components over a range of sweep rates which varied with analyte concentration. The Ru(I) complex was also subject to a follow-up reaction having a rate constant of 1.0 s-1. Detailed explicit finite difference simulations of the CV curves allowed solution of the electron-transfer parameters for the two one-electron couples in CH3CN at Pt. The average values from 15 simulations over a scan rate range of 0.4-100 V s-1 and a concentration range of 0.50-1.3 mM were as follows: Ru(II/I), E(o)1 = -1.43 V, k(s1) greater-than-or-equal-to 2 cm s-1; Ru(I/0), E(o)2 = -1.40 V, k(s2) = 4.5 X 10(-4) cm s-1, alpha(o2) = 0.50, d(alpha-2)/dE = 0.22 V-1. The equilibrium constant and rate constant for the disproportionation reaction 2Ru(I) half arrow right over half Ru(II) + Ru(0) were 2.0 and 6.3 x 10(4) M-1 s-1, respectively. The diffusion coefficient of the Ru(II) complex was only about 0.45 times that of the Ru(0) complex. This redox system obeyed an E(rev)E(qrev) model down to experiment times of 10-mu-s. This is believed to be the first recognized example of kinetic discrimination between one-electron processes of a two-electron EE wave.