Proton-coupled electron-transfer reactions. Mechanisms of two-electron reduction of trans-dioxoruthenium(VI) to trans-aquooxoruthenium(IV) and disproportionation of trans-dioxoruthenium(V)

The kinetics and mechanism of the reduction of trans-[Ru vI(TMC)(O) 2] 2+ to trans-[Ru Iv(TMC)O(H 2O)] 2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) in aqueous solution by outer-sphere one-electron reductants (R), trans-[Ru VI(TMC)(O) 2] 2+ + 2R + 2H + → trans-[Ru Iv(TMC)O(H 2O)] 2+ + 2R +, have been studied. The first step of the reduction is a simple outer-sphere electron-transfer process, trans-[Ru VI(TMC)(O) 2] 2+ + R → trans-[Ru v(TMC)(O) 2] + + R + with rate = k 2[Ru(VI)] [R]. The rate constants (k 2) with cis-[Ru II(NH 3) 4bpy] 2+ (bpy = 2,2′-bipyridine), [Ru II(NH 3) 5isn] 2+ (isn = isonicotinamide), and [Ru II(NH 3) 5py] 2+ (py = pyridine) have been measured and can be correlated with the Marcus cross relation. The estimated self-exchange rate constant of the trans-[Ru VI(TMC)(O) 2] 2+/ trans-[Ru v(TMC)(O) 2]+ couple is 1.5 × 10 5 mol -1 dm 3 s -1 (I = 0.1 M) at 298 K. When trans-[Ru VI(TMC)(O) 2] 2+ is in excess, the trans-[Ru V(TMC)(O) 2]+ species formed will undergo rapid disproportionation, 2trans-[Ru v(TMC)(O) 2] + + 2H + → trans-[Ru VI(TMC)(O) 2] 2+ + trans-[Ru IV(TMC)O(H 2O)] 2+. The disproportionation follows the rate law: rate = K dis[Ru(V)] 2 with K dis = 2k eK p[H +]/(1 + K p[H +]) 2. The K p and k e are referred to the equilibrium constant and rate constant for the respective reactions, trans-[Ru v(TMC)(O) 2] 2+ + H +⇄ trans-[Ru v(TMC)O(OH)] 2+ and trans-(Ru v(TMC)O(OH)] 2+ + trans-[Ru v(TMC)(O) 2] + → trans-[Ru VI(TMC)(O) 2] 2+ + trans-[Ru IV(TMC)O(OH)] +. At 299 K and I = 0.1 M, K p and k c have been determined to be 615 ± 50 mol -1 dm 3 and (2.72 ± 0.34) × 10 6 mol -1 dm 3 s -1, respectively. The self-exchange rate constant of the trans-[Ru v(TMC)O(OH)] 2+/trans-[Ru IV(TMC)O(OH)] + has been estimated to be 5 × 10 3 mol -1 dm 3 s -1 at 298 K and I = 0.1 M.