OXIDATION OF HYDROQUINONES BY [(BPY)2(PY)RU(IV)(O)]2+ AND [(BPY)2(PY)RU(III)(OH)]2+ - PROTON-COUPLED ELECTRON-TRANSFER

The oxidations of hydroquinone, 2-chlorohydroquinone, and 2,6-dichlorohydroquinone by cis-[(bpy)2(py)RuIV(O)]2+ (bpy is 2,2'-bipyridine; py is pyridine) to their corresponding quinones occur in distinct 1e- steps and follow pH dependent rate laws. Within the pH range 1-8 the rate laws have the form -d [Ru(IV) = O2+]/dt = [RU(IV) = O2+][QH2] {k1'[H+] + k2 + k3'[H+]-1}. The term inverse in [H+] involves the hydroquinone anion, QH-, as the reductant. For hydroquinone there is a small rate enhancement below pH 3, which arises from the capture of the semiquinone intermediate by [(bpy)2(py)RuIV(O)]2+, leading to formation of the long-lived (t1/2 = 48 s) intermediate, [(bpy)2(py)RuII(p-benzoquinone)]2+. Below pH 1 there is another source of rate enhancement which appears to arise from a pathway involving the hydroquinone cation, QH3+. Fits to the pH dependent kinetic data (T = 20.0 +/- 0.2-degrees-C, mu = 0.1 M), under conditions which mask the semiquinone reaction, give k1' = 1.1 X 10(7) M-2 s-1, k2 = 9.2 X 10(5) M-1 s-1, and k3' = 1.2 X 10(-2) s-1. The k2 pathway displays a large H2O/4D2O kinetic isotope effect of 30 +/- 1 at 20-degrees-C, which arises largely from the temperature dependence of the reaction: DELTA-H (H2O) = 0.64 +/- 0.04 and DELTA-H(D2O) = 2.12 +/- 0.06 kcal mol-1; DELTA-S(H2O) = -29.0 +/- 0.1 and DELTA-S (D2O) = -30.7 +/- 0.2 cal deg-1 mol-1. A similar rate law was found for cis-[(bpy)2(py)Ru(III)(OH)]2+ as the oxidant with k1' = 1.2 X 10(8) M-2 s-1, k2 = 1.1 X 10(6) M-1 s-1, and k3' = 3.7 x 10(-2) s-1. In strongly acidic solution it is concluded that the oxidation of hydroquinone involves [(bpy)2(py)RuIII(OH2)]3+ with k1 = k1'. K(a)III = 1.7 X 10(7) M-1 s-1. The k2 pathway displays an H2O/D2O kinetic isotope effect of 9.7 +/- 0.1 at 20-degrees-C which also arises almost entirely from the temperature dependence of the reaction: DELTA-H (H2O) = 1.80 +/- 0.05 and DELTA-H(D2O) = 3.28 +/- 0.03 kcal mol-1; DELTA-S(H2O) = -24.7 +/- 0.2 and DELTA-S (D2O) = -24.2 +/- 0.1 cal deg-1 mol-1. Both oxidants display a linear H2O/D2O Mole fraction dependence for k2 suggesting that the isotope effects for these pH independent pathways involve a single proton transfer. With regard to mechanism, it is concluded that (1) the k3 pathway for both oxidants occurs by oxidation of the hydroquinone monoanions, (2) the k, pathway for RuIII involves outer-sphere electron transfer with the aqua ion, [(bpy)2(py)RuIII(OH2)]3+, as oxidant, (3) the k2 pathway for both oxidants occurs by proton-coupled electron transfer for which, in contrast to H-atom transfer, the electron and proton donor functions of the reducing agent reside at different sites within the molecule.