Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex

The kinetics and mechanisms of the oxidation of I (-) and Br (-) by trans-[Ru (VI)(N 2O 2)(O) 2] (2+) have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru (VI)(N 2O 2)(O) 2] (2+) + 3X (-) + 2H (+) --> trans-[Ru (IV)(N 2O 2)(O)(OH 2)] (2+) + X 3 (-) (X = Br, I). In the oxidation of I (-) the I 3 (-)is produced in two distinct phases. The first phase produces 45% of I 3 (-) with the rate law d[I 3 (-)]/dt = ( k a + k b[H (+)])[Ru (VI)][I (-)]. The remaining I 3 (-) is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I (-)], [H (+)], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru (VI) and I (-), which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru (IV)(N 2O 2)(O)(OHI)] (2+), and a one electron transfer to give [Ru (V)(N 2O 2)(O)(OH)] (2+) and I (*). [Ru (V)(N 2O 2)(O)(OH)] (2+) is a stronger oxidant than [Ru (VI)(N 2O 2)(O) 2] (2+) and will rapidly oxidize another I (-) to I (*). In the second phase the [Ru (IV)(N 2O 2)(O)(OHI)] (2+) undergoes rate-limiting aquation to produce HOI which reacts rapidly with I (-) to produce I 2. In the oxidation of Br (-) the rate law is -d[Ru (VI)]/d t = {( k a2 + k b2[H (+)]) + ( k a3 + k b3[H (+)]) [Br (-)]}[Ru (VI)][Br (-)]. At 298.0 K and I = 0.1 M, k a2 = (2.03 +/- 0.03) x 10 (-2) M (-1) s (-1), k b2 = (1.50 +/- 0.07) x 10 (-1) M (-2) s (-1), k a3 = (7.22 +/- 2.19) x 10 (-1) M (-2) s (-1) and k b3 = (4.85 +/- 0.04) x 10 (2) M (-3) s (-1). The proposed mechanism involves initial oxygen atom transfer from trans-[Ru (VI)(N 2O 2)(O) 2] (2+) to Br (-) to give trans-[Ru (IV)(N 2O 2)(O)(OBr)] (+), which then undergoes parallel aquation and oxidation of Br (-), and both reactions are acid-catalyzed.     

Kinetics and mechanism of the oxidation of alkylaromatic compounds by a trans-dioxoruthenium(VI) complex

The oxidations of a series of 21 alkylaromatic compounds by trans-[Ru(VI)(L)(O)(2)](2+) (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in CH(3)CN. Toluene is oxidized to benzaldehyde and a small amount of benzyl alcohol. 9,10-Dihydroanthracene is oxidized to anthracene and anthraquinone. Other substrates give oxygenated products. The kinetics of the reactions were monitored by UV-vis spectrophotometry, and the rate law is: -d[Ru(VI)]/dt = k(2)[Ru(VI)][ArCH(3)]. The kinetic isotope effects for the oxidation of toluene/d(8)-toluene and fluorene/d(10)-fluorene are 15 and 10.5, respectively. A plot of Delta H(++) versus Delta S(++) is linear, suggesting a common mechanism for all the substrates. In the oxidation of para-substituted toluenes, a linear correlation between log k(2) and sigma(0) values is observed, consistent with a benzyl radical intermediate. A linear correlation between Delta G(++) and Delta H(0) (the difference between the strength of the bond being broken and that being formed in a H-atom transfer step) is also found, which strongly supports a hydrogen atom transfer mechanism for the oxidation of these substrates by trans-[Ru(VI)(L)(O)(2)](2+). The slope of (0.61 +/- 0.06) is in reasonable agreement with the theoretical slope of 0.5 predicted by Marcus theory.     

Kinetics and mechanism of the oxidation of hydroquinones by a trans-dioxoruthenium(VI) complex

The kinetics of the oxidation of hydroquinone (H(2)Q) and its derivatives (H(2)Q-X) by trans-[Ru(VI)(tmc)(O)(2)](2+) (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) have been studied in aqueous acidic solutions and in acetonitrile. In H(2)O, the oxidation of H(2)Q has the following stoichiometry: trans-[Ru(VI)(tmc)(O)(2)](2+) + H(2)Q --> trans-[Ru(IV)(tmc)(O)(OH(2))](2+) + Q. The reaction is first order in both Ru(VI) and H(2)Q, and parallel pathways involving the oxidation of H(2)Q and HQ(-) are involved. The kinetic isotope effects are k(H(2)O)/k(D(2)O) = 4.9 and 1.2 at pH = 1.79 and 4.60, respectively. In CH(3)CN, the reaction occurs in two steps, the reduction of trans-[Ru(VI)(tmc)(O)(2)](2+) by 1 equiv of H(2)Q to trans-[Ru(IV)(tmc)(O)(CH(3)CN)](2+), followed by further reduction by another 1 equiv of H(2)Q to trans-[Ru(II)(tmc)(CH(3)CN)(2)](2+). Linear correlations between log(rate constant) at 298.0 K and the O-H bond dissociation energy of H(2)Q-X were obtained for reactions in both H(2)O and CH(3)CN, consistent with a H-atom transfer (HAT) mechanism. Plots of log(rate constant) against log(equilibrium constant) were also linear for these HAT reactions.     

Imido transfer from bis(imido)ruthenium(VI) porphyrins to hydrocarbons: effect of imido substituents, C-H bond dissociation energies, and Ru(VI/V) reduction potentials

[Ru(VI)(TMP)(NSO2R)2] (SO2R = Ms, Ts, Bs, Cs, Ns; R = p-C6H4OMe, p-C6H4Me, C6H5, p-C6H4Cl, p-C6H4NO2, respectively) and [Ru(VI)(Por)(NTs)2] (Por = 2,6-Cl2TPP, F20-TPP) were prepared by the reactions of [Ru(II)(Por)(CO)] with PhI=NSO2R in CH2Cl2. These complexes exhibit reversible Ru(VI/V) couple with E(1/2) = -0.41 to -0.12 V vs Cp2Fe(+/0) and undergo imido transfer reactions with styrenes, norbornene, cis-cyclooctene, indene, ethylbenzenes, cumene, 9,10-dihydroanthracene, xanthene, cyclohexene, toluene, and tetrahydrofuran to afford aziridines or amides in up to 85% yields. The second-order rate constants (k2) of the aziridination/amidation reactions at 298 K were determined to be (2.6 +/- 0.1) x 10(-5) to 14.4 +/- 0.6 dm3 mol(-1) s(-1), which generally increase with increasing Ru(VI/V) reduction potential of the imido complexes and decreasing C-H bond dissociation energy (BDE) of the hydrocarbons. A linear correlation was observed between log k' (k' is the k2 value divided by the number of reactive hydrogens) and BDE and between log k2 and E(1/2)(Ru(VI/V)); the linearity in the former case supports a H-atom abstraction mechanism. The amidation by [Ru(VI)(TMP)(NNs)2] reverses the thermodynamic reactivity order cumene > ethylbenzene/toluene, with k'(tertiary C-H)/k'(secondary C-H) = 0.2 and k'(tertiary C-H)/k'(primary C-H) = 0.8.     

Oxygen atom transfer from a trans-dioxoruthenium(VI) complex to nitric oxide

In aqueous acidic solutions trans-[Ru(VI)(L)(O)(2)](2+) (L=1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) is rapidly reduced by excess NO to give trans-[Ru(L)(NO)(OH)](2+). When ≤1 mol equiv NO is used, the intermediate Ru(IV) species, trans-[Ru(IV)(L)(O)(OH(2))](2+), can be detected. The reaction of [Ru(VI)(L)(O)(2)](2+) with NO is first order with respect to [Ru(VI)] and [NO], k(2)=(4.13±0.21)×10(1) M(-1) s(-1) at 298.0 K. ΔH(≠) and ΔS(≠) are (12.0±0.3) kcal mol(-1) and -(11±1) cal mol(-1) K(-1), respectively. In CH(3)CN, ΔH(≠) and ΔS(≠) have the same values as in H(2)O; this suggests that the mechanism is the same in both solvents. In CH(3)CN, the reaction of [Ru(VI)(L)(O)(2)](2+) with NO produces a blue-green species with λ(max) at approximately 650 nm, which is characteristic of N(2)O(3). N(2)O(3) is formed by coupling of NO(2) with excess NO; it is relatively stable in CH(3)CN, but undergoes rapid hydrolysis in H(2)O. A mechanism that involves oxygen atom transfer from [Ru(VI)(L)(O)(2)](2+) to NO to produce NO(2) is proposed. The kinetics of the reaction of [Ru(IV)(L)(O)(OH(2))](2+) with NO has also been investigated. In this case, the data are consistent with initial one-electron O(-) transfer from Ru(IV) to NO to produce the nitrito species [Ru(III)(L)(ONO)(OH(2))](2+) (k(2)>10(6) M(-1) s(-1)), followed by a reaction with another molecule of NO to give [Ru(L)(NO)(OH)](2+) and NO(2)(-) (k(2)=54.7 M(-1) s(-1)).     

Hydrocarbon oxidation by beta-halogenated dioxoruthenium(VI) porphyrin complexes: effect of reduction potential (RuVI/V) and C-H bond-dissociation energy on rate constants

beta-Halogenated dioxoruthenium(VI) porphyrin complexes [Ru(VI)(F(28)-tpp)O(2)] [F(28)-tpp=2,3,7,8,12,13, 17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato(2-)] and [Ru(VI)(beta-Br(8)-tmp)O(2)] [beta-Br(8)-tmp=2,3,7,8,12,13,17,18-octabromo-5,10,15,20- tetrakis(2,4,6-trimethylphenyl)porphyrinato(2-)] were prepared from reactions of [Ru(II)(por)(CO)] [por=porphyrinato(2-)] with m-chloroperoxybenzoic acid in CH(2)Cl(2). Reactions of [Ru(VI)(por)O(2)] with excess PPh(3) in CH(2)Cl(2) gave [Ru(II)(F(20)-tpp)(PPh(3))(2)] [F(20)-tpp=5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato(2-)] and [Ru(II)(F(28)-tpp)(PPh(3))(2)]. The structures of [Ru(II)(por)(CO)(H(2)O)] and [Ru(II)(por)(PPh(3))(2)] (por=F(20)-tpp, F(28)-tpp) were determined by X-ray crystallography, revealing the effect of beta-fluorination of the porphyrin ligand on the coordination of axial ligands to ruthenium atom. The X-ray crystal structure of [Ru(VI)(F(20)-tpp)O(2)] shows a Ru=O bond length of 1.718(3) A. Electrochemical reduction of [Ru(VI)(por)O(2)] (Ru(VI) to Ru(V)) is irreversible or quasi-reversible, with the E(p,c)(Ru(VI/V)) spanning -0.31 to -1.15 V versus Cp(2)Fe(+/0). Kinetic studies were performed for the reactions of various [Ru(VI)(por)O(2)], including [Ru(VI)(F(28)-tpp)O(2)] and [Ru(VI)(beta-Br(8)-tmp)O(2)], with para-substituted styrenes p-X-C(6)H(4)CH=CH(2) (X=H, F, Cl, Me, MeO), cis- and trans-beta-methylstyrene, cyclohexene, norbornene, ethylbenzene, cumene, 9,10-dihydroanthracene, xanthene, and fluorene. The second-order rate constants (k(2)) obtained for the hydrocarbon oxidations by [Ru(VI)(F(28)-tpp)O(2)] are up to 28-fold larger than by [Ru(VI)(F(20)-tpp)O(2)]. Dual-parameter Hammett correlation implies that the styrene oxidation by [Ru(VI)(F(28)-tpp)O(2)] should involve rate-limiting generation of a benzylic radical intermediate, and the spin delocalization effect is more important than the polar effect. The k(2) values for the oxidation of styrene and ethylbenzene by [Ru(VI)(por)O(2)] increase with E(p,c)(Ru(VI/V)), and there is a linear correlation between log k(2) and E(p,c)(Ru(VI/V)). The small slope (approximately 2 V(-1)) of the log k(2) versus E(p,c)(Ru(VI/V)) plot suggests that the extent of charge transfer is small in the rate-determining step of the hydrocarbon oxidations. The rate constants correlate well with the C-H bond dissociation energies, in favor of a hydrogen-atom abstraction mechanism.     

Stepwise oxidation of anilines by cis-[RuIV(bpy)2(py)(O)]2+

The net six-electron oxidation of aniline to nitrobenzene or azoxybenzene by cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (bpy is 2,2'-bipyridine; py is pyridine) occurs in a series of discrete stages. In the first, initial two-electron oxidation is followed by competition between oxidative coupling with aniline to give 1,2-diphenylhydrazine and capture by H(2)O to give N-phenylhydroxylamine. The kinetics are first order in aniline and first order in Ru(IV) with k(25.1 degrees C, CH(3)CN) = (2.05 +/- 0.18) x 10(2) M(-1) s(-1) (DeltaH(++) = 5.0 +/- 0.7 kcal/mol; DeltaS(++) = -31 +/- 2 eu). On the basis of competition experiments, k(H)2(O)/k(D)2(O) kinetic isotope effects, and the results of an (18)O labeling study, it is concluded that the initial redox step probably involves proton-coupled two-electron transfer from aniline to cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (Ru(IV)=O(2+)). The product is an intermediate nitrene (PhN) or a protonated nitrene (PhNH(+)) which is captured by water to give PhNHOH or aniline to give PhNHNHPh. In the following stages, PhNHOH, once formed, is rapidly oxidized by Ru(IV)=O(2+) to PhNO and PhNHNHPh to PhN=NPh. The rate laws for these reactions are first order in Ru(IV)=O(2+) and first order in reductant with k(14.4 degrees C, H(2)O/(CH(3))(2)CO) = (4.35 +/- 0.24) x 10(6) M(-1) s(-1) for PhNHOH and k(25.1 degrees C, CH(3)CN) = (1.79 +/- 0.14) x 10(4) M(-1) s(-1) for PhNHNHPh. In the final stages of the six-electron reactions, PhNO is oxidized to PhNO(2) and PhN=NPh to PhN(O)=NPh. The oxidation of PhNO is first order in PhNO and in Ru(IV)=O(2+) with k(25.1 degrees C, CH(3)CN) = 6.32 +/- 0.33 M(-1) s(-1) (DeltaH(++) = 4.6 +/- 0.8 kcal/mol; DeltaS(++) = -39 +/- 3 eu). The reaction occurs by O-atom transfer, as shown by an (18)O labeling study and by the appearance of a nitrobenzene-bound intermediate at low temperature.     

Oxidative homolysis of a nitrosylchromium complex

Metal(III)-polypyridine complexes [M(NN)(3)](3+) (M = Ru or Fe; NN = bipyridine (bpy), phenanthroline (phen), or 4,7-dimethylphenanthroline (Me(2)-phen)) oxidize the nitrosylpentaaquachromium(III) ion, [Cr(aq)NO](2+), with an overall 4:1 stoichiometry, 4 [Ru(bpy)(3)](3+) + [Cr(aq)NO](2+) + 2 H(2)O --> 4 [Ru(bpy)(3)](2+) + [Cr(aq)](3+) + NO(3)(-) + 4 H(+). The kinetics follow a mixed second-order rate law, -d[[M(NN)(3)](3+)]/dt = nk[[M(NN)(3)](3+)][[Cr(aq)NO](2+)], in which k represents the rate constant for the initial one-electron transfer step, and n = 2-4 depending on reaction conditions and relative rates of the first and subsequent steps. With [Cr(aq)NO](2+) in excess, the values of nk are 283 M(-1) s(-1) ([Ru(bpy)(3)](3+)), 7.4 ([Ru(Me(2)-phen)(3)](3+)), and 5.8 ([Fe(phen)(3)](3+)). In the proposed mechanism, the one-electron oxidation of [Cr(aq)NO](2+) releases NO, which is further oxidized to nitrite, k = 1.04x10(6) M(-1) s(-1), 6.17x10(4), and 1.12x10(4) with the three respective oxidants. Further oxidation yields the observed nitrate. The kinetics of the first step show a strong correlation with thermodynamic driving force. Parallels were drawn with oxidative homolysis of a superoxochromium(III) ion, [Cr(aq)OO](2+), to gain insight into relative oxidizability of coordinated NO and O(2), and to address the question of the "oxidation state" of coordinated NO in [Cr(aq)NO](2+).     

Dichlororuthenium(IV) complex of meso-tetrakis(2,6-dichlorophenyl)porphyrin: active and robust catalyst for highly selective oxidation of arenes, unsaturated steroids, and electron-deficient alkenes by using 2,6-dichloropyridine N-oxide

[Ru(IV)(2,6-Cl2tpp)Cl2], prepared in 90 % yield from the reaction of [Ru(VI)(2,6-Cl2tpp)O2] with Me3SiCl and structurally characterized by X-ray crystallography, is markedly superior to [Ru(IV)(tmp)Cl2], [Ru(IV)(ttp)Cl2], and [Ru(II)(por)(CO)] (por=2,6-Cl2tpp, F20-tpp, F28-tpp) as a catalyst for alkene epoxidation with 2,6-Cl2pyNO (2,6-Cl2tpp=meso-tetrakis(2,6-dichlorophenyl)porphyrinato dianion; tmp=meso-tetramesitylporphyrinato dianion; ttp=meso-tetrakis(p-tolyl)porphyrinato dianion; F20-tpp=meso-tetrakis(pentafluorophenyl)porphyrinato dianion; F28-tpp=2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato dianion). The "[Ru(IV)(2,6-Cl2tpp)Cl2]+2,6-Cl2pyNO" protocol oxidized, under acid-free conditions, a wide variety of hydrocarbons including 1) cycloalkenes, conjugated enynes, electron-deficient alkenes (to afford epoxides), 2) arenes (to afford quinones), and 3) Delta5-unsaturated steroids, Delta4-3-ketosteroids, and estratetraene derivatives (to afford epoxide/ketone derivatives of steroids) in up to 99 % product yield within several hours with up to 100 % substrate conversion and excellent regio- or diastereoselectivity. Catalyst [Ru(IV)(2,6-Cl2tpp)Cl2] is remarkably active and robust toward the above oxidation reactions, and turnover numbers of up to 6.4x10(3), 2.0x10(4), and 1.6x10(4) were obtained for the oxidation of alpha,beta-unsaturated ketones, arenes, and Delta5-unsaturated steroids, respectively.     

Kinetics and mechanism of chromate reduction with hydrogen peroxide in base

[Cr(VI)O(4)](2)(-) is reduced to [Cr(V)(O(2))(4)](3)(-) by hydrogen peroxide in strongly basic media where the acid dissociation of H(2)O(2) (pK(a) = 11.65) is appreciable. The reaction is first order in chromium(VI) and inhibited by hydroxide. The hydrogen peroxide dependence is defined by the form of the effective pseudo-first-order rate constant: k(eff) = [H(2)O(2)](3)/(K(1) + K(2)[H(2)O(2)] + K(3)[HO(2)(-)]) with K(1) = 175(43) s x M(3), K(2) = 403(18) s x M(2), and K(3) = 1422(34) s x M(2). Hydrogen peroxide anion initially attacks chromate, and subsequent equilibrium steps that exchange oxo groups for three peroxo groups precede a rate-determining, one-electron, intramolecular reduction step.     

Oxidation of nitrite by a trans-dioxoruthenium(VI) complex: direct evidence for reversible oxygen atom transfer

Reaction of trans-[Ru(VI)(L)(O)(2)](2+) (1, L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane, a tetradentate macrocyclic ligand with N(2)O(2) donor atoms) with nitrite in aqueous solution or in H(2)O/CH(3)CN produces the corresponding (nitrato)oxoruthenium(IV) species, trans-[Ru(IV)(L)(O)(ONO(2))](+) (2), which then undergoes relatively slow aquation to give trans-[Ru(IV)(L)(O)(OH(2))](2+). These processes have been monitored by both ESI/MS and UV/vis spectrophotometry. The structure of trans-[Ru(IV)(L)(O)(ONO(2))](+) (2) has been determined by X-ray crystallography. The ruthenium center adopts a distorted octahedral geometry with the oxo and the nitrato ligands trans to each other. The Ru=O distance is 1.735(3) A, the Ru-ONO(2) distance is 2.163(4) A, and the Ru-O-NO(2) angle is 138.46(35) degrees . Reaction of trans-[Ru(VI)(L)((18)O)(2)](2+) (1-(18)O(2)) with N(16)O(2)(-) in H(2)O/CH(3)CN produces the (18)O-enriched (nitrato)oxoruthenium(IV) species 2-(18)O(2). Analysis of the ESI/MS spectrum of 2-(18)O(2) suggests that scrambling of the (18)O atoms has occurred. A mechanism that involves linkage isomerization of the nitrato ligand and reversible oxygen atom transfer is proposed.     

Hydrogen atom transfer reactions of imido manganese(V) corrole: one reaction with two mechanistic pathways

Hydrogen atom transfer (HAT) reactions of (tpfc)MnNTs have been investigated (tpfc = 5,10,15-tris(pentafluorophenyl)corrole and Ts = p-toluenesulfonate). 9,10-Dihydroanthracene and 1,4-dihydrobenzene reduce (tpfc)MnNTs via HAT with second-order rate constants 0.16 +/- 0.03 and 0.17 +/- 0.01 M(-1) s(-1), respectively, at 22 degrees C. The products are the respective arenes, TsNH(2) and (tpfc)Mn(III). Conversion of (tpfc)MnNTs to (tpfc)Mn by reaction with dihydroanthracene exhibits isosbestic behavior, and formation of 9,9',10,10'-tetrahydrobianthracene is not observed, suggesting that the intermediate anthracene radical rebounds in a second fast step without accumulation of a Mn(IV) intermediate. The imido complex (tpfc)Mn(V)NTs abstracts a hydrogen atom from phenols as well. For example, 2,6-di-tert-butyl phenol is oxidized to the corresponding phenoxyl radical with a second-order rate constant of 0.32 +/- 0.02 M(-1) s(-1) at 22 degrees C. The other products from imido manganese(V) are TsNH(2) and the trivalent manganese corrole. Unlike reaction with dihydroarenes, when phenols are used isosbestic behavior is not observed, and formation of (tpfc)Mn(IV)(NHTs) is confirmed by EPR spectroscopy. A Hammett plot for various p-substituted 2,6-di-tert-butyl phenols yields a V-shaped dependence on sigma, with electron-donating substituents exhibiting the expected negative rho while electron-withdrawing substituents fall above the linear fit (i.e., positive rho). Similarly, a bond dissociation enthalpy (BDE) correlation places electron-withdrawing substituents above the well-defined negative slope found for the electron-donating substituents. Thus two mechanisms are established for HAT reactions in this system, namely, concerted proton-electron transfer and proton-gated electron transfer in which proton transfer is followed by electron transfer.     

Structural determination of ruthenium-porphyrin complexes relevant to catalytic epoxidation of olefins

A reproducible synthesis of a competent epoxidation catalyst, [Ru(VI)(TPP)(O)2)] (TPP = tetraphenylporphyrin dianion), starting from [Ru(II)(TPP)(CO)L] (L = none or CH3OH), is described. The molecular structure of the complex was determined by using ab initio X-ray powder diffraction (XRPD) methods, and its solution behavior was in detail investigated by NMR techniques such as PGSE (pulsed field gradient spin-echo) measurements. [Ru(IV)(TPP)(OH)]2O, a reported byproduct in the synthesis of [Ru(VI)(TPP)(O)2], was synthesized in a pure form by oxidation of [Ru(II)(TPP)(CO)L] or by a coproportionation reaction of [Ru(VI)(TPP)(O)2] and [Ru(II)(TPP)(CO)L], and its molecular structure was then determined by XRPD analysis. [Ru(VI)(TPP)(O)2] can be reduced by dimethyl sulfoxide or by carbon monoxide to yield [Ru(II)(TPP)(S-DMSO)2] or [Ru(II)(TPP)(CO)(H2O)], respectively. These two species were characterized by conventional single-crystal X-ray diffraction analysis.     

Primary and secondary phosphane complexes of metalloporphyrins: isolation, spectroscopy, and X-ray crystal structures of ruthenium and osmium porphyrins binding phenyl- or diphenylphosphane

[Ru(II)(por)(PH(n)Ph(3-n))2], [Os(II)(por)(CO)(PH(n)Ph(3-n))] (n=1, 2), and [Os(II)(F20-tpp){P(OH)Ph2}(PHPh2)] (F20-tpp=5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato dianion) were prepared from the reaction of [M(II)(por)(CO)] (M=Ru, Os) or [Os(VI)(por)O2] with the respective primary/secondary phosphane and characterized by 1H NMR, 31P NMR, UV/Vis, and IR spectroscopy, mass spectrometry, and elemental analysis. The reaction of [Os(VI)(por)O2] with PHPh2 also gave minor amounts of [Os(II)(por){P(OH)Ph2}2]. [Ru(II)(F20-tpp)(PH2Ph)2] exhibits a remarkable stability toward air and shows a reversible metal-centered oxidation couple at E(1/2)=0.39 V versus [Cp2Fe](+/0) in the cyclic voltammogram. The structures of [Ru(II)(F20-tpp)(PH2Ph)2] x 2CH2Cl2, [Ru(II)(4-Cl-tpp)(PHPh2)2] x 2CH2Cl2 (4-Cl-tpp=5,10,15,20-tetrakis(p-chlorophenyl)porphyrinato dianion), [Ru(II)(F20-tpp)(PHPh2)2], and [Os(II)(F20-tpp){P(OH)Ph2}2] were determined by X-ray crystallography and feature Ru-P distances of 2.3397(11)-2.3609(9) A and an Os-P distance of 2.369(2) A.     

Mechanism of oxidation of benzaldehyde by polypyridyl oxo complexes of Ru(IV)

The oxidation of benzaldehyde and several of its derivatives to their carboxylic acids by cis-[Ru(IV)(bpy)2(py)(O)]2+ (Ru(IV)=O2+; bpy is 2,2'-bipyridine, py is pyridine), cis-[Ru(III)(bpy)2(py)(OH)]2+ (Ru(III)-OH2+), and [Ru(IV)(tpy)(bpy)(O)]2+ (tpy is 2,2':6',2'-terpyridine) in acetonitrile and water has been investigated using a variety of techniques. Several lines of evidence support a one-electron hydrogen-atom transfer (HAT) mechanism for the redox step in the oxidation of benzaldehyde. They include (i) moderate k(C-H)/k(C-D) kinetic isotope effects of 8.1 +/- 0.3 in CH3CN, 9.4 +/- 0.4 in H2O, and 7.2 +/- 0.8 in D2O; (ii) a low k(H2O/D2O) kinetic isotope effect of 1.2 +/- 0.1; (iii) a decrease in rate constant by a factor of only approximately 5 in CH3CN and approximately 8 in H2O for the oxidation of benzaldehyde by cis-[Ru(III)(bpy)2(py)(OH)]2+ compared to cis-[Ru(IV)(bpy)2(py)(O)]2+; (iv) the appearance of cis-[Ru(III)(bpy)2(py)(OH)]2+ rather than cis-[Ru(II)(bpy)2(py)(OH2)]2+ as the initial product; and (v) the small rho value of -0.65 +/- 0.03 in a Hammett plot of log k vs sigma in the oxidation of a series of aldehydes. A mechanism is proposed for the process occurring in the absence of O2 involving (i) preassociation of the reactants, (ii) H-atom transfer to Ru(IV)=O2+ to give Ru(III)-OH2+ and PhCO, (iii) capture of PhCO by Ru(III)-OH2+ to give Ru(II)-OC(O)Ph+ and H+, and (iv) solvolysis to give cis-[Ru(II)(bpy)2(py)(NCCH3)]2+ or the aqua complex and the carboxylic acid as products.     

Why Static O-H Bond Parameters Cannot Characterize the Free Radical Scavenging Activity of Phenolic Antioxidants: ab initio Study

Recently, selecting high efficient phenolic antioxidants with low toxicity was paid much attention1-4. Moreover, quantitative structure-activity relationships (QSAR) for phenolic antioxidants have been investigated to accelerate the selection process5-7. Hence, how to theoretically characterize the free radical scavenging activity of phenolic antioxidants is important and significant. Although the parameters characterizing O-H bond dissociation energy or enthalpy (BDE) correlate well with …     

Hydride ion transfer from ruthenium(II) complexes in water: kinetics and mechanism

Reactions of hydride complexes of ruthenium(II) with hydride acceptors have been examined for Ru(terpy)(bpy)H(+), Ru(terpy)(dmb)H(+), and Ru(η(6)-C(6)Me(6))(bpy)(H)(+) in aqueous media at 25 °C (terpy = 2,2';6',2'-terpyridine, bpy = 2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine). The acceptors include CO(2), CO, CH(2)O, and H(3)O(+). CO reacts with Ru(terpy)(dmb)H(+) with a rate constant of 1.2 (0.2) × 10(1) M(-1) s(-1), but for Ru(η(6)-C(6)Me(6))(bpy)(H)(+), the reaction was very slow, k ≤ 0.1 M(-1) s(-1). Ru(terpy)(bpy)H(+) and Ru(η(6)-C(6)Me(6))(bpy)(H)(+) react with CH(2)O with rate constants of (6 ± 4) × 10(6) and 1.1 × 10(3) M(-1) s(-1), respectively. The reaction of Ru(η(6)-C(6)Me(6))(bpy)(H)(+) with acid exhibits straightforward, second-order kinetics, with the rate proportional to [Ru(η(6)-C(6)Me(6))(bpy)(H)(+)] and [H(3)O(+)] and k = 2.2 × 10(1) M(-1) s(-1) (μ = 0.1 M, Na(2)SO(4) medium). However, for the case of Ru(terpy)(bpy)H(+), the protonation step is very rapid, and only the formation of the product Ru(terpy)(bpy)(H(2)O)(2+) (presumably via a dihydrogen or dihydride complex) is observed with a k(obs) of ca. 4 s(-1). The hydricities of HCO(2)(-), HCO(-), and H(3)CO(-) in water are estimated as +1.48, -0.76, and +1.57 eV/molecule (+34, -17.5, +36 kcal/mol), respectively. Theoretical studies of the reactions with CO(2) reveal a "product-like" transition state with short C-H and long M-H distances. (Reactant) Ru-H stretched 0.68 ?; (product) C-H stretched only 0.04 ?. The role of water solvent was explored by including one, two, or three water molecules in the calculation.     

Photocatalytic Oxidation of Aromatic Hydrocarbons

In acidic aqueous solutions UO(2)(2+) serves as a photocatalyst (lambda(irr) >/= 425 nm) for the oxidation of benzene by H(2)O(2). Under conditions where 50% of the excited state UO(2)(2+) is quenched by H(2)O(2) (k = 5.4 x 10(6) M(-)(1) s(-)(1)) and 50% by benzene (k = 2.9 x 10(8) M(-)(1) s(-)(1)), the quantum yield for the formation of phenol is 0.70. The yield does not change when benzene is replaced by benzene-d(6), but decreases by a factor of approximately 4 upon the change of solvent from H(2)O to D(2)O. Photocatalytic oxidation of toluene by UO(2)(2+)/H(2)O(2) produces PhCHO, PhCH(2)OH, and a mixture of cresols with a total quantum yield of 0.28 under conditions where 50% of UO(2)(2+) is quenched by H(2)O(2). The quenching of UO(2)(2+) by benzene and substituted benzenes takes place with k > 10(8) M(-)(1) s(-)(1). The system UO(2)(2+)/t-BuOOH/C(6)H(6)/hnu does not result in the oxidation of benzene, but instead yields methane and ethane.     

Acid-catalyzed oxidation of iodide ions by superoxo complexes of rhodium and chromium

Superoxometal complexes L(H(2)O)MOO(2+) (L = (H(2)O)(4), (NH(3))(4), or N(4)-macrocycle; M = Cr(III), Rh(III)) react with iodide ions according to the stoichiometry L(H(2)O)MOO(2+) + 3I(-) + 3H(+) --> L(H(2)O)MOH(2+) + 1.5I(2) + H(2)O. The rate law is -d[L(H(2)O)MOO(2+)]/dt = k [L(H(2)O)MOO(2+)][I(-)][H(+)], where k = 93.7 M(-2) s(-1) for Cr(aq)OO(2+), 402 for ([14]aneN(4))(H(2)O)CrOO(2+), and 888 for (NH(3))(4)(H(2)O)RhOO(2+) in acidic aqueous solutions at 25 degrees C and 0.50 M ionic strength. The Cr(aq)OO(2+)/I(-) reaction exhibits an inverse solvent kinetic isotope effect, k(H)()2(O)/k(D)2(O) = 0.5. In the proposed mechanism, the protonation of the superoxo complex precedes the reaction with iodide. The related Cr(aq)OOH(2+)/I(-) reaction has k(H)2(O)/k(D)2(O) = 0.6. The oxidation of (NH(3))(5)Rupy(2+) by Cr(aq)OO(2+) exhibits an [H(+)]-dependent pathway, rate = (7.0 x 10(4) + 1.78 x 10(5)[H(+)])[Ru(NH(3))(5)py(2+)][Cr(aq)OO(2+)]. Diiodine radical anions, I(2)(*)(-), reduce Cr(aq)OO(2+) with a rate constant k = 1.7 x 10(9) M(-1) s(-1).     

Water oxidation intermediates applied to catalysis: benzyl alcohol oxidation

Four distinct intermediates, Ru(IV)═O(2+), Ru(IV)(OH)(3+), Ru(V)═O(3+), and Ru(V)(OO)(3+), formed by oxidation of the catalyst [Ru(Mebimpy)(4,4'-((HO)(2)OPCH(2))(2)bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) and 4,4'-((HO)(2)OPCH(2))(2)bpy = 4,4'-bismethylenephosphonato-2,2'-bipyridine] on nanoITO (1-PO(3)H(2)) have been identified and utilized for electrocatalytic benzyl alcohol oxidation. Significant catalytic rate enhancements are observed for Ru(V)(OO)(3+) (~3000) and Ru(IV)(OH)(3+) (~2000) compared to Ru(IV)═O(2+). The appearance of an intermediate for Ru(IV)═O(2+) as the oxidant supports an O-atom insertion mechanism, and H/D kinetic isotope effects support net hydride-transfer oxidations for Ru(IV)(OH)(3+) and Ru(V)(OO)(3+). These results illustrate the importance of multiple reactive intermediates under catalytic water oxidation conditions and possible control of electrocatalytic reactivity on modified electrode surfaces.