R.S. Shukla

Ruthenium(III) and ruthenium(III)-aminopolycarboxylic acid chelate-catalyzed oxidation of ascorbic acid by molecular oxygen

The kinetics of Ru(III) ion, Ru(III)-EDTA (1:1) and Ru(III)-IMDA (1:1) catalyzed oxidation of ascorbic acid by molecular oxygen are investigated at 25°C, μ = 0.1 M KNO3, in the pH range 1.50 to 2.75. First-order kinetics were observed with respect to the concentrations of Ru(III) ion, Ru(III)-EDTA, Ru(III)-IMDA and ascorbic acid. The rate of oxidation was found to be inversely proportional to the hydrogen ion concentration. One-half-order and zero-order dependences with respect to the concentration of molecular oxygen were found in the cases of Ru(III) ion and Ru(III)-amino-polycarboxylic acid chelate-catalyzed oxidations, respectively. An inverse relationship was found between the stability and catalytic activity of the Ru(III) chelates of aminopolycarboxylic acids. The catalytic activities of Ru(III) ion and its chelates increase in the order Ru(III)-EDTA < Ru(III)-IMDA < Ru(III). The mechanistic implications of the oxidations catalyzed by Ru(III) ion and its chelates are discussed.

Ru(III)-EDTA catalyzed oxidation of cyclohexane by molecular oxygen

Abstract The oxidation of cyclohexane to cyclohexanol and cyclohexanone by molecular oxygen catalyzed by Ru(III)-EDTA in the presence and absence of the micelle cetyltrimethylammonium bromide (CTAB) is reported. The investigation was carried out in the pH range 2.00 – 3.50 varying the temperature from 298 – 318 K in a 1:1 water-dioxane mixture (μ = 0.1 M KNO 3 ). The dependence of the rate of oxidation on factors such as catalyst, substrate, molecular oxygen and pH were determined. The reaction is first order with respect to catalyst and substrate, and one-half order with respect to molecular oxygen concentrations. The rate of oxidation was found to be independent of hydrogen ion concentration. The rate of oxidation of cyclohexane increases in the presence of CTAB. Based on the kinetic data, a mechanism is proposed for the oxidation of cyclohexane to cyclohexanol and cyclohexanone. The activation energy E a corresponding to the observed rate constants were calculated for the reaction in the presence and absence of the micelle. The activation energy of the oxidation was found to be favourable by about 13 Kcal mol −1 in the presence of CTAB.

Thermodynamics of the homogeneous oxidation of L-ascorbic acid by molecular oxygen catalyzed by ruthenium ion and ruthenium chelates

Abstract The homogeneous oxidation of ascorbic acid by molecular oxygen catalyzed by RuCl2(H2O)4+, Ru(III)-IMDA and Ru(III)-EDTA has been investigated in the temperature range 278 – 308 K, μ = 0.1 M KNO3. The rate of oxidation of ascorbic acid in the presence of RuCl2(H2O)4+ was found to be first order in RuCl2(H2O)4+ and ascorbate concentrations and one-half order in molecular oxygen concentration. Based on the kinetic parameters, a μ-peroxoruthenium(IV)-ascorbate complex was suggested as an active intermediate in the RuCl2(H2O)4+-catalyzed oxidation of ascorbic acid by molecular oxygen. The thermodynamic parameters corresponding to the formation of the μ-peroxoruthenium(IV)-ascorbate complex and the formation of various mixed ligand complexes with ascorbic acid were computed. The activation parameters for the oxidation steps corresponding to the rate constants were also calculated. The rate of oxidation of ascorbic acid in the presence of Ru(III) chelates was found to depend only on the metal chelate and ascorbate concentrations and was independent of the concentration of molecular oxygen. A mixed ligand complex of Ru(III)-EDTA and Ru(III)-IMDA with ascorbic acid is proposed as an active intermediate for electron transfer to the metal ion. The reduced metal ion is reoxidized in a fast step by molecular oxygen. As in the case of Fe(III) and Cu(II) aminopolycarboxylic acid chelate-catalyzed oxidation of ascorbic acid, the Ru(III)-IMDA and Ru(III)-EDTA chelates act as oxidase models in the oxidation of ascorbic acid by molecular oxygen. The thermodynamic parameters are discussed in terms of the oxidation potentials E 1 2 of the metal complexes.

Thermodynamics of Ru(III)-edta catalyzed oxidation of saturated organic compounds by molecular oxygen

Abstract The thermodynamics of Ru(III)-EDTA catalyzed oxidation of cyclohexane to cyclohexanol and of cyclohexanol to cyclohexanone by molecular oxygen in acidic medium has been investigated in the temperature range 288–318 K, μ = 0.1 M KNO 3 in a 1:1 water:1,4-dioxane mixture (v/v). The dependence of the rate of oxidation on factors such as temperature (288–318 K), catalyst, substrates, molecular oxygen concentrations and pH was studied. The order of the reaction with respect to the concentrations of substrate and catalyst is unity. The dependence in molecular oxygen was found to be one-half and zero for the oxidations of cyclohexane and cyclohexanol, respectively. No dependence on [H + ] was observed for oxidation of cyclohexane. The rate of oxidation of cyclohexanol is inversely proportional to the hydrogen ion concentration. The thermodynamic parameters corresponding to equilibrium constants and activation parameters corresponding to rate constants were computed. The mechanisms proposed on the basis of the kinetic data are discussed in terms of thermodynamic stability and reactivity. The activation energy for the oxidation of cyclohexane is 7.3 kcal mol −1 higher than that of cyclohexanol. Thermodynamic factors are more favourable for the oxidation of cyclohexanol, which proceeds by a rate higher than that of cyclohexane.

Kinetic and spectroscopic study of the formation of an intermediate ruthenium(III) ascorbate complex in the oxidation of L-ascorbic acid

Abstract The formation of an intermediate ruthenium(III) ascorbate complex (1) by the interaction of L -ascorbic acid and dichlorotetraaquoruthenium(III) is reported in the temperature range 25–40°C. The kinetics and mechanism of the formation of 1 were studied as a function of [RuIIICl2(H2O)4]+, [ascorbic acid], pH, ionic strength and temperature. The rate of formation of 1 was found to be first order in [RuIIICl2(H2O)4]+ and [ascorbic acid]. The rate has an inverse dependence on hydrogen ion concentration. Ionic strength dependence indicated the formation of 1 by monoanionic and cationic species in solution. Decomposition of 1 takes place slowly and gives the products dehydroascorbic acid and [RuIICl2(H2O)4]. A detailed discussion of the kinetic data and a comparison of rate and equilibrium constants are presented with activation and thermodynamic parameters.

Inner sphere oxidation of L-ascorbic acid by Ru(III) ion and its complexes in aqueous acidic medium

Abstract The kinetics of electron transfer from L-ascorbic acid [H 2 A] to oxidants, dichlorotetraaquoruthenium(III) [RuCl 2 (H 2 O) 4 ] + , iminodiacetatoruthenium(III) [Ru(III)IMDA] + and ethylenediaminetetraacetatoruthenate(III) [Ru(III)EDTA] − exhibit a first order dependence both on L-ascorbic acid and oxidants and inverse first order dependence on hydrogen ion concentration. Kinetic, spectroscopic and thermodynamic parameters are reported for the formation of intermediate Ru(III) (1:1) and Ru(III)chelateascorbate (1:1:1) complexes during the oxidation of L-ascorbic acid. The results are interpreted in terms of a mechanism involving a rate-determining inner sphere one electron transfer from L-ascorbic acid to the oxidants used in the present investigation, followed by a subsequent and kinetically rapid transfer of the second electron of ascorbic acid to another molecule of the oxidant. A detailed discussion of the kinetic data, temperature and ionic strength dependence of the oxidation reactions is presented.

Ru(III)-EDTA catalyzed oxidation of ascorbic acid by hydrogen peroxide in aqueous solution

Abstract The kinetics of oxidation of ascorbic acid to dehydroascorbic acid by hydrogen peroxide catalyzed by ethylenediaminetetraacetatoruthenate(III) has been studied over the pH range 1.50 – 2.50, at 30°C and μ = 0.1 M KNO3. The reaction has a first-order dependence on ascorbic acid and Ru(III)-EDTA concentrations, an inverse first-order dependence on hydrogen ion concentration, and is independent of hydrogen peroxide concentration in the pH range studied. A mechanism has been proposed in which ascorbate anion forms a kinetic intermediate with the catalyst in a pre-equilibrium step. Ruthenium(III) is reduced to ruthenium(II) in a rate-determining step and is reoxidized with hydrogen peroxide back to the Ru(III) complex in a fast step.

Kinetics and mechanism of the hydroxylation of toluene catalyzed by a ruthenium(III) analogue of the udenfriend system: ru(III)-edta-ascorbate-molecular oxygen

Abstract The hydroxylation of toluene to cresols by molecular oxygen is catalyzed by the Ru(III) analogue of the model Udenfriend system: Ru(III)-EDTA-ascorbic acid. The hydroxylation was investigated at 30 °C and μ = 0.1 M KNO3 in a 50% (v/v) mixture of 1,4-dioxane and water in the pH range 1.50–2.50. The kinetic study of the reaction shows a first-order dependence in the concentrations of Ru(III)-EDTA, ascorbic acid, molecular oxygen and the substrate toluene. The rate of reaction was found to increase linearly with an increase in pH, and is inverse first order in hydrogen ion concentration in the pH range 1.50 – 2.50. The kinetic and stability parameters of the complicated reaction system were determined by potentiometric, spectrophotometric, manometric and gas Chromatographie techniques. The reaction proceeds via an oxygen-dependent pathway by the formation of an intermediate Ru(IV) dioxygen complex. A mechanism for hydroxylation is proposed on the basis of these results. The system Ru(III)-EDTA-ascorbic acid-molecular oxygen selectively oxidizes toluene to o- and p-cresols.

Dioxygen affinities of some ruthenium(III) schiff base complexes

Abstract The synthesis and dioxygen affinities of some ruthenium(III) Schiff base complexes in DMF solution in the presence of different axial bases are reported. The ligands used are bis(salicylaldehyde)ethylenediimine (salen), bis(salicylaldehyde)diethylenetriimine (saldien), bis(picolinaldehyde)- o -phenylenediimine (picoph), bis(picolinaidehyde)ethylenediimine (picen) and bis(picolinaldehyde)diethylenetriimine (picdien). The axial ligands employed are chloride (Cl − ), imidazole (Im) and 2-methylimidazole (2-MeIm). From the oxygenation constants it is found that electron donating substituents on the Schiff bases increase the affinity for dioxygen. Equilibrium dioxygen uptake measurements at 278, 288 and 303 K provide values of Δ H ° and Δ S ° of oxygenation that fall in the range − 6.1 to −13.3 kcal mol − 1 for Δ H ° and − 10 to − 31 cal deg − 1 mol − 1 for Δ S °. The dioxygen adducts of Ru III were characterized by electrochemistry, UV–vis, IR and EPR techniques as Ru IV superoxo complexes.

Thermodynamic aspects of the Ru(III)—EDTA—ascorbate—molecular oxygen system for the oxidation of saturated and unsaturated organic compounds

Abstract The activation and thermodynamic parameters corresponding to rate and equilibrium constants, respectively, for the homogeneous oxidation of the saturated substrates, cyclohexane to cyclohexanol, cyclohexanol to cis -1,3-cyclohexane diol and olefin, cyclohexene to epoxide by Ru(III)—EDTA—ascorbateO 2 system were determined by measuring the various rates and equilibrium constants at four different temperatures in the range 288–313 K and μ = 0.1 M KNO 3 in a 50% (V/V) mixture of 1,4-dioxane and water in acidic medium. The kinetics of the oxidation of these substrates at each particular temperature was studied as a function of the concentration, the substrates, hydrogen ion, catalyst, ascorbic acid and molecular oxygen. The orders of the reaction in cyclohexanol and cyclohexene concentrations are one, and those in cyclohexane and hydrogen ion concentration are fractional and inverse first-order, respectively. For all substrates the reaction is first order with respect to the concentrations of molecular oxygen, ascorbic acid and catalyst. The source of the oxygen atom transferred to the substrates was confirmed by 18 O 2 isotope studies in which the 18 O was incorporated in the oxidized products. The kinetics and solvent isotope effect were studied for the oxidation of C 6 H 12 , C 6 D 12 , C 6 H 11 OH and C 6 D 11 OD. The order of the reactivity observed in the oxidation of the substrates studied is cyclohexene > cyclohexanol > cyclohexane. A comparison of the rates of oxidation of the substrates and the corresponding activation parameters with the catalytic systems Ru(III)—EDTAO 2 and Ru(III)—EDTA—ascorbateH 2 O 2 indicated that activation parameters become more favourable in the presence of ascorbic acid, where the system acts as a mono-oxygenase and the activation energies are drastically reduced. Highly negative entropies are associated with all oxygen atom transfer reactions, indicating that the oxidation process is associative in nature.