M.M.Taqui Khan

Synthesis and characterization of some ruthenium(V) oxo complexes of the Schiff base, bis(salicylaldehyde)-o-phenylenediamine, with appended Cl−, imidazole and 2-methylimidazole: the first examples of stable oxo complexes via direct oxygenation

Abstract Oxygenation of ruthenium(III) Schiff base complexes of the composition K[Ru III (Saloph)Cl 2 ] and [Ru III (Saloph)XCl] (Saloph=bis(salicylaldehyde)- o - phenylenediamine; X=imidazole (Im), 2-methylimidazole (2-MeIm)) with molecular oxygen gives the oxo derivatives of the composition, [Ru V (Saloph)X- (O)] + (X=Cl, Im or 2-MeIm). The oxo complexes were also synthesized by the reaction of [Ru III - (Saloph)XCl] with PhIO or H 2 O 2 . The complexes thus obtained were characterized by analytical data, molar conductance, magnetic susceptibility, spectroscopic and electrochemical methods. Kinetics of the oxygenation reaction have also been investigated.

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.

Reduction of CO2 by molecular hydrogen to formic acid and formaldehyde and their decomposition to CO and H2O

Abstract The reduction of carbon dioxide by hydrogen is catalyzed by K[RuIII-(EDTA-H)Cl]·2H2O in aqueous medium at milder pressures (1–4 atm CO2 or H2) and 40°C. The reduction of CO2 gives formic acid and formaldehyde as the initial reaction products, which later decompose to give CO and H2O as the final products. The rates of formation of formic acid and formaldehyde exhibited first-order dependence with respect to catalyst and dissolved CO2 and H2 concentrations, respectively. The rates of decomposition of formic acid and formaldehyde studied under the same reaction conditions also showed first-order dependence with respect to catalyst and substrate concentrations, respectively. The effect of temperature on the rates of formation and decomposition of formic acid and formaldehyde was also studied in the temperature range 30–50°C, and from the Arrhenius plots activation energies were evaluated. Based on the kinetic data, a mechanism is proposed for the formation of formic acid and formaldehyde and their decomposition to CO and H2O, the end products of the reverse water-gas shift reaction.

Hydration of acetylene to acetaldehyde using K[RuIII(EDTA―H)Cl]2H2O

The hydration of acetylene catalyzed by water-soluble K[RuIII(EDTA-H)C1]2H2O at 80° and 1 atm gave a clean product, acetaldehyde. Under the reaction conditions studied, the rate of hydration of acetylene exhibits a first-order dependence with respect to both catalyst and dissolved acetylene concentrations. Based on our observations, a mechanism has been proposed involving a π-complex intermediate with a water molecule coordinated to the metal ion. From the study of rate of hydration of acetylene as a function of temperature, the activation energy evaluated for the reaction is 33.4 kcal mol−1.

Reductive carbonylation of nitrobenzene to phenylurethane catalyzed by Ru(III)-schiff base complexes

Abstract Ruthenium complexes containing Schiff bases with N 2 O 2 , N 4 and N 5 donor groups with the general formula [Ru III (X)Cl 1 or 2 ], where X = Schiff base such as bis(salicylaldehyde)- o -phenylenediimine (saloph), bis(salicylaldehyde)ethylenediimine (salen), bis(picolaldehyde)ethylenediimine (picen), bis(picolaldehyde)- o -phenylenediimine (pic-opd), bis(picolaldehyde)diethylenetriimine (pic-dien), were tested for their catalytic activity towards the reductive carbonylation of nitrobenzene in ethanol to give phenylurethane. The five Ru(III) complexes tested towards reductive carbonylation showed different catalytic activities in the range 160 – 200 °C and CO partial pressure of 15 atm. Among the complexes tested, [Ru(saloph)Cl 2 ] showed the highest catalytic activity with a turnover rate greater than 80 mol product per mol catalyst per hour at 160 °C and 15 atm CO. [Ru(pic-en)Cl 2 ]Cl and [Ru(picopd)Cl 2 ]Cl complexes with N 4 donor systems were found to be less active towards carbonylation of nitrobenzene, as indicated by their turnover rates of 20 and 15 mol product per mol catalyst per hour, respectively, at 200 °C and 15 atm CO. The complex [Ru(pic-dien)Cl]Cl 2 N 5 donor system was completely inactive even at 200 °C and 15 atm CO, and no conversion of nitrobenzene was seen even after 12 h contact time.

Oxygen atom transfer in the epoxidation of cyclohexene catalysed by Ru(III)—EDTA

Abstract The oxidation of cyclohexene to the epoxide is catalysed by Ru(III)—EDTA in a 50% ethanol/water solution at 30 °C in the pH range 1 – 3 (μ = 0.1 M KCl). The observed reaction rates for the oxidation of cyclohexene are first order with respect to Ru(III)–EDTA and cyclohexene, and one-half order with respect to molecular oxygen. A mechanism has been proposed for the oxidation of cyclohexene by molecular oxygen catalysed by Ru(III)–EDTA. The turnover number for the epoxidation of cyclohexene by molecular oxygen catalysed by Ru(III)—EDTA is 200 mol (mol catalyst) −1 min −1 .

An efficient water-soluble ruthenium hydroformylation catalyst for the exclusive formation of 1-heptaldehyde from 1-hexene

Abstract Hydroformylation of 1-hexene was carried out in a high pressure reactor using water soluble [Ru(III)-EDTA] catalyst, synthesis gas (1:1) CO + H2 at 50 atm, in 80:20 ethanol-water mixture and at 130 °C. The reaction proceeds with a turnover rate of 11.83 mol product per mol catalyst per hour with the conversion of 1-hexene exclusively (100%) to linear 1-heptaldehyde.

Homogeneous hydrogenation of cyclohexene catalyzed by complexes of rhodium and iridium

Abstract The synthesis and characterization of RhClHN(CH2CH2PPh2)2 and IrClHN(CH2CH2PPh2)2 and their catalytic activity in the homogeneous hydrogenation of cyclohexene over the temperature range 20 – 50 °C and 0.4 – 1 atm H2 partial pressure have been investigated. The dependence of the rate of hydrogenation on factors such as the catalyst concentration, the substrate concentration, H2 pressure and the temperature is reported. A mechanism has been proposed in which the catalysts activate molecular hydrogen by forming dihydrido species of the type MClL(H)2 [M = Rh(I), Ir(I) and L = HN(CH2CH2PPh2)2] followed by transfer of hydrogen to the olefin to form the saturated product. The experimental data are in accordance with a rate expression of the form : Rate = where [H2], [S] and [C] are the concentration of H2, substrate and catalyst, respectively. The activation parameters of the reaction, ΔH‡ and ΔS‡, have also been evaluated.

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.