Kinetics and mechanism of Os(VIII)-catalyzed hexacyanoferrate(III) oxidation of α amino acids in alkaline medium

The kinetics of the Os(VIII)-catalyzed oxidation of glycine, alanine, valine, phenylalanine, isoleucine, lycine, and glutamic acid by alkaline hexacyanoferrate(III) reveal that these reactions are zero order in hexacyanoferrate(III) and first order in Os(VIII). The order in amino acid as well as in alkali is 1 at [amino acid] ?2.5 × 10^?2M and [OH^?] ?1.3 × 10^?M, but less than unity at higher concentrations of amino acids or alkali. The active oxidizing species under the experimental conditions is OsO₄(H₂O) (OH)^?. The ferricyanide is merely used up to regenerate the Os(VIII) species from Os(VI) formed during the reaction. The structural influence of amino acids on the reactivity has been discussed. The amino acids during oxidation are shown to be degraded through intermediate keto acids. The kinetic data are accommodated by considering the interaction between the conjugate base of the amino acids and the active oxidizing species of Os(VIII) to form a transient complex in the primary rate-determining step. The catalytic effect of hexacyanoferrate(II) has been rationalized.     

Kinetics of oxidation of reducing sugars by catalytic amount of osmium(VIII) in presence of periodate

The kinetics of oxidation of some reducing sugars viz. glucose, galactose, fructose, maltose, and lactose by osmium(VIII) in presence of sodium metaperiodate in alkaline medium have been investigated. The reactions are zero order in periodate. The order of reaction in substrate and OH^? decreases from unity to zero at higher [substrate] or [OH^?], respectively. Rate of oxidation is proportional to [Osmium(VIII)]. Osmium(VIII) serves as an effective oxidant which oxidizes reducing sugars and itself reduces to osmium(VI). Role of IO₄^? is to regenerate osmium(VIII) from osmium(VI). An evidence for the complex formation between osmium(VIII) and reducing sugar has also been obtained. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36:441–448, 2004.     

Kinetics and mechanisms of oxidation by metal ions,part XII [1] oxidation of formaldehyde by alkaline osmium(VIII)

The stopped-flow measurements on the disappearance of alkaline osmium(VIII) at 402 nm indicated a first-order dependence each in [Os(VIII)] and [HCHO]. The pseudo first-order rate constant k_obs ([HCHO] ? [Os(VIII)]) decreased with increasing [OH^?]. The ionic strength, however, had no effect on k_obs. The rapid scan spectra of the reaction mixture indicated the formation of an inert complex which absorbs at 319 nm. Therefore the rate determining step is considered to involve the bimolecular collision between OsO₄(OH) and hydrated formaldehyde. The values of the rate limiting constant k and the equilibrium constant K_ha for the formation of the alkoxide ion from the reaction of hydrated formaldehyde with OH^? are evaluated. The equilibrium constant K_ha, within the experimental limits, is independent of temperature. The pK_a value, calculated from K_ha, is 13.62 ± 0.05 which is in good agreement with the pK_a value 13.27 for formaldehyde. The activation parameters, ΔH? = 40 ± 2 kJ mol^?1 and ΔS? = ?51 ± 6 JK^?1 mol^?1, for the rate limiting constant k are determined.     

Osmium(VIII)-Catalyzed Oxidation of Some Cyclic Amines by Potassium Hexacyanoferrate(III) in Alkaline Media: a Kinetics and Mechanistic Study

Reactions of morpholine, piperidine, and piperazine with Os(VIII)-catalyzed hexacyanoferrate(III) in alkaline media to produce the corresponding lactam have been studied at constant temperature and ionic strength. The reactions followed first-order kinetics with respect to [amine] and [Os(VIII)] but were independent of [Fe(CN)₆ ^3-] and [OH^-]. The effects of introduced electrolytes, potassium hexacyanoferrate(II), relative permitivity, and temperature have also been studied. A mechanism accounting for these results has been proposed.     

Spectral and Mechanistic Investigation of the Osmium(VIII) Catalyzed Oxidation of Diclofenac Sodium by the Copper(III) Periodate Complex in Aqueous Alkaline Medium

The kinetics of the osmium(VIII) (Os(VIII)) catalyzed oxidation of diclofenac sodium (DFS) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium has been studied spectrophotometrically at a constant ionic strength of 1.0 mol⋅dm⁻³. The reaction showed first order kinetics in [Os(VIII)] and [DPC] and less than unit order with respect to [DFS] and [alkali]. The rate decreased with increase in [periodate]. The reaction between DFS and DPC in alkaline medium exhibits 1:2 [DFS]:[DPC] stoichiometry. However, the order in [DFS] and [OH⁻] changes from first order to zero order as their concentration increases. Changes in the ionic strength and dielectric constant did not affect the rate of reaction. The oxidation products were identified by LC-ESI-MS, NMR, and IR spectroscopic studies. A possible mechanism is proposed. The reaction constants involved in the different steps of the mechanism were calculated. The catalytic constant (K _C) was also calculated for Os(VIII) catalysis at the studied temperatures. From plots of log ₁₀ K _C versus 1/T, values of activation parameters have been evaluated with respect to the catalytic reaction. The activation parameters with respect to the slow step of the mechanism were computed and discussed, and thermodynamic quantities were also determined. The active osmium(VIII) and copper(III) periodate species have been identified.     

Catalytic action of CTAB/KBr/C9OH micellar media on the basic hydrolysis of crystal violet

The kinetics of basic hydrolysis of crystal violet (CV) in CTAB/KBr/C₉OH micellar media was investigated under pseudo-first-order conditions. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of CV at 590?nm. It was observed that the pseudo-first-order rate constant increases with increase in C₀. The enhancement of reaction rate with C₀ is explained on the basis of dependence of reaction rate on micellar morphology. Further, the viscosity and DLS analysis supports nonanol-induced morphological transitions. Fluorescence spectroscopy has been used to understand dye–micelles interactions. The enhancement of fluorescence intensity of CV with C₀ suggests an increase in dye–micelles interaction with C_0. The concentration of surfactant and salt had a marked effect on reaction rate. The inhibition of reaction rate at high concentration of surfactant and salt is due to the ionic competition of OH^? and Br^? ions for the reaction center. The influence of [OH^?] on CV hydrolysis was also investigated. The results show that the pseudo-first-order rate constant, k’, increases linearly with hydroxide ion concentration, indicating first-order dependence on [OH^?].     

Mechanistic study of osmium(VIII) promoted oxidation of crotonic acid by aqueous alkaline solution of potassium bromate

Kinetics and mechanism of the Os(VIII) catalysed oxidation of crotonic acid (CA) by KBrO₃ in alkaline medium have been investigated. Zero order dependence in [KBrO₃] was observed, while first order with respect to CA in its lower concentration range tends to zero order at its higher concentration range. The order in [Os(VIII)] was found to be unity and a positive effect of [OH⁻] was observed. Variation of the ionic strength (μ) and dielectric constant of the medium and addition of Hg(OAc)₂ (used as Br⁻ scavenger) had an insignificant effect on the rate of reaction. Thermodynamic parameters have also been calculated and reported. A suitable mechanism consistent with the observed kinetic results has been suggested and the related rate law deduced.     

A kinetic and thermodynamic study of the unexpected comproportionation reaction between cis-[Os(VIII)O4(OH)2](2-) and trans-[Os(VI)O2(OH)4](2-) to form a postulated [Os(VII)O3(OH)3](2-) complex anion

A kinetic study of [OsO(4)] reduction by aliphatic alcohols (MeOH and EtOH) was performed in a 2.0 M NaOH matrix at 298.1 K. The rate model that best fitted the UV-VIS data supports a one-step, two electron reduction of Os(VIII) (present as both the [Os(VIII)O(4)(OH)](-) and cis-[Os(VIII)O(4)(OH)(2)](2-) species in a ratio of 0.34:0.66) to form the trans-[Os(VI)O(2)(OH)(4)](2-) species. The formed trans-[Os(VI)O(2)(OH)(4)](2-) species subsequently reacts relatively rapidly with the cis-[Os(VIII)O(4)(OH)(2)](2-) complex anion to form a postulated [Os(VII)O(3)(OH)(3)](2-) species according to: cis-[Os(VIII)O(4)(OH)(2)](2-) + trans-[Os(VI)O(2)(OH)(4)](2-) (k+2) (k-2) 2[Os(VII)O(3)(OH)(3)](2-). The calculated forward, k(+2), and reverse, k(-2), reaction rate constants of this comproportionation reaction are 620.9 ± 14.6 M(-1) s(-1) and 65.7 ± 1.2 M(-1) s(-1) respectively. Interestingly, it was found that the postulated [Os(VII)O(3)(OH)(3)](2-) complex anion does not oxidize MeOH or EtOH. Furthermore, the reduction of Os(VIII) with MeOH or EtOH is first order with respect to the aliphatic alcohol concentration. In order to corroborate the formation of the [Os(VII)O(3)(OH)(3)](2-) species predicted with the rate model simulations, several Os(VIII)/Os(VI) mole fraction and mole ratio titrations were conducted in a 2.0 M NaOH matrix at 298.1 K under equilibrium conditions. These titrations confirmed that the cis-[Os(VIII)O(4)(OH)(2)](2-) and trans-[Os(VI)O(2)(OH)(4)](2-) species react in a 1:1 ratio with a calculated equilibrium constant, K(COM), of 9.3 ± 0.4. The ratio of rate constants k(+2) and k(-2) agrees quantitatively with K(COM), satisfying the principle of detailed balance. In addition, for the first time, the molar extinction coefficient spectrum of the postulated [Os(VII)O(3)(OH)(3)](2-) complex anion is reported.     

Kinetics and mechanism of osmium (VIII) and ruthenium(III) catalyzed oxidation of aliphatic amines by chloramine-T in alkaline and perchloric acid media

The kinetics of oxidation of aliphatic amines viz., ethylamine, n-butylamine, isopropylamine (primary amines), diethylamine (secondary amine), and triethylamine (tertiary amine) by chloramine-T have been studied in NaOH medium catalyzed by osmium (VIII) and in perchloric acid medium with ruthenium(III) as catalyst. The order of reaction in [Chloramine-T] is always found to be unity. A zero order dependence of rate with respect to each [OH^?] and [Amine] has been observed during the osmium(VIII) catalyzed oxidation of diethylamine and triethylamine while a retarding effect of [OH^?] or [Amine] on the rate of oxidation is observed in case of osmium(VIII) catalyzed oxidation of primary aliphatic amines. The ruthenium(III) catalyzed oxidation of amines follow almost similar kinetics. The order of reactions in [Amine] or [Acid] decreases from unity at higher amine or acid concentrations. The rate of oxidation is proportional to {k′ and k″ [Ruthenium(III)] or [Osmium(VIII)]} where k′ and k″ (having different values in case of ruthenium(III) and osmium(VIII)) are the rate constants for uncatalyzed and catalyzed path respectively. The suitable mechanism consisting with the kinetic data is proposed in each case and discussed.     

Spectrophotometric determination of osmium with phthalimide dithiosemicarbazone by means of complex formation and catalytic reactions

Phthalimide dithiosemicarbazone forms a 1:1 complex with osmium at pH 3.3–4.5 (?₄₅₀ = 1.3 · 10⁴ l mol^?1 cm^?1 ) which is applied to the photometric determination of osmium; Beer's law is obeyed for the range 1–12 μg Os ml^?1. The oxidation of the reagent with cerium(IV) is catalyzed by osmium(VIII), and this reaction allows a more sensitive procedure for the determination of osmium; the calibration curve is linear over the range 0.05–0.4 μg Os ml^?1. The interferences in both procedures are described.     

Synthesis, characterization, and crystal structures of the osmium triflate complexes CpOs(P-P)(OTf) and [CpOs(P-P)(OH2)][OTf]

Treatment of the osmium(II) hydrides Cp^∗Os(P-P)H (Cp^∗ = pentamethylcyclopentadienyl) with methyl trifluoromethanesulfonate (MeOTf) affords osmium(II) triflate complexes with the general formula Cp^∗Os(P-P)(OTf), where P-P = bis(dimethylphosphino)methane (dmpm), bis(diphenylphosphino)methane (dppm), or 1,2-bis(dimethylphosphino)ethane (dmpe). The aqua complexes [Cp^∗Os(dmpm)(OH₂)][OTf] and [Cp^∗Os(dppm)(OH₂)][OTf] are synthesized by the addition of water to the corresponding anhydrous triflates. The complexes Cp^∗Os(dppm)(OTf) and [Cp^∗Os(dmpm)(OH₂)][OTf] have been examined crystallographically, and all compounds have been characterized by NMR spectroscopy.     

Kinetic-Spectrophotometric Determination of Traces of Osmium by Its Catalytic Effect on the Oxidation of Pyrogallol Red by Hydrogen Peroxide

Abstract

A Kinetic-spectrophotometric method for the determination of ultra-trace amounts of osmium(VIII) is described. It is based on the catalytic action of osmium(VIII) on the oxidation of pyrogallol red with hydrogen peroxide, yielding a colorless product in neutral medium. The reaction is followed spectrophotometrically by measuring the rate of change in absorbance at 540 nm and 30°C. Os(VIII) in the range 0.005 -100 ng.ml^?1 can be determined. The proposed method is hardly subject to interferences. The relative standard deviation is 1.5% for 1 ng.ml^?1 of Os(VIII). The kinetic parameters of the catalyzed and uncatalyzed reactions are reported. The detection limit is 1 pg.ml^?1.     

Speciation Analysis of Osmium(VIII) and Osmium(IV) by UV‐VIS Spectrophotometry Using Quercetin as the Reagent

Abstract

The UV‐VIS spectrophotometric methods for the determination of Os(VIII) (as OsO₄) and Os(IV) (as OsCl₆ ^2? complex) in their mixtures were developed. Quercetin (Q), a flavonoid compound, was used as a chromogenic reagent. Both direct and derivative spectrophotometry can be employed for the determination of Os(VIII). The calculation of the first‐derivative spectrum of the examined mixture and the use of the signal at 285.1 nm allows reaching a better detection limit (0.01 µg mL^?1 Os) as compared with direct spectrophotometry (0.1 µg mL^?1 Os). Relative standard deviations of the results are in the range of 0.87%–4.65% and 0.45%–1.15% for direct and derivative mode, respectively. Selective redox reaction of OsO₄ with Q under the conditions used (0.05 M HCl, 1×10^?4 M Q, 15 min heating at 70°C) makes the basis of its determination in mixtures with the OsCl₆ ^2? complex. Quercetin does not react with the OsCl₆ ^2? complex. The signals of the OsCl₆ ^2? complex can be isolated from the examined mixtures by the calculation of the third‐order derivative spectra and the use of the values at 340.0 nm. The effectiveness of the reduction of OsO₄ in chloride solutions has been studied by the developed method.     

A rapid method for the spectrophotometric determination of palladium(II) and osmium(VIII)

Propionyl promazine phosphate is proposed as a new reagent for the rapid spectrophotometric determination of microgram amounts of Pd(II) and Os(VII). PPP instantaneously forms an orange-red 1:1 complex with Pd(II) in sodium acetate-hydrochloric acid buffer of pH 0.8 to 4.0 at room temperature. The reagent also forms an orange-red radical cation with Os(VIII) in 0.5 to 2.0 M hydrochloric acid. The Pd-PPP complex exhibits an absorption maximum at 490–500 nm with molar absorptivity of 7.1 × 10³ liter mol^?1 cm^?1. The Os-PPP radical cation has an absorption maximum at 505–515 nm with molar absorptivity of 2.21 × 10⁴ liters mol^?1 cm^?1. The Sandell sensitivity is 0.022 μg/cm² (Pd) and 0.008 μg/ cm² (Os). Beer's law is valid over the concentration range 0.2 to 21 ppm (Pd) and 0.2 to 4.2 ppm (Os). The proposed method offers the advantages of simplicity, rapidity, and without the need for heating or extraction. The reagent is used for the determination of Pd in the synthetic mixtures corresponding to Pd alloys used in jewelery and Os in osmiridium alloy.     

Equilibrium investigation of complex formation reactions involving copper(II), nitrilo-tris(methyl phosphonic acid) and amino acids,peptides or DNA constitutents. The kinetics,mechanism and correlation of rates with complex stability for metal ion promoted hydrolysis of glycine methyl ester

The complex formation reactions of [Cu(NTP)(OH₂)]^4? (NTP?=?nitrilo-tris(methyl phosphonic acid)) with some selected bio-relevant ligands containing different functional groups, are investigated. Stoichiometry and stability constants for the complexes formed are reported. The results show that the ternary complexes are formed in a stepwise mechanism whereby NTP binds to copper(II), followed by coordination of amino acid, peptide or DNA. Copper(II) is found to form Cu(NTP)H _n species with n?=?0, 1, 2 or 3. The concentration distribution of the various complex species has been evaluated. The kinetics of base hydrolysis of glycine methyl ester in the presence of copper(II)-NTP complex is studied in aqueous solution at different temperatures. It is proposed that the catalysis of GlyOMe ester occurs by attack of OH^? ion on the uncoordinated carbonyl carbon atom of the ester group. Activation parameters for the base hydrolysis of the complex [Cu(NTP)NH₂CH₂CO₂Me]^4? are, ΔH^±?=?9.5?±?0.3?kJ?mol^?1 and ΔS^±?=??179.3?±?0.9?J?K^?1?mol^?1. These show that catalysis is due to a substantial lowering of ΔH^±.     

Kinetics of the oxidation of aliphatic aldehydes by Os(VIII) in alkaline solution

Kinetics of oxidation of six aliphatic aldehydes by Os(VIII) in alkaline solutions have been studied. The reaction is of first order with respect to each of the aldehyde and Os(VIII). The pseudo-first order rate constants decreased with an increase in the concentration of hydroxyl ions. The oxidation of deuterioacetaldehyde (MeCDO) exhibited a substantial primary kinetic isotope effect. Separate rate constants for the oxidation of hydrate and free aldehyde forms have been evaluated. The aldehyde hydrate is postulated as the active reductant. Ionic strength has no noticable effect on the rate. The rate-determining step is, therefore, postulated to be a bimolecular reaction between the aldehyde hydrate and [OsO₄(OH)₂]^?2. The value of the limiting rate constant exhibited an excellent correlation with Taft σ* values; reaction constant being negative. A mechanism involving transfer of a hydride ion from the aldehyde hydrate to Os(VIII) has been proposed.     

Kinetics and mechanism of osmium(VIII)-catalyzed oxidation of D-proline and L(–)-methionine by hexacyanoferrate(III)

The osmium(VIII)-catalyzed oxidation of D -proline and L (–)-methionine by alkaline hexacyanoferrate(III) has been studied spectrophotometrically. The reactions follow kinetics different from those of the oxidation of many amino acids investigated earlier, being first order in hexacyanoferrate(III) and osmium(VIII). The order in proline or methionine and OH^? decreases from unity to zero at higher concentrations of proline or methionine and OH^?, respectively. A mechanism consistent with the kinetic data is proposed and discussed.     

Spectrophotometric Determination of Osmium at the PPB Level Using Pyrocatechol and Hydroxyamidine

Abstract

A sensitive method for the spectrophotometric determination of osmium at the ppb level is described. The method is based on the formation of a brown-coloured complex by heating the reaction mixture containing Os(VIII), pyrocatechol and a hydroxyamidine at pH 8.5 over a boiling water bath, with subsequent extraction of the coloured species into chloroform. The molar absorptivity of the coloured species with N-hydroxy-N,N′-diphenylbenzamidine is 3.95 × 10⁶ 1 mol^?1 cm^?1 at λ_max = 410 nm. The method is free from interferences for almost all ions tested.     

Thermodynamic quantities for the different steps involved in the mechanism of osmium(VIII) catalysed oxidation of l-lysine by a new oxidant,diperiodatoargentate(III) (stopped flow technique)

The kinetics of Os(VIII) catalysed oxidation of l-lysine by diperiodatoargentate(III) (DPA) in alkaline medium at T = 298 K and a constant ionic strength of 0.50 mol · dm^?3 was studied spectrophotometrically. The oxidation products are aldehyde (5-aminopentanal) and Ag(I). The stoichiometry is i.e. [l-lysine]:[DPA] = 1:1. The reaction is of first order in [Os(VIII)] and [DPA] and is less than unit order in both [l-lys] and [alkali]. Addition of periodate has no effect on the reaction. Effect of added products, ionic strength, and dielectric constant of the reaction medium have been investigated. The oxidation reaction in alkaline medium has been shown to proceed via a Os(VIII)-l-lysine complex, which further reacts with one molecule of deprotonated DPA in a rate determining step followed by other fast steps to give the products. The main products were identified by spot test, IR, and GC-MS. The reaction constants involved in the different steps of the mechanism are calculated at different temperatures. The catalytic constant (K_C) was also calculated at different temperatures. From the plots of lg K_C versus 1/T, values of activation parameters with respect to the catalyst have been evaluated. The activation parameters with respect to slow step of the mechanism are computed and discussed and thermodynamic quantities are also determined. The active species of catalyst and oxidant have been identified.     

Ultratrace Osmium,Ruthenium and Lead in Airborne Particulate Matter: Peak Area as Instrumental Datum to Improve Their Simultaneous Voltammetric Determination

Peak area as instrumental datum for determining the concentration of metals in solution instead of peak height is proposed for the simultaneous voltammetric determination in particulate matter of ultratrace Os(VIII), Ru(III) and Pb(II), species linked to vehicle emissions. In the case of species present at ultratrace concentration level or having low reversibility degree of the electrodic processes, the employment of peak area, instead of peak current, permits to achieve limits of detection lower even more of one order of magnitude. The method is based on the catalytic current of the Os(VIII)‐, Ru(III)‐ and Pb(II)‐bromate system by differential pulse voltammetry. 0.3 mol L^?1 acetate buffer pH 4.5+6.9×10^?2 mol L^?1 NaBrO₃+2.3×10^?4 mol L^?1 EDTA‐Na₂ was employed as the supporting electrolyte. For all the elements, the accuracy, expressed as relative error e%, and the precision, expressed as relative standard deviation s_r%, were satisfactory being lower than 6 %. To better validate the analytical procedure, a comparison with spectroscopic (electrothermal atomic absorption spectroscopy, ET‐AAS) is also reported.