Kinetics and mechanism of chromium(III) catalyzed oxidation of 1,10-phenanthroline by alkaline permanganate

The kinetics of chromium(III) catalyzed oxidation of 1,10-phenanthroline by permanganate in alkaline medium at a constant ionic strength has been studied spectrophotometrically. The reaction between permanganate and 1,10-phenanthroline in alkaline medium exhibits 4:1 stoichiometry (KMnO₄:1,10-phenanthroline). The reaction shows first order dependence on [permanganate] and [chromium(III)] and less than unit order dependence in 1,10-phenanthroline, zero order in alkali concentrations. The results suggest the formation of a complex between the 1,10-phenanthroline and the chromium(III) which reacts further with one mole of permanganate species in the rate-determining step, resulting in the formation of a free radical, which again reacts with three moles of permangante species in a subsequent fast step to yield the products. The reaction constants involved in the mechanism were evaluated. The activation parameters were computed with respect to the slow step of the mechanism.This revised version was published online in December 2005 with corrections to the Cover Date.     

Kinetics and mechanism of the oxidation of chromium(III)–dipicolinic acid complex by N-bromosuccinimide

The kinetics of oxidation of the chromium(III)–dipicolinic acid complex [Cr^III(DPA)₂(H₂O)₂]^– by N-bromosuccinimide (NBS) in aqueous solution to Cr^VI have been studied spectrophotometrically over the 20–40 °C range. The reaction is first order with respect to both [NBS] and [Cr^III], and increases with pH over the 5.92–6.93 range. Thermodynamic activation parameters were calculated. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of [NBS] to chromium(III).     

A comparative study of ruthenium(III) catalysed oxidation of L-leucine and L-isoleucine by alkaline permanganate. A kinetic and mechanistic approach

The kinetics of the Ru^III-catalysed oxidation of L-leucine and L-isoleucine by alkaline permanganate were studied and compared, spectrophotometrically using a rapid kinetic accessory. The reaction is first order with respect to [oxidant] and [catalyst] with an apparently less than unit order in [substrate] and [alkali] respectively. The results suggest the formation of a complex between the amino acid and the hydroxylated species of ruthenium(III). The complex reacts further with the alkaline permanganate species in a rate-determining step, resulting in the formation of a free radical, which again reacts with the alkaline permanganate species in a subsequent fast step to yield the products. The reaction constants involved in the mechanism were calculated. There is a good agreement between observed and calculated rate constants under different experimental conditions. The activation parameters with respect to the slow step of the mechanism for both the amino acids were calculated and discussed. Of the two amino acids, leucine is oxidised at a faster rate than isoleucine.     

Kinetics and mechanism of the oxidation of nitrilotris(methylenephosphonato)chromium(III) by periodate

The kinetics of oxidation of nitrilotris(methylenephosphonato)chromium(III), Cr^IIINTMP, by periodate to yield Cr^VI have been studied spectrophotometrically over the 5.80–6.85 pH range at 22–33 °C. The reaction rate, which is first-order with respect to [Cr^IIINTMP] and [IO^– ₄] and inversely dependent on [H⁺], obeys the rate law:-d[Cr^IIINTMP/dt=kKK_h[IO^- ₄] [Cr^III]_T/K_h+ [H⁺] +KK_h[IO^- ₄] The values of the intramolecular electron transfer, k, and the formation constant of the intermediate complex, K, were determined at various temperatures. The hydrolysis constant for Cr^IIINTMP, K _h , was determined spectrophotometrically and is in agreement with the value estimated from the kinetic data. The activation parameters were calculated from the temperature dependence of the specific rate constants. A mechanism is proposed in which the hydroxo complex, [CrHNTMP(OH)]^3–, is the reactive species. The results support a mechanism where intramolecular electron transfer is the rate-determining step.     

Kinetic studies on the oxidation of [N-phenylethylethylenediaminetriacetate]chromium(III) by periodate

Summary Kinetic studies of the oxidation of [Cr^IIIZ(H₂O)](Z=N-phenylethylethylenediaminetriacetate) by periodate ion, to produce chromium(VI), were carried out in aqueous solutions. The reaction is first order with respect to both total chromium(III) and total periodate, and the rate is inversely dependent upon H⁺ in the 5.43–7.02 pH range. The reaction may follow a two-step inner-sphere electron transfer mechanism. The activated parameters are reported. Steric effects of the phenyl ring account for the smaller electron-transfer rate constants for [Cr^IIIZ(H₂O)] compared to [Cr^III(TOH)(H₂O)], (TOH=N-(2-hydroxyethyl)ethylenediaminetriacetate).     

Kinetics and mechanism of a trans-tetraazamacrocyclic chromium(III) complex oxidation by hexacyanoferrate(III) in strongly alkaline media

Oxidation of the trans-[Cr(cyca)(OH)₂]⁺ complex, where cyca = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, by [Fe(CN)₆ ]^3- ion in strongly alkaline media, leading to [Cr^V O(cyca_ox )]³⁺ ion, has been studied using electronic and e.p.r. spectroscopy. The kinetics of the Cr^III → Cr^IV transformation have been studied using a large excess of the reductant and OH^- ion over the oxidant. The reaction is a second order process: first order in [Cr^III] and [Fe^III] at constant [OH^-]. The second order rate constant is higher than linearly dependent on the OH^- concentration. The mechanism of the reaction has been discussed. A relatively inert intermediate chromium(V) species was detected based on characteristic bands in the visible region and the e.p.r. signal at g_iso = 1.987 for the systems where an excess of oxidant was used. The hyperfine structure of the main e.p.r. signal is consistent with the d¹ -electron interactions with four equivalent nitrogen nuclei and [Cr^V = O(cyca_ox)]³⁺ formula, where cyca_ox = oxidized cyca, can be postulated for the intermediate Cr^V complex.     

Kinetics and mechanism of ruthenium(III)-catalysed oxidation of tellurium(IV) by alkaline diperiodatonickelate(IV)

The kinetics of Ru^III-catalysed oxidation of tellurium(IV) by alkaline diperiodatonickelate(IV) were studied spectrophotometrically using a rapid kinetic accessory. The reaction is a two stage process. In both the stages, the reaction is first-order with respect to [oxidant] and to [catalyst] with an apparent less than unit order, each in [substrate] and [alkali]. Periodate has a retarding effect on the reaction rate. A mechanism involving monoperiodatonickelate(IV) (MPN) as the reactive oxidant species is proposed. The data suggest that oxidation proceeds via formation of a complex between the active species of Ru^III and Te^IV, which then reacts with 1 mol of MPN in a slow step to yield the products. The reaction constants involved in the mechanism were evaluated. There is good agreement between the observed and calculated rate constants under varying experimental conditions for both the stages of reaction. The activation parameters for the slow step were calculated and discussed.     

Free radical intervention,deamination and decarboxylation in the ruthenium(III)-catalysed oxidation of L-arginine by alkaline permanganate – a kinetic study

The kinetics of the Ru^III catalysed oxidation of L-arginine by alkaline permanganate was studied spectrophotometrically using a rapid kinetic accessory. The reaction follows a two stage process. In both the stages the reaction is first order with respect to [oxidant] and [catalyst] with an apparent less than unit order in [substrate] and [alkali]. The data suggest that oxidation proceeds via formation of a complex between the active Ru^III species and L-arginine, which then reacts with one mole of permanganate in a slow step to yield a L-arginine free radical, followed by a fast step to form the products. The reaction constants involved in the mechanism were evaluated. There is a good agreement between observed and calculated rate constants under different experimental conditions for both stages of reaction. The activation parameters for the slow step were calculated and are discussed.     

Kinetics and mechanism of oxidation of (aqua-2-aminomethyl-pyridine) chromium(III) by N-bromosuccinimide

Summary The kinetics of oxidation of (aqua-2-aminomethyl-pyridine) Cr^III by N-bromosuccinimide (NBS) in aqueous solution to yield chromium(VI) has been studied spectrophotometrically over the 25–40 °C range. The reaction rate is first order with respect to both [NBS] and [Cr^III], and increases with increasing pH between 7.6 and 8.6. The thermodynamic activation parameters were calculated. The experimental rate law is consistent with a mechanism in which the deprotonated [Cr(L)₂(OH)]²⁺ was considered to be the most reactive form compared to its conjugate acid. It is assumed that electron transfer takes place via an inner-sphere mechanism.     

Kinetics and mechanism of oxidation of chromium(III)-tetraoxalylurea complex by periodate

A novel chromium(III) complex of tetraoxalylurea was prepared. In aqueous solutions, [Cr^III(H₂L)(H₂O)]⁺ (H₂L = diprotonated tetraoxalylurea) is oxidized by IO ₄ ^– according to the rate law

Biphasic oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) by N-bromosuccinimide and the formation of chromium(IV) and chromium(V)

The kinetics of oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) complex, [Cr^III(phen)(H₂O)₂Cl₂]⁺, by N-bromosuccinimide (NBS) is biphasic. The first faster step involves the oxidation of Cr(III) to Cr(IV). The second slower step is due to the oxidation of Cr(IV) to Cr(V). The reaction product is isolated and characterized by electron spin resonance (ESR), IR, and elemental analysis. The chromium(V) product is consistent with the formula [Cr^V(phen)Cl₂(O)]Br. The rate constants k_f and k_s, for the faster and the slower steps respectively, were obtained using an Origin 9.0 software program. Values of both k_f and k_s, varied linearly with [NBS] at constant reaction conditions. The effect of pH on the reaction rate is investigated over the pH (4.11–6.01) range at 25.0°C. The rate constants k_f and k_s increased with increasing pH. This is consistent with hydroxo forms of the chromium species being more reactive than the aqua forms. Chromium(III) complexes, more often than not, are inert. The oxidation of the Cr(III) complex to Cr(IV), most likely, proceeds by an outer sphere mechanism. Since chromium(IV) is labile the mechanism of its oxidation to chromium(V) is not certain.     

Dinitrogen complexes of chromium(III) with chelating agents

Summary Two new dinitrogen complexes of Cr^III with EDTA and CDTA have been prepared from reaction of [Cr(Y-H)H₂O] (Y = EDTA, CDTA) with sodium azide in aqueous solution. Both complexes exhibit characteristic bands at 2070 cm^–1 in the i.r. spectra corresponding to (N₂). The 10 Dq and B parameters were calculated from the u.v. visible spectra.     

Oxidative degradation and deamination of atenolol by diperiodatocuprate(III) in aqueous alkaline medium: A mechanistic study

The kinetics of oxidation of atenolol (ATN) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.10 mol dm⁻³ was studied spectrophotometrically. The reaction between DPC and ATN in alkaline medium exhibits 1:2 stoichiometry (ATN:DPC). The reaction is of first order in [DPC] and has less than unit order in both [ATN] and [alkali]. However, the order in [ATN] and [alkali] changes from first order to zero order as their concentration increase. Intervention of free radicals was observed in the reaction. Increase in periodate concentration decreases the rate. The oxidation reaction in alkaline medium has been shown to proceed via a monoperiodatocuprate(III)–ATN complex, which decomposes slowly in a rate-determining step followed by other fast steps to give the products. The main oxidative products were identified by spot test, IR, NMR and LC–ESI-MS studies. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to slow step of the mechanism are computed and discussed, and thermodynamic quantities are also determined.     

Kinetics and mechanism of reduction of oxo-chromium(V) salen by vanadium(IV)

The reduction of oxo-chromium(V) salen with a 40–160-fold excess of oxovanadium(IV) ([H⁺] = 0.02–0.1 M) at 25 °C has been investigated. The observed absorbance changes fitted a pseudo-first-order process. The nature of the intermediate, final product and reaction mechanism have been proposed on the basis of reaction conditions and observed rate constants. E.s.r. data support 1:1 stoichiometry with VO²⁺ in a deficiency. With an excess of VO²⁺ a Cr^III product corresponding to a two electron reduction process has been obtained. The spectral and ion exchange properties of the chromium product correspond to that of the N,N-ethylene-bis(salicylideneimine) derivative of Cr^III. The rate of formation of the final product increases with decreasing [H⁺]. The observed kinetic behavior is consistent with a mechanism involving the formation of a Cr^IV—V^V intermediate in an equilibrium step prior to the electron transfer step. The equilibrium constant for the formation of the intermediate has been estimated to be 11.2 ± 0.8 M^–1. The second-order-rate constants for the reduction of Cr^V species have been estimated to be 0.14 × 10², 0.10 × 10² and 0.05 × 10² M^–1 S^–1 at [H⁺] = 0.02, 0.05 and 0.1 M respectively. Like the Fe^II—Cr^V redox couple, the V^IV—Cr^V redox reaction also follows an inner-sphere process.     

One-step three-electron oxidation of tartaric and glyoxylic acids by chromium(VI) in the absence and presence of manganese(II)

Kinetics of the initial stages of oxidation of tartaric acid (TA) in the absence and presence of manganese(II) ions have been studied spectrophotometrically. The rate of the induction was slow and then gradually increased with increasing [TA] and/or [HClO₄]. The reaction followed third-order kinetics; first-order with respect to each of [TA], [HClO₄] and [Cr^VI]. The kinetic and manganese(II) effect studies are consistent with a one-step three-electron mechanism (Cr^VI Cr^III without passing through Cr^IV as an intermediate) in which a termolecular complex is formed between TA, Mn^II and HCrO₄ ^–. In order to obtain further insight, oxidation of glyoxylic acid (GA), an oxidation product of TA, was also studied under the similar conditions. Details of the process are discussed.     

Ruthenium(III) catalyzed oxidation of sulfanilic acid by diperiodatocuprate(III) in aqueous alkaline medium. A kinetic and mechanistic approach

The kinetics of ruthenium(III) catalyzed oxidation of sulfanilic acid by diperiodatocuprate(III) (DPC) in alkaline medium at a constant ionic strength of (0.50 mol dm⁻³) has been studied spectrophoto-metrically. The reaction between sulfanilic acid and DPC in alkaline medium exhibits 1: 4 stoichiometry (sulfanilic acid: DPC). The reaction is first order with respect to [DPC] and [Ru^III] and has less than unit order both in [sulfanilic acid] and [alkali]. The active species of catalyst and oxidant have been identified. Intervention of free radicals was observed in the reaction. The main products were identified by spot test and IR. Probable mechanism is proposed and discussed. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to the slow step of the mechanism are computed and discussed. Thermodynamic quantities are also determined.     

Mechanistic aspects of the reduction of ruthenium(III) catalyzed

The kinetics of Ru_III catalyzed reduction of hexacyanoferrate(III) [Fe(CN)₆]^3–, by atenolol in alkaline medium at constant ionic strength (0.80 mol dm^–3) has been studied spectrophotometrically, using a rapid kinetic accessory. The reaction between atenolol and [Fe(CN)₆]^3– in alkaline medium exhibits 1:2 stoichiometry [atenolol:Fe(CN)₆ ^3–]. The reaction showed first order kinetics in [Fe(CN)₆]^3– concentration and apparent less than unit order dependence, each in atenolol and alkali concentrations. Effect of added products, ionic strength and dielectric constant of the reaction medium have been investigated. A retarding effect was observed by one of the products i.e., hexacyanoferrate(II). The main products were identified by i.r., n.m.r., fluorimetric and mass spectral studies. A mechanism involving the formation of a complex between the atenolol and the hydroxylated species of ruthenium(III) has been proposed. The active species of oxidant and catalyst were [Fe(CN)₆]^3–and [Ru (H₂O)₅OH]²⁺, respectively. The reaction constants involved in the mechanism were evaluated. The activation parameters were computed with respect to the slow step of the mechanism, and discussed.     

Kinetic studies of the oxidation ofN-(2-hydroxycyclohexyl)-ethylenediaminetriacetatochromate(III) by periodate

The kinetics of the oxidation of [Cr^III(H₂O)(XOH)], (XOH=N-(2-hydroxycyclohexyl)ethylenediaminetriacetate) to Cr^VI by periodate have been investigated in aqueous solution at various pH values (6.00–7.20) and temperatures (15.0–35.0°C). The reaction follows the rate law:

Kinetics and mechanism of oxidation of cis-diaquabis(glycinato)chromium(III) by periodate ion in aqueous solutions

The kinetics of oxidation of cis-[Cr^III(gly)₂(H₂O)₂]⁺ (gly = glycinate) by $ {\text{IO}}_{ 4}^{ - } $ has been studied in aqueous solutions. The reaction is first order in the chromium(III) complex concentration. The pseudo-first-order rate constant, k _obs, showed a small change with increasing $ \left[ {{\text{IO}}_{ 4}^{ - } } \right] $ . The pseudo-first-order rate constant, k _obs, increased with increasing pH, indicating that the hydroxo form of the chromium(III) complex is the reactive species. The reaction has been found to obey the following rate law: $ {\text{Rate}} = 2k^{\text{et}} K_{ 3} K_{ 4} \left[ {{\text{Cr}}\left( {\text{III}} \right)} \right]_{t} \left[ {{\text{IO}}_{ 4}^{ - } } \right]/\left\{ {\left[ {{\text{H}}^{ + } } \right] + K_{ 3} + K_{ 3} K_{ 4} \left[ {{\text{IO}}_{ 4}^{ - } } \right]} \right\} $ . Values of the intramolecular electron transfer constant, k ^et, the first deprotonation constant of cis-[Cr^III(gly)₂(H₂O)₂]⁺, K ₃ and the equilibrium formation constant between cis-[Cr^III(gly)₂(H₂O)(OH)] and $ {\text{IO}}_{ 4}^{ - } $ , K ₄, have been determined. An inner-sphere mechanism has been proposed for the oxidation process. The thermodynamic activation parameters of the processes involved are reported.     

Binary and ternary chromium(III) complexes with cellular reductants and DNA components isolated from redox type systems. Part 3. Chromium(VI) – <Emphasis Type="SmallCaps">L</Emphasis>-ascorbic acid – adenine (adenosine,ATP)

The reduction of CrO3 with an excess of L-ascorbic acid and its interaction with DNA fragments (adenine, adenosine) and the ATP nucleotide was studied by the analysis of the isolated solid products. The precipitates were characterised by elemental analyses, FAB-mass spectral data, spectroscopic methods (u.v.–vis., i.r) and magnetic measurements. The Cr^III complexes obtained from ternary systems appeared to be [Cr^III–L-ascorbic acid] and [Cr^III–L-ascorbic acid–adenine (adenosine, ATP)] species. The structure of ternary complexes has been proposed assuming hydrogen bond formation between the [Cr^III–ascorbic acid] complex and the DNA components. The effect of main cellular reductants: cysteine, glutathione and L-ascorbic acid on the composition and structure of isolated products has been discussed.