The oxidative degradation of dibenzoazepine derivatives by cerium(IV) complexes in acidic sulfate media

The kinetics of the oxidation of imipramine and desipramine using cerium(IV) complexes were studied in the presence of a large excess of azepine derivative (TCA) in acidic sulfate media using UV-Vis spectroscopy. The reaction proceeds via dibenzoazepine radical formation, identified by EPR measurements. The kinetics of the first degradation step were studied independently of the further slower degradation reactions. Linear dependences, with zero intercept, of the pseudo-first-order rate constants (k(obs)) on [TCA] were established for both dibenzoazepine radical formation processes. Rates of reactions decreased with increasing concentration of the H(+) ion indicating that cerium(IV) as well as both reductants exist in an equilibrium with their protolytic forms. The activation parameters for the degradation of dibenzoazepine derivatives in the first oxidation stage were as follows: ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 8 J K(-1) mol(-1) for imipramine and ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 6 J K(-1) mol(-1) for desipramine, respectively. Imipramine and desipramine radicals dimerized leading to an intermediate radical dimer, which decayed in a first-order consecutive decay process. These two further reactions proceed with rates which are characterized by non-linear dependences of the pseudo-first-order rate constants (k(obs)) on [TCA]. The degradation reaction of the intermediate radical dimer leads to an uncharged dimer as a final product. Mechanistic consequences of all the results are discussed.     

Oxidation of thiosulfate by manganese(III) and nickel(III) complexes: A kinetic and mechanistic investigation

The kinetics of the oxidation of thiosulfate ion by [Mn(Y)]^? (H₄Y = trans-cyclohezane-1,2-dimine-N,N,N′,N′-tetraacetic acid) and [Ni(L¹)]²⁺ (HL¹ = 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) have been investigated at 20.0, 30.0, and 40.0°C in aqueous solution and found to follow the general rate law ?d/dt [complex] = k_ox[S₂O₃^2?] [complex]. The large negative entropy (ΔS?) of activation for the reactions and the results of Marcus calculation provide support for inner-sphere mechanism to be operative. © 1995 John Wiley & Sons, Inc.     

Kinetics of the manganese(III)-oxalate reaction in acidic sulphate media

The kinetics of the reaction of manganese(III) with oxalic acid (OA) has been studied in H₂SO₄ solutions. Under the experimental conditions of 6 × 10^–3 <>₀ < 0.4=" mol=">^–3 and [H₂SO₄]₀ 0.2 mol dm^–3 the observed pseudo-first order rate constant k _obs follows the expression

Kinetic and mechanistic studies on the electron-transfer reactions between diaquabisethylenediaminecobalt(III) and ethylenediaminetetraacetatocobaltate(III) with promazine in acidic aqueous media

The kinetics of the electron-transfer reactions between promazine (ptz) and [Co(en)₂(H₂O)₂]³⁺ in CF₃SO₃H solution ([Co^III] = (2–6) × 10⁻³ m, [ptz] = 2.5 × 10⁻⁴ m, [H⁺] = 0.02 − 0.05 m, I = 0.1 m (H⁺, K⁺, CF₃SO ₃ ⁻ ), T = 288–308 K) and [Co(edta)]⁻ in aqueous HCl ([Co^III] = (1 − 4) × 10⁻³ m, [ptz] = 1 × 10⁻⁴ m, [H⁺] = 0.1 − 0.5 m, I = 1.0 m (H⁺, Na⁺, Cl⁻), T = 313 − 333 K) were studied under the condition of excess Co^III using u.v.–vis. spectroscopy. The reactions produce a Co^II species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (k _obs) on [Co^III] with a non-zero intercept was established for both redox processes. The rate of reaction with the [Co(en)₂(H₂O)₂]³⁺ ion was found to be independent of [H⁺]. In the case of the [Co(edta)]⁻ ion, the k _obs dependence on [H⁺] was linear and the increasing [H⁺] accelerates the rate of the outer-sphere electron-transfer reaction. The activation parameters were calculated as follows: ΔH ^≠ = 105 ± 4 kJ mol⁻¹, ΔS ^≠ = 93 ± 11 J K⁻¹mol⁻¹ for [Co(en)₂(H₂O)₂]³⁺; ΔH ^≠ = 67 ± 9 kJ mol⁻¹, ΔS ^≠ = − 54 ± 28 J K⁻¹mol⁻¹ for [Co(edta)]⁻.     

Oxidation of phenothiazine dyes by manganese(III) in sulfuric acid solution

Manganese(III) sulfato complexes cause the oxidative degradation of methylene blue and its partially and fully N-demethylated derivatives, azure B and thionine dyes, respectively, in sulfuric acid media. The reaction proceeds through a colored reactive organic radical generated in the first stage via one-electron oxidation of the starting material, leading to a mixture of N-demethylated and/or deaminated species. The rates of formation of the methylene blue and azure B radicals are much higher than those of their further decomposition, whereas the generation of the thionine radical is much slower than its immeasurably fast decay. The kinetics of decomposition of all three dyes and the methylene blue and azure B radicals were studied spectrophotometrically under isolation conditions at 298 K. The first stage of each reaction proceeds according to a second-order rate expression, being first order in the dyes and in the manganese(III) concentrations. Dependence of the pseudo-first-order rate constants on the oxidant concentration for the second stage exhibits a saturation effect under the applied conditions. It is postulated that electron transfer takes place between the [Mn(SO₄)₃]³⁻ complex and the protonated form of the dye. The reactivity order of the dyes as determined from the second-order rate constants for the first reaction stage corresponds to the order of their HOMO energies.     

A kinetic study of the homogeneous oxidation of hydroxylamine by manganese(III)-bis(salicylaldimine) complexes

Summary The kinetics of the oxidation of hydroxylamine by manganese(III)-bis (salicylaldimine) complexes have been studied over the 5.2–8.4 pH range. The reaction is first order in both hydroxylamine and oxidant, and inversely proportional to [H⁺]. The [complex]: [hydroxylamine] stoichiometric ratio is 11 in both acidic and neutral media, and 21 in an alkaline medium. The second-order rate constant increased in the sequence: [Mn^III(L²)OH₂]-ClO₄·2H₂O > [Mn^III(L¹)OH₂]ClO₄ > [Mn^IIIL¹)OAc]-H₂O. The reactivity of unprotonated hydroxylamine is much higher than that of the protonated form. The reaction rate decreased significantly with addition of chloride ions. A plausible mechanism is proposed.     

Oxidation of manganese(II) by ozone and reduction of manganese(III) by hydrogen peroxide in acidic solution

Manganese(II) is oxidized by ozone in acid solution, k=(1.5±0.2)×10³ M⁻¹ s⁻¹ in HClO₄ and k=(1.8±0.2)×10³M⁻¹ s⁻¹ in H₂SO₄. The plausible mechanism is an oxygen atom transfer from O₃ to Mn²⁺ producing the manganyl ion MnO²⁺, which subsequently reacts rapidly with Mn²⁺ to form Mn(III). No free OH radicals are involved in the mechanism. The spectrum of Mn(III) was obtained in the wave length range 200–310 nm. The activation energy for the initial reaction is 39.5 kJ/mol. Manganese(III) is reduced by hydrogen peroxide to Mn(II) with k(Mn(III)+H₂O₂)=2.8×10³M⁻¹ s⁻¹ at pH 0–2. The mechanism of the reaction involving formation of the manganese(II)-superoxide complex and reaction of H₂O₂ with Mn(IV) species formed due to reversible disproportionation of Mn(III), is suggested. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 207–214, 1998.     

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.     

A mechanistic study of the dynamic quenching of the excited state of europium(III) and terbium(III) macrocyclic complexes by charge- or electron transfer

Dynamic quenching of the metal-based excited state of Eu(III) and Tb(III) complexes of sixteen different macrocyclic ligands has been studied. Quenching by urate, ascorbate and selected catechols is most effective for Tb(III) systems, and involves intermediate formation of an excited state complex (exciplex) between the electron-poor heterocyclic sensitising moiety incorporated into the ligand (tetraazatriphenylene, azaxanthone or a pyrazoyl-azaxanthone) and the electron-rich reductant. The process is sensitive to steric inhibition created by the local ligand environment; quenching is reduced as temperature increases as exciplex formation is entropically disfavoured. In contrast, iodide quenches each complex studied according to a classical collisional encounter model; increasing temperature enhances the rate of quenching, and the process is more sensitive to local electrostatic fields generated by ligand substitution, conforming to a traditional Stern-Volmer kinetic model. Quenching may be inhibited by protein association, allowing the identification of candidates for use as optical imaging probes in cellulo.     

Catalysis and kinetics of hydrogen peroxide disproportionation by dinuclear manganese(III) complexes of 1,5-bis(salicylidenamino)pentan-3-ol and the 5-bromophenyl-substituted derivative

The dinuclear MnIII complex [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br has been prepared and its structure and reactivity toward H2O2 studied in comparison with [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br (salpent-OH = 1,5-bis(salicylidenamino)pentan-3-ol and 5-Br-salpentOH = 1,5-bis(5-bromesalicylidenaminopentan-3-ol). The X-ray diffraction analysis of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br (monoclinic, P21/n, a = 13.081(2) A, b = 13.429(2) A, c = 17.375(2) A, beta = 102.31(1) degrees, V = 2982.0 A3, Z = 4) revealed a mu-alkoxo, mu-acetatodimanganese(III) core with a Mn...Mn separation of 2.932(1) A. The ligand lies in the meridional plane, and the sixth coordination position of each manganese atom is occupied by a methanol molecule providing two substitution-labile sites in the cis position. The two complexes showed catalytic activity toward disproportionation of H2O2 in methanol and dimethylformamide in the 0-25 degrees C temperature range. The initial rate of oxygen evolution in the presence of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br or [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]-Br is first order in catalyst concentration. The two complexes show saturation kinetics in methanol, with the higher kcat = 0.98 s-1 and kcat/KM = 70 M-1 s-1 observed for [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br.     

A spectral and electrochemical study of mononuclear manganese(III) complexes of a polybenzimidazolyl ligand

Summary A series of Mn^III complexes with stoichiometry [MnLX](Y)₂ have been synthesized utilizing an exogenous anionic ligand, X^- = OAc, SCN or N₃, and the hexadentate ligand N,N,N,N-tetrakis[2(benzimidazolyl) methyl-1,2-ethanediamine] and its N-octyl derivative. The vis. spectra of the compounds are in keeping with a pseudo-C _4v , symmetry for the Mn^III ion, implying that all the pendant arms of the hexadentate ligand do not bind to the Mn^III centre. Cyclic voltammetric studies reveal that the E _1/2 for the Mn^III/Mn^IV couple shifts to positive values with SCN^- as the anionic ligand, implying that this anion stabilizes the Mn^III oxidation state, whereas E _1/2 data for N₃ ^- reveals that the anion destabilizes the Mn^III state.     

Oxidation of ethylenediaminetetraacetate by tris(diimine) iron(III) complexes and the dodecatungstocobaltate(III) ion: a comparative kinetic and mechanistic study

Summary The stoichiometry and kinetics of the oxidation of ethyl-enediaminetetraacetate by [Fe(phen)₃]³⁺, [Fe(bipy)₃]³⁺ and [Co^IIIW₁₂O₄₀]⁵⁻ were studied in aqueous HClO₄. Reaction rates were first order with respect to the oxidants and the reductant, and the dependence of the second order rate constant k ₂ on [H⁺] is given by k ₂ = a + b[H^{+ −}. The primary products were CO₂, CH₂O and (CH₂NH₂)₂. Schuster treatment is employed to show that the reactions occur by the outersphere mechanism.     

Kinetics and mechanism of oxidation of quinols by tris(1,10-phenanthroline) iron(III) complexes in aqueous acidic perchlorate media

The kinetics of oxidation of several substituted quinols by a series of Tris(1,10-phenanthroline)iron(III) complexes has been investigated with a stopped-flow technique at 6.0 and 20.0°C. The reactions were found to be first order on both reactants and independent of acidity. The second-order specific rate constants were strongly dependent on free energy of reaction. An interpretation of the mechanism in the light of Marcus theory has been developed. The first electron abstraction with semiquinone radical formation has been suggested as the rate-determining step, and on this basis, intrinsic parameters of the reactions have been derived. A good agreement was found between experimental and computed data.     

Thermal decomposition studies on manganese(III) selenito complexes

The heats of immersion of the systems B₂O₃P₂O₅ and Na₂OB₂O₃P₂O₅ in pyridine have been measured by the use of a solution microcalorimeter. In the composition range R R = atomic ratio, B/P) the heat of immersion was relatively small and was almost independent of R. The heat increased markedly with boron content in R > 1. The increase of heat qualitatively parallels that of surface acidity. Introduction of the Na₂O component reduced both heat of immersion and acidity. MgO was immersed in a benzene solution of benzoic acid. The heat of immersion increased with an increase of surface basicity.     

Outer-Sphere electron-transfer and the effect of linkage isomerism in the titanium(III) reduction of nitro-pentacyanocobaltate(III) and nitro-, and nitrito-pentaamminecobalt(III) complexes in aqueous acidic media

The reductions of [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺, by Ti^III in aqueous acidic solution have been studied spectrophotometrically. Kinetic studies were carried out using conventional techniques at an ionic strength of 1.0 mol dm⁻³ (LiCl/HCl) at 25.0 ± 0.1 °C and acid concentrations between 0.015 and 0.100 mol dm⁻³. The second-order rate constant is inverse—acid dependent and is described by the limiting rate law:- k₂ ≈ k₀ + k[H⁺]⁻¹,where k=k′K_a and K_a is the hydrolytic equilibrium constant for [Ti(H₂O)₆]³⁺. Values of k₀ obtained for [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺ are (1.31 ± 0.05) × 10⁻² dm³ mol⁻¹ s⁻¹, (4.53 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ and (1.7 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ respectively, while the corresponding k′ values from reductions by TiOH²⁺ are 10.27 ± 0.45 dm³ mol⁻¹ s⁻¹, 14.99 ± 0.70 dm³ mol⁻¹ s⁻¹ and 17.93 ± 0.78 dm³ mol⁻¹ s⁻¹ respectively. Values of K _a obtained for the three complexes lie in the range (1–2) × 10⁻³ mol dm⁻³ which suggest an outer-sphere mechanism.     

A kinetic and mechanistic study of oxidation of arsenic(III) by hexacyanoferrate(III) in aqueous ethanoic acid

The kinetics of oxidation of As^IIIby Fe(CN)₆ ^3– has been studied spectrophotometrically in 60% AcOH–H₂O containing 4.0moldm^–3HCl. The oxidation is made possible by the difference in redox potentials. The reaction is first order each in [Fe(CN)₆ ^3–] and [As^III]. Amongst the initially added products, Fe(CN)₆ ^4– retards the reaction and As^Vdoes not. Increasing the acid concentration at constant chloride concentration accelerates the reaction. At constant acidity increasing chloride concentration increases the reaction rate, which reaches a maximum and then decreases. H₂Fe(CN)₆ ^–, is the active species of Fe(CN)₆ ^3–, while AsCl₅ ^2– in an ascending portion and AsCl₂ ⁺ in a descending portion are considered to be the active species of As^III. A suitable reaction mechanism is proposed and the reaction constants of the different steps involved have been evaluated.     

Manganese(III,IV) and manganese(III) oxide clusters trapped by copper(II) complexes

Reactions of quinquedentate Schiff base ligands with Mn and Cu ions afforded icosa- and hexadecanuclear mixed-metal clusters in which dinuclear CuII complexes trapped oxo-bridged [MnIII8MnIV4O12] and [MnIII6O6] cores, respectively. Maximum entropy method analysis for synchrotron X-ray diffraction data was used to determine the oxidation states of the Mn ions.     

Synthesis and mechanistic studies on the aquation of selenitoaquobis(diamine)cobalt(III) complexes

Summary The kinetics of the aquation of four selenitoaquobisdiamine)cobalt(III) complexes to their respective diaquabis-(diamine)complexes (diamine=ethylenediamine en, propylenediamine pn, dimethylethylenediamine me₂en and trimethylenediamine tmd) have been carried out conductimetrically in the 25–45 °C range. All reactions exhibit simple first order kinetics, and the rates increase with increasing temperature. In aqueous solution, the complex species exist in equilibrium with their respective hydroxo species, but only the hydroxo species are involved in the aquation process. This result is also reflected in the linear Arrhenius plot. The rates are higher in a 10% alcoholic solution than in water, but then decrease with increase in the alcohol content. A Grunwald-Winstein plot of rates in methanol yields slopes of 0.18 ± 0.04, 0.27±0.03, 0.43±0.03 and 0.34±0.02 for the four amines respectively. A dissociation mechanism is proposed for the aquation of all these complexes.     

Kinetic and mechanistic studies on the formation of bis(dicarboxylato)carbonatochromate(III) complexes

The reaction of the Cr(xx)₂(H₂O)₂ ⁻ (xx = oxalate, malonate and methylmalonate) complexes with dissolved CO₂ was studied by stopped-flow spectrophotometry in the 7 < pH < 9 range and between 20 to 30°C at an ionic strength of 0.5 mol dm⁻³ (NaCl). Under the experimental conditions the aqua complex ion consists of a pH-dependent mixture of Cr(xx)₂(H₂O)₂ ⁻, Cr(xx)₂(OH) (H₂O)²⁻ and Cr(xx)₂(OH)₂ ³⁻. The monohydroxo and dihydroxo species undergo CO₂ uptake and subsequent intramolecular carbonate ligand chelation independently, at rates which are readily distinguishable and are governed by the uptake rate constants k ₁ and k ₂ and chelation rate constants k ₃ and k ₄, respectively. Only the k ₁ values for oxalato, malonato and methylmalonato complexes could be calculated; k ₁ = 1084 and 1333 and 1650 mol⁻¹ dm³ s⁻¹, respectively. The results obtained were compared with those obtained from other systems that have either cobalt(III), iridium(III) or rhodium(III) as central atoms. This revised version was published online in June 2006 with corrections to the Cover Date.     

Predominance-zone diagrams of Fe(III) and Fe(II) sulfate complexes in acidic media. Voltammetric and spectrophotometric studies

A thermodynamic study based on concepts of generalized species and equilibria, was used to understand the distribution of Fe(III) and Fe(II) species in the Fe(III)/Fe(II)/H(2)SO(4)/H(2)O system. The two-dimensional predominance zone diagrams (TDPZ) and Pourbaix type diagrams thus obtained permitted the selection of optimum experimental conditions, to differentiate the chemical species involved in this system. The existence of the different predominant chemical species for Fe(III): Fe(3+), FeSO(+)(4) and Fe(SO(4))(-)(2) was evidenced by spectrophotometrical studies for pSO'(4) values from 4 to 0 units in a buffered solution of pH 0.5. Additionally, voltammetric studies performed on Pt, Au and carbon paste electrodes confirmed that the electron exchange between Fe(III) Fe(II) in H(2)SO(4) media occurs by an inner Helmholtz layer mechanism. It was also possible to show that the representative couples at pSO'(4) = 0 (buffered) are: (a) for pH < 0 FeSO (+)(4)FeHSO (+)(4) and (b) for pH > 1.0: Fe(SO (4)) (-)(2)FeHSO (+)(4). The last couple presents a coupled chemical reaction in the electrochemical mechanism; this reaction is associated with the different coordination numbers of Fe(III) and Fe(II).