Palladium(II) inhibition during cerium(IV) oxidation of aldoses: a kinetic study

Summary The kinetics of oxidation of aldoses, namely xylose, arabinose, galactose and glucose, by Ce^IV have been studied in HClO₄ + H₂SO₄ medium and in the presence of Pd^II. The reactions exhibit a first order rate dependence with respect to oxidant. The rate is inversely dependent on the [HSO _inf4 ^sup– ][H⁺] ratio. The order of reaction with respect to aldose decreases at higher [aldose]. Due to the formation of a complex between Ce^IV and Pd^II, a retarding effect of [Pd^II] on the rate of disappearance of [Ce^IV] has been observed. A mechanism consistent with the observed kinetic data is proposed.     

Osmium(VIII)/palladium(II) catalysis of cerium(IV) oxidation of allyl alcohol in aqueous acid

Summary Catalysis of the Ce^IV-allyl alcohol (AA) reaction in acid solution depends both on the of rate enhancement and product distribution on the catalyst used: Os^VIII results mainly in acrolein, whereas Pd^II gives acrylic acid. The rate laws in the two cases also differ:viz., Equations 1 and 2K₁ is the equilibrium constant of formation of the Os^VIII-allyl alcohol complex and k₁ is the rate constant of its oxidation by Ce^IV; K₂ is the equilibrium constant for the formation of the Ce^IV-Pd^II-allyl alcohol complex and k₂ is its rate constant of decomposition. Rate = K₁k₁[Ce^IV][AA][Os^VIII]/(1+K₁[AA]) (1) Rate = K₁k₁[Ce^IV][Pd^II]/(1+K₂[Ce^IV]) (2)While Os^VIII is effective in H₂SO₄ solution, aqueous HClO₄ is needed for Pd^II. Both reactions proceed through formation of catalyst-allyl alcohol complexes with participation of free radicals. The details of these observations are discussed.     

Kinetics of oxidative degradation of pantothenic acid by cerium(IV) in aqueous perchloric acid

The kinetics of oxidation of pantothenic acid (PA), Me₂C(CH₂OH)CH(OH)C(O)NHCH₂CH₂CO₂H, by cerium(IV) in aqueous HClO₄ medium at constant ionic strength, 2.0 mol dm^–3, has been studied spectrophotometrically. The reaction showed first-order kinetics in Ce^IV concentration, an apparent less than unit order dependence in [PA] and an inverse fractional order in [H⁺]. Initial addition of products had no significant effect on the rate of the reaction. A possible mechanism is proposed, and the reaction constants involved in the mechanism have been computed. There is good agreement between the observed and calculated rate constants under different experimental conditions. The activation parameters were calculated with respect to the slow step of the proposed mechanism.     

Mechanistic Study of Oxidation of Palladium(II) by Cerium(IV) in Aqueous Acid

The kinetics of oxidation of Pd^II by Ce^IV have been studied spectrophotometrically in HClO₄ media at 40 °C. The reaction is first order each in [Ce^IV] and [Pd^II] at constant [H⁺]. Increasing [H⁺] accelerates the reaction rate with fractional order in [H⁺]. The initially added products, palladium(IV) and cerium(III) do not have any significant effect on the reaction rate. At constant acidity, increasing the added chloride concentration enhances the rate of reaction. H₃Ce(SO₄)₄⁻ and PdCl₄²⁻ are the active species of oxidant and reductant respectively. The possible mechanisms are proposed and the reaction constants involved have been determined.     

Facile and Reversible Formation of Iron(III)–Oxo–Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

Ceric ammonium nitrate (CAN) or Ce^IV(NH₄)₂(NO₃)₆ is often used in artificial water oxidation and generally considered to be an outer‐sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)Fe^III‐O‐Ce^IV(OH₂)(NO₃)₄]⁺ ( 3 ), a complex obtained from the reaction of [(N4Py)Fe^II(NCMe)]²⁺ with 2 equiv CAN or [(N4Py)Fe^IV=O]²⁺ ( 2 ) with Ce^III(NO₃)₃ in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe^IV and Ce^IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S =1 Fe^IV in 2 to S =5/2 in 3 , which is found to be facile despite the formal spin‐forbidden nature of this process. This observation suggests that Fe^IV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.     

Oxidation of N-(2-hydroxyethyl)ethylenediamine triacetate by tris(polypyridy)iron(III) complexes and the dodecatungstocobaltate(III) ion

Summary The stoichiometry, kinetics and mechanism of oxidation of N-(2-hydroxyethyl)ethylenediamine triacetate by K₅Co^IIIW₁₂O₄₀, Fe(phen) _inf3 ^sup3+ and Fe(bipy) _inf3 ^sup3+ have been studied. Each reaction is first order with respect to the oxidant and the reductant, but retarded by [H⁺] in the 0.20–1.60 mol dm ^–3 range. Simple electron exchanges are thought to occur, leading to the decarboxylation of the substrate.     

Kinetics and mechanism of ruthenium(III) catalyzed oxidation of tetrahydrofurfuryl alcohol by cerium(IV) in sulfuric acid media

The kinetics and mechanism of ruthenium(III) catalyzed oxidation of tetrahydrofurfuryl alcohol (THFA) by cerium(IV) in sulfuric acid media have been investigated spectrophotometrically in the temperature range 298–313 K. It is found that the reaction is first-order with respect to Ce^IV, and exhibits a positive fractional order with respect to THFA and Ru^III. The pseudo first-order ([THFA]≫[Ce^IV]≫[Ru^III]) rate constant k _obs decreases with the increase of [HSO ₄ ⁻ ]. Under the protection of nitrogen, the reaction system can initiate polymerization of acrylonitrile, indicating the generation of free radicals. On the basis of the experimental results, a reasonable mechanism has been proposed and the rate equations derived from the mechanism can explain all the experimental results. From the dependence of k _obs on the concentration of HSO ₄ ⁻ , has been found as the kinetically active species. Furthermore, the rate constants of the rate determining step together with the activation parameters were evaluated.     

Kinetic study of the Ce(III)‐, Mn(II)‐ or Fe(phen)32+‐catalyzed Belousov‐Zhabotinsky reaction with 2‐ketoglutaric acid

The Belousov‐Zhabotinsky (BZ) reaction of bromate ion with 2‐ketoglutaric acid (KGA) in aqueous sulfuric acid catalyzed by Ce(III), Mn(II), or Fe(phen)₃²⁺ ion exhibits sustained barely damped oscillations under aerobic conditions. In general, the reaction oscillates without an induction period. Fe(phen)₃²⁺ ion behaves differently from Ce(III) and Mn(II) ions in catalyzing this oscillating system. The gem‐diol form of KGA exhibits different behavior from that of the keto form of KGA in the BZ reaction. The kinetics and mechanism of the reaction of KGA with Ce(IV), Mn(III), or Fe(phen)₃³⁺ ion was investigated. The order of relative reactivities of metal ions toward reaction with KGA is Mn(III) > Ce(IV) ≫ Fe(phen)₃³⁺. Experimental results are rationalized. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 101–107, 2001     

Complexation kinetics of Fe2+ with 1,10‐phenanthroline forming ferroin in acidic solutions

The formation kinetics of ferroin is studied under varied acid conditions at 25°C and fixed ionic strength (0.48 mol dm^?3) under pseudo‐first‐order conditions with respect to Fe²⁺ by using the stopped‐flow technique. The reaction followed is first and third order with respect to Fe²⁺ and 1,10‐phenanthroline (phen)_T, respectively. Increasing the acid concentration retarded the reaction, and the reaction rate showed a positive salt effect. The rate‐limiting step involved the complexation of the phen or protonated phen with [Fe(phen)₂]²⁺ complex ion, leading to formation of [Fe(phen)₃]²⁺ ion. The observed retardation of the reaction rate with increasing [H⁺]₀ is due to the increased [phenH⁺]_eq and low reactivity of phenH⁺ with [Fe(phen)₂]²⁺ complex ion. Simulated curves for the acid variation experiments agreed well with the corresponding experimental curves and the estimated rate coefficients supporting the proposed mechanism. Relatively low energy of activation (26 kJ mol^?1) and high negative entropy of activation (?159.8 J K^?1 mol^?1) agree with the proposed mechanism and the formation of compact octahedral complex ion. © 2008 Wiley Periodicals, Inc. 40: 515–523, 2008     

Kinetics of reactions of N1-phenylbiguanidine with CeIV and [MnO4]− in aqueous sulphuric acid media

Summary In the title reaction each mole of N¹-phenylbiguanidine, R–HNC(=X)NHC(=NH)NH₂ (R=Ph, X=NH), consumes 4 moles of Ce^IV and produces guanylurea (R=H, X=O), 1,4-benzoquinone and ammonia. On the other hand, the reaction of N¹-phenylbiguanidine (pbg) with [MnO₄]^– proceeds with variable stoicheiometry which depends on reaction conditions. In the case of [MnO₄]^– no benzoquinone is detected among the reaction products; instead, carbon dioxide, guanylurea, and ammonia were identified. Pbg itself in acid solution slowly hydrolyses to aniline which rapidly reacts with Ce^IV and [MnO₄]^–. The kinetics for the reactions of pbg with the oxidants is consistent with the rate law –d[oxidant]/dt=k[pbg].The k values and the corresponding activation enthalpies and entropies for the reaction of bpg with Ce^IV, [MnO₄]^–, and Cr^VI lie within a narrow range. These results are interpreted in terms of rate-determining hydrolysis of pbg in all the three cases.     

Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from mono(aminocarboxylato)iron(III) complexes

Summary The kinetics and mechanism of the system: [FeL(OH)]^2–n + 5 CN^– [Fe(CN)₅(OH)]^3– + L^n–, where L=DTPA or HEDTA, have been investigated at pH= 10.5±0.2, I=0.25 M and t=25±0.1 C.As in the reaction of [FeEDTA(OH)]^2–, the formation of [Fe(CN)₅(OH)]^3– through the formation of mixed ligand complex intermediates of the type [FeL(OH)(CN)_x]^2–n–x, is proposed. The reactions were found to consist of three observable stages. The first involves the formation of [Fe(CN)₅(OH)]^3–, the second is the conversion of [Fe(CN)₅(OH)]^3– into [Fe(CN)₆]^3– and the third is the reduction of [Fe(CN)₆]^3– to [Fe(CN)₆]^4– by oxidation of L^n– The first reaction exhibits a variable order dependence on the concentration of cyanide, ranging from one at high cyanide concentration to three at low concentration. The transition between [FeL(OH)]^2–n and [Fe(CN)₅(OH)]^3– is kinetically controlled by the presence of four cyanide ions around the central iron atom in the rate determining step. The second reaction shows first order dependence on the concentration of [Fe(CN)₅(OH)]^3– as well as on cyanide, while the third reaction follows overall second order kinetics; first order each in [Fe(CN)₆]^3– and L^n–, released in the reaction. The reaction rate is highly dependent on hydroxide ion concentration.The reverse reaction between [Fe(CN)₅(OH)]^3– and L^n– showed an inverse first order dependence on cyanide concentration along with first order dependence each on [Fe(CN)_5– (OH)]^3– and L^n–. A five step mechanism is proposed for the first stage of the above two systems.     

Kinetics of oxidation of thiocyanate ion by manganese(III) in pyrophosphate medium

Summary Mn^III is stabilized by pyrophosphate in weakly acidic solutions. The nature of the complex formed was elucidated spectrophotometrically. The kinetics of Mn^III oxidation of thiocyanate in pyrophosphate medium was investigated over the pH range 2–3. The oxidation followed first order kinetics with [Mn^III]. The effects of varying [Mn^III], [NCS^–], added Mn^II and metal ions, pH, total [P₂O _f7 ^p4– ] and added ClO _f4 ^p– , Cl^– and SO _f4 ^p2– were studied. The order in [NCS^–] was unity, and increasing [H⁺] increased the rate. Retardations with added P₂O _f7 ^p4– and Mn^II were observed. Complexation of NCS^– as K₂Zn(NCS)₄ decreased the reactivity without any change in overall mechanism. The dependence of the reaction rate on temperature was examined, and activation parameters were computed from Arrhenius and Eyring plots. A mechanism consistent with the results is proposed.     

Tris-o-phenanthrolineiron(lll) Complex as a Photoinitiator of Acrylamide Polymerization

Abstract

The blue Tris-o-phenanthrolineiron(m) complex, Fe(phen)₃ ³⁺, formed in aqueous solution by oxidation of the red ferrous phenanthroline complex by nitric acid, is found to be a good photo-initiator of acrylamide (A.AM) polymerization. Systematic study of the kinetics of polymerization of acrylamide by Fe(phen) ₃ ³⁺ in aqueous nitric acid solution in the presence of light of Δ = 365 nm at room temperature showed that the rate of polymerization R_p was dependent on [A.AM]^1,5, [C] ^0,5, I^0,5, and [HCOOH], and the rate of Fe(phen) ₃ ³⁺disappearance, -R_c, was found to be proportional to [A.AM], [C], I, and [HCOOH], where [A.AM], [C] and [HCOOH] refer to the concentrations of acrylamide, Fe(phen) ₃ ³⁺, and formic acid, respectively, and I refers to light intensity. The kinetic observations are consistent with the interaction of the excited Fe(phen) ₃ ³⁺ with acrylamide molecule to produce a radical R. capable of initiation and also reduction of Fe(phen) ₃ ³⁺, to the stable Fe(phen) ₃ ²⁺. The results of the present study differ from those reported for photo initiation by ion pairs of the type Fe ³⁺X ^n- where ×= CI^?, Br^?, C₂O₄ ^2- OH^?, etc., which may be attributed to differences in the photo-behavior of the two systems.     

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.     

Mechanistic study of iodide catalysed oxidation of l-glutamic acid by cerium(IV) in aqueous sulphuric acid medium

A minute quantity (10⁻⁶ mol dm⁻³) of iodide catalysed oxidation of l-glutamic acid by Ce^IV has been studied in H₂SO₄ and SO ₄ ²⁻ media. The reaction was first order each in [Ce^IV] and [I⁻]. The order with respect to [l-glutamic acid] was less than unity (0.71). Increase in [H₂SO₄] decreased the reaction rate. The added HSO ₄ ⁻ and SO ₄ ²⁻ decreased the rate of reaction. The added product, succinic acid, had no effect on the reaction rate, whereas added Ce^III retarded the reaction. The ionic strength and dielectric constant did not have any significant effect on the rate of reaction. The active species of oxidant was Ce(SO₄)₂. A suitable mechanism was proposed. The activation parameters were determined with respect to the slow step of the mechanism. The thermodynamic quantities were also determined and discussed.     

Effect of cosolvent on the volume of activation for base hydrolysis of tris(1.10‐phenanthroline)‐ and tris (2.2′‐bipyridine)iron(II) complexes

The kinetics of the base hydrolysis of Fe(phen)₃²⁺ and Fe(bipy)₃²⁺ (phen = 1,10‐phenanthroline and bipy = 2,2'‐bipyridine) in some aqueous alcohol mixtures at ambient and elevated pressures (up to 1kbar) have been monitored spectrophotometrically at 25.0°C. For a given pressure, the alcohol cosolvent increases the rate of reaction relative to the reaction in a wholly aqueous medium. In all cases, increasing pressure causes rate retardation and derived volumes of activation for the reactions in aqueous solvent mixtures vary between +15 and +25 cm³ mol⁻¹, indicating that solvation changes of a different magnitude occur upon reaching the transition state from those occurring for the reactions in aqueous medium. Since the reaction has been established earlier to be nucleophilic attack of the incoming hydroxide ion, the volumes of activation signify marked increases in the loss of electrostricted solvent from the vicinities of the hydroxide ion and the iron(II) complex ions. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 263–270, 2000     

Synthesis,features and thermal decomposition of hexachlorostannates and hexachloroplatinates of tris(2,2′-dipyridyl)metal(II) and tris(1,10-phenanthroline)metal(II) (metal=Ru,Fe, Ni)

Several complex salts of general formula [M^II(dipy)₃]M^IVCl₆ or [M^II(phen)₃]M^IVCl₆ (whereM ^II=Ru, Fe, Ni andM ^IV=Sn, Pt) were synthetized and subjected to thermal analyses. Heating of these derivatives leads to the release of organic fragments and chlorine, which are often involved in oxidation processes. The residues comprise metal oxides or pure metals (e.g. Pt). Differences in the structures and features of the ligand molecules, revealed on the basis of quantum-chemistry calculations, account qualitatively for the differences in behaviour and stability of the complex compounds studied.This work was financed by the Polish State Committee for Scientific Research (KBN) under grant 2 0679 91 01 (contract no. 1156/2/91).     

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.     

Kinetics and mechanism of oxidation of 8-hydroxyquinoline and its derivatives by cerium(IV) through precursor complex formation

Summary In acid (HClO₄) medium Ce^IV is reduced to Ce^III by 8-hydroxyquinoline (1) and its derivatives, the 5-sulphonate (2) and 7-iodo-5-sulphonate (3), through precursor complex formation. The rates of precursor complex formation have been studied by stopped-flow spectrophotometry for both (1) and (2). Formation of the precursor complex occurs in two concurrent paths involving Ce _aq ⁴⁺ and [Ce(OH)] _aq ³⁺ reacting by I_a and I_d processes respectively. The reaction with (3) is, however, too fast for the stopped-flow method. The rates for subsequent intramolecular electron transfer processes in the precursor complexes are sufficiently slow for all the three species(1)–(3) for evaluation by conventional spectrophotometry. For the electron transfer process the rate varies in the sequence:(2)>(1)(3) due to electron-withdrawing character of the substituent SO ₃ ^– and electron-releasing character of I.     

Ce(iv)-centered charge-neutral perovskite layers topochemically derived from anionic [CeTa2O7]− layers

Layered perovskites have been extensively investigated in many research fields, such as electronics, catalysis, optics, energy, and magnetics, because of the fascinating chemical properties that are generated by the specific structural features of perovskite frameworks. Furthermore, the interlayers of these structures can be chemically modified through ion exchange to form nanosheets. To further expand the modification of layered perovskites, we have demonstrated an advance in the new structural concept of layered perovskite “charge-neutral perovskite layers” by manipulating the perovskite layer itself. A charge-neutral perovskite layer in [Ce^IVTa₂O₇] was synthesized through a soft chemical oxidative reaction based on anionic [Ce^IIITa₂O₇]⁻ layers. The Ce oxidation state for the charge-neutral [Ce^IVTa₂O₇] layers was found to be tetravalent by X-ray absorption fine structure (XAFS) analysis. The atomic arrangements were determined through scattering transmission electron microscopy and extended XAFS (EXAFS) analysis. The framework structure was simulated through density functional theory (DFT) calculations, the results of which were in good agreement with those of the EXAFS spectra quantitative analysis. The anionic [Ce^IIITa₂O₇]⁻ layers exhibited optical absorption in the near infrared (NIR) region at approximately 1000 nm, whereas the level of NIR absorption decreased in the [Ce^IVTa₂O₇] charge-neutral layer due to the disappearance of the Ce 4f electrons. In addition, the chemical reactivity of the charge-neutral [Ce^IVTa₂O₇] layers was investigated by chemical reduction with ascorbic acid, resulting in the reduction of the [Ce^IVTa₂O₇] layers to form anionic [Ce^IIITa₂O₇]⁻ layers. Furthermore, the anionic [Ce^IIITa₂O₇]⁻ layers exhibited redox activity which the Ce in the perovskite unit can be electrochemically oxidized and reduced. The synthesis of the “charge-neutral” perovskite layer indicated that diverse features were generated by systematically tuning the electronic structure through the redox control of Ce; such diverse features have not been found in conventional layered perovskites. This study could demonstrate the potential for developing innovative, unique functional materials with perovskite structures.

This study proposed a new layer modification technique, “layer charge control”, for layered perovskites, and the structures of the obtained charge neutral [CeTa₂O₇] perovskite sheet were characterized theoretically and experimentally.