A kinetic study of the oxidation of thiourea by octacyanomolybdate(V) and octacyanotungstate(V) ions in an alkaline medium

The kinetics of the oxidation of thiourea to sulfate by octacyanomolybdate(V) and octacyanotungstate(V) ions have been studied in an alkaline medium. The reactions are first order with respect to thiourea, octacyanomolybdate(V) or octacyanotungstate(V), hydroxide and sodium ions. The observed fourth order rate constants are ca. 1.20×10⁷ and ca. 9.80×10³ M^–3 min^–1 for oxidation by Mo(CN) ₈ ^3– and W(CN) ₈ ^3– , respectively at 298K. A reaction mechanism is proposed.

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(V) (V) . , (V) (V), . 1,2·10⁷ 9,8·10³ M^{–3 –1} Mo(CN)₈ ^–3 W(CN)₈ ^–3 298 . .

     

Quantitative study of the reaction between quinones and iron(II) in phosphoric acid medium

A simple method is described for the determination of some quinones with ammonium iron(II) sulphate, by titration potentiometrically or with cacotheline, Methylene Blue, Methylene Green, thionine, Toluidine Blue, Azure A, Azure C or Azure-2-eosin as redox indicators. The reverse titrations can be used for estimation of iron(II). Alternatively, excess of iron (II) can be added and the iron(III) produced determined colorimetrically with thiocyanate.     

Reporting a new class of divanadium(V) compounds connected by an unsupported hydroxo bridge

Dinuclear oxovanadium(V) compounds [LV(V)O(mu-OH)OV (V)L](PF6) [H2L = N,N'-tert-ethylene bis(salicylideneimine) (H 2Salen) and its derivatives] ( 1- 3) have been obtained by aerial oxidation of V (IV)OL precursors in THF in the presence of added NH 4PF 6. The oxidized vanadium(V) probably extracts an OH (-) ligand from the residual moisture in the solvent and is retained as an unsupported hydroxo-bridge between the metal centers of these compounds as confirmed by single-crystal X-ray diffraction analyses. The molecules of 1- 3 have centrosymmetric structures with each vanadium center having a distorted octahedral geometry. The bridging OH (-) group is located trans to the terminal VO t bond. The latter exerts strong trans labilizing influence to set the participating vanadium centers apart by about 4.1 A. These separations are by far the largest (e.g., V...V#, 4.131 A in 1) among all binuclear compounds containing an unsupported hydroxo bridge reported to date. The complexes retain their identity also in solution as established by (1)H NMR spectroscopy. Electrochemically, the behaviors of 1-3 are quite interesting as studied by cyclic voltammetry in acetonitrile, each undergoing three (except 3) nearly reversible metal-based reductions, all in the positive potential range (e.g., at E (1/2) = 0.57, 0.39, and 0.04 V versus Ag/AgCl reference for 1) as indicated by steady state voltammetry. The electrode process at 0.39 V appears to involve a single-step two-electron transfer as revealed from the normal and differential pulse voltammetric data and probably includes a combination of V(V)-V(IV) <--> V(III)-V(IV) mixed oxidation states. Compounds 1-3 thus provide a unique example of divanadium compounds in which the metal centers are linked by an unsupported hydroxo-bridge.     

Spectrophotometric and potentiometric study of the cobalt(II)-thiocyanate system in aqueous medium

The cobalt(II)—thiocyanate system was spectrophotometrically studied at 2.0 M ionic strength (NaClO₄) and 25°C. The following formation constants were obtained: β₁ = 6.9 M^?, β₂ = 28.9 M^?2, β₃ = 12.1 M^?3 and β₄ = 1.30 M^?4. Three wavelengths were considered, 515, 590 and 615 nm, and the molar absorptivities of each species were calculated. Linear relationships were obtained for $\text{ε}$ vs $\text{n}$ and α_i. There is strong evidence that the tetrahedral [Co(SCN)₄]^2? is virtually the only species absorbing at 590 and 615 nm. An indirect potentiometric method led to comparable equilibrium constants. The cadmium(II)—thiocyanate formation constants used in the indirect method, under the same conditions, were found to be β₁ = 21.51 ± 0.09 M^?1, β₂ = 123 ± 1 M^?2, β₃ = 130 ± 3 M^?3 and β₄ = 173 ± 1.2 M^?4, in good agreement with earlier literature data.     

On the formation of iron(III) hydroxo acetate complexes

The formation of hydroxo acetate complexes of iron (III) ion has been studied at 25 degrees C in 3 M (Na)ClO4 ionic medium by measuring with a glass electrode the hydrogen ion concentration in Fe(ClO4)3-HClO4-NaAc mixtures (Ac = acetate ion). The acetate/metal ratio ranged from 0 to 6, the metal concentration varied from 0.005 to 0.06 M, whereas [H+] was stepwise decreased from 0.1 M to initial precipitation of hydroxo-acetates. This occurred, depending on the acetate/metal ratio, in the -log[H+] range 1.85-2.7. The potentiometric data are consistent with the presence of Fe3(OH)3Ac3(3+), Fe2(OH)2(4+), Fe3(OH)4(5+), Fe3(OH)5(4+) and, as minor species, of Fe3(OH)2Ac6+, FeAc2+, FeAc2+, FeOH2+ and Fe(OH)2+. Previously published EMF measurements with redox and glass half-cells were recalculated to refine the stability constants of FeAc2+, FeAc2+ and Fe3(OH)2Ac6+. Formation constants *beta pqr for pFe(3+)+(q-r)H2O + rHAc reversible Fep(OH)(q-r)(Ac)r3p-q + qH+ (in parenthesis the infinite dilution value): log*beta 111 = -1.85 +/- 0.02 (-0.67 +/- 0.15), log*beta 122 = -3.43 +/- 0.02 (-1.45 +/- 0.15); log*beta 363 = -5.66 +/- 0.03 (-2.85 +/- 0.40), log*beta 386 = -8.016 +/- 0.006 (-4.06 +/- 0.15), log*beta 220 = -2.88 +/- 0.02 (-2.84 +/- 0.05), log*beta 340 = -6.14 +/- 0.18 (-6.9 +/- 0.4), log*beta 350 = -8.44 +/- 0.09 (-7.65 +/- 0.15).     

The thermodynamic characteristics of step complex formation in the (5,10,15,20-tetraphenylporphyrinato)(chloro)indium(III)-pyridine-toluene system

The thermodynamic characteristics, step mechanism, and spectral manifestations of step reactions between (5,10,15,20-tetraphenylporphyrinato)(chloro)indium(III) ((Cl)InTPP) and organic base pyridine (Py) in inert toluene were studied using spectrophotometric titration and physicochemical analysis of intermediate forms. The coordination of Py molecule (K ₁ = 9.45 × 10² l/mol at 298 K) with the formation of (Cl)(Py)InTPP, the substitution of Cl^? in (Cl)(Py)InTPP with the second Py molecule (K ₂ = 2.3 × 10² l/mol), and the coordination of the third Py molecule to produce [(Py)₃ InTPP]⁺ · Cl^? (K ₃ = 8.7 l/mol) were observed at the first, second, and third steps, respectively. The fourth reaction stage most likely corresponds to π?π complex formation equilibrium between the aromatic complex and pyridine with a 1: 2 stoichiometry. The thermodynamic parameters of step reactions ΔH ^° and ΔS ^° well correspond to the stoichiometric mechanism of the reaction. Prospects for the use of the metalloporphyrin as a receptor of organic N-bases are considered.     

The state of 5,10,15,20-tetraphenyl-21H,23H-porphine rhenium(V) complexes in solutions of acids

The spectral properties (UV/Vis, IR, ¹H NMR) and stability of diverse forms of 5,10,15,20-tetraphenyl-21H,23H-porphine rhenium(V) complexes in neutral and protolytic solvents have been studied. Quantitative characteristics have been obtained for the reactions of formation and interconversion of the μ-oxo dimeric and monomeric rhenium(V) complex species in the benzene-AcOH system and dissociation at the coordination center of the H⁺-associated form of the monomeric rhenium(V) complex in mixed H₂O-H₂SO₄ solvents in a wide range of component concentrations. It has been shown that the stability of the coordination center of the rhenium(V) complexes sharply depends on the nature of a second acido ligand, in addition to the coordinated porphyrin.     

Quantitative analysis of chromium(V) by EPR spectroscopy

A procedure to accurately quantitate chromium(V) in environmental and medicinal chemistry samples was developed using electron paramagnetic resonance spectroscopy (EPRS) as the method of detection. It was found to have an error in the order of +/-10% and a detection limit of 0.010 mM (0.5 mg l(-1)) chromium(V). The method has been used to quantitate the formation of chromium(V) in the interaction of chromium(VI) with fulvic acid and a simple model of this acid, viz, 1.2-dihydroxybenzene. Analysis of solutions obtained from the reaction of 1,2-dihydroxybenzene with chromium(VI) demonstrated that even when the organic substrate was present in a 182-fold excess, the maximum chromium(V) concentration attained represented just 1.44% of the initial chromium(VI). Reactions between chromium(VI) and fulvic acid yielded similar results. It was therefore concluded that at background environmental concentrations of chromium and fulvic acid, the production of chromium(V) is insignificant, however, its possible importance in contaminated systems cannot be disregarded on this basis alone. The method for quantitative analysis reported in this paper should be an invaluable tool for investigations into the significance of chromium(V) in the toxicological mechanism of chromium(VI) and its role as a mutagenic agent.     

Hydroxyoxo(5,10,15,20-tetraphenylporphinato)tungsten(V) as a receptor for foodstuff and drug components. Thermodynamics of supramolecular complexation

The thermodynamics of stepwise reactions of hydroxyoxo(5,10,15,20-tetraphenylporphinato)tungsten(V) (O=W(OH)TPP) with biologically active base molecules was studied. For the reaction of O=W(OH)TPP with benzimidazole, four steps were found and studied; the equilibrium constants K _n were determined to be 2.55 × 10⁴, 1.87 × 10³ L/mol, 1.06 × 10⁶, 4.09 × 10⁴ L²/mol². The reaction of O=W(OH)TPP with pyrazine was a one-step reversible process of Pyz coordination to the eighth coordination site (K = 399 L/mol). The correlation equations relating the stability of supramolecular complexes to pK of the bases were obtained. The thermodynamic data were used to demonstrate the prospects of applying the metal porphyrin as a receptor for N-bases in membranes, sensors, and analytical devices for quality control of foodstuffs, drugs, and gas mixtures for the content of VOCs.     

Application of 5,10,15,20-tetrakis(4-carboxyphenyl)porphine for cadmium preconcentration in flow-injection system.

The suitability of 5,10,15,20-tetrakis(4-carboxyphenyl)porphine as a complexing agent for the on-line preconcentration of cadmium using an anion exchanger (Amberlite IRA-904) in a microcolumn has been tested. Various parameters which affect complex formation and its sorption, as well as elution into the nebulizer of flame atomic absorption spectrometry (FAAS), were optimized. A 5 x 10(-4) mol 1(-1) reagent was on-line mixed with an aqueous sample solution and flowed through the microcolumn for 2 min. The sorbed complexes were then eluted with 2 mol 1(-1) of nitric acid into the nebulizer of FAAS. A good precision (2.1% RSD for 40 microg 1(-1) cadmium) and a high enrichment factor (36) with a detection limit (3sigma) of 1.4 microg 1(-1) were obtained. The achieved analytical results for a standard reference material (SRM 1643d) were in good agreement with the certified values.     

Thermal analysis study of 5,10,15,20-tetrakis (methoxyphenyl) porphyrins and their nickel complexes

The thermal stability in air of 5,10,15,20-tetrakis (4-methoxyphenyl) porphyrin (4-TMPP), 5,10,15,20-tetrakis (3-methoxyphenyl) porphyrin (3-TMPP), and their nickel metallo-complexes (4-TMPP-Ni and 3-TMPP-Ni) has been investigated by thermogravimetry (TG). 4-TMPP and 4-TMPP-Ni exhibit higher thermal stability compared to 3-TMPP and 3-TMPP-Ni. Nickel complexes exhibit a little higher thermal stability than the corresponding porphyrins. The thermal behavior including melting temperature and enthalpy of fusion was determined by differential scanning calorimetry (DSC) and infrared spectra (IR). The activation energies of thermal decompositions of 4-TMPP-Ni and 3-TMPP-Ni were obtained by integral model-free method; the mechanism functions and pre-exponential factors were determined by master plots method. The kinetic models follow the same mechanism function, Avramie-Erofeev equations with integral forms g() = [−ln (l − )]^0.278 and g() = [−ln (l − )]^0.260, respectively.     

Theoretical study of the interaction between a high-valent manganese porphyrin oxyl-(hydroxo)-Mn(IV)-TMPyP and double-stranded DNA

Cationic porphyrin derivatives such as meso-tetrakis(4-N-methylpyridinium)porphyrin, TMPyP, have been shown to interact with double-stranded DNA. The manganese derivative, Mn(III)-TMPyP, activated by an oxygen donor like potassium monopersulfate, provides an efficient DNA-cleaving system. Previous experimental work1 has shown that DNA cleavage by the Mn(III)-TMPyP/KHSO(5) system was due to an oxidative attack, within the minor groove of B-DNA, at the C5' or C1' carbons of deoxyribose units. The aim of this study was to use molecular modeling to elucidate the specificity of the interactions between the transient active species oxyl-Mn(IV)-TMPyP and the DNA target. Geometric parameters, charges, and force field constants consistent with the AMBER 98 force field were calculated by DFT methods. Molecular modeling (mechanics and dynamic simulations) were performed for oxyl-(hydroxo)-Mn(IV)-TMPyP bound in the minor groove of the dodecamer d(5'-TCGTCAAACCGC)-d(5'-GCGGTTTGACGA). Geometry, interactions, and binding energy of the metalloporphyrin located at the A.T triplet region of the dodecamer were analyzed. These studies show no significant structural change of the DNA structure upon ligand binding. Mobility of the metalloporphyrin in the minor groove was restrained by the formation of a hydrogen bond between the hydroxo ligand trans to the metal-oxyl and a DNA phosphate, restricting the access of the oxyl group to the (pro-S) H atom at C5'.     

Hydrogen peroxide oxidation of di(hydroxo)platinum(II) species in carboxylic acids

A facile procedure for synthesizing the mono(hydroxo)tris(carboxylato)platinum(IV) species has been achieved. The reaction of [Pt^II(OH)₂(dmpda)] (dmpda=2,2-dimethyl-1,3-propanediamine) with a 30% aqueous solution of H₂O₂ in the presence of a carboxylic acid produces a stable [Pt^IV(OOCR)₃(OH)(dmpda)] (R=Me, Et) complex in high yield. The crystal structures of [Pt^IV(OOCMe)₃(OH)(dmpda)] . H₂O (triclinic P1 bar, a=8.761(2) Å, b=9.245(3) Å, c=10.659(2) Å, =106.25(2)°, =93.90(2)°, =98.92(2)°, V=813.1(3) ų, Z=2, R= 0.0474) and [Pt^IV(OOCEt)₃(OH)(dmpda)] (monoclinic P2₁/c, a=12.777(4) Å, b=10.514(2) Å, c=14.971(3) Å, =107.40(2)°, V=1919.2(8) ų, Z=4, R=0.0611) show that the hydroxyl group has been selectively positioned at an axial site. The intramolecular hydrogen bond between the OH and C=O moiety exists (O(H)...=C, 2.83 Å for [Pt^IV(OOCMe)₃(OH)(dmpda)] · H₂O; 2.72 Å for [Pt^IV(OOCEt)₃(OH)(dmpda)]. Formation of the axial-mono(hydroxo)tris(carboxylato)platinum(IV) species may be ascribed to a combination of `reactive-equatorial effects' with `cis-addition' in the carboxylic acid.     

Kinetics of oxidation of ethyldigol by vanadium(V) in aqueous acidic medium

The kinetics of oxidation of ethyldigol by vanadium(V) in aqueous acidic medium has been carried out. The reaction is first order with respect to vanadium(V) and the substrate and is acid catalysed.Hammett acidity function (H ₀) andBunnett hypothesis have been applied. The formation of free radicals during the course of the reaction has been indicated. A probable reaction mechanism is proposed.

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Die Kinetik der Oxidation von Ethyldigol mit Vanadium(V) in wäßrigem saurem Medium

Zusammenfassung Es wurde die Kinetik der Oxidation von Ethyldigol mittels Vanadium(V) in wäßriger saurer Lösung untersucht. Die Reaktion ist erster Ordnung bezüglich Vanadium(V) und Substrat und ist säurekatalysiert. Es wurden dieHammett-Aciditätsfunktion (H ₀) und dieBunnett-Hypothese angewandt. Die Bildung von freien Radikalen während der Reaktion konnte bestätigt werden. Es wird ein Reaktionsmechanismus vorgeschlagen.

     

A kinetic study of the reaction of O(3P) with toluene

The absolute rate constant of the reaction of O(³P) with toluene was measured by the microwave discharge-fast-flow method to obtain k_T = 10^{9.7–2.7/2.303RT} l./mole·sec at 100–375°C. This was in good agreement with the rate constant calculated from the combination of the relative rate constant obtained by Jones and Cvetanovic with the recently determined absolute rate constants of the reaction of O(³P) + olefins. The extrapolation of the above Arrhenius plot to 27°C was also in good agreement with the absolute value of k_T = 4.5 × 10⁷ l./mole·sec determined recently by Atkinson and Pitts at 27°C. The rate constant of the reaction of chlorobenzene with O(³P), obtained at 238°C as 10^8.3 l./mole·sec by a competitive method, was smaller than k_T by a factor of about two at the same temperature.     

The oxidation of thiosulphate ions by octacyanotungstate(V) in weak acidic medium

The kinetics of the oxidation of thiosulphate ions by octacyanotungstate(V) ions has been studied in the pH range 3.9–5.0. The reaction showed zero-order kinetics with respect to [W(CN)₈^3?] and is consistent with the rate law R = k[H⁺][S₂O₃^2?]². A reaction mechanism is proposed for the reaction with a third-order rate constant of 0.26 M^?2 s^?1 at 25°C.