Biphasic Oxidation of cis‐Diaquabis(1,10‐phenanthroline)chromium(III) by N‐Bromosuccinimide and the Formation of Chromium(IV) and Chromium(V)

The oxidation of cis‐diaquabis(1,10‐phenanthroline)chromium(III) [cis‐Cr^III(phen)₂(H₂O)₂]³⁺ by ‐bromosuccinimide (NBS) to yield cis‐dioxobis(1,10‐phenanthroline)chromium(V) has been studied spectrophotometrically in the pH 1.57–3.56 and 5.68–6.68 ranges at 25.0°C. The reaction displayed biphasic kinetics at pH < 4.0 and a simple first order at the pH > 5.0. In the low pH range, the reaction proceeds by two successive steps; the first faster step corresponds to the oxidation of Cr(III) to Cr(IV), and the second slower one corresponds to the oxidation of Cr(IV) to Cr(V), the final product of the reaction. The formation of both Cr(IV) and Cr(V) has been detected by electron spin resonance (ESR). The ESR clearly showed the formation and decay of Cr(IV) as well as the formation of Cr(V). Each oxidation process exhibited a first‐order dependence on the initial [Cr(III)]. The pseudo–first‐order rate constants k₃₄ and k₄₅, for the faster and slower steps, respectively, were obtained by a computer program using Origin7.0. Both rate constants showed first‐order dependence on [NBS] and increased with increasing pH.     

Factors controlling electron transfer reactions and stabilisation of uncommon oxidation states of chromium

Some experimental approaches to seek semi-quantitative understanding of factors controlling outer sphere electron transfer reactions of some transition metal complexes have been made. The relative importance of nuclear and electronic factors to outer sphere processes has been examined. By the manipulation of Franck-Condon or nuclear factors, it has now been possible to gain access into the chemistry of chromium in unusual oxidation states. An example of a reorganisation controlled electron transfer reaction involving Cr(IV)-Cr(III) system has been demonstrated. The bimolecular rate of reduction of diperoxoaquaethylenediamine chromium(IV) and diperoxodiethylenetriamine chromium(IV) is independent of the nature of th reductant employed viz. Fe²⁺ or VO²⁺ indicating that the generation of6 coordinate Cr(IV) species from7 coordinate of diperoxochromium(IV) reactant may be rate limiting. Similarly by increasing the barrier for the6 coordinate to4 coordinate structures through equatorial coordination of macrocyclic ligands, it has now been possible to detect through cyclic voltommogram the formation of relatively stable Cr(IV) species in the electrochemical oxidation of Cr(Me₄[14] tetraene)(H₂O) ₂ ³⁺ in aqueous sulphuric acid media. The kinetics and mechanism of the cerium(IV) and iodosyl benzene oxidation of Cr(salen)(H₂O) ₂ ⁺ and Cr(salprn)(H₂O) ₂ ⁺ have been investigated and kinetic and spectroscopic evidence for the formation of Cr(IV) transients and stable Cr(V) products has been presented. The relative importance of Franck-Condon factors in the oxidation of Cr(III) to Cr(IV) and Cr(V) states in different macrocyclic and multidentate ligand environments has been discussed.     

Kinetics and mechanism of oxidation of the binary and ternary complexes of chromium(III) involving inosine and glycine by N -bromosuccinimide

The kinetics of oxidation of the chromium(III) complexes, [Cr(Ino)(H₂O)₅]³⁺ and [Cr(Ino)(Gly)(H₂O)₃]²⁺ (Ino?=?Inosine and Gly?=?Glycine) involving a ligands of biological significance by N-bromosuccinimide (NBS) in aqueous solution to chromium(VI) have been studied spectrophotometrically over the 25–45°C range. The reaction is first order with respect to both [NBS] and [Cr], and increases with pH over the 6.64–7.73 range in both cases. The experimental rate law is consistent with a mechanism in which the hydroxy complexes [Cr(Ino)(H₂O)₄(OH)]²⁺ and [Cr(Ino)(Gly)(H₂O)₂(OH)]⁺ are significantly more reactive than their conjugate acids. The value of the intramolecular electron transfer rate constant, k ₁, for the oxidation of the [Cr(Ino)(H₂O)₅]³⁺ (6.90?×?10^?4?s^?1) is lower than the value of k ₂ (9.66?×?10^?2?s^?1) for the oxidation of [Cr(Ino)(Gly)(H₂O)₂]²⁺ at 35°C and I?=?0.2?mol?dm^?3. The activation parameters have been calculated. Electron transfer apparently takes place via an inner-sphere mechanism.     

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).     

Electron transfer to oxochromate(V) mediated through a sulphur bond

Reduction of carboxylato bound chromium(V) by several reducing agents such as N₂H₄, NH₂OH, Ti(III), Fe(II), VO²⁺, U(IV) formed part of earlier studies in which electron transfer to Cr(V) seems to occur through nitrogen or by inner or outer sphere path. In the chromium(V) oxidation of lactic acid, saturation kinetics has been observed and the association constant,K, evaluated from a double reciprocal plot, indicates weak interaction between reactants. In contrast, the reduction of Cr(V) by thiolactic acid exhibits total second order kinetics-first order with respect to each reactant. Probably the Cr^v-S bond is weaker than the Cr^v-O bond and electron transfer mediated through the sulphur system is facile. Under identical conditions Cr(VI) seems to be a better oxidant for lactic than for thiolactic acid.     

Kinetic studies of the oxidation of chromium(III) by N-bromosuccinimide

Summary A kinetic study of the oxidation of chromium(III) by N-bromosuccinimide (NBS) in aqueous solutions and H₂O-MeOH solvent mixtures were performed. The kinetics in aqueous solutions obeyed the rate law: d[Cr^VI]/dt = {k ₄ K _h K ₂[NBS][Cr^III]_T}/[₊]{1 + K _h/[H⁺] + (K ₁ + K _h K ₂/[H⁺][NBS])} where K _h, K ₁ and K ₂ are the hydrolysis constant of [Cr^III(H₂O)₆]³⁺, and pre-equilibrium formation constants for the protonated and deprotonated precursor complexes, respectively. An innersphere mechanism is proposed. An argument based on isokinetic correlations among activation parameters for the oxidation of a series of cobalt(II) and chromium(III) complexes including [Cr(H₂O)₆]³⁺ is presented in support of a common mechanism for these reactions. Abstracted from the Ph.D. Thesis (Ain Shams University) of A. E.-D. M. Abdel-Hady.     

Kinetics of the Temperature-Dependent eaq− and ⋅OH Radical Reactions with Cr(III) Ions in Aqueous Solutions

The reactivity of chromium(III) species with the major oxidizing and reducing radiolysis products of water was investigated in aqueous solutions at temperatures up to 150 °C. The reaction between the hydrated electron (e_aq⁻) and Cr(III) species showed a positive temperature dependence over this temperature range. The reaction was also studied in pH 2.5 and 3.5 solutions for the first time. This work also studied the reaction between acidic Cr(III) species and the hydroxyl radical (⋅OH). It was found that Cr³⁺ did not react significantly with the ⋅OH radical, but the first hydrolysis species, Cr(OH)²⁺, did with a rate coefficient of k= (7.2±0.3)×10⁸ M⁻¹ s⁻¹ at 25 °C. The oxidation of Cr(OH)²⁺ by the ⋅OH radical formed an absorbing product species that ultimately oxidized to give Cr(VI). These newly measured reaction rates allow for the development of improved models of aqueous chromium speciation for the effective remediation of liquid high-level nuclear waste via vitrification processes.     

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.     

Electron transfer. 109. The reaction of dimeric molybdenum(V) with carboxylato-bound chromium(V). A third mechanistic variation

The bridged dimer of molybdenum(V), Mo₂O₄²⁺ (aq) is oxidized to Mo(VI) by carboxylato-bound chromium(V). Reaction of bis(chelated) Cr(V) with excess (Mo^V)₂ yields a chelated Cr(III) complex, but this conversion proceeds through a pink Cr(IV) intermediate, indicating that the oxidation of (Mo^V)₂ entails a series of le^? steps, passing through a reactive transient, the mixed valence complex, Mo^VMo^VI. When experiments are carried out in buffers of the ligating acid, 2-ethyl-2-hydroxybutanoic acid, two stages of ligation of (Mo^V)₂ by the ligand anion, characterized by rate constants near 10⁴ and 0.14 M^?1 s^?1 (19°C; pH 3.0; μ = 0.6 M) must be considered. In quick mixing experiments, the first, but not the second, of these proceeds before the redox reaction gets under way, and autocatalytic redox profiles are observed. If the slower ligation is allowed to reach completion before Cr(V) is added, reduction to Cr(IV) is greatly accelerated and conforms to the superposition of two processes, whereas the reduction of Cr(IV) to Cr(III) is slow and exhibits a rate independent of [Cr^IV]. A proposed sequence applicable to the latter conditions includes reductions of Cr(V) at two ligation levels, slow unimolecular conversion of (Mo^V)₂ to an activated form, and rapid reduction of the latter with Cr(IV). Here Cr(IV) has assumed the role of a scavenger for the reactive form of (Mo^V)₂.     

Photochemistry of macromolecular metal complexes. II. Synthesis,thermal, and photochemistry of some polypyridyl complexes of chromium (III) bound to macromolecules

Polymer coordinated chromium(III) complexes [Cr(bpy)₂(PAA)₂]⁺, 1 , [Cr(bpy)₂-(PMA)₂]⁺, 2 , [Cr(phen)₂(PAA)₂]⁺, 3 , and [Cr(phen)₂(PMA)₂]⁺, 4 , [where bpy, phen, PAA and PMA are, respectively, 2,2′-bipyridine, 1,10-phenanthroline, poly(acrylic acid), and poly(methacrylic acid)] were synthesized. The polymer–chromium(III) complexes were characterized by elemental and spectroscopic analyses. Thermal substitution reactions of these macromolecular chromium(III) complexes in basic solutions lead to the replacement of the polypyridyl ligand by hydroxide ion while in strong acidic solutions the polymer complexes precipitate out. The photochemical reactions are qualitatively similar to that of the thermal reactions and the quantum yields are dependant on the pH of the medium. Further, lower quantum yields were observed for the aquation of the polymer complexes in comparison with the monomeric chromium(III) complexes and the results are discussed in terms of the effect of the polymer environment. Flash photolysis of 1 and 3 results in the formation of transients with maxima at 480 nm for 1 and 470 nm, 580 nm for 3 . The decay of the transients were found to obey first order kinetics and the rate constants were determined. The transients were suggested to be the alkyl-chromium complexes. Flash photolysis of 2 and 4 does not produce transients which is interpreted to be due to the presence of a methyl group in the ligand which hinders the formation of the carbonchromium bond.     

One‐step,two‐electron oxidation of cis‐diaquabis(1,10‐phenanthroline)chromium(III) to cis‐dioxobis(1,10‐phenanthroline)chromium(V) by periodate in aqueous acidic solutions

The kinetics of oxidation of cis‐[Cr^III(phen)₂(H₂O)₂]³⁺ (phen = 1,10‐phenanthro‐ line) by IO₄^? has been studied in aqueous acidic solutions. In the presence of a vast excess of [IO₄^?], the reaction is first order in the chromium(III) complex concentration. The pseudo‐first‐order rate constant, k_obs, showed a very small change with increasing [IO₄^?]. The dependence of k_obs on [IO₄^?] is consistent with Eq. (i). (i) 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. An inner‐sphere mechanism has been proposed for the oxidation process. The thermodynamic activation parameters of the processes involved are also reported. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 563–568, 2011     

Rate equation of gelation of chromium(III)-polyacrylamide sol

During gelation, the time dependence of the apparent viscosity has been used to analyze the kinetics of the gelation of polyacrylamide (PA) sol with chromium (III) ions. The investigations have stablished the following facts: Before gelation the relation between the viscosity, η, of Cr(III)-PA sol and the PA concentration is η = f([PA]) = 2.36×10¹⁵ [PA]^3.15 and the rate equation of gelation in the steady-state stage during gelation is expressed as V_n = k_s f([PA]) [PA]² [Cr(III)]².     

Crystal Structure and Spin Frustration Behavior of an Oxo‐centered Trinuclear Chromium(III) Complex

A trinuclear chromium(III) complex, [Cr₃O(HCO₂)₆(CH₃OH)₃]NO₃·H₂O·CH₃OH ( 1 ), is synthesized and structurally characterized by single‐crystal X‐ray diffraction. Three chromium(III) ions are bridged by one oxygen atom in the center, forming a triangular structure. The HCOO ^? anion acts as bidentate ligand and bridges couples of Cr(III) ions. Magnetic susceptibility measurement indicates that a strong antiferromagnetic interaction is operative between chromium(III) ions, and the S = 1/2 ground state reveals normal spin frustration behavior.     

Synthesis,characterization and reactions of 2,2,2-trichloroethoxy derivatives of chromium(III)

Tris(2,2,2-trichloroethoxy)chromium(III) tetrahydrofuran solvate, Cr(OCH₂CCl₃)₃ ·THF; Chlorobis(2,2,2-trichloroethoxy)chromium(III)tetrahydrofuran solvate, CrCl(OCH₂CCl₃)₂ ·THF and dichloro(2,2,2-trichloroethoxy) chromium(III) tetrahydrofuran solvate, CrCl₂(OCH₂CCl₃) ·THF have been prepared. These compounds react with various oxygen and nitrogen donor ligands to form adducts of 1:2 composition. 2,2,2-trichloroethoxy bridged structures have been proposed on the basis of their infrared and¹H nmr spectra. The diffuse reflectance spectral results are consistent with an octahedral geometry for chromium(III) whereas their low magnetic moment values suggest polymeric structures exhibiting antiferromagnetic coupling between chromium(III) atoms. The mass spectral data of Cr(OCH₂CCl₃)₃ ·THF and CrCl(OCH₂CCl₃)₂ ·THF support the dimeric structures for these complexes.

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Synthese, Charakterisierung und Reaktionen von 2,2,2-Trichlorethoxyderivaten von Chrom(III)

Zusammenfassung Es wurde Tris(2,2,2-trichlorethoxy)chrom(III)-tetrahydrofuransolvat, Cr(OCH₂CCl₃)₃ ·THF, Chlorbis(2,2,2-trichlorethoxy)chrom(III)tetrahydrofuransolvat, CrCl(OCH₂CCl₃)₂ ·THF, und Dichlor(2,2,2-trichlorethoxy)chrom(III)tetrahydrofuransolvat, CrCl₂(OCH₂CCl₃ ·THF, hergestellt. Diese Verbindungen reagieren mit verschiedenen Sauerstoff- und Stickstoffdonorliganden unter Bildung von Addukten mit der Zusammensetzung 1:2. Basierend auf IR- und¹H-NMR-Daten wurden 2,2,2-trichlorethoxyüberbrückte Strukturen vorgeschlagen. Die Resultate aus Diffuse-Reflectance-Messungen sind mit einer oktaedrischen Geometrie um Cr(III) in Übereinstimmung, während die niedrigen Werte für die magnetischen Momente polymere Strukturen mit antiferromagnetischer Kopplung zwischen den Chrom(III)-Atomen nahelegen. Die massenspektroskopischen Daten für Cr(OCH₂CCl₃)₃ ·THF und CrCl(OCH₂CCl₃)₂ ·THF sprechen für eine dimere Struktur dieser Komplexe.

     

Microcalorimetric Study on Effect of Chromium(III) and Chromium(VI) Species on the Growth of Escherichia coli

The effects of Cr(III) and Cr(VI) species (Cr₂O₇^2?, CrO₄^2? and Cr³⁺) on the growth of Escherichia coli (E. coli) have been investigated in detail by microcalorimetry at 37 °C. Parameters including the growth rate constant (k), inhibitory ratio (I), half‐inhibitory concentration (IC₅₀), total heat output (Q_total), time of the maximum heat production (t_log) in the log phase have been obtained. The results showed that Cr(VI) and Cr(III) had the inhibition effect on the growth of E. coli in aquatic environment; however, the inhibitory ratio of Cr(III) to E. coli was smaller than that of Cr(VI). The k values of E. coli in the presence of Cr(VI) and at high concentrations of Cr(III) were decreased with increasing the concentrations of these chromium species. Among the three chromium species investigated, Cr₂O₇^2? was found to be the most poisonous species against E. coli with an IC₅₀ value of 35.52 µg·mL^?1. CrO₄^2? exhibited moderate toxicity on E. coli with an IC₅₀ of 50.24 µg·mL^?1, and Cr³⁺ had the lowest toxicity with an IC₅₀ of 84.30 µg·mL^?1. Microcalorimetry can provide a convenient, sensitive and reliable method to study the effect of various metal species on the growth of bacteria or other microorganisms.     

Solubilities of chromium(III) in molten potassium hydrogensulfate

The behavior of chromium(III) in molten potassium hydrogensulfate (220° C, P(H₂O) = 0.04 atm) is reported. Chromium(III) exists as Cr³⁺ in acidic medium, and as chromium(III) sulfate, which is insoluble in neutral medium but soluble in basic medium as Cr(SO₄)^?₂. The values of acidity constants and the solubilities of Cr(III) are reported.     

Kinetics of cerium(IV) oxidation of macrocyclic complexes of chromium(III) and fate of chromium(IV) intermediates

Kinetics and mechanism of the cerium(IV) oxidation of Cr(III) complexes of a series of macrocyclic (or pseudomacrocyclic) ligands with [14]-membered intraligand ring-sizes have now been investigated at I = 1.0 M (LiClO₄) Temp. 30°C. The complexes of the formulation Cr(macrocycle)(X)(H₂O) where X = CHCl₂ and H₂O, n = 0 or 1 undergo oxidation to Cr(VI) with the formation of chromium(IV) intermediates. The observed kinetic parameters for the Ce(IV) oxidation of Cr(III) macrocyclic complexes have been discussed in terms of changes brought about by the macrocyclic ligands on the Cr(III)—Cr(IV) redox potentials and in specific rates for Cr(IV)—Cr(V) conversion. On the basis of this study, it has been suggested that the trapping of Cr(IV) is easier when a macrocyclic ligand having a symmetrical intra-ligand ring size and unsaturation in the cyclic structure is coordinated equatorially. Cyclic voltammetric studies indicate the formation of Cr(IV) transient in the case of electrochemical oxidation of trans-Cr(Me₄[14]tetraene)(H₂O).     

Oxidative degradation of non-ionic surfactant (TritonX-100) by chromium(VI)

Degradation of polyoxyethylene chain of non-ionic surfactant (TritonX-100) by chromium(VI) has been studied spectrophotometrically under different experimental conditions. The reaction rate bears a first-order dependence on the [Cr(VI)] under pseudo-first-order conditions, [TritonX-100]  [Cr(VI)] in presence of 1.16 mol dm⁻³ perchloric acid. The observed rate constant (k_obs) was 3.3 × 10⁻⁴ to 3.5 × 10⁻⁴ s⁻¹ and the half-life (t_1/2) was 33–35 min for chromium(VI). The effects of total [TritonX-100] and [H⁺] on the reaction rate were determined. Reducing nature of non-ionic TritonX-100 surfactant is found to be due to the presence of –OH group in the polyoxyethylene chain. It was observed that monomeric and non-ionic micelles of TritonX-100 were oxidized by chromium(VI). When [TritonX-100] was less than its critical micelle concentration (cmc) the k_obs values increased from 0.76 × 10⁻⁴ to 1.5 × 10⁻⁴ s⁻¹. As the [TritonX-100] was greater than the cmc, the k_obs values increases from 2.1 × 10⁻⁴ to 8.2 × 10⁻⁴ s⁻¹ in presence of constant [HClO₄] (1.16 mol dm⁻³) at 40 °C. A comparison was made of the oxidative degradation rates of TritonX-100 with different metal ion oxidants. The order of the effectiveness of different oxidants was as follows: permanganate > diperiodatoargentate(III) > chromium(VI) > cerium(IV).     

Kinetics and mechanism of the reaction between dimethylformamide and chromium(VI)

The kinetics of oxidation of N,N‐dimethylformamide by chromium(VI) has been studied spectrophotometrically in aqueous perchloric acid media at 20°C. The rate showed a first‐order dependence on both [Cr(VI)] and [DMF], and increased markedly with increasing [H⁺]. The order with respect to [HClO₄] was found to lie between 1 and 2. The rate was found to be independent of ionic strength as well as of any inhibition effect of Mn(II). The formation of superoxochromium(III) ion was detected in an aerated solution of chromium(VI), DMF and HClO₄. The proposed mechanism, involving two reaction pathways, leads to the rate law, rate = K_a1 [HCrO₄⁻] [DMF] (k_I K_a2 [H⁺]²+k_II[H⁺]). The first pathway, with rate constant k_I, involves the formation of chromium(V) and a free radical. The second pathway, with rate constant k_II, involves the formation of Cr(IV), CO₂ and dimethylamine. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 409–415, 1999     

铬(III)离子催化铈(IV)离子氧化四氢糠醇的反应动力学及机理

在酸性介质中用氧化还原滴定法研究了铈(IV)离子在铬(III)离子催化作用下, 于25～40 ℃区间氧化四氢糠醇的反应动力学. 结果表明反应对铈(IV)和四氢糠醇均为一级. 准一级速率常数k_obs随催化剂[Cr(III)]增加而增大, 亦随[H^＋]增加而增大, 而随增加而减小. 在氮气保护下, 反应不能引发丙烯酰胺聚合, 说明在反应中没有自由基产生. 提出了催化剂、底物和氧化剂间生成双核加合物的反应机理. 通过k_obs与的依赖关系, 并结合Ce(IV)在溶液中的平衡, 找到了本反应体系的动力学活性物种是Ce(SO₄)₂. 还计算出一些速率常数及相应的活化参数.