Micellar catalysis on 1,10-phenanthroline promoted hexavalent chromium oxidation of ethanol

The kinetics and mechanism of Cr(VI) oxidation of ethanol in the presence and absence of 1,10-phenanthroline in aqueous acid media have been carried out. Monomeric species of Cr(VI) are kinetically active in the absence of phen, while in the phen catalyzed path, the Cr(VI)-phen complex has been suggested as the active oxidant. In the catalyzed path, the Cr(VI)-phen complex participates in the oxidation of ethanol and ultimately is converted into the Cr(III)-phen complex. In the uncatalyzed path, the Cr(VI)-substrate ester experiences an acid catalyzed redox decomposition in the rate-determining step. The uncatalyzed path shows a second-order dependence on [H⁺], while the phen catalyzed path shows a first-order dependence on [H⁺]. Both the uncatalyzed and phen-catalyzed paths show first-order dependence on [ethanol]_T and [Cr(VI)]_T. The phen-catalyzed path is first order in [phen]_T. These observations remain unaltered in the presence of externally added surfactants. CPC inhibits the reactions while SDS catalyzes the reactions. The observed miceller effects have been explained by considering partitioning of the reactants between the miceller and aqueous phase.     

Stabilities of the Ternary Complexes of Copper(II) with Substituted 1,10-Phenanthrolines and Some Amino Acids in Aqueous Solution

In this study the binary and ternary complexes of copper(II) with substituted 1,10-phenanthrolines [s-phen: 1,10-phenanthroline (phen), 4,7-dimethyl-1,10-phenanthroline (dmphen) and 5-nitro-1,10-phenanthroline (nphen)] and l-amino acids [aa: l-phenylalanine (phe), l-tyrosine (tyr) and l-tryptophan (trp)] have been investigated using potentiometric methods in 0.1 mol·L^?1 KCl aqueous ionic media at 298.2 K. The protonation constants of the ligands and the stability constants of the binary and ternary complexes of Cu(II) with the ligands were calculated from the potentiometric data using the “BEST” software package. It was inferred that the aromatic 1,10-phenanthrolines act as a primary ligand in the ternary complexes, while the oxygen and nitrogen donor-containing amino acids are secondary ligands. The observed values of Δlog₁₀ K indicate that the ternary complexes are more stable than the binary ones, suggesting no interaction takes place between the ligands in the ternary complexes. The magnitudes of the measured stability constants of all of the ternary complexes are in the order [Cu(s-phen)(trp)]⁺ > [Cu(s-phen)(tyr)]⁺ > [Cu(s-phen)(phe)]⁺, which is identical to the sequence found for the binary complexes of Cu(II) with the amino acids. When the substituted 1,10-phenanthroline is changed, the stability constants of the ternary complexes decrease in the following order: [Cu(dmphen)(aa)]⁺ > [Cu(phen)(aa)]⁺ > [Cu(nphen)(aa)]⁺.     

Micellar catalysis of chromium(VI) oxidation of ethane-1,2-diol in the presence and absence of 2,2′-bipyridine in aqueous acid media

The kinetics and mechanism of the Cr(VI) oxidation of ethane-1,2-diol in the presence and absence of 2,2′-bipyridine (bipy) in aqueous acid media were studied under the conditions [ethane-1,2-diol]_T ? [Cr(VI)]_T. Under the kinetic conditions, monomeric Cr(VI) was found to be kinetically active in the absence of bipy, whereas in the bipy-catalyzed path the Cr(VI)-bipy complex was the active oxidant. In this path, the Cr(VI)-bipy complex undergoes nucleophilic attack by the substrate to form a ternary complex which subsequently undergoes redox decomposition (through 2e transfer) leading to hydroxyethanol and the Cr(IV)-bipy complex. The Cr(IV)-bipy complex then participates further in oxidation of organic substrate, ultimately converted into inert Cr(III)-bipy complex. The uncatalyzed path shows a second-order dependence on [H⁺], while the bipy-catalyzed path shows a first-order dependence on [H⁺]. Both the uncatalyzed and bipy-catalyzed paths show first-order dependence on [ethane-1,2-diol]_T and on [Cr(VI)]_T. The bipy-catalyzed path is first-order in [bipy]_T. All these patterns remain unaltered in the presence of externally added surfactants. The effects of a cationic surfactant, N-cetylpyridinium chloride (CPC), and an anionic surfactant, sodium dodecyl sulfate (SDS), on both the uncatalyzed and bipy-catalyzed paths were studied. CPC inhibits both the uncatalyzed and bipy-catalyzed paths, whereas SDS catalyzes the reactions. The observed micellar effects are explained by considering a distribution pattern of the reactants between the micellar and aqueous phases.     

Micellar Effect on the Reaction of Chromium(VI) Oxidation of L‐Sorbose in the Presence and Absence of Picolinic Acid in Aqueous Acid Media: A Kinetic Study

The kinetics and mechanism of chromic acid oxidation of L‐sorbose in the presence and absence of picolinic acid (PA) have been studied under the conditions, [L‐sorbose]_T » [PA]_T » [Cr(VI)]_T, at different temperatures. In the absence of PA, the monomeric chromic acid undergoes esterification with the substrate followed by the acid catalysed redox decomposition of the Cr(VI)‐substrate ester through glycol splitting to formaldehyde and the lactone of C₅‐aldonic acid and Cr(IV) which subsequently participates in the faster reactions. In the presence of PA, the Cr(VI)‐PA complex produced in a pre‐equilibrium step experiences a nucleophilic attack by the substrate to produce a ternary complex which decomposes through glycol splitting giving rise to the organic products and Cr(IV)‐PA complex. Both the uncatalysed and PA‐catalysed paths show the first‐order dependence on [L‐sorbose]_T and [Cr(VI)]_T. The PA‐catalysed path is first‐order in [PA]_T and it shows a fractional order in [H⁺]. The uncatalysed path shows a second‐order dependence on [H⁺]. In the presence of the surfactants like N‐cetylpyridinium chloride (CPC, a cationic surfactant) and sodium dodecyl sulfate (SDS, an anionic sulfate), the reaction orders remain unchanged. CPC has been found to inhibit both the uncatalysed and PA‐catalysed paths while SDS shows the rate accelerating effect for both the uncatalysed and PA‐catalysed paths. The observed micellar effects have been rationalised by considering the distribution of the reactants between the micellar and aqueous phases in terms of the proposed reaction mechanism.     

Potentiometric Studies on Complexation of Cu(II) Ion with Methyl/Nitro-Substituted 1,10-Phenanthrolines and Selected Amino Acids

Protonation constants of methyl/nitro substituted 1,10-phenanthrolines {(m/n-sphen): 4-methyl-phenanthroline (4-mphen), 5-methyl-1,10-phenanthroline (5-mphen), 4,7-dimethyl-1,10-phenanthroline (dmphen), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen) and 5-nitro-1,10-phenanthroline (5-nphen)] and the amino acids (aa) l-tyrosine (tyr) and glycine (gly), and their corresponding binary and ternary stability constants with Cu(II), were determined in aqueous 0.1 mol·L^?1 KCl ionic media at 298.15 K. The protonation constants of the ligands and the stability constants of the binary and ternary complexes of Cu(II) with the ligands were calculated from the potentiometric data using the “BEST” software package. The species distribution diagrams were obtained using the “SPE” software package under the experimental conditions described. The order of stability of the ternary complexes in terms of the primary ligands is [Cu(tmphen)(aa)]⁺ > [Cu(dmphen)(aa)]⁺ > [Cu(4-mphen)(aa)]⁺ > [Cu(5-mphen)(aa)]⁺ > [Cu(5-nphen)(aa)]⁺. The stability constants of the ternary complexes decrease in the following order: [Cu(m/n-sphen)(gly)]⁺ > [Cu(m/n-sphen)(tyr)]⁺, which is identical to the sequence found for the binary complexes of Cu(II) with gly and tyr.     

Kinetics and mechanism of chromic acid oxidation of oxalic acid in absence and presence of different acid media. A kinetic study

Kinetics and mechanism of the reaction of Cr(VI) with oxalic acid have been studied in presence and absence of H₂SO₄, HClO₄, and CH₃COOH by monitoring the formation of Cr(III)-oxalic acid complex at 560 nm. The effect of total [oxalic acid], [Cr(VI)], [H₂SO₄], [HClO₄], and [CH₃COOH] on the reaction rate was determined at 30°C. Formation of carbon dioxide was also confirmed. The oxidation rate increases with [oxalic acid] and [CH₃COOH] while it decreases with [H₂SO₄], [HClO₄], and pH. The rate law governing the oxidation of oxalic acid over a wide range of conditions is rate=k₁ K_es1 [oxalic acid]_T [Cr(VI)]_T 1+K_es1 [oxalic acid]_T, where only undissociated oxalic acid is kinetically active. Kinetic evidence for the formation of a Cr(VI)(SINGLEBOND)oxalic acid 1:1 complex has been obtained and the equilibrium constant for their formation has been determined. The 1:1 complex exists most likely in an open chain form. The rate-limiting step of the oxidation reaction involves the breaking of the C(SINGLEBOND)C bond in the 1:2 complex. Oxidizing ability of Cr(VI) species have been discussed. Mechanism with the associated reaction kinetics is assigned. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 335–340, 1998     

Combined effect of promoter and surfactant on the chromium(VI) oxidation of D-ribose in aqueous media at room temperature

Effect of polypyridine derivatives on chromium(VI) oxidation of D-ribose in aqueous media was studied spectrophotometrically. The oxidized product D-erythrose was detected by paper chromatography. The promoters 1,10-phenanthroline, 2,2?-bipyridine, 2-picolinic acid, and 2,3-dipicolinic acid accelerated the oxidation, whereas isomeric 4,7-phenanthroline, 4,4?-bipyridine, 4-picolinic acid, and 2,6-dipicolinic acid did not influence the oxidation. Formation of Cr(VI)-promoter complex was identified through fluorescence spectroscopy. Rate constants depended on promoter concentration. SDS and TX-100 enhanced the D-ribose oxidation, while CPC retarded the reaction. Location of D-ribose inside micelles was observed through ¹H NMR. DLS study showed that the relative size of SDS and TX-100 micelles expanded in presence of chromium(VI).     

Micellar catalysis on picolinic acid promoted hexavalent chromium oxidation of glycerol

Under pseudo-first-order conditions, monomeric Cr(VI) was found to be kinetically active in the absence of picolinic acid (PA), whereas in the PA-promoted path, the Cr(VI)–PA complex undergoes nucleophilic attack by the substrate to form a ternary complex which subsequently experiences redox decomposition, leading to glyceraldehydes and Cr(IV)–PA complex. The uncatalyzed path shows a second-order dependence on [H⁺], whereas the PA-catalyzed path shows zero-order dependence on [H⁺]. Both the uncatalyzed and PA-catalyzed path show a first-order dependence on [glycerol]_T and [Cr(VI)]_T. The PA-catalyzed path is first order in [PA]_T. All these observations remain unaltered in the presence of externally added surfactants. The effect of the cationic surfactant cetyl pyridinium chloride (CPC) and anionic surfactant sodium dodecyl sulfate (SDS) on the PA-catalyzed path have been studied. CPC inhibits, whereas SDS accelerates the reaction. Here, SDS is a catalyst for glyceraldehydes production and at the same time reduction of carcinogenic hexavalent chromium to nontoxic trivalent chromium. The reaction proceeds simultaneously in both aqueous and micellar phase. Micellar effects have been explained by considering the preferential partitioning of reactants between the micellar and aqueous phase. The Menger–Portnoy model, Piszkiewicz cooperative model, and pseudo-phase ion exchange model have been tested to explain the observed micellar effect.     

Immobilization of calix[6]arene bearing carboxylic acid and amide groups on aminopropyl silica gel and its sorption properties for Cr(VI)

An aminopropyl silica gel-immobilized calix[6]arene (C[6]APS) containing both amide and acid moieties was prepared from p-tert-butylcalix[6]arene hexacarboxylate derivative and aminopropyl silica gel in the presence of N,N′-diisopropyl carbodiimide coupling reagent. C[6]APS was used to evaluate the sorption properties of Cr(VI) as a sorbent material. In sorption studies, it was observed that C[6]APS was highly effective at pH 1.5 for Cr(VI). The effect of parameters such as pH, sorbent dosage, contact time, initial Cr(VI) concentration and temperature on Cr(VI) sorption; the sorption isotherms were also studied. Maximum sorption capacity was obtained as 3.1 mg g^?1 at pH 1.5 and 25 °C for 1 h and 10.4 mg L^?1 initial Cr(VI) concentration. Thermodynamic parameters such as change in free energy, enthalpy, and entropy were also determined. In the isotherm studies, Langmuir and Freundlich isotherm models were applied and it was found that the experimental data confirmed to Freundlich isotherm model, and the batch sorption capacity of C[6]APS was calculated as 37.66 mg g^?1.     

A Novel 12-Tungstovanadate Doubly-Grafted by Zn-Complexes: Synthesis, Crystal Structure and Properties

A novel vanadium-centered tungstovanadate, {[Zn(phen)₂(H₂O)]₂[VW₁₂O₄₀]}·3H₂O (1) (phen = 1,10-phenanthroline), has been hydrothermally synthesized and characterized by elemental analysis, single-crystal X-ray diffraction, IR spectroscopy and thermogravimetric analysis. Single-crystal X-ray diffraction at 293 K reveals that 1 consists of a [VW₁₂O₄₀]^4? Keggin unit doubly-grafted by a [Zn(phen)₂(H₂O)]²⁺ complex. The modified carbon-paste electrode of this tungstovanadate displays good electrocatalytic activity not only towards normal inorganic molecules NO₂ ^?, BrO₃ ^? and H₂O₂, but also biological molecule vitamin C. To some extent, this work shows that vanadium can be introduced into a polyoxometalate-modified electrode to facilitate catalytic oxidation or reduction of some chemical materials. Furthermore, this tungstovanadate shows high photoactivity with respect to degradation of methylene blue.     

Simultaneous voltammetric determination of Mo(VI) and W(VI) by adsorptive differential pulse stripping method using adaptive neuro-fuzzy inference system

An adsorptive differential pulse stripping method is proposed for the simultaneous determination of molybdenum and tungsten based on the formation of their complexes with a novel proton transfer compound, [phenH]⁺[pyzdc]^?, containing both pyrazine-2,3-dicarboxylic acid and 1,10-phenanthroline. The optimum experimental conditions were obtained using 0.052 mM [phenH]⁺[pyzdc]^? ligand, pH 3.3, accumulation potential of ?0.1 V versus Ag/AgCl, accumulation time of 60 s and scan rate of 33.3 mV/s. An adaptive neuro-fuzzy inference system (ANFIS) was utilized for the analysis of the voltammogram data. The detection limits were 4.0 ng/mL for Mo(VI) and 3.0 ng/mL for W(VI). The influence of potential interfering ions on the determination of molybdenum and tungsten was studied. The procedure was applied to the simultaneous determination of molybdenum and tungsten in some water samples.     

Mechanism and kinetics of the 2,2′-bipyridyl catalysis of the chromium(VI) oxidation of malic acid

A spectrophotometric study of the kinetics and mechanism of the oxidation of malic acid (Mal) by chromium(VI) catalyzed by 2,2′-bipyridyl (bpy) in aqueous acidic medium was conducted in a temperature range of ～298 to 313 K. This reaction was found to be pseudo-first order with respect to Cr(VI) and first order with respect to malic acid. Under the conditions of the pseudo-first order ([Mal]_o ? [Cr(VI)]_o), the observed rate constant (k _obs) increased with the increase in [H⁺] and [bpy]. There was a weak negative salt effect. Based on the experimental results, a possible reaction mechanism for this oxidation catalyzed by bpy is proposed. The rate equation derived from this mechanism can explain all the experimental phenomena.     

Micellar effect on the reaction of picolinic acid catalyzed chromium(VI) oxidation of dimethyl sulfoxide in aqueous acidic media: A kinetic study

The kinetics and mechanism of picolinic acid (PA) catalyzed oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone by chromium(VI) in both aqueous H₂SO₄ and HClO₄ media have been studied in the absence and presence of surfactants at different temperatures. Cr(VI)–PA complex formed in preequilibrium steps is the active oxidant that experiences the nucleophilic attack by DMSO to form a positively charged intermediate ternary complex. Within the proposed ternary complex, an oxygen transfer or a ligand coupling or both occurs to generate the product, dimethyl sulfone. Cr(VI) is ultimately converted to Cr(III)–PA complex. Under the experimental conditions, the process shows a first‐order dependence on each of the reactants (i.e., [Cr(VI)]_T, [PA]_T, [DMSO]_T, and [H⁺]). HCrO₄⁻ has been found kinetically active. The reaction is catalyzed by sodium dodecyl sulfate (SDS, a representative anionic surfactant) monotonically, while cetylpyridinium chloride (CPC, a representative cationic surfactant) retards the reaction continuously. The observed micellar effects have been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. A pseudo‐phase ion exchange (PIE) model has been applied to explain the micellar effect. The Piszkiewicz cooperative model has been applied to determine the kinetic parameters, and it indicates the existence of catalytically productive submicellar aggregates. Because of this reactant‐promoted micellization of the surfactant before or below the cmc value, the present systems do not show any discontinuity at the respective reported cmc values of the surfactants. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 173–181, 2001     

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     

Oxovanadium phenanthroimidazole derivatives: synthesis,DNA binding and antitumor activities

Four unsymmetrical oxovanadium phenanthroimidazole complexes, [VO(hntdtsc)(NPIP)] (1), [VO(hntdtsc)(CPIP)] (2), [VO(hntdtsc)(MEPIP)] (3) and [VO(hntdtsc)(HPIP)] (4) (hntdtsc = 2-hydroxy-1-naphthaldehyde thiosemicarbazone, NPIP = 2-(4-nitrophenyl)-imidazo[4,5-f]1,10-phenanthroline, CPIP = 2-(4-chlorphenyl)-imidazo[4,5-f]1,10-phenanthroline), MEPIP = 2-(4-methylphenyl)-imidazo[4,5-f]1,10-phenanthroline), HPIP = 2-(4-hydroxylphenyl)-imidazo[4,5-f] 1,10-phenanthroline), have been synthesized and characterized. Their DNA binding and antitumor activities were determined by biochemical methods. All four oxovanadium complexes can bind with CT-DNA by an intercalation model and can also cleave supercoiled plasmid DNA in the presence of H₂O₂. The antitumor properties and mechanism of the complexes have been analyzed by MTT assay, cell cycle analysis, apoptosis assay and Western blot analysis. The results showed that the free ligands and their corresponding complexes all possess antiproliferative activities with very low IC₅₀ values against Hela, BIU-87 and SPC-A-1 cell lines. Complex 1, which has a strongly electron-withdrawing nitro group, exhibited the best antiproliferative activities. Complex 1 caused G₀/G₁ phase arrest of the cell cycle and induced apoptosis in Hela cells. Additionally, complex 1 attenuated the phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2).This indicates that inhibition of the ERK1/2 signaling pathway may contribute to the antitumor effects of these complexes.     

New Zn(II) complexes based on biologically active substituted acetic acid and nitrogen donor ligands: synthesis,crystal structure and biological applications

The complexes [Zn(phenylacetato)₂(2-aminopyridin)₂] (3), [Zn(phenylacetato)₂(1,10-phenanthroline)]·H₂O (4), and [Zn(phenylacetato)₂(2,9-dimethyl-1,10-phenanthroline)]·0.5 H₂O (5) were prepared and characterized by IR-, UV–Visible, ¹H and ¹³C NMR spectroscopy, and single crystal X-ray diffraction. BNPP hydrolysis of the complexes and their parent nitrogen ligands showed that the hydrolysis rate of bis-(4-nitrophenyl) phosphate (BNPP) was 1.7 × 10⁵ L mol^?1 s^?1 for 3, 3.1 × 10⁵ L mol^?1 s^?1 for 4 and 4.3 × 10⁴ L mol^?1 s^?1 for 5. Antibacterial activities show the effect of complexation on activity against Gram-positive (S. epidermidis, S. aureus, E. faecalis, M. luteus and B. subtilis) and Gram-negative (K. pneumonia, E. coli, P. mirabilis and P. aeruginosa) bacteria using the agar well diffusion method. Complex 4 showed good activity against G? bacteria except P. aeruginosa, and against G+ bacteria except E. ferabis. Complex 5 showed no activity against G? bacteria, low activity against M. luteus and B. subtilis bacteria and high activity against S. epidemidis and S. aureus. Complex 3 did not show any activity against G? or G+ bacteria.     

Mono-, di-, and trinuclear phosphonate oxygen-bridged copper(II) complexes: syntheses,structures, and properties

Reactions of copper salts, zoledronic acid, and 2,2′-bipyridine/1,10-phenanthroline in aqueous ethanolic solutions afforded four phosphonate oxygen-bridged copper complexes, Cu(bipy)(H₄zdn)(HSO₄) (1), [Cu₂(bipy)₂(H₂zdn)(H₂O)(Cl)]·4H₂O (2), [Cu₂(phen)₂(H₂zdn)(H₂O)(Cl)]·2.5H₂O (3), and [Cu₃(bipy)₃(H₄zdn)(H₂zdn)(SO₄)]·5H₂O (4) (H₅zdn = zoledronic acid, bipy = 2,2′-bipyridine, phen = 1,10-phenanthroline). The copper centers of 1–4 have square pyramidal coordination geometries. The Cu(II) ions are coordinated to bipy/phen, zoledronate, and HSO₄^?/Cl^? forming mononuclear units for 1, dinuclear for 2 and 3, and trinuclear for 4. These building units are further extended into 3-D supramolecular networks via multiple hydrogen bond interactions. Temperature-dependent magnetic properties of 2 and 4 suggest weak antiferromagnetic coupling (J = ?4.53(8) cm^?1 for 2, J = ?1.69(4) cm^?1 for 4). The antitumor activity of 2 was evaluated against the human lung cancer cell line and indicates effective time- and dose-dependent cytotoxic effects.     

Inner-sphere oxidation of a ternary dipicolinatochromium(III) complex involving a malonic acid co-ligand

The oxidation of a ternary complex of chromium(III), [Cr^III(DPA)(Mal)(H₂O)₂]^?, involving dipicolinic acid (DPA) as primary ligand and malonic acid (Mal) as co-ligand, was investigated in aqueous acidic medium. The periodate oxidation kinetics of [Cr^III(DPA)(Mal)(H₂O)₂]^? to give Cr(VI) under pseudo-first-order conditions were studied at various pH, ionic strength and temperature values. The kinetic equation was found to be as follows: \( {\text{Rate}} = {{\left[ {{\text{IO}}_{4}^{ - } } \right]\left[ {{\text{Cr}}^{\text{III}} } \right]_{\text{T}} \left( {{{k_{5} K_{5} + k_{6} K_{4} K_{6} } \mathord{\left/ {\vphantom {{k_{5} K_{5} + k_{6} K_{4} K_{6} } {\left[ {{\text{H}}^{ + } } \right]}}} \right. \kern-0pt} {\left[ {{\text{H}}^{ + } } \right]}}} \right)} \mathord{\left/ {\vphantom {{\left[ {{\text{IO}}_{4}^{ - } } \right]\left[ {{\text{Cr}}^{\text{III}} } \right]_{\text{T}} \left( {{{k_{5} K_{5} + k_{6} K_{4} K_{6} } \mathord{\left/ {\vphantom {{k_{5} K_{5} + k_{6} K_{4} K_{6} } {\left[ {{\text{H}}^{ + } } \right]}}} \right. \kern-0pt} {\left[ {{\text{H}}^{ + } } \right]}}} \right)} {\left\{ {\left( {\left[ {{\text{H}}^{ + } } \right] + K_{4} } \right) + \left( {K_{5} \left[ {{\text{H}}^{ + } } \right] + K_{6} K_{4} } \right)\left[ {{\text{IO}}_{4}^{ - } } \right]} \right\}}}} \right. \kern-0pt} {\left\{ {\left( {\left[ {{\text{H}}^{ + } } \right] + K_{4} } \right) + \left( {K_{5} \left[ {{\text{H}}^{ + } } \right] + K_{6} K_{4} } \right)\left[ {{\text{IO}}_{4}^{ - } } \right]} \right\}}} \) where k ₆ (3.65 × 10^?3 s^?1) represents the electron transfer reaction rate constant and K ₄ (4.60 × 10^?4 mol dm^?3) represents the dissociation constant for the reaction \( \left[ {{\text{Cr}}^{\text{III}} \left( {\text{DPA}} \right)\left( {\text{Mal}} \right)\left( {{\text{H}}_{2} {\text{O}}} \right)_{2} } \right]^{ - } \rightleftharpoons \left[ {{\text{Cr}}^{\text{III}} \left( {\text{DPA}} \right)\left( {\text{Mal}} \right)\left( {{\text{H}}_{2} {\text{O}}} \right)\left( {\text{OH}} \right)} \right]^{2 - } + {\text{H}}^{ + } \) and K ₅ (1.87 mol^?1 dm³) and K ₆ (22.83 mol^?1 dm³) represent the pre-equilibrium formation constants at 30 °C and I = 0.2 mol dm^?3. Hexadecyltrimethylammonium bromide (CTAB) was found to enhance the reaction rate, whereas sodium dodecyl sulfate (SDS) had no effect. The thermodynamic activation parameters were estimated, and the oxidation is proposed to proceed via an inner-sphere mechanism involving the coordination of IO₄ ^? to Cr(III).     

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.