Kinetics and mechanism of the oxidation of diaquabis(1,10-phenanthroline) iron(II) by periodate in aqueous and micellar media

The kinetics of oxidation of [Fe^II(phen)₂(H₂O)₂]²⁺ (phen = 1,10-phenanthroline) by periodate were investigated in aqueous acidic medium at different [H⁺] over a temperature range of 20–40 °C. The reaction was studied under pseudo-first-order conditions by taking [IO ₄ ^? ] > tenfold over [Fe^II(phen)₂(H₂O) ₂ ²⁺ ]. The reaction rate increases with increasing [H⁺], and the kinetics of oxidation obeyed the following rate law:

$$ {\text{Rate}} = \left[ {{\text{Fe}}^{\text{II}} ({\text{phen}})_2({\text{H}}_{2} {\text{O}})_{2}^{2 + } } \right]\left[ {{\text{IO}}_{4}^{ - } } \right]\left\{ {k_{4} K_{2} + k_{5} K_{1} K_{3} [{\text{H}}^{ + } ]} \right\} $$

The surfactant sodium dodecyl sulfate was found to enhance the rate, whereas cetyltrimethylammonium bromide had little effect. Activation parameters associated with k ₂ and k ₃ were calculated. An electron transfer from Fe(II) to I(VII) is identified as the rate-determining step. The I(VI) species thus generated reacts in a fast step with another Fe(II) complex.

     

Kinetics and mechanism of chromium(III) catalyzed oxidation of 1,10-phenanthroline by alkaline permanganate

The kinetics of chromium(III) catalyzed oxidation of 1,10-phenanthroline by permanganate in alkaline medium at a constant ionic strength has been studied spectrophotometrically. The reaction between permanganate and 1,10-phenanthroline in alkaline medium exhibits 4:1 stoichiometry (KMnO₄:1,10-phenanthroline). The reaction shows first order dependence on [permanganate] and [chromium(III)] and less than unit order dependence in 1,10-phenanthroline, zero order in alkali concentrations. The results suggest the formation of a complex between the 1,10-phenanthroline and the chromium(III) which reacts further with one mole of permanganate species in the rate-determining step, resulting in the formation of a free radical, which again reacts with three moles of permangante species in a subsequent fast step to yield the products. The reaction constants involved in the mechanism were evaluated. The activation parameters were computed with respect to the slow step of the mechanism.This revised version was published online in December 2005 with corrections to the Cover Date.     

Kinetics and mechanism of silver(I) catalyzed oxidation of tris(1,10-phenanthroline)iron(II) with peroxodiphosphate in acetate buffers

A kinetic study of the silver(I)-catalyzed oxidation of tris(1,10-phenanthroline)iron(II) with peroxodiphosphate was carried out by estimating tris-complex at 510 nm. The reaction is found to conform to the rate law (i). with K₂ and K₃ being the acid dissociation constants of H₃P₂O₈^? and H₂P₂O₈^2?, respectively. The silver(I) catalysis in the reaction has been explained on the basis of complex formation between pdp and silver(I).     

Oxidation of phenol by tris(1,10-phenanthroline)osmium(III)

Outer-sphere oxidation of phenols is under intense scrutiny because of questions related to the dynamics of proton-coupled electron transfer (PCET). Oxidation by cationic transition-metal complexes in aqueous solution presents special challenges because of the potential participation of the solvent as a proton acceptor and of the buffers as general base catalysts. Here we report that oxidation of phenol by a deficiency of [Os(phen)(3)](3+), as determined by stopped-flow spectrophotometry, yields a unique rate law that is second order in [osmium(III)] and [phenol] and inverse second order in [osmium(II)] and [H(+)]. A mechanism is inferred in which the phenoxyl radical is produced through a rapid PCET preequilibrium, followed by rate-limiting phenoxyl radical coupling. Marcus theory predicts that the rate of electron transfer from phenoxide to osmium(III) is fast enough to account for the rapid PCET preequilibrium, but it does not rule out the intervention of other pathways such as concerted proton-electron transfer or general base catalysis.     

Kinetics of the reaction of chromium(VI) with tris(1,10-phenanthroline)iron(II) ions in acidic solutions. Anion and medium effects: perchlorate versus triflate

Reinvestigation of the reaction between title reagents in aqueous acidic triflate and perchlorate media revealed an unusual difference: the reaction is strictly first-order with respect to the concentration of Fe(phen)3(2+) (phen = 1,10-phenanthroline) in the triflate medium but shows an additional, but we believe artifactual, higher-order term in the perchlorate medium. We postulate that the apparent orders with respect to [Fe(phen)3(2+)] in (H/Li)ClO4 do not indicate the actual chemical mechanism but, in whole or in part, the orders, particularly the higher-order component, reflect an interaction specific to Fe(phen)3(2+) or Fe(phen)3(3+) and ClO4(-) in solution. Data in (H/Li)O3SCF3 solutions indicate that, in the absence of added Fe(phen)3(3+), the first of the three sequential electron-transfer steps is rate controlling. Reactions started in the presence of the product Fe(phen)3(3+) occur somewhat more slowly, suggesting the first electron transfer is reversible. This finding allows the relative rate constants for Cr(V) oxidation and reduction to be evaluated, with limited precision, by two methods of analysis. The dependences on [Cr(VI)] can be resolved into contributions from the species HCrO4(-) and Cr2O7(2-), each of which in turn depends on [H+]. The reaction mechanism is discussed in light of the data obtained in the triflate medium. Further, the rate constants for certain steps can be considered in light of E0 for the Cr(VI)/(V) couple.     

Kinetics and mechanism of oxidation of glycine by iron(iii)-1,10-phenanthroline complex in perchloric acid medium

Kinetics and mechanism of oxidation of glycine by iron (III)-1,10-phenanthroline complex has been studied in perchloric acid medium. The reaction is first order with respect to iron(III) and glycine. An increase in (phenanthroline) increases the rate, while increase in [H⁺] decreases the rate. Hence it can be inferred that the reactive species of the substrate is the zwitterionic form and that of the oxidant is [Fe(phen)₂(H₂O)₂]³⁺. The proposed mechanism leads to the rate law as elucidated.     

Kinetics of the dissociation of tris(1,10-phenanthroline) iron(III) complex ion in aqueous acid solutions. A mechanistic model for product specificity

Summary The natural decay of Fe(phen) _f3 ^p3+ , where no Ce^IV is employed for scavenging the side reaction product, Fe(phen) _f3 ^p2+ , is now treated as a complex reaction involving two parallel processes, and the experimental kinetics are consistent with the rate laws derived from a mechanism that simultaneously explains the composition of the products as a function of acidity. In terms of the proposed mechanism the dissociation rate of the complex ion in acid solutions containing Ce^IV as scavenging agent is to be regarded as a Ce^IV retarded aquation rate, and OH^– is to be assigned a catalytic role in the kinetics of basic reduction.     

Thermal racemization of tris(1,10-phenanthroline) chromium(III)

The thermal racemization of (+)₅₈₉-Cr phen₃(ClO₄)₃ has been examined under a variety of medium conditions. The racemization rate constant in water at 75°C, for example, is 8·5 × 10⁻⁵ sec⁻¹, with an energy and entropy of activation of 23·5 kcal mole⁻¹ and −10·9 e.u., respectively. The effect of acid, hydroxide, and various other added ions on the rate constant has been investigated, and the results compared with other tris-phenanthroline systems. An intramolecular twist process is considered the most probable mechanism of racemization.     

Transient bleaching of tris(1,10-phenanthroline) iron(II)

Absorption at the excitation wavelength recovers in a sub-nanosecond, two stage process following bleaching of tris(1,10-phenanthroline) iron(II) by a single picosecond pulse at 530 nm. Absorption coefficients and decay times suggest that a CT and a dd excited state are consecutively occupied before ground state repopulation.     

The reaction of p-benzenediols with Co(III) in aqueous perchlorate media. Kinetics of oxidation

The kinetics of redox reactions between Co(III) and a series of 1,4-benzenediol derivatives has been investigated in aqueous perchlorate media by means of a stopped-flow technique. Reaction rates dependences on oxidant, organic substrate (S), and acidity, were found to conform with: $\text{rate}\text{= (k + k′[}\text{H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{-1}}\text{)[}\text{Co(III)}\text{][S]}$ for all the investigated compounds. Substituents effects on the rate of the single reaction pathways are discussed with reference to Co(III) reactive species and reaction mechanisms.     

Kinetic spectrophotometric determination of antimony(III) based on induced oxidation of tris(1,10-phenanthroline)iron(II) by chromium(VI)

The method involves the measurement of the extent of the induced reaction, which ceases a few seconds after initiation. Antimony(III) can be determined in the range 0.4–10 μg ml^-1. The standard deviation is ±0.25 μg. The method is applied to marine sediments.     

Spectrocoulometric study of the hydrolysis of the tris (1,10-phenanthroline)iron(III) complex ion

The spectrocoulometric technique reported earlier is applied to verify the mechanism and to evaluate the contributions k_Bi of the individual bases to the total rate constant k of the hydrolysis of the tris (1,10-phenanthroline) iron(III) complex, Fe (phen)³⁺₃. Both normal and “open-circuit” spectrocoulometric experiments are used. Partial rate constants for four bases in the acetate-buffered solutions are k_H2O=(3.4±1.2) × 10^?4s^?1 (k_H2O includes the H₂O concentration), k_OH=(1.20±0.06)×10⁷ mol^?1dm³s^?1, k_phen=(1.4±0.2) mol^?1dm³s^?1, k_Ac=(3.8±0.3)×10^?2 mol^?1dm³s^?1, at 25°C and ionic strength 0.5 mol dm^?3. The Fe(phen)³⁺³ hydrolysis, with (phen)₂ (H₂O) Fe-O-Fe (H₂O) (phen)⁴⁺² formation, is first order with respect to Fe (phen)³⁺³ and the bases present in the solution. The rate-determining step in the hydrolysis is the entry of a base to the coordinating sphere of the complex, as in the hydrolysis of the analogous 2,2′-bipyridyl complex.     

Effects of alkyltrimethylammonium bromide surfactants on the kinetics of the oxidation of tris(1,10-phenanthroline)iron(II) by azido-pentacyanocobaltate(III) complex

The redox reaction between tris(1,10-phenanthroline)iron(II), [Fe(phen)₃]²⁺, and azido-pentacyanocobaltate(III), [Co(CN)₅N₃]^3? was investigated in three cationic surfactants: dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB) and cetyltrimethylammonium bromide (CTAB) in the presence of 0.1?M NaCl at 35°C. Second-order rate constant in the absence and presence of surfactant, k_w and k_ψ, respectively, were obtained in the concentration ranges DTAB?=?0???4.667?×?10^?4?mol?dm^?3, TTAB?=?0–9.364?×?10^?5?mol?dm^?3, CTAB?=?0???6.220?×?10^?5?mol?dm^?3. Electron transfer rate was inhibited by the surfactants with premicelllar activity. Inhibition factors, k_w/k_ψ followed the trend CTAB?>?TTAB?>?DTAB with respect to the surfactant concentrations used. The magnitudes of the binding constants obtained suggest significant electrostatic and hydrophobic interactions. Activation parameters ΔH^≠, ΔS^≠, and E_a have larger positive values in the presence of surfactants than in surfactant-free medium. The electron transfer is proposed to proceed via outer-sphere mechanism in the presence of the surfactants.     

Kinetics and mechanism of oxidation of tris(ethylenediamine)-chromium(III) by N-bromosuccinimide

Summary In aqueous solutions, [Cr(en)₃]³⁺ aquates to [Cr(en)₂-(H₂O) ₂]³⁺. A kinetic study of the oxidation of [Cr(en)₃]³⁺ by N-bromosuccinimide (NBS) in aqueous solutions and water-alcohol solvent mixtures was performed. The reaction is first order with respect to both total [Cr^III] and [NBS]. The rate is inversely dependent upon [H⁺] in the 7.0–7.9 pH range, and varies with the co-solvent according to the order: MeOH > EtOH > PrOH. An appropriate mechanism, in which the deprotonated [Cr(en)₂(OH)(H₂O)]²⁺ is the reactive species, is suggested. Thermodynamic activation parameters have been calculated.Abstracted from the PhD thesis (Ain Shams University) of A. E.- D. M. Abdel-Hady.     

Electron transfer reactions of tris(1,10-phenanthroline)cobalt(II) with copper(III) imine-oxime complexes

Summary The kinetics of oxidation of [Co^II(phen)₃]²⁺ (phen = 1,10-phenanthroline) by copper(III) imine-oxime complexes are first-order in each reactant. All reactions proceed via two parallel pathways; one pH independent, the other pH dependent. The second-order rate constant varies with [H⁺] as k₂ = k _inf2 ^supo + k _inf2 ^supH [H⁺]. The rapidity of the electron transfer step, coupled with the relative inertness of [Co^II(phen)₃]²⁺ over the pH range studied and the absence of a bridging atom on the phen ligand, supports an outer-sphere mechanism for this process. Reasonably good agreement between the experimental rate constants and those calculated using the Marcus equation has been obtained.     

Kinetics of oxidation of pyruvic acid by [ethylenebis(biguanide)]silver(III) in aqueous acidic media

The oxidation of pyruvic acid by the title silver(III) complex in aqueous acidic (pH, 1.1–4.5) media is described. The reaction products are MeCO₂H and CO₂, together with a colourless solution of the Ag⁺ ion. The free ligand, ethylenebis(biguanide) is released in near-quantitative yield upon completion of the reduction. The parent complex, [Ag(H₂L)]³⁺ and one of its conjugate bases, [Ag(HL)]²⁺, participate in the reaction with both pyruvic acid (HPy) and the pyruvate anion (Py^–) as the reactive reducing species. Ag⁺ was found to be catalytically inactive. At 25.0°C, I=1.0moldm^–3, rate constants for the reactions [Ag(H₂L)]³⁺+HPy (k ₁), [Ag(H₂L)]³⁺+Py^– (k ₂), [Ag(HL)]²⁺+HPy (k ₃) and [Ag(HL)]²⁺+Py^– (k ₄) arek ₁=(94±6)×10^–5dm³mol^–1s^–1, (k ₂ K _a+k ₃ K _a1)= (1.3±0.1)×10^–5s^–1 and k ₄=(58±4)×10^–5dm³mol^–1s^–1, respectively, where K _a1is the first acid dissociation constant of the [Ag(H₂L)]³⁺ and K _a is for pyruvic acid. A comparison between the k ₁ and k ₄ values is indicative of the judgement that k ₂k ₃. A one-electron inner-sphere redox mechanism seems more justified than an outer-sphere electron-transfer between the redox partners.     

Kinetics and mechanism of Ru(III) catalysed oxidation of some polyhydric alcohols by N-bromosuccinimide in acidic media

Kinetics of oxidation of ethylene glycol, glycerol, erythritol and dulcitol by acidic solution of N-bromosuccinimide (NBS) in presence of ruthenium(III) chloride as a homogeneous catalyst and mercuric acetate as scavenger in the temperature range of 30–50°C have been reported. The reactions follow identical kinetics, being first order in each NBS, substate and Ru(III). Zero effect of [H⁺], [mercuric acetate] and ionic strength has been observed. A negative effect of succinimide and acetic acid is observed while [Cl^-] shows the positive effect on reaction velocity. Various activation parameters have been computed. The products of the reaction were identified as the coresponding acids. A suitable mechanism consistent with the experimental results has been proposed.