Inner sphere oxidation of L-ascorbic acid by Ru(III) ion and its complexes in aqueous acidic medium

A kinetic study and mechanisms of reduction of N, N'-phenylenebis(salicyalideneiminato)cobalt(III) by L-ascorbic acid in DMSO-water medium

The kinetics of reduction of N, N¹-phenylenebis-(salicylideneiminato)cobalt (III), referred to as [Co(Salophen)]⁺ by L-ascorbic acid (H₂A) was studied in mixed aqueous medium (DMSO:H₂O; 1:4 v/v) under pseudo-first-order conditions at 33 ± 1 °C, μ = 0.1 mol dm⁻³ (NaCl) and λ_max = 470 nm. L-ascorbic acid was oxidized to dehydroascorbic acid with kinetics that was first order in both the [H₂A] and [Co(Salophen)⁺] and second-order overall. The reaction involves two parallel reaction pathways; an acid-dependent and the inverse acid-dependent pathways. The inverse acid pathway shows that there is a pre-equilibrium step before the rate determining-step in which a proton is lost. The kinetics followed negative Brønsted–Debye salt effect. Evidence was obtained for the presence of free radicals but none to support the formation of an intermediate complex of significant stability during the reaction. Overall, the data obtained suggest an outer-sphere mechanism for the reaction. A plausible mechanism is proposed.

Electronic and Structural Effects of Inner Sphere Coordination of Chloride to a Homoleptic Copper(II) Diimine Complex

The reaction of CuCl₂ with 2,9-dimethyl-1,10-phenanthroline (dmp) does not lead to the formation of [Cu(dmp)₂](Cl)₂ but instead to [Cu(dmp)₂Cl]Cl, a 5-coordinated complex, in which one chloride is directly coordinated to the metal center. Attempts at removing the coordinated chloride by changing the counterion by metathesis were unsuccessful and resulted only in the exchange of the noncoordinated chloride, as confirmed from a crystal structure analysis. Complex [Cu(dmp)₂Cl]PF₆ exhibits a reversible cyclic voltammogram characterized by a significant peak splitting between the reductive and oxidative waves (0.85 and 0.60 V vs NHE, respectively), with a half-wave potential E_1/2 = 0.73 V vs NHE. When reduced electrochemically, the complex does not convert into [Cu(dmp)₂]⁺, as one may expect. Instead, [Cu(dmp)₂]⁺ is isolated as a product when the reduction of [Cu(dmp)₂Cl]PF₆ is performed with l-ascorbic acid, as confirmed by electrochemistry, NMR spectroscopy, and diffractometry. [Cu(dmp)₂]²⁺ complexes can be synthesized starting from Cu(II) salts with weakly and noncoordinating counterions, such as perchlorate. Growth of [Cu(dmp)₂](ClO₄)₂ crystals in acetonitrile results in a 5-coordinated complex, [Cu(dmp)₂(CH₃CN)](ClO₄)₂, in which a solvent molecule is coordinated to the metal center. However, solvent coordination is associated with a dynamic decoordination-coordination behavior upon reduction and oxidation. Hence, the cyclic voltammogram of [Cu(dmp)₂(CH₃CN)]²⁺ is identical to the one of [Cu(dmp)₂]⁺, if the measurements are performed in acetonitrile. The current results show that halide ions in precursors to Cu(II) metal-organic coordination compound synthesis, and most likely also other multivalent coordination centers, are not readily exchanged when exposed to presumed strongly binding and chelating ligand, and thus special care needs to be taken with respect to product characterization.

Kinetic, mechanistic, and thermodynamic investigations on ferrofluid-catalyzed oxidation of L-ascorbic acid by hydrogen peroxide in acidic aqueous solution

The reduction of ferrofluid (FF) by L-ascorbic acid (H(2)A) and the reoxidation of the reduced FF to its original form by hydrogen peroxide have been investigated in aqueous acidic medium. The rate of reduction of FF was found to be first order with respect to [H(2)A] and [FF] and independent with respect to [H(2)O(2)] and ionic strength. The rate of reduction of FF increased with increasing pH (2.5-4.0), having an inverse first order dependence in [H(+)]. With increasing temperature (15-45 degrees C), the rate of reduction was increased in line with the Arrhenius equation. Based on experimental evidence and results a mechanism, operative to reduce FF and reoxidize the reduced FF by H(2)O(2), which makes the system catalytic, is suggested. Thermodynamic quantities associated with FF-catalyzed oxidation of H(2)A by H(2)O(2) were determined and compared with other closely related systems.

Kinetics and Mechanism of Ruthenium (III) Chloride Catalysed Oxidation of Propanol by N-Chloro-P-Toluene Sulfonamide (Chloramine-T) in Aqueous Acid Medium

Kinetics of ruthenium (III) chloride catalysed oxidation of propanal has been studied.

Kinetics and Mechanism of Two-Electron Reduction of Surfacial Ferric Ions of Colloidal Ferrofluid by l-Ascorbic Acid in Aqueous Acidic Medium

The kinetics of electron transfer from l-ascorbic acid (H2A) to ferrofluid (FF) in dilute aqueous acidic solutions (pH 2.5-4.0) was investigated spectrophotometrically in detail as a function of [H2A], [FF], [H+], ionic strength, and temperature. FF was found to be reduced and the reduction was first-order dependent on both H2A and FF concentrations. An inverse first order dependence in H+ ion concentration and no dependence on ionic strength were observed. Kinetic, electrochemical, spectrophotometric, and thermodynamic results are interpreted in terms of a mechanism involving a rate-determining outer-sphere one-electron transfer from H2A to FF followed by a subsequent and kinetically rapid transfer of the second electron of H2A to the same molecule of FF, which was already reduced by one electron in rate-determining step. This overall two-electron transfer process from H2A to the same molecule of FF is discussed and compared with other closely related reactions. Copyright 1999 Academic Press.