Kinetics and mechanism of the oxidation of nitrilotris(methylenephosphonato)chromium(III) by periodate

The kinetics of oxidation of nitrilotris(methylenephosphonato)chromium(III), Cr^IIINTMP, by periodate to yield Cr^VI have been studied spectrophotometrically over the 5.80–6.85 pH range at 22–33 °C. The reaction rate, which is first-order with respect to [Cr^IIINTMP] and [IO^– ₄] and inversely dependent on [H⁺], obeys the rate law:-d[Cr^IIINTMP/dt=kKK_h[IO^- ₄] [Cr^III]_T/K_h+ [H⁺] +KK_h[IO^- ₄] The values of the intramolecular electron transfer, k, and the formation constant of the intermediate complex, K, were determined at various temperatures. The hydrolysis constant for Cr^IIINTMP, K _h , was determined spectrophotometrically and is in agreement with the value estimated from the kinetic data. The activation parameters were calculated from the temperature dependence of the specific rate constants. A mechanism is proposed in which the hydroxo complex, [CrHNTMP(OH)]^3–, is the reactive species. The results support a mechanism where intramolecular electron transfer is the rate-determining step.     

Kinetics of the oxidation of iodide by copper(III)-imine-oxime complexes

Summary The copper(III)-imine-oxime complexes [Cu^III(Enio)]⁺ and [Cu^III(Pre)]⁺ {EnioH₂ =N,N-ethylene bis(isonitrosoacetylacetoneimine) and PreH₂ = N,N-propylene bis (isonitrosoacetylacetoneimine)} react very rapidly with iodide. The rate law under fixed conditions for the reaction is given by the equation: –d[Cu^III]/dt = (2k₂[I^–] + 2k₃[I^–]²)[Cu^III] The [Cu^III(Enio)]⁺ reaction was pH-independent whereas the [Cu (Pre)]⁺ reaction rate increased with increasing pH. Both the k₂ and the k₃ pathways are believed to involve one-electron transfer. An inner-sphere mechanism may operate in the pathway, first-order in [I^–].     

Rhodium(III) as a homogeneous catalyst for the oxidative decolorization of ethyl orange with aqueous acidic chloramine-T: a spectrophotometric, kinetic and mechanistic study

The kinetics and mechanism of sodium N-chloro-p-toluenesulfonamide oxidative decolorization of ethyl orange (EO) in aqueous perchloric acid have been studied at 303 K in the presence of rhodium(III) chloride as catalyst. The reaction exhibits first-order dependence on [EO]_o and a fractional-order dependence on [CAT]_o, [H⁺] and [Rh^III]. The dielectric effect is positive. The stoichiometry of the reaction was found to be 1:1, and the oxidation products of EO were identified as N-(4-diethylamino-phenyl)-hydroxylamine and 4-nitroso-benzenesulfonic acid. The rhodium(III)-catalyzed reaction is about fourfold faster than the uncatalyzed reaction. The proposed mechanism and derived rate law are in agreement with the observed kinetic results.     

Kinetic studies on H+- and OH−-catalysed aquation of bis-aspartatochromium(III)

A potential synthetic biochromium source, bis-aspartatochromium(III) ion (where Asp is a tridentate N,O,O′-ligand, bonded via amine nitrogen and carboxylate oxygen atoms) has been obtained and characterized in aqueous solution. Kinetics of partial dechelation of the complex catalysed by H⁺ and OH^? ions has been studied spectrophotometrically within 0.1–1.0 M HClO₄ and 0.1–1.0 M NaOH ranges under first-order conditions. A linear dependence of the k _obs,H on [H⁺] and independence of the k _obs,OH on [OH^?] were established. The derived rate expression and identification of components of the reaction mixture provide evidence for a reaction mechanism, where the key role in the overall process is the formation of an intermediate species with bidentate N,O-bonded Asp via both spontaneous and H⁺(OH^?)-catalysed reaction paths. The intermediate is meta-stable and at pH 5–7 restores the substrate.     

A linear Cu-Gd-Cu-Gd complex derived from the assembly reaction of [NaCuH3L] and [Gd(thd)3(H2O)2] (H3L = meso-1,2-diphenyl-1-(2-oxybenzamido)-2-(2-oxy-3-ethoxybenzylideneamino)ethane and Hthd = 2,2,6,6-tetra-methyl-3,5-heptanedione)

A linear tetranuclear Cu^II-Gd^III-Cu^II-Gd^III complex [Cu^IIL^dpen(meso)Gd^III(thd)₂(H₂O)]₂ was synthesized from the reaction of [NaCu^IIL^dpen(meso)(DMF)] with [Gd^III(thd)₃(H₂O)₂], and the structures and magnetic properties were investigated, where H₃L^dpen(meso) = meso-1,2-diphenyl-1-(2-hydroxybenzamido)-2-(2-hydroxy-3-ethoxybenzylideneamino)ethane and Hthd = 2,2,6,6-tetramethyl-3,5-heptanedione. The Cu^II complex component [NaCu^IIL^dpen(meso)(DMF)] has a one-dimensional (1D) chain structure, in which the Na⁺ ion is coordinated by two phenoxo and an ethoxy oxygen atoms of a Cu^II complex and an amido oxygen atom of the adjacent Cu^II unit to produce the 1D structure, in which the diphenylethylenediamine moieties have the array of {(1R,2S)-Na-(1S,2R)}_1∞. The assembly reaction of the Cu^II and Gd^III components gave a linear complex with the array of Cu(1)-Gd(1)-Cu(2)-Gd(2), in which two diphenylethylenediamine moieties have the same chirality of (1R,2S)-(1R,2S) or (1S,2R)-(1S,2R). Two linear Cu(1)-Gd(1)-Cu(2)-Gd(2) units are linked by hydrogen bonds through two water molecules to give a cyclic structure with a center of symmetry. The temperature dependence of the magnetic susceptibilities and field-dependent magnetization revealed the ferromagnetic interaction between the Cu^II and Gd^III ions within the linear chain.     

Inner-sphere oxidation of cis-diaquabis(oxalato)chromate(III) by periodate in aqueous weakly acidic solutions

The kinetics of oxidation of cis-[Cr^III(ox)₂(H₂O)₂]^– (ox = C₂O₄ ^2–) by IO₄ ^– showed a first-order dependence on the initial Cr^III complex concentration in the presence of a vast excess of [IO₄ ^–]. The dependence of the pseudo-first-order rate constant on [IO₄ ^–] is complex and is consistent with the formation of a precursor complex. It is proposed that this complex is formed through the coordination of the two carbonyl oxygens of the ox ligand with the IO₄ ^– ion, forming a cyclic intermediate. The kinetics are consistent with the hydroxo form of the Cr^III complex being the reactive species, whereas the aqua species forms an unreactive complex.     

Kinetics and mechanism of oxidation of chromium(III)-tetraoxalylurea complex by periodate

A novel chromium(III) complex of tetraoxalylurea was prepared. In aqueous solutions, [Cr^III(H₂L)(H₂O)]⁺ (H₂L = diprotonated tetraoxalylurea) is oxidized by IO ₄ ^– according to the rate law

Copper(I) coordination compounds with closo-dodecaborate anion

Copper(I) complexes with a closo-dodecaborate anion were synthesized: Cat[CuB₁₂H₁₂], where Cat = Cs⁺, Ph₄P⁺, Ph₄As⁺, or R _x NH _4?x ⁺ (R = Me, Et, Pr, or Bu; x = 3 or 4). The complexes were synthesized from the copper(II)-closo-dodecaborate-sulfur dioxide (sodium sulfite) system. The structure of [Cu₂(NCCH₃)₄B₁₂H₁₂] was determined using X-ray crystallography.     

Kinetics and mechanism of oxidation of N,N-bis(salicylaldehyde-1,2-diaminoethane) cobalt(II) by N-bromosuccinimide in aqueous acidic medium

The kinetics of oxidation of N,N-bis(salicylaldehyde-1,2-diaminoethane) cobalt(II) complex by N-bromosuccinimide (NBS) in aqueous acid and H₂O–MeOH solvent mixtures were studied spectrophotometrically over the 20–40 °C range, 0.1–0.5 mol dm^?3 ionic strength, 2.2–2.8 pH range and 0–40 wt% MeOH–H₂O solvent mixtures for a range of NBS and complex concentrations. The rate shows first-order dependence on both [NBS] and [complex] and decreases with pH over the range studied. The protonated form of N-bromosuccinimide was identified as the main reactive species. An inner-sphere mechanism involving free radicals is proposed.     

A kinetic study of the 1, 10-phenanthroline-catalysed iron(III) oxidation of epimeric aldo-, and D-, and L-ketohexoses

Summary Fe^III in H₂SO₄ medium does not oxidize sugars even at the reflux temperature, however, the reaction is catalysed by trace amounts of 1,10-phenanthroline (phen). A kinetic study shows that the oxidation rate increases as [phen] increases and exhibits a fractional order dependence upon [phen]. The orders with respect to [Fe^III] and [carbohydrate] are unity. The oxidation rate decreases as [HSO^4/–] and [H₂SO₄] increase. A plausible mechanism is proposed involving participation of an [Fe^III-phen] precursor and sugar in the rate-determining step.     

Inner-sphere oxidation of iron(II) by tris(malonato)cobaltate(III)

The kinetics of oxidation of Fe²⁺ by [Co(C₃H₂O₄)₃]^3? in acidic solutions at 605 nm showed a simple first-order dependence in each reactant concentration. The second-order rate constant dependence on [H⁺] is in accordance with eqn (i) k₂ = k′₂ + k₃[H⁺] (i) where k′₂ and k₃ have values of 73.4 ± 14.0 M ^?1 s^?1 and 353 ± 41 M^?2 s^?1, respectively, at 1.0 M ionic strength (NaClO₄) and 25°C. At 310 nm the formation and decomposition of an intermediate, believed to be [FeC₃H₂O₄]⁺, was observed. The increase in the rate of oxidation with increasing [H⁺] was interpreted in terms of a “one-ended” dissociation mechanism which facilitates chelation of Fe₂₊ by the carbonyl oxygens of malonate in the transition state.     

Kinetics and mechanism of oxidation of iodide with a (μ‐oxo)diiron(III,III) complex in weakly acidic media

The complex ion [Fe^III₂(μ‐O)(phen)₄(H₂O)₂]⁴⁺ ( 1 ) (phen = 1,10‐phenanthroline) and its hydrolytic derivatives [Fe^III₂(μ‐O)(phen)₄(H₂O)(OH)]³⁺ ( 1a ) and [Fe^III₂(μ‐O)(phen)₄‐ (OH)₂]²⁺ ( 2a ) coexist in rapid equilibria in the range pH 4.23–5.35 in the presence of excess phenanthroline (pK_a1 = 3.71±0.03, pK_a2 = 5.28± 0.07). The solution reacts quantitatively with I⁻ to produce [Fe(phen)₃]²⁺ and I₂. Only 1 but none of its hydrolytic derivatives is kinetically active. Both inner and outer sphere pathways operate. The observed rate constants show second‐order dependence on the concentration of iodide, while the dependence on [H⁺] is complex in nature. Added Cl⁻ inhibits the formation of adduct with I⁻ and thus retards the rate of inner sphere path, leading to a rate saturation at high [Cl⁻], where only the outer sphere mechanism is active. Kinetic data indicate that simultaneous presence of two I⁻ in the vicinity of diiron core is necessary for the reduction of 1 . © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 737–743, 2005     

Kinetics of oxidation of thiocyanate ion by manganese(III) in pyrophosphate medium

Summary Mn^III is stabilized by pyrophosphate in weakly acidic solutions. The nature of the complex formed was elucidated spectrophotometrically. The kinetics of Mn^III oxidation of thiocyanate in pyrophosphate medium was investigated over the pH range 2–3. The oxidation followed first order kinetics with [Mn^III]. The effects of varying [Mn^III], [NCS^–], added Mn^II and metal ions, pH, total [P₂O _f7 ^p4– ] and added ClO _f4 ^p– , Cl^– and SO _f4 ^p2– were studied. The order in [NCS^–] was unity, and increasing [H⁺] increased the rate. Retardations with added P₂O _f7 ^p4– and Mn^II were observed. Complexation of NCS^– as K₂Zn(NCS)₄ decreased the reactivity without any change in overall mechanism. The dependence of the reaction rate on temperature was examined, and activation parameters were computed from Arrhenius and Eyring plots. A mechanism consistent with the results is proposed.     

A kinetic study of the homogeneous oxidation of hydroxylamine by manganese(III)-bis(salicylaldimine) complexes

Summary The kinetics of the oxidation of hydroxylamine by manganese(III)-bis (salicylaldimine) complexes have been studied over the 5.2–8.4 pH range. The reaction is first order in both hydroxylamine and oxidant, and inversely proportional to [H⁺]. The [complex]: [hydroxylamine] stoichiometric ratio is 11 in both acidic and neutral media, and 21 in an alkaline medium. The second-order rate constant increased in the sequence: [Mn^III(L²)OH₂]-ClO₄·2H₂O > [Mn^III(L¹)OH₂]ClO₄ > [Mn^IIIL¹)OAc]-H₂O. The reactivity of unprotonated hydroxylamine is much higher than that of the protonated form. The reaction rate decreased significantly with addition of chloride ions. A plausible mechanism is proposed.     

A new octahedral cobalt(III) complex of (1,8)- bis (2-hydroxybenzamido)-3,6-diazaoctane incorporating phenolate-amide-amine coordination: synthesis, X-ray crystal structure, ligand substitution and redox activity with sulfur(IV) and l -ascorbic acid

The octahedral complex, [Co^III(HL)]·9H₂O (H₄L = (1,8)-bis(2-hydroxybenzamido)-3,6-diazaoctane) incorporating bis carboxamido-N-, bis sec-NH, phenolate, and phenol coordination has been synthesized and characterized by analytical, NMR (¹H, ¹³C), e.s.i.-Mass, UV–vis, i.r., and Raman spectroscopy. The formation of the complex has also been confirmed by its single crystal X-ray structure. The cyclic voltammetry of the sample in DMF ([TEAP] = 0.1 mol dm⁻³, TEAP = tetraethylammonium perchlorate) displayed irreversible redox processes, [Co^III(HL)] → [Co^IV(HL)]⁺ and [Co^III(HL)] → [Co^II(HL)]⁻ at 0.41 and −1.09 V (versus SCE), respectively. A slow and H⁺ mediated isomerisation was observed for the protonated complex, [Co^III(H₂L)]⁺ (pK = 3.5, 25 °C, I = 0.5 mol dm⁻³). H₂Asc was an efficient reductant for the complex and the reaction involved outer sphere mechanism; the propensity of different species for intra molecular reduction followed the sequence: [{[Co^III(HL)],(H₂Asc)}–H]⁻ <<< {[Co^III(H₂L)],(H₂Asc)}⁺ < {[Co^III(HL)],(H₂Asc)}. A low value (ca. 3.7 × 10⁻¹⁰ dm³ mol⁻¹ s⁻¹, 25 °C, I = 0.5 mol dm⁻³) for the self exchange rate constant of the couple [Co^III(HL)]/[Co^II(HL)]⁻ indicated that the ligand HL³⁻ with amido (N-) donor offers substantial stability to the Co^III state. HSO₃⁻ and [Co^III(HL)] formed an outer sphere complex {[Co^III(HL)],(HSO₃⁻)}, which was slowly transformed to an inner sphere S-bonded sulfito complex, [Co^III(H₂L)(HSO₃)] and the latter was inert to reduction by external sulfite but underwent intramolecular S^IV → Co^III electron transfer very slowly. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Kinetics and mechanism of oxidation of cis-diaquabis(glycinato)chromium(III) by periodate ion in aqueous solutions

The kinetics of oxidation of cis-[Cr^III(gly)₂(H₂O)₂]⁺ (gly = glycinate) by $ {\text{IO}}_{ 4}^{ - } $ has been studied in aqueous solutions. The reaction is first order in the chromium(III) complex concentration. The pseudo-first-order rate constant, k _obs, showed a small change with increasing $ \left[ {{\text{IO}}_{ 4}^{ - } } \right] $ . 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. The reaction has been found to obey the following rate law: $ {\text{Rate}} = 2k^{\text{et}} K_{ 3} K_{ 4} \left[ {{\text{Cr}}\left( {\text{III}} \right)} \right]_{t} \left[ {{\text{IO}}_{ 4}^{ - } } \right]/\left\{ {\left[ {{\text{H}}^{ + } } \right] + K_{ 3} + K_{ 3} K_{ 4} \left[ {{\text{IO}}_{ 4}^{ - } } \right]} \right\} $ . Values of the intramolecular electron transfer constant, k ^et, the first deprotonation constant of cis-[Cr^III(gly)₂(H₂O)₂]⁺, K ₃ and the equilibrium formation constant between cis-[Cr^III(gly)₂(H₂O)(OH)] and $ {\text{IO}}_{ 4}^{ - } $ , K ₄, have been determined. An inner-sphere mechanism has been proposed for the oxidation process. The thermodynamic activation parameters of the processes involved are reported.     

Kinetics and mechanism of oxidation of 2‐mercaptosuccinic acid by bis(μ‐oxo)‐ manganese(III,IV)‐cyclam complex in aqueous medium: Influence of externally added copper(II)

Kinetic studies on the oxidation of 2‐mercaptosuccinic acid by dinuclear [Mn₂^III/IV(μ‐O)₂(cyclam)₂](ClO₄)₃] ( 1 ) (abbreviated as Mn^III–Mn^IV) (cyclam = 1,4,8,11‐tetraaza‐cyclotetradecane) have been carried out in aqueous medium in the pH range of 4.0–6.0, in the presence of acetate buffer at 30°C by UV–vis spectrophotometry. In the pH region, two species of complex 1 (Mn^III–Mn^IV and Mn^III–Mn^IVH, the later being μ‐O protonated form) were found to be kinetically significant. The first‐order dependence of the rate of the reactions on [Thiol] both in presence and absence of externally added copper(II) ions, first‐order dependence on [Cu²⁺] and a decrease of rate of the reactions with increase in pH have been rationalized by suitable sequence of reactions. Protonation of μ‐O bridge of 1 is evidenced by the perchloric acid catalyzed decomposition of 1 to mononuclear Mn(III) and Mn(IV) complex observed by UV–vis and EPR spectroscopy. The kinetic features have been rationalized considering Cu(RSH) as the reactive intermediate. EPR spectroscopy lends support for this. The formation of a hydrogen bonded outer‐sphere adduct between the reductant and the complex in the lower pH range prior to electron transfer reactions is most likely to occur. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 170–177 2004     

Kinetic and mechanistic studies on the electron-transfer reactions between diaquabisethylenediaminecobalt(III) and ethylenediaminetetraacetatocobaltate(III) with promazine in acidic aqueous media

The kinetics of the electron-transfer reactions between promazine (ptz) and [Co(en)₂(H₂O)₂]³⁺ in CF₃SO₃H solution ([Co^III] = (2–6) × 10⁻³ m, [ptz] = 2.5 × 10⁻⁴ m, [H⁺] = 0.02 − 0.05 m, I = 0.1 m (H⁺, K⁺, CF₃SO ₃ ⁻ ), T = 288–308 K) and [Co(edta)]⁻ in aqueous HCl ([Co^III] = (1 − 4) × 10⁻³ m, [ptz] = 1 × 10⁻⁴ m, [H⁺] = 0.1 − 0.5 m, I = 1.0 m (H⁺, Na⁺, Cl⁻), T = 313 − 333 K) were studied under the condition of excess Co^III using u.v.–vis. spectroscopy. The reactions produce a Co^II species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (k _obs) on [Co^III] with a non-zero intercept was established for both redox processes. The rate of reaction with the [Co(en)₂(H₂O)₂]³⁺ ion was found to be independent of [H⁺]. In the case of the [Co(edta)]⁻ ion, the k _obs dependence on [H⁺] was linear and the increasing [H⁺] accelerates the rate of the outer-sphere electron-transfer reaction. The activation parameters were calculated as follows: ΔH ^≠ = 105 ± 4 kJ mol⁻¹, ΔS ^≠ = 93 ± 11 J K⁻¹mol⁻¹ for [Co(en)₂(H₂O)₂]³⁺; ΔH ^≠ = 67 ± 9 kJ mol⁻¹, ΔS ^≠ = − 54 ± 28 J K⁻¹mol⁻¹ for [Co(edta)]⁻.     

Reactions of 1,4-diaza-3-methylbutadien-2-ylpalladium(II) derivatives with chloro-bridged rhodium(I) complexes

The reactions of the organometallic 1,4-diazabutadienes, RN=C(R′)C(Me)=NR″ [R = R″ = p-C₆H₄OMe, R′ = trans-PdCl(PPh₃)₂ (DAB); R = p-C₆H₄OMe, R″ = Me, R′ = trans-PdCl(PPh₃)₂ (DAB^I; R = R″ = p-C₆H₄OMe, R′ = Pd(dmtc)-(PPh₃), dmtc = dimethyldithiocarbamate (DAB^II); R = R″ = p-C₆H₄OMe, R′ = PdCl(diphos), diphos = 1,2-bis(diphenylphosphino)ethane (DAB^III)] with [RhCl(COD)]₂ (COD = 1,5-cyclooctadiene, Pd/Rh ratio = $\text{1}\text{2}$) depend on the nature of the ancillary ligands at the Pd atom in group R′. In the reactions with DAB and DAB^I transfer of one PPh₃ ligand from Pd to Rh occurs yielding [RhCl(COD)(PPh₃)] and the new binuclear complexes [Rh(COD) {RN=C(R?)-C(Me)=NR″}], in which the diazabutadiene moiety acts as a chelating bidentate ligand. Exchange of ligands between the two different metallic centers also occurs in the reaction with DAB^II. In this case, the migration of the bidentate dmtc anion yields [Rh(COD)Pdmtc] and [Rh(COD) {RN=C(R?)C(Me)=NR″}]. In contrast, the reaction with DAB^III leads to the ionic product [Rh(COD)- (DAB^III)][RhCl₂(COD)], with no transfer of ligands. The cationic complex [Rh(COD)(DAB^III)]⁺ can be isolated as the perchlorate salt from the same reaction (Pd/Rh ratio = 1/1) in the presence of an excess of NaClO₄. In all the binuclear complexes the coordinated 1,5-cyclooctadiene can be readily displaced by carbon monoxide to give the corresponding dicarbonyl derivatives. The reaction of [RhCl(CO)₂]₂ with DAB and/or DAB^I yields trinuclear complexes of the type [RhCl(CO)₂]₂(DAB), in which the diazabutadiene group acts as a bridging bidentate ligand. Some reactions of the organic diazabutadiene RN=C(Me)C(Me)=NR (R = p-C₆H₄OMe) are also reported for comparison.     

Oxidation of thiourea and thioacetamide by octacyanomolybdate(v) anion in perchloric acid

Summary The kinetics of the reduction of octacyanomolybdate(V) anion by thiourea and thioacetamide have been studied in aqueous HClO₄ at constant ionic strengthI=0.10 mol dm^–3 (NaClO₄). The rate of oxidation of these substrates by the oxidant shows a first order dependence in both the oxidant and the substrates and while the thiourea system exhibits an inverse first-order dependence on [H⁺] that of thioacetamide is found to be first-order in [H⁺]. The variation observed in [H⁺] dependences in these reactions is attributed to the nature of the thiourea in the pH range used in this study and the inductive effect of the methyl group in thioacetamide. A mechanistic interpretation of these observations is advanced.