Homogeneous Palladium Nanoparticles Surface Hosts Catalyzed Reduction of the Chromophoric Azo (–N=N–) Group of Dye,Acid Orange 7 by Borohydride in Alkaline Media

In alkaline media, well‐characterized gelatin‐stabilized palladium (GPd) nanoparticles catalyze the reduction of the azo group containing pollutant dye, Acid Orange 7 (AO7) by sodium borohydride (NaBH₄) to 1‐amino‐2‐napthol and sulfanilic acid. Kinetic observations and detailed FTIR studies suggests that the reaction follows Langmuir–Hinshelwood kinetic model, where during the reaction both AO7 and borohydride are adsorbed on the GPd surface. Plots of lnk_o versus ln[AO7] or ln[NaBH₄] show that the order of reaction with respect to AO7 and NaBH₄ remains almost same over different molar ratios of [NaBH₄]/[AO7]. The catalyzed reaction shows an initial induction period (t₀) due to a surface‐restructuring process of GPd nanoparticles, and (1/t₀) can be defined as the rate of surface restructuring. The activation energy of the catalyzed reaction and energy of the surface‐restructuring process of GPd are estimated as 22 ± 3 and 25 ± 7 kJ M^?1, respectively.     

Cu(II)‐catalyzed oxidation of thiols by superoxide ligated to CoIII2

Copper(II) dramatically catalyzes the oxidation of thiols by a superoxide bridging two Co^III ions. The catalyzed path overwhelmingly dominates over the uncatalysed path and is first order in the superoxo complex concentration. The first‐order rate constants show a first‐order dependence in [Cu²⁺], a second‐order dependence in [thiol] and linearly varies with [H⁺]^?3. On the basis of observed kinetics reported here, it is proposed that Cu(II) reacts with two thiol molecules to form a Cu^II(thiol)₂ complex, an electron is transferred from one ligated thiol to the Cu^II center to form Cu^I(thiol) and a thiyl radical. The copper(I)‐thiol complex is oxidized by the conjugate base of the title complex to regenerate Cu^II(thiol). A Cu^II/I catalytic cycle is thus believed to be responsible for the observed catalysis. Copyright © 2012 John Wiley & Sons, Ltd.     

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 and mechanism of reaction between zirconium‐lactate and pentane‐2,4‐dione in excess lactate

The kinetics of the reaction of pentane‐2,4‐dione (HA) with Zr^IV has been studied at 25.0°C in an excess lactate (L^?) media. The equilibrium reaction was found to be: Zr₂L₆+HA?HL+Zr₂AL₅. The equilibrium was approached from either direction and a plausible mechanism has been proposed with kinetic constants, but individual reactivities of the keto and the enol tautomers of pentane‐2,4‐dione could not be apportioned. However, it was found that both the uncatalyzed and acid‐catalyzed paths contribute to the reverse reaction. But 2‐thenoyltrifluoroacetone (HT) forms a stronger chelate with Zr^IV, so its reaction with less reactive Zr₂L₅(OH₂) could not be detected; reactivity of the more reactive Zr₂L₅(OH₂)(OH) could be found. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 725–729, 2011     

Oxidation of l‐Methionine by Bisperoxo(1,10‐phenanthroline)oxovanadate(V): A Mechanistic Study

In phosphate buffer media (pH 5.8–8.0), bisperoxo(1,10‐phenanthroline)oxovanadate(V) ( 1 ) oxidizes l ‐methionine to methionine sulfoxide. The stoichiometry of the reaction is 1:1. The reaction occurs in two subsequent first‐order steps. In the first step, one of the peroxo ligands of 1 gets substituted by l ‐methionine. The observed first‐order rate constants for both steps increase linearly with increasing [H⁺] as well as with increasing [l ‐methionine]. The EPR spectra prove that the reaction involves a cysteinyl radical‐type intermediate and that V^V gets reduced to a V^IV species.     

Penicillamine and captopril: mechanistic exploration of defensive actions of thiol drugs against a metal bound-superoxo complex

In acid-media ([H⁺] = 0.01–0.06 M), each of the thiol compounds, D-penicillamine (PEN, L_PH₂) and captopril (CAP, L_CH₂) exist in several proton-dependent forms which can reduce the superoxo complex [(en)(dien)Co^III(O₂)Co^III(en)(dien)]⁵⁺ (1) to the corresponding peroxo [(en)(dien)Co^III(O₂)Co^III(en)(dien)]⁴⁺ (2) or the hydroperoxo complex [(en)(dien)Co^III(OOH)Co^III(en)(dien)]⁵⁺ (3). The observed first-order rate constants, k_o,P and k_o,C for PEN and CAP increase with the increase in [T_PEN] and [T_CAP] (which are the analytical concentrations of the respective thiols) but decrease with the increase in the media-acidity ([H⁺]) and the media ionic strength (I). The protolytic equilibria in aqueous solution allow several potentially reducing forms to coexist for both PEN (L_PH₃⁺, L_PH₂, L_PH^?, and L_P^2?) and CAP (L_CH₂, L_CH^?, L_C^2?) but the kinetic analyses reveal that the order of reactivity for the species are L_PH₃⁺ ~ L_PH₂ <<< L_PH^? and L_CH₂ < L_CH^? <<< L_C^2?, respectively. The predominance and higher reactivities of the anionic species, L_PH^? and L_C^2? are supported by the negative slopes of the plots of k_o,P or k_o,C versus I. Moreover, a large value of k_H/k_D for PEN suggests an inner-sphere electroprotic reaction pathway while the absence of such effect for CAP strongly supports an outer-sphere electron transfer reaction. These propositions are supported by the structural features of L_PH^? and L_C^2?.     

Kinetics of uncatalysed and Mn2+ -catalysed oxidation of hydrazine by [MnIV 3 (μ-O)4(bipy)4(H2O)2]4+ ion in aqueous acid

In aqueous acid, hydrazine reduces [Mn^IV ₃(-O)₄(bipy)₄(H₂O)₂]⁴⁺, (1), quantitatively to [Mn^III,IV ₂(-O)₂(bipy)₄]³⁺, (2), and Mn²⁺ if [N₂H⁺ ₅] 2 × (stoichiometric amount). At higher [N₂H⁺ ₅], reduction proceeds up to Mn²⁺. The reduction of (1) to (2) is strongly catalysed by Mn²⁺ and the absorbance (A _t ) versus time (t) graphs have sigmoidal shapes. The graphs become steeper with increasing amounts of added Mn²⁺ and N₂H⁺ ₅, but remain unchanged when [H⁺] is changed. The A _t – t graphs, under various experimental conditions, can all be simulated with a single set of second order rate constants, estimated for the individual steps in a proposed reaction scheme, in which the catalytic action of Mn²⁺ involves a one-electron and a two-electron reduced form of (1), but not (1) itself. The absence of any proton-dependence of the reaction rate refutes an electroprotic mechanism and an inner-sphere mechanism appears to be most likely for the reduction of (1) by N₂H⁺ ₅     

Kinetics and mechanism of oxidation of thiosulphate by the complex ion [MnIV3(μ-O)4(phen)4(H2O)2]4+ (phen = 1,10-phenanthroline) in weakly acidic aqueous media

The trinuclear complex ion [Mn^IV ₃(-O)₄(phen)₄(H₂O)₂]⁴⁺ (1) is quantitatively reduced by an excess of S₂O₃ ^2– to Mn^II, but the binuclear complex [Mn^IIIMn^IV(-O)₂(phen)₄]³⁺, (2) is the only manganese product when [S₂O₃ ^2–] = 1.5 [(1)]. With an excess of S₂O₃ ^2– biphasic kinetics, (1) k ₁ ⁰ (2) k ₂ ⁰ Mn^II is observed, while the reaction with S₂O₃ ^2– = 1.5 [(1)], follows one-step second order kinetics with the second order rate constant k = k ⁰ ₁/[S₂O₃ ^2–]. The rate constant k ⁰ ₁ is independent of c _phen ( = [phen] + [Hphen⁺]) but directly proportional to [H⁺] and [S₂O^2– ₃]. Rapid formation of an adduct between (1) and S₂O^2– ₃, followed by rate-determining one-electron, one-proton reduction of the adduct, appears logical. A comproportionation reaction in one of the subsequent rapid steps leads to (2) without any Mn^II co-product. Kinetic dependences for the second step are same to those for an authentic complex of (2), and further support the assigned reaction sequence.