PVP stabilized Pt nano particles catalyzed de-oxygenation of phenoxazine group by hydrazine in physiological buffer media: surfactant competes with reactants for the same surface sites.

PVP capped platinum nano particles (PNP) of 5 nm diameter were prepared and characterized as homogeneous and of spherical nature. At physiological pH range (6.0-8.0), these PNP catalyze the deoxygenation of phenoxazine group containing resazurin (1) by hydrazine. The observed rate constants (k(o)), increase linearly with [PNP] at constant [1] and [Hydrazine]; but first increase and then after reaching a maximum it decrease with increase in [1] as well as in [Hydrazine]. The k(o) values increase linearly with 1/[H(+)] indicating N(2)H(4) as the reducing species that generates from the PNP assisted deprotonation of N(2)H(5)(+). The kinetic observations suggest Langmuir-Hinshelwood type surface reaction mechanism where both 1 and hydrazine are adsorbed on nano particles surface and compete for the same sites. Interestingly, the surfactant molecules, polyvinylpyrrolidone (PVP), though do not take part into reduction reaction but having same type of functional groups as reactants, competes with them for the same surface sites. Adsorption on PNP with same type of functional group is further supported by the FTIR spectra of Pt-PVP and Pt-1. Thus on increasing [PVP], k(o) decreases linearly and only when [PVP] is held constant, the plot of k(o) vs. [PNP] passes through the origin indicating the insignificance of uncatalyzed reaction. The plot of ln k(o) vs. [1] or [Hydrazine] shows two different linear zones with different exponent values with respect to [1] and [Hydrazine]. This indicates that along with the complex heterogeneous surface adsorption processes, the mutual interactions between the reactants are also changing with the relative concentrations of reactants or, in general, with the molar ratio ([Hydrazine]/[1]).

Gold nano particles catalyzed oxidation of hydrazine by a metallo-superoxide complex: experimental evidences for surface activity of gold nano particles.

Gelatin-capped gold nano particles (GNPs) of diameter 23, 28 and 36 nm were prepared and characterized as almost monodispersed, near-spherical solids. In acidic media, these GNPs at their very low concentration level (∼10(-13) M) catalyze the oxidation of hydrazine by the metallo-superoxide, [(NH(3))(4)Co(III)(µ-NH(2),µ-O(2))Co(III)(NH(3))(4)](NO(3))(4) (1). In the presence of a large excess of hydrazine over [1], the catalyzed oxidation is first-order in [1], [GNPs] and media alkalinity. The pure first-order dependence implies that the size as well as the nature of the catalyst remained unchanged during the reaction. The catalytic efficacies increased with increased total surface area of the GNPs. Increasing T(Hydrazine) (T(Hydrazine) is the analytical concentration of hydrazine) tends to saturate the first-order rate constant (k(o)) for hydrazine oxidation and a plot of 1/k(o)versus T(Hydrazine) was found to be linear at a particular [GNPs], indicating the GNPs assisted deprotonation of N(2)H(5)(+) to N(2)H(4). The rate constants show a non-linear behavior with temperature studied in the range 288-308 K. At a lower temperature interval, viz. 288-298 K, k(o) increases with increasing temperature whereas at temperature interval, viz. 303-308 K, k(o) decreases with temperature. Such a variation indicates the important process of absorption and desorption of the reactants on and from the surface. A plausible mechanism for the GNPs catalyzed oxidation of hydrazine is suggested.

Kinetics of oxidation of nitrosodisulfonate anion radical with a metallo-superoxide.

The metal bound superoxide in µ-superoxo-bis[pentaamminecobalt(III)](5+) (1) oxidizes the nitrosodisulfonate anion radical (NDS(2-)) by two electrons. Oxidized NDS(2-) quickly decomposes to SO(4)(2-) and NO. 1 is itself reduced to the corresponding hydroperoxo complex which also decomposes fast to Co(ii), NH(4)(+) ions and oxygen. 1.5 moles of volatile products formed per mole of 1 mixed with excess NDS(2-). In the absence of superoxide in a bridged complex, e.g. the µ-amido-bis[pentaamminecobalt(III)](5+) complex fails to oxidize the nitroxyl radicals, NDS(2-), TEMPO and 4-oxo TEMPO. With excess NDS(2-) over 1, the reaction is first-order with respect to [1], [NDS(2-)] and inverse first order in [H(+)]. The activation entropy, ΔS(≠), is largely negative, increased ionic strength decreased the rate and a Brønsted plot is fairly linear with a negative slope. Oxidant µ-superoxo-bis[(ethylenediamine)(diethylenetriamine)cobalt(III)](5+) has ligands sterically more crowded though more basic than ammonia in 1. It oxidizes NDS(2-) much more slowly. No solvent kinetic isotope effect (k(H(2)O/D(2)O)≈ 1) could be seen; a spin-adduct formation by the conjugate base of 1 followed by electron transfer is postulated.

Reduction mechanism of a coordinated superoxide by thiols in acidic media.

In weakly acidic media ([H(+)], 0.01-0.06 M), 2-mercaptoethanol (mercap, RSH), thioglycolic acid (tga, R'SH) and L-cysteine (cys, R''SH) reduce the superoxo ligand of the complex ion, {mu-amido-mu-superoxo-bis[tetraamminecobalt(III)]}(4+) (1) to its corresponding hydroperoxo complex, {mu-amido-mu-hydroperoxo-bis[tetraamminecobalt(III)]}(3+) (2). During this act, RSH and R'SH are quantitatively oxidized to their respective disulfides. However, cysteine (R''SH) is converted to a mixture of approximately 80% of the disulfide, cystine and approximately 20% to cystine sulfinic acid. Cystine itself is not a source of the sulfinic acid. Dissolved copper, even at the impurity level, dramatically catalyzes the reaction such that the direct reactions are inaccessible. Nevertheless, the catalyzed path can be masked completely with 0.20 mM dipicolinic acid and it can be determined for the first time that, the direct reactions are first-order in [1], in [total thiol] and in basicity. Rate decreases linearly with increasing mol% of D(2)O in the solvent. H-atom (H(+) + e) transfer from thiols to superoxide in 1 seems logical for the conversion of 1 to 2.

Reaction of hydrogen peroxide with coordinated superoxide in [(NH3)5CoIII(micro-O2)CoIII(NH3)5]5+: a mechanistic study.

In aqueous acetate buffer media, hydrogen peroxide reduces the bridging superoxide in [(NH3)5CoIII(micro-O2)CoIII(NH3)5]5+ (1) to the corresponding peroxide in the complex, [(NH3)5CoIII(micro-O2H)CoIII(NH2)(NH3)4]4+ (2), itself being oxidized to HO2*. The complex 2 thus produced decomposes rapidly to the final products, CoII, NH3, etc. instead of reacting with a second molecule of hydrogen peroxide. In the presence of excess [H2O2] over (1), the reaction obeyed first-order kinetics and exhibited inverse proton dependence. [(NH3)5CoIII(micro-O2)CoIII((NH2)(NH3)4]4+ (3), a conjugate base of 1, seems to be the kinetically reactive species and the cause for the observed inverse proton dependence. Kinetics is little affected when one of the hydrogen atoms from hydrogen peroxide is replaced with an alkyl group, as in tert-butyl hydroperoxide. But replacement of both the H atoms with alkyl groups halts the reaction as seen with di-tert-butyl peroxides, and peroxodisulfate ion. The reaction rate with hydrogen peroxide significantly decreases with increasing proportion of D2O replacing water in the solvent and the rate-limiting step seems to be an H-atom transfer.