Sphere Electron Transfer Mechanism Research Articles

Redox Kinetics and Mechanism of the Reactions of µ –Oxobis [aquobis (2,2’ – Bipyridine)] diruthenium (III) Ion and some Aliphatic Alcohols in Acidic Medium

The redox kinetics and mechanisms of the reactions of μ – oxobis[aquobis(2,2’ - bipyridine)]diruthenium (III) ion, [(bipy)2(H2O)RuORu(H2O)(bipy)2]4+ (hereafter denoted as Ru2O4+ or [(H2O)2(bipy)4Ru2O]4+) and some primary aliphatic alcohols was studied in perchloric acid medium, [H+] = 5.0 x 10-3 mol dm-3, ionic strength (I), = 0.5 mol dm–3 (NaClO4), temperature (T) = 31 ± 1°C and wavelength ,λmax = 660 nm. The reactions, which led to the formation of corresponding aldehydes had a stoichiometry of 1:1, were all first order with respect to each reactant, and second order overall. The reactions proceeded in the absence of acid but when acid was added, the reactions ceased. Varying the ionic strength and dielectric constants of the reaction medium had no effect on the reaction. Added ions catalysed the reaction and free radicals were identified in the reaction mixtures in the course of the reactions. There was no evidence for the formation if intermediate complex in the course of the reaction. The order of reactivity is of the order CH3OH > C2H5OH > C3H7OH Based on the results, it is suggested that all the reactions proceeded through the outer – sphere electron transfer mechanism and a plausible mechanism that represented all the reactions is proposed.

Outer Sphere Electron Transfer Enabling High-Voltage Aqueous Electrolytes.

Aqueous electrolytes with a low voltage window (1.23 V) and prone side reactions, such as hydrogen evolution reaction and cathode dissolution, compromise the advantages of high safety and low cost of aqueous metal-ion batteries. Herein, introducing catechol (CAT) into the aqueous electrolyte, an outer sphere electron transfer mechanism is initiated to inhibit the water reactivity, achieving an electrochemical window of 3.24 V. In a typical Zn-ion battery, the outer sphere electrons jump from CAT to Zn2+-H2O at a geometrically favorable situation and between the solvation molecules without breaking or forming chemical bonds as that of the inner sphere electron transfers. The excited state π-π stacking further leads to the outer sphere electron transfer occurring at the electrolyte/electrode interface. This high-voltage electrolyte allows achieving an operating voltage two times higher than that of the usual aqueous electrolytes and provides almost the highest energy density and power density for the V2O5-based aqueous Zn-ion full batteries. The Zn//Zn symmetric battery delivers a 4000 h lifespan, and the Zn//V2O5 full battery achieves a ∼380 W h kg-1 energy density and a 92% capacity retention after 3000 cycles at 1 A g-1 and a 2.4 V output voltage. This outer sphere electron transfer strategy paves the way for designing high-voltage aqueous electrolytes.

On the effects of Br−, ClO4−, NO3−anions on the oxygen reduction reaction electrocatalysis at Fe phthalocyanine modified electrodes

Fe phthalocyanine with carbon nanotubes (FePc-CNT) has been used as catalysts for ORR using the following supporting electrolyte: 0.1 M NaOH, 1 M NaOH, 1 M HNO3, 1 M HClO4, and 1 M HBr. The thermodynamic and kinetic information was acquired from cyclic voltammetry in N2 saturated media, and then polarization curves saturating with O2. The rate-determining step is modulated by the formal potential (E0’Fe(III)/(II)) in basic media, coherent with the inner sphere electron transfer mechanism. However, in strong acid electrolytes, the Tafel slope value is ∼ -0.090 V dec−1 at low overpotentials and ∼ -0.120 V dec−1 at high overpotentials indicating that a poisoning process is hampering the ORR, and causing an outer sphere electron transfer mechanism. Impedance spectroscopy and poisoning test (recovery of the activity of the catalyst in NaOH after initial use in acid) support the thesis of ion adsorption/poisoning for the low electrocatalytic activity of FePc based catalysts in acid media.

Electronic structure modulation of molybdenum-iron double-atom catalyst for bifunctional oxygen electrochemistry

Exploration of cost effective and high performance electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is becoming the paramount interest towards the emerging renewable energy technologies such as fuel cells, metal air-batteries and water electrolysers, etc. One of such catalysts is single atom catalyst that exhibits well defined active sites with very high atom utilization efficiency but selective mainly towards ORR. We demonstrate one-step synthesis of carbon–nitrogen (CN)-coordinated Fe-Mo double atom catalyst with high atom utilization efficiency towards both ORR and OER. The catalyst exhibits enhanced activity with lower overpotential associated with ORR and OER (ΔE = Ej=10(OER) - E1/2(ORR)) and better mass activity in comparison to that of Mo-N-C and Fe-N-C, even better than the state-of-the-art Pt/C and RuO2 catalysts for ORR and OER, respectively. This place the double atom catalysts in the series of non-precious metal based bifunctional catalysts for overall oxygen electrochemistry and is due to the electronic structure modification around the metal atoms induced by other neighbouring metal atom. The double atom Fe-Mo-N-C exhibits a remarkable and highly durable ORR performance with suppressed peroxide generation due to the muted outer sphere electron transfer mechanism in comparison with the commercially available state-of-the-art Pt/C electrocatalyst. Moreover, Fe-Mo-N-C shows remarkable enhanced overall oxygen electrochemistry at higher temperature (323 K - a temperature 25 K above the room temperature), widening its applicability. Overall, this work paves the way to design multi-atom catalysts towards multi-functional electrocatalytic activities.

Micellar and Polymer Catalysis in the Kinetics of Oxidation of L-lysine by Permanganate Ion in Perchloric Acid Medium

ABSTRACT Kinetics of oxidation of L-lysine by permanganate ion in a perchloric acid medium was investigated to explore the order of the reaction with respect to oxidant and substrate and to study the catalytic behaviour of sodium lauryl sulphate (SLS) and polyethylene glycol (PEG). The reaction was found to be first-order with respect to the oxidant and the substrate and zero-order with respect to hydrogen ion. Changes in the sodium sulphate concentration produce a non-significant variation in the rate of the reaction. SLS and PEG were found to catalyze the reaction. Surfactant catalysis was modelled by Piszkiewicz's cooperativity model, while polymer catalysis was explained with the help of the Benesi-Hildebrand equation. The temperature dependence of the rate of the reaction was elucidated, and activation parameters were obtained. Interestingly, the reaction was found to possess positive activation entropy indicating the dissociative nature of the transition state and outer-sphere electron transfer mechanism. A mechanism of the reaction that is supported by the experimental findings was suggested. Keywords: L-lysine, permanganate ion, micellar catalysis, polymer catalysis, outer sphere electron transfer mechanism.

Inner Sphere Electron Transfer Promotion on Homogeneously Dispersed Fe-Nx Centers for Energy-Efficient Oxygen Reduction Reaction.

The study reports the optimized incorporation of pyridinic nitrogen in nitrogen-doped carbon nanotubes (CNTs) to realize effective Fe-Nx centers throughout the framework. The study unveils nitrogen as a valuable asset to promote the homogeneous dispersion of Fe moieties throughout the CNT framework, which is a necessary component to institute uniform Fe-Nx centers. In addition, pyridinic nitrogen causes disruption in strongly delocalized π-electrons, which impart electron-withdrawing nature in the carbon matrix, resulting in an anodic shift in oxygen reduction reaction (ORR) onset potential (Eonset). The direct interaction of Fe-Nx with O2, as evidenced by poisoning and computational studies, ensures the preferential inner sphere electron transfer mechanism. Despite the alkaline medium, the outer sphere electron transfer mechanism was muted, with suppressed HO2- generation, preferential 4e- reduction pathways, and excellent cyclic stability. The study indicates the dependency of ORR half-wave potential on the electron transfer mechanism. The poisoning study unveils the direct involvement of Fe-Nx electroactive centers in facilitating ORR in alkaline medium. It further indicates a noticable increase (up to ∼25%) in peroxide generation-an unwanted ORR intermediate-and concomitant reduction in average electron transfer no. per oxygen molecule.

Constructing Multi‐Stimuli‐Responsive Luminescent Materials through Outer Sphere Electron Transfer in Ion Pairs

AbstractStimuli‐responsive luminescent materials are of scientific and technological interest due to their wide range of optoelectronic applications. The utilization of photoinduced electron transfer (PET) reactions is an effective strategy to design various stimuli‐responsive luminescent materials. To date, most of these materials are based on the inner sphere electron transfer mechanism, which refers to a redox chemical reaction that proceeds via a covalent bridge. In contrast, the method of using outer sphere electron transfer mechanism is superior to the covalent approach as it does not require tedious synthesis. Ion pairs, which are composed of two oppositely charged components, are ideal for the development of stimuli‐responsive materials based on outer sphere electron transfer mechanism. Here, it is demonstrated that the emission intensity of an ion pair, consisting of an electron‐deficient phosphor ([Ir(ttpy)2]3+, ttpy = 4′‐(p‐tolyl)‐2,2′:6′,2″‐terpyridine) and an electron‐rich quencher (BH4−, tetrabutylammonium borohydride), is significantly enhanced by applying an electrical stimulus or under UV light irradiation. As far as it is known, this work represents the first example of luminescent material showing smart responses to both electrical and light stimuli. The present results indicate that the use of suitable ion pairs can pave the way toward the construction of multi‐stimuli‐responsive smart materials through outer sphere electron transfer mechanism.

Oxygen Activation Switch in the Copper Amine Oxidase of Escherichia coli.

Copper amine oxidases (CuAOs) are metalloenzymes that reduce molecular oxygen to hydrogen peroxide during catalytic turnover of primary amines. In addition to Cu2+ in the active site, two peripheral calcium sites, ∼32 Å from the active site, have roles in Escherichia coli amine oxidase (ECAO). The buried Ca2+ (Asp533, Leu534, Asp535, Asp678, and Ala679) is essential for full-length protein production, while the surface Ca2+ (Glu573, Tyr667, Asp670, and Glu672) modulates biogenesis of the 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor. The E573Q mutation at the surface site prevents calcium binding and TPQ biogenesis. However, TPQ biogenesis can be restored by a suppressor mutation (I342F) in the proposed oxygen delivery channel to the active site. While supporting TPQ biogenesis (∼60% WTECAO TPQ), I342F/E573Q has almost no amine oxidase activity (∼4.6% WTECAO activity). To understand how these long-range mutations have major effects on TPQ biogenesis and catalysis, we employed ultraviolet–visible spectroscopy, steady-state kinetics, inhibition assays, and X-ray crystallography. We show that the surface metal site controls the equilibrium (disproportionation) of the Cu2+-substrate reduced TPQ (TPQAMQ) Cu+-TPQ semiquinone (TPQSQ) couple. Removal of the calcium ion from this site by chelation or mutagenesis shifts the equilibrium to Cu2+-TPQAMQ or destabilizes Cu+-TPQSQ. Crystal structure analysis shows that TPQ biogenesis is stalled at deprotonation in the Cu2+-tyrosinate state. Our findings support WTECAO using the inner sphere electron transfer mechanism for oxygen reduction during catalysis, and while a Cu+-tyrosyl radical intermediate is not essential for TPQ biogenesis, it is required for efficient biogenesis.

Synthesis, characterization, cytotoxicity effect and DNA cleavage study of symmetric dinuclear chloro and azido bridged copper(II) complexes of napthyl-pyrazole based ligand

Symmetric dinuclear chloro copper(II) complex [Cu(L)(Cl)(µ-Cl)]2 (1) and azo dinuclear azido copper(II) complex [Cu2(L)2(N3)3(µ2-N3)]n (2) [where L represents (5-methyl-pyrazol-1-ylmethyl)-napthalen-1-ylmethyl-amine] have been synthesized to examine the effect of napthyl group in the structure of pyrazole based dinuclear copper(II) complexes in DNA nuclease activity. The structure of 1 and 2 are characterized by X-ray crystallography, electrochemistry and various spectroscopic techniques. Coordinating ligand L is generated in situ from bis(3,5-dimethyl-pyrazol-1-ylmethyl)-napthalen-1-ylmethyl-amine (A) during complexation. Cytotoxic potential of free ligand (A), synthesized complexes 1, 2 and one cobalt(II) complex derived from ligand A, CoII(A)Cl2 (3) are analyzed using MTT cytotoxicity assay in U937 human monocytic cell line. Complexes 1 and 2 show very potent cytotoxicity (IC50 = 13–17 μM); the best IC50 value is found for 1. LDH assay revealed that A and 3 has greater necrotic activity than the copper complexes. However, the results of DNA cleavage study clearly demonstrated that symmetric bridged dinuclear complexes with napthyl group lead to high level of nuclease activity 72–75% in the presence of glutathione. The bridged dinuclear copper(II) complexes undergo facile transformation to Cu(I) centre through inner sphere electron transfer mechanism (ISET) in presence of glutathione which facilitate the formation of free radicals/ions for DNA cleavage. Lacking of any reducible metal center in mononuclear cobalt(II) complex make it inactive towards free radicals generation in DNA cleavage activity.

Electron transfer mechanism of catalytic superoxide dismutation via Cu(ii/i) complexes: evidence of cupric-superoxo/-hydroperoxo species.

To understand the electron transfer mechanisms (outer versus inner sphere) of catalytic superoxide dismutation via a Cu(ii/i) redox couple such as occur in the enzyme copper-zinc superoxide dismutase, the Cu(ii/i) complexes [(L1)2Cu](ClO4)2·CH3CN, (1·CH3CN) and [(L1)2Cu](ClO4), (2) supported by a bis-N2Sthioether ligand, 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine (L1) have been synthesized and structurally characterised. Both 1 and 2 display the same cyclic voltammogram (CV) featuring a quasireversible response at E1/2 = +0.33 V vs. SCE that falls in the SOD potential window of -0.04 V to +0.99 V. These complexes catalytically dismutate superoxide radicals at 298 K in aqueous medium (the IC50 for 1 is 2.15 μM). Electronic absorption spectra (233 K and 298 K), FTIR, ESI mass spectra, CV (233 K and 298 K) and DFT calculations collectively indicate formation of [(L1)2Cu(O2˙(-))](+), [(L1)2Cu(O2(2-))] and [(L1)2Cu(OOH(-))](+) species and help to elucidate the electron transfer mechanism for the SOD function of 1 and 2. Once O2˙(-) binds to Cu(II) (evident at 233 K), the first step of the catalytic cycle (Cu(II) + O2˙(-)→ Cu(I) + O2) does not follow but the second step (Cu(I) + O2˙(-) + 2H(+)→ H2O2 + Cu(II)) does follow. Therefore, the catalytic disproportionation of superoxide radicals via1 and 2 at 298 K indicates that the first and second steps of the catalytic cycle proceed through outer and inner sphere electron transfer mechanisms, respectively. Feasibility of the first step to occur in pure aprotic solvent (where 18-crown-6-ether is used to solubilise KO2) was tested and also supports the same notion of the electron transfer mechanisms as stated above.

Electron Transfer Reaction of Ion Pairs: 1. Surfactant Cobalt(III) Complexes by Fe(CN)6 4− in Microheterogeneous Media

Abstract The kinetic study of reduction of single chain surfactant octahedral cobalt(III) complexes, cis-[Co(en)2(4AMP)(DA)](ClO4)3 and cis-[Co(trien)(4CNP)(DA)](ClO4)3 (where en = ethylenediamine; trien = triethylenetetramine; 4AMP = 4-aminopyridine; 4CNP = 4-cyanopyridine; DA = dodecylamine) with [Fe(CN)6]4– has been carried out by spectrophotometric method under pseudo first order conditions using an excess of a reductant in different media namely, micelles, β-cyclodextrim (β-CD), liposome vesicles (DPPC) and ionic liquids ((BMIM)Br) at different temperatures in order to have deep insight into the molecular behavior during the electron transfer reaction. The analysis of kinetic results strongly suggests that the reduction reaction between the surfactant complex and [Fe(CN)6]4– occurs via a second order outer sphere electron transfer mechanism. The remarkable increase in the rate of electron transfer from the oxidant to reductant is observed in ionic liquid medium as compared to the rest of all other media studied in the present investigation. This can be ascribed due to the fact that the formation of aggregated and constrained geometry of the surfactant cobalt(III) complexes (oxidants) in ionic liquid medium renders the rate of one electron transfer from the oxidants to the reductant, [Fe(CN)6]4– to be higher. Furthermore, the rate constant value increases with increase in the concentration of micelles, however in the presence of β-cyclodextrin medium the rate of electron transfer substantially decreases due to the inclusion of long aliphatic chain of the surfactant cobalt(III) complexes into β-cyclodextrin. In liposome media, second order rate constant for this electron transfer reaction from both the oxidants was found to decrease with increase in the concentration of the liposome below the phase transfer temperature of DPPC. However, above the phase transition temperature the reaction was found to increase with increase in the concentration of the DPPC. The main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction between the surfactant complexes and vesicular surface. The activation parameters (ΔS‡ and ΔH‡) of this outer sphere electron transfer reaction have been calculated by varying temperatures and the analysis of the data corroborates the kinetics of the reaction.

Electrocatalytic Pathways for Oxygen Reduction and Evolution in Aqueous and Non Aqueous Electrolytes

Oxygen reduction (ORR) and evolution reaction (OER) at an electrochemical interface has important connotations with several key technologies ranging from the use of oxygen depolarized cathodes in chlorine generation to fuel cells, electrolyzers, water treatment, hybrid batteries (including Li/Air) and even life support. In all the decades of research conducted in a wide range of pH (aqueous) and non aqueous electrolytes some very interesting threads have emerged pointing to revolutionary materials development. For example, recent advances in our understanding of alternative active sites for oxygen reduction has provided for the basis of its molecular design. This presentation will put these developments in the context of more than two decades of effort devoted to engendering such non noble metal electrocatalysts. In this presentation we will present our latest data on the most active analogs which comprise of a FeNx coordinated active site in close concert with Fe nano-particles either present in some polymer composite or more ideally as edge defects close to the Fe-Nx coordinated structure. We will show how these active sites evolve on carbon supports as graphene defect structures. Such active site determinations are made with the use of a special in situ synchrotron x-ray absorption method using the near edge spectra referred to as x-ray absorption near edge structure (XANES), in a subtractive mode wherein the signal contribution from the bulk is successfully subtracted from the effect of the surface adsorbed species. When combined with our ability to simulate the same signatures using models with specifically adsorbed moieties, a powerful tool emerges to study electrochemical interfaces under actual in situ and operando conditions. This technique, commonly called the ‘Delta Mu (Dμ) Technique’ has been applied to a wide variety of transition metal surfaces and alloys[i],[ii] including non-Pt based metal electrocatalysts with element specificity. EXAFS data taken concurrently provide information on the changes in short range atomic order around the absorber atom thereby providing structural information such as bond distances and coordination numbers (thereby information on average cluster size, homogeneity and surface segregation etc.). In this presentation among other things we will provide a picture of the electrocatalytic pathways in aqueous (at both the extreme edges of the pH scale) and non-aqueous environments. In the latter, our prior efforts to understand the effect of electrolyte both in terms of solvent[iii] and choice of salt will be presented in terms of engendering inner our outer sphere charge transfer[iv] during ORR and OER reactions. The technological consequence of such materials in power generation, electrolysis and energy storage will be described. Acknowledgement. The authors deeply appreciate the financial assistance from the Army Research Office (Single Investigator award, W911NF-09-1) and the Department of Energy (EERE, DE-EE-000459). Authors also express their thanks to the Department of Energy, Materials Science Division for building and maintaining the National Synchrotron Light Source, in Brookhaven National Laboratory, Upton, NY.[i] Contrast to Metal Ligand Effects on PtnM Electrocatalysts with M equal to Ru vs. Mo and Sn as Exhibited by in situ XANES and EXAFS Measurements in Methanol’, F. C. Scott, S. Mukerjee, and D. E. Ramaker, J. Phys. Chem. C., 114, 442 (2010). [ii] ‘Electrochemical Kinetics and X-ray Absorption Spectroscopic Investigation of Oxygen Reduction on Chalcogen-Modified Ru Catalysts in Alkaline Medium’, N. Ramaswamy, R. J. Allen and S. Mukerjee, J. Phys. Chem. C., 115, 12650 (2011).[iii] ‘Oxygen Reduction Reactions in Ionic Liquids and the Formulation of General ORR Mechanism for Li-Air Batteries,’ C. J. Allen, H. Hwang, R. Kautz, S. Mukerjee, E. J. Plichta, M. A. Hendrickson and K. M. Abraham, J. Phys. Chem. C., 116, 20755 (2012). [iv] Influence of Inner and Outer Sphere Electron Transfer Mechanisms during Electrocatalysis of Oxygen in Alkaline Medium’, N. Ramaswamy and S. Mukerjee, J. Phys. Chem. C., 115, 18015 (2011).

Manifestation of Polar Reaction Pathways of 2,3-Dichloro-5,6-dicyano-p-benzoquinone

Reactions of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) with silyl enol ethers, silyl ketene acetals, allylsilanes, enamino esters, and diazomethanes have been studied in CH3CN and CH2Cl2 solutions. The second-order rate constants for C attack at DDQ (log k(C)) correlate linearly with the nucleophile-specific parameters N and s(N) and are 2-5 orders of magnitude larger than expected for SET processes, which strongly supports the polar mechanism for C-C bond formation. The second-order rate constants for O attack agree well with the calculated rate constants for rate-determining single electron transfer (SET). As a radical clock experiment ruled out outer sphere electron transfer, an inner sphere electron transfer mechanism is suggested for O attack.

Engineering and Tuning of Oxygen Reactivity in Heme Protein Maquettes

There is a dearth of quantitative data related to reactive oxygen species (ROS) production, particularly superoxide, from protein systems. Understanding of ROS chemistry in proteins is necessary for avoiding damaging oxidative reactions involved in biological processes including aging and ischemic injury. These reactions are also necessary to control in order to develop robust artificial enzymes and photosystems for alternative energy production.Here we show multi-faceted control over the oxygen reactivity of ferrous heme containing designed protein maquettes. We demonstrate control over superoxide production through both inner and outer sphere electron transfer (ET) mechanisms. Inner sphere ET occurs through oxygen binding and both oxygen on and off rates effect detectible superoxide production rates and total yields. On rates are controlled via helical strain and core packing while off rates are controlled via water accessibility to the heme. The faster the on rate, the less superoxide produced due to more complete formation of the oxyferrous state. Stable oxyferrous states with half times on the order of seconds produce superoxide below the level of detection and biological relevancy. When the oxyferrous state cannot be formed due to lack of helical strain, outer sphere ET occurs at rapid rates on the order of NADPH oxidases. Faster rates of ET are observed for more solvent-exposed hemes. In all cases when the superoxide production rate is faster, we observe lower total yields. This effect is due to increased dismutation or peroxide when there are higher local concentrations of superoxide. The engineering principles we learned for superoxide control could be used to understand natural ROS processes as well as develop durable artificial ET protein systems.

Electron Transfer Properties of Alkoxyl Radicals. A Time-Resolved Kinetic Study of the Reactions of thetert-Butoxyl, Cumyloxyl, and Benzyloxyl Radicals with Alkyl Ferrocenes

A time-resolved kinetic study on the reactions of the tert-butoxyl (t-BuO*), cumyloxyl (CumO*), and benzyloxyl (BnO*) radicals with alkylferrocenes has been carried out in MeCN solution. With all radicals, clear evidence for an electron transfer (ET) process has been obtained, and with the same ferrocene donor, the reactivity has been observed to increase in the order t-BuO* < CumO* < BnO*, with the difference in reactivity approaching 3 orders of magnitude on going from t-BuO* to BnO*. With BnO*, an excellent fit to the Marcus equation has been obtained, from which a value of the reduction potential of BnO* (E degrees(BnO*/BnO(-)) = 0.54 V/SCE) has been derived. The latter value appears, however, to be significantly higher than the previously determined reduction potential values for alkoxyl radicals and in contrast with the differences in the computed solution-phase electron affinities determined for t-BuO*, CumO*, and BnO*, indicating that the reaction of BnO* with ferrocene donors may not be described in terms of a straightforward outer sphere ET mechanism. From these data, and taking into account the available value of the reduction potential for CumO*, a value of E degrees (BnO*/BnO(-)) = -0.10 V/SCE has been estimated. On the basis of computational evidence for the formation of a pi-stacked prereaction complex in the reaction between BnO* and DcMFc, an alternative ET mechanism is proposed for the reactions of both CumO* and BnO*. In these cases, the delocalized nature of the unpaired electron allows for the aromatic ring to act as an electron relay by mediating the ET from the ferrocene donor to the formal oxygen radical center. This hypothesis is also in line with the observation that both BnO* and CumO* react with the ferrocene donors with rate constants that are in all cases at least 2 orders of magnitude higher than those measured for t-BuO*, wherein the radical is well-localized.

Activation-controlled outer-sphere electron transfer reactions: reduction of heteropoly 11-tungstovanadophosphate and heteropoly 10-tungstodivanado phosphate by thiourea in aqueous acid medium

The kinetics of reduction of heteropoly 11-tungstovanadophosphate, [PVVW11O40]4−, (HPA1) and heteropoly 10-tungstodivanadophosphate, [PVVVVW10O40]5−, (HPA2) by thiourea has been investigated in HClO4/phthalate/acetate buffer solutions spectrophotometrically at 25 °C in aqueous medium. The stoichiometry of the reaction is 1:1 in both cases. The HPAs are converted into the corresponding one-electron reduced heteropoly blues, namely, [PVIVW11O40]5− and [PVIVVVW10O40]6−, and thiourea is oxidised to formamidine disulphide. The reaction shows first-order dependence in both [HPA] and [thiourea] at constant pH. The rate–pH profile shows the participation of both the neutral and deprotonated forms of thiourea in the reaction. The reaction proceeds through an outer sphere electron transfer mechanism in which activation-controlled electron transfer is the rate-determining step. Self-exchange rate constants for the couples [PVVW11O40]4−/[PVIVW11O40]5−, [PVVVVW10O40]5−/[PVIVVVW10O40]6− and H2NCSNH2/H2NCS·+NH2 have been evaluated by Marcus theory.

Water exchange rates of water-soluble manganese(iii) porphyrins of therapeutical potential

The activation parameters and the rate constants of the water-exchange reactions of Mn(III)TE-2-PyP(5+) (meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin) as cationic, Mn(III)TnHex-2-PyP(5+) (meso-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin) as sterically shielded cationic, and Mn(III)TSPP(3-) (meso-tetrakis(4-sulfonatophenyl)porphyrin) as anionic manganese(iii) porphyrins were determined from the temperature dependence of (17)O NMR relaxation rates. The rate constants at 298 K were obtained as 4.12 x 10(6) s(-1), 5.73 x 10(6) s(-1), and 2.74 x 10(7) s(-1), respectively. On the basis of the determined entropies of activation, an interchange-dissociative mechanism (I(d)) was proposed for the cationic complexes (DeltaS(double dagger) = approximately 0 J mol(-1) K(-1)) whereas a limiting dissociative mechanism (D) was proposed for Mn(III)TSPP(3-) complex (DeltaS(double dagger) = +79 J mol(-1) K(-1)). The obtained water exchange rate of Mn(III)TSPP(3-) corresponded well to the previously assumed value used by Koenig et al. (S. H. Koenig, R. D. Brown and M. Spiller, Magn. Reson. Med., 1987, 4, 52-260) to simulate the (1)H NMRD curves, therefore the measured value supports the theory developed for explaining the anomalous relaxivity of Mn(III)TSPP(3-) complex. A magnitude of the obtained water-exchange rate constants further confirms the suggested inner sphere electron transfer mechanism for the reactions of the two positively charged Mn(iii) porphyrins with the various biologically important oxygen and nitrogen reactive species. Due to the high biological and clinical relevance of the reactions that occur at the metal site of the studied Mn(iii) porphyrins, the determination of water exchange rates advanced our insight into their efficacy and mechanism of action, and in turn should impact their further development for both diagnostic (imaging) and therapeutic purposes.

Kinetics of N-substituted phenothiazines and N-substituted phenoxazines oxidation catalyzed by fungal laccases

AbstractLaccase-catalyzed oxidation of N-substituted phenothiazines and N-substituted phenoxazines was investigated at pH 5.5 and 25°C. The recombinant laccase from Polyporus pinsitus (rPpL) and the laccase from Myceliophthora thermophila (rMtL) were used. The dependence of initial reaction rate on substrate concentration was analyzed by applying the laccase action scheme in which the laccase native intermediate (NI) reacts with a substrate forming reduced enzyme. The reduced laccase produces peroxide intermediate (PI) which in turn decays to the NI. The calculated constant (kox) values of the PI formation are (6.1±3.1)×105 M−1s−1 for rPpL and (2.5±0.9)×104 M−1s−1 for rMtL. The bimolecular constants of the reaction of the native intermediate with electron donor (kred) vary in the interval from 2.2×105 to 2.1×107 M−1s−1 for rPpL and from 1.3×102 to 1.8×105 M-1s−1 for rMtL. The larger reactivity of rPpL in comparison to rMtL is associated with the higher redox potential of type I Cu of rPpL. The variation of kred values for both laccases correlates with the change of the redox potential of substrates. Following outer sphere (Marcus) electron transfer mechanism the calculated activationless electron transfer rate and the apparent reorganization energy are 5.0×107 M−1s−1 and 0.29 eV, respectively.

Oxidation of the thiosulfate ion by some iron(III) complexes with phenolate - amide-amine coordination: A comparative kinetic study

The redox reactions of thiosulfate with four iron(III) complexes having phenolate-amide-amine coordination, FeIII(L){L = 1,2-bis(2-hydroxybenzamido)ethane, L1; 1,3-bis(2-hydroxybenzamido)propane, L2; 1,5-bis(2-hydroxybenzamido)3-azapentane, L3; and 1,8-bis(2-hydroxybenzamido)3,6-diazaoctane, L4} have been investigated in 10% v/v MeOH + H2O and I = 0.3 mol dm−3. At constant pH (~ 4.8) and under pseudo-first order conditions of [S2O32−] the reaction obeyed the rate law : − d[FeIII(L)]/dt = kobs [FeIII(L)] + kobs′ where kobs′ denotes the observed rate constant of thiosulfate decomposition; kobs = a[S2O32−] + b[S2O32−]T2 is valid for all the complexes, particularly at pH < 6, while kobs′ = [H+][S2O32−]T2 is consistent with the rate law for thiosulfate decomposition proposed earlier. The rate data (kobs) were analysed on the basis of the reactivities of various species of FeIII(L) generated by the equilibrium protonation of the sec-NH of dien and trien spacer units resulting in the ring opening (for [FeIII(L3/L4)]), and acid base equilibrium of the aqua ligand bound to the iron(III) centre ([FeIII(L)(OH2)n]). The redox activities, both for second and third order paths, show the ligand dependencies : L4<L3<L1<L2 conforming to the fact that the complexes tend to be less susceptible to electron transfer from S2O32− with (i) the increase of the number of chelate rings, (ii) the decrease of overall charge, and (iii) the decrease of ring size offered by the amine moiety (from six membered to five membered one as for [FeIII(L1/L2)(OH2)2]+. There was no evidence for the formation of inner sphere thiosulfato complex, the possibility of the formation of the outer sphere ion-pairs, [Fe(L/HL)(OH2)n+/2+, S2O32−] with low equilibrium constant value may not be excluded. In view of this, the outer sphere electron transfer (ET) mechanism is the most likely possibility.

N-compound reduction and actinide immobilisation in surficial fluids by Fe(II): the surface Fe IIIOFe IIOH° species, as major reductant

Soluble Fe(II) is an important reductant in anoxic surficial fluids, due to fast redox kinetics of the Fe(II)/Fe(III) couple. However, its availability is limited in neutral and alkaline pH range by the solubility of FeS(s), Fe 3(PO 4) 2, FeCO 3(s) and other Fe(II) rich minerals. The adsorption of Fe(II) on a variety of mineral phases has been studied. It is shown that, provided enough surface area is available, adsorption is completed before the onset of precipitation, leading to Fe(II) surface species which are able to reduce compounds present in solution in a very efficient way. The abiotic reduction of a variety of N rich compounds (nitrites and nitrobenzenes) by sorbed Fe(II) has been reported in the literature. The observed initial rate of such reduction reactions is shown to be proportional to the Fe IIIOFe IIOH° species concentration, in the same manner that the homogeneous oxygenation rate of Fe(II) is proportional to Fe(OH) 2° (aq) concentration. The electron transfer in these reactions, appears to occur dominantly via an outer sphere mechanism. In contrast, the abiotic reduction of inorganics, such as U(VI) and Tc(VII), by sorbed Fe(II) involves inner sphere electron transfer mechanism. In the case of uranium reduction, three kinetic steps can be distinguished: the adsorption of the uranyl ion (formation of an inner sphere surface complex), followed by two reductive steps which lead to the formation of a UO 2/Fe(OH) 3 mixed solid phase. These surface-catalysed reduction reactions may have led to the formation of uranium mineral ores and to the removal of uranium from reducing surface waters.