One-electron Transfer Steps Research Articles

The Abiotic Nitrite Oxidation by Ligand-Bound Manganese (III): The Chemical Mechanism

Given their environmental abundances, it has been long hypothesized that geochemical interactions between reactive forms of manganese and nitrogen may play important roles in the cycling of these elements. Indeed, recent studies have begun shedding light on the possible role of soluble, ligand-bound Mn(III) in promoting abiotic transformations under environmentally relevant conditions. Here, using the kinetic data of Karolewski et al. (Geochim Cosmochim Acta 293:365–378, 2021), we provide the chemical mechanism for the abiotic oxidation of nitrite (NO2−) by Mn(III)-pyrophosphate, MnIIIPP, to form nitrate (NO3−). Nitrous acid (HNO2), not NO2−, is the reductant in the reaction, based on thermodynamic and kinetic considerations. As soluble Mn(III) complexes react in a one-electron transfer reaction, two one-electron transfer steps must occur. In step one, HNO2 is first oxidized to nitrogen dioxide, ·NO2, a free radical via a hydrogen atom transfer (HAT) reaction. We show that this inner sphere reaction process is the rate-limiting step in the reaction sequence. In step two, ·NO2 reacts with a second MnIIIPP complex to form the nitronium ion (NO2+), which is isoelectronic with CO2. Unlike the poor electron-accepting capability of CO2, NO2+ is an excellent electron acceptor for both OH− and H2O, so NO2+ reacts quickly with water to form the end-product NO3− (step 3 in the reaction sequence). Thus, water provides the O atom in this nitrification reaction in accordance with the O-isotope data. This work provides mechanistic perspective on a potentially important interaction between Mn and nitrogen species, thereby offering a framework in which to interpret kinetic and isotopic data and to further investigate the relevance of this reaction under environmental conditions.

Treatment of ethylmercury chloride by heterogeneous photocatalysis with TiO2

Ethylmercury chloride (C2H5HgCl) was treated by UV/TiO2 photocatalysis in the presence of O2 and under N2 at pH 4.2. No report exists on C2H5Hg+ degradation by heterogeneous photocatalysis. The adsorption of C2H5Hg+ over TiO2 (no irradiation) was studied and fitted to the Freundlich isotherm. The photocatalytic evolution of C2H5Hg+ was adjusted to a two parameter Langmuir-Hinshelwood model, modified to include a third parameter attributed to the deactivation caused by the deposition of Hg(0). Hg(II) in solution, Hg(0) and Hg2Cl2 (detected only under N2) were the products of the photocatalytic degradation; the organic moiety was degraded but no organic by-product could be detected. Experiments in the absence of O2 showed a higher conversion rate, indicating that C2H5Hg+ is removed both by oxidative and reductive pathways, being this last step partially inhibited by O2. A degradation mechanism considering both oxidative and reductive one-electron transfer steps is proposed.

Watching Intermolecular Light-Induced Charge Accumulation on Naphthalene Diimide by Tris(bipyridyl)ruthenium(II) Photosensitizer

Performing photocatalytic reactions to produce solar fuels requires the coupling of multiple photoinduced one-electron transfer steps on multielectronic catalysis. Understanding the light-driven ch...

Synthesis, Spectroscopy and Electrochemistry in Relation to DFT Computed Energies of Ferrocene- and Ruthenocene-Containing -Diketonato Iridium(III) Heteroleptic Complexes. Structure of [(2-Pyridylphenyl)2Ir(RcCOCHCOCH3

A series of new ferrocene- and ruthenocene-containing iridium(III) heteroleptic complexes of the type [(ppy)2Ir(RCOCHCOR′)], with ppy = 2-pyridylphenyl, R = Fc = FeII(η5-C5H4)(η5-C5H5) and R′ = CH3 (1) or Fc (2), as well as R = Rc = RuII(η5-C5H4)(η5-C5H5) and R′ = CH3 (3), Rc (4) or Fc (5) was synthesized via the reaction of appropriate metallocene-containing β-diketonato ligands with [(ppy)2(μ-Cl)Ir]2. The single crystal structure of 3 (monoclinic, P21/n, Z = 4) is described. Complexes 1–5 absorb light strongly in the region 280−480 nm the metallocenyl β-diketonato substituents quench phosphorescence in 1–5. Cyclic and square wave voltammetric studies in CH2Cl2/[N(nBu)4][B(C6F5)4] allowed observation of a reversible IrIII/IV redox couple as well as well-resolved ferrocenyl (Fc) and ruthenocenyl (Rc) one-electron transfer steps in 1−5. The sequence of redox events is in the order Fc oxidation, then IrIII oxidation and finally ruthenocene oxidation, all in one-electron transfer steps. Generation of IrIV quenched phosphorescence in 6, [(ppy)2Ir(H3CCOCHCOCH3)]. This study made it possible to predict the IrIII/IV formal reduction potential from Gordy scale group electronegativities, χR and/or ΣχR′ of β-diketonato pendent side groups as well as from DFT-calculated energies of the highest occupied molecular orbital of the species involved in the IrIII/IV oxidation at a 98% accuracy level.

Effect of Manganese Oxide Aging and Structure Transformation on the Kinetics of Thiol Oxidation.

The kinetics and mechanism of thiol oxidation by Mn oxides undergoing dynamic structural transformation under environmentally relevant conditions remain poorly understood. In this study, thiol/disulfide pair concentrations were simultaneously determined in situ using voltammetric microelectrodes during the interaction of four common thiols (cysteine, homocysteine, cysteamine, and glutathione) with fresh and aged δ-MnO2 at pH 7.0. The reaction kinetics was first order with respect to thiol and zero order with respect to Mn oxides. A transient intermediate sulfur surface species observed during the reaction provides evidence for a mechanism involving two successive one-electron transfer steps. The reaction kinetics of fresh and aged δ-MnO2 was investigated with cysteine and compared to that of manganite, a Mn(III) oxyhydroxide phase. The reactivity of aged δ-MnO2 decreased as a result of structural transformation to cryptomelane but remained higher than that of manganite, suggesting the potential roles of transient Mn(III) surface intermediate in promoting the reduction of Mn(IV) in δ-MnO2 and cryptomelane (compared to Mn(III) in manganite). This study demonstrates the importance of correlating Mn oxide mineral structure and redox reactivity and extends the potential for thiols commonly found in sedimentary environments to be utilized as electron shuttles during dissimilatory Mn reduction.

Electrochemistry of Porphyrazines with Exocyclic Pended Pyridyl and N-Methylpyridyl Rings

Two new series of porphyrazines with exocyclic pended 2-pyridyl and 2-N-methylpyridyl rings were investigated as to their electrochemical properties. The structures of the investigated compounds are shown in Chart 1a and represented in one case as [Py8PzM], where M = 2H, MgII(H2O), CoII, CuII or ZnII and in the other as [(2-Mepy)8PzM]8+, where M = MgII(H2O) or ZnII. The redox behavior of these new compounds is then compared to that of earlier synthesized porphyrazines with p-extended systems which are represented as [Py8PyzPzM] (Chart 1b), and [Py8QxPzM] where Qx = quinoxaline. Each porphyrazine undergoes multiple one-electron reductions in DMSO, benzonitrile, pyridine or CH2Cl2 while the two octacationic porphyrazines, [(2-Mepy)8PzM]8+, also exhibit 2 or 3 one electron reductions in water containing 0.1 M NaOAc or 0.1 M KCl as supporting electrolyte. The potentials for each redox reaction were measured by cyclic voltammetry and thin-layer UV-visible spectroelectrochemistry was used to characterize some of the redox reactions. Special emphasis was placed on the cobalt(II) derivative [Py8PzCoII]+, which is oxdized in a single step to give [Py8PzCoIII] or reduced in up to five one-electron transfer steps, with the exact potentials and site of electron transfer depending upon the solvent. Pyridine was shown to strongly coordinate to the metal center of [Py8PzCoII] and [Py8PzCoIII] and binding constants were calculated using standard equations. Finally, comparisons are made between redox behavior of the three types of compounds in Chart 1, [Py8QxPzM] and the well-characterized phthalocyanines, [PcM], having the same central metal ions. Chart 1. Structures for (a) newly investigated porphyrazines and (b) related porphyrazines with p-extended structures.(In the figure : 1) change formula: [(2-Mepy)8PzM]8+ instead of [(CH3-Py)8PzM] ; 2) change the order of metal centers and put the oxidation state: M = 2H, MgII(H2O), CoII, CuII, ZnII instead of M = Mg(H2O), Cu, Zn, 2H, Co ; 3) put M = MgII(H2O), ZnII instead of M = Mg(H2O), Zn) Figure 1

(Invited) Understanding the Chemistry of Photocatalytic Processes

Any enhancement of the efficiency of the utilization of photogenerated charge carriers on the surface of photocatalysts requires an enhanced understanding of the entire photocatalytic process. The reduction potentials of most substrates, as well as those of the intermediates formed during the photocatalytic reaction(s), are well known; nevertheless, it is essential to realize that thermodynamic properties may change upon the adsorption of these molecules at the photocatalyst surface. Therefore, a detailed understanding of the processes occurring on the photocatalyst surface before, during, and after light absorption is of utmost importance. On the other hand, the charge carriers generated upon light absorption that survive recombination and reach the semiconductor surface may suffer surface recombination processes or recombination via an electron shuttle mechanism (Z scheme deactivation mechanism), thus reducing the total efficiency of the photocatalytic system. Moreover, since the overall efficiency of a photocatalytic process will be determined by the efficiency of the slowest reaction step it is crucial to know whether this is the reductive or oxidative half-reactions. Besides the obvious one-electron transfer steps these reactions entail in particular multi-electron transfer processes, e.g. , the four-electron oxidation of water or the eight-electron reduction of CO2. Hence, this lecture provides some necessary tools for the understanding and the development of the photocatalytic reaction mechanism.

Redox chemistry of coenzyme Q—a short overview of the voltammetric features

Quinones constitute a big family of organic redox active compounds that are overwhelmingly involved in important physiological processes. The most important members in the class of quinones are, indeed, the plastoquinones and the coenzyme Q (CoQ) derivatives. Voltammetry of coenzyme Q family members attracts significant attention since 50 years ago. In this work, we refer to some of the most important voltammetric features of coenzyme Qs studied in aprotic and in aqueous media. While the redox chemistry of coenzyme Q members in non-aqueous aprotic organic solvents can be described by two consecutive one-electron transfer steps, more complex situation exists in the voltammetry of coenzyme Qs performed in aqueous media. Although it has been claimed for a while that the voltammetric processes of coenzyme Qs in aqueous solutions proceed via formation of semiquinone radical intermediate species, it has been recently proven that this can be not completely true. Intensive voltammetric and spectroscopic studies of coenzyme Q systems in buffered and non-buffered aqueous media revealed that hydrogen bonding between electrochemically created CoQ species and the water molecules plays an important role in stabilizing electrochemically generated species of these systems. We also pay attention to the amazing redox chemistry of coenzyme Qs in strong alkaline media, while we refer to the chemical features of novel coenzyme Q derivatives obtained under such conditions. Hints are presented about the antioxidant capacity of some of the novel hydroxylated coenzyme Q systems. Also, the possibility of these systems to bind and transfer earth-alkaline cations across biomimetic membranes is shortly elaborated. In the end, we refer to some relevant theoretical works that describe closely the voltammetric behavior of various coenzyme Q systems. We believe that this short review will contribute towards better understanding of the amazing chemistry of coenzyme Q derivatives.

Europium triple-decker complexes containing phthalocyanine and nitrophenyl-corrole macrocycles.

A series of europium triple-decker complexes containing phthalocyanine and nitrophenyl-corrole macrocycles were synthesized and characterized by spectroscopic and electrochemical methods in nonaqueous media. The examined compounds are represented as Eu2[Pc(OC4H9)8]2[Cor(Ph)n(NO2Ph)3-n], where n varies from 0 to 3, Pc(OC4H9)8 represents the phthalocyanine macrocycle, and Cor indicates the corrole macrocycle having phenyl (Ph) or nitrophenyl (NO2Ph) meso substituents. Three different methods were used for syntheses of the target complexes, two of which are reported here for the first time. Each examined compound undergoes five reversible one-electron oxidations and 3-5 one-electron reductions depending upon the number of NO2Ph substituents. The nitrophenyl groups on the meso positions of the corrole are highly electron-withdrawing, and this leads to a substantial positive shift in potential for the five oxidations and first reduction in CH2Cl2, PhCN, or pyridine as compared to the parent triple-decker compound with a triphenylcorrole macrocycle. The measured E1/2 values are linearly related to the number of NO2Ph groups on the corrole, and the relative magnitude of the shift in potential for each redox reaction was used in conjunction with the results from thin-layer spectro-electrochemistry to assign the initial site of oxidation or reduction on the molecule. The nitrophenyl substituents are also redox-active, and each is reduced to [C6H4NO2](-) in a separate one-electron transfer step at potentials between -1.12 and -1.42 V versus saturated calomel electrode.

Electrochemical Behavior of Cadmium in 1-Butyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide Room-temperature Ionic Liquid

Electrodeposition of cadmium (Cd) was investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA) containing Cd(TFSA)2 as a Cd source with cyclic voltammetry and chronoamperometry on a glassy carbon electrode. The infrared spectroscopy showed Cd(II) in BMPTFSA is octahedrally coordinated with three TFSA–. Electrochemical reduction of Cd(II) to Cd(0) was suggested to be an electrochemically irreversible process involving 2 one-electron transfer steps. The diffusion coefficient of Cd(II) at 25°C was estimated to be 0.95×10−7cm2s−1. Electrodeposits obtained by galvanostatic and potentiostatic electrolysis were confirmed to be metallic Cd by X-ray diffraction and energy dispersive X-ray analysis. The nucleation process and the surface morphology of the electrodeposited Cd were found to depend on the applied potential, probably due to the change in the electric double layer structure by accumulation of BMP+ at the negatively polarized electrode surface. Furthermore, Cd nanoparticles were obtained in the ionic liquid by potentiostatic cathodic reduction at −2.6V vs. Ag/Ag(I).

Nitric oxide reduction to ammonia by TiO₂ electrons in colloid solution via consecutive one-electron transfer steps.

The reaction mechanism of nitric oxide (NO) reduction by excess electrons on TiO2 nanoparticles (e(TiO2)(-)) has been studied under anaerobic conditions. TiO2 was loaded with 10-130 electrons per particle using γ-irradiation of acidic TiO2 colloid solutions containing 2-propanol. The study is based on time-resolved kinetics and reactants and products analysis. The reduction of NO by e(TiO2)(-) is interpreted in terms of competition between a reaction path leading to formation of NH3 and a path leading to N2O and N2. The proposed mechanism involves consecutive one-electron transfers of NO, and its reduction intermediates HNO, NH2O(•), and NH2OH. The results show that e(TiO2)(-) does not reduce N2O and N2. Second-order rate constants of e(TiO2)(-) reactions with NO (740 ± 30 M(-1) s(-1)) and NH2OH (270 ± 30 M(-1) s(-1)) have been determined employing the rapid-mixing stopped-flow technique and that with HNO (>1.3 × 10(6) M(-1) s(-1)) was derived from fitting the kinetic traces to the suggested reaction mechanism, which is discussed in detail.

Electrochemistry and spectroelectrochemistry of β-pyrazino-fused tetraarylporphyrins in nonaqueous media

Bis-porphyrins containing a β,β′-fused pyrazino (Pz) linking group were examined by electrochemistry and thin-layer UV-visible spectroelectrochemistry in PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte. The investigated compounds are represented as M(TPP)-Pz-(TPP)M, where TPP is the dianion of tetraphenylporphyrin and M = Zn(II) , Cu(II) or Ag(II) . The effect of the linking Pz group on the redox potentials and UV-visible spectra of the neutral, electroreduced and electrooxidized bis-porphyrins is discussed and the data compared to what is observed for related monoporphyrins and earlier characterized bis-porphyrins containing a tetraazaanthracene (TA) linking group and the same central metal ions. The Cu(II) and Zn(II) Pz linked bis-porphyrins exhibit a relatively strong interaction between the two equivalent porphyrin macrocycles as evidenced by UV-visible spectra of the neutral compounds and characteristic splitting of redox processes for three of the four electron transfer reactions, the one exception being the first oxidation, where no splitting of potentials is observed in the formation of the bisporphyrin bis-cation radical, [M(TPP)-Pz-(TPP)M]2+. The first oxidation and first reduction of the bis-porphyrin with two Ag(II) central metal ions occurs via two overlapping one-electron transfer steps in each process indicating equivalent, but non-interacting redox centers. This difference in redox behavior is due to differences in the site of electron transfer, metal centered for the Ag(II) bisporphyrins, which undergo Ag(II) / Ag(III) and Ag(II) / Ag(I) processes as compared to only ring centered electron transfers for the Cu(II) and Zn(II) derivatives. The Pz-linked Zn(II) bis-porphyrins have a larger average HOMO–LUMO gap (2.01 V) as compared to related tetraazaanthracene linked bis-porphyrins (1.67 V), which were earlier characterized in the literature, but the gap is smaller than that for the mono-porphyrins with the same central metal ions (2.17 V). Each redox reaction of the investigated bisporphyrins was characterized by thin-layer spectroelectrochemistry.

Linear Free Energy Relationship Analysis of Chlorinated Hydrocarbons in Cement Slurries

The present study is to study linear free energy relationships (LFERs) for dechlorination ratesin cement/Fe(II) slurries of eight chlorinated hydrocarbons including carbon tetrachloride, chloroform produced from carbon tetrachloride, 1,1,1-trichloroethane, trichloroethylene produced from 1,1,2,2-tetrachloroethane, tetrachloroethylene, trichloroethylene, 1,1-dichloroethylene, and vinyl chloride, with six descriptor variables in pursuit of finding a reasonable descriptor showing the highest coefficient of determination (R2). Seventeen descriptors by various computational methods were screened into six descriptors. In addition, this study correlated logarithms of rate constants (log k) with multiple descriptors. The results have shown that the bond dissociation energy, overall free energy change of the one-electron transfer step, and lowest unoccupied molecular orbital energies calculated by ab initio methods with 6-31G* parameters were strongly correlated with the log k values of eight compounds by Fe(II) in cement slurries. The combinations of descriptors that had the highest correlation coefficient in multiple regression were one-electron reduction potential and gasphase homolytic-dissociation energy (E1 and DR-X), which two variables are commonly based on that a dissociative one-electron transfer is the initial and rate-limiting step in the reaction.

Intramolecular interactions in a new tris-dithizonatocobalt(III) complex

The reactions between dithizone (H2Dz (1)) or potassium dithizonate (KHDz (3)), and [Co(H2O)6]2+ (6), in acetone or methanol to liberate tris-dithizonatocobalt(III), Co(HDz)3 (5), are described. The structure of (5) was confirmed by single crystal X-ray analyses and shows bidentate coordination to CoIIIvia S and N donor atoms for all three HDz− ligands. A comparative voltammetric and spectro-electrochemical study revealed that (1) can be oxidised in two one-electron transfer steps, to generate a disulphide first and then HDz+. In contrast, upon complexation with cobalt, the free mercaptan group of (1) becomes a stable “metal thioether”, Co-S-C, which effectively prevents disulphide formation in all three ligands of (5) upon electrochemical oxidation. As a result, each ligand of Co(HDz)3 shows just one oxidation process. Intramolecular communication between ligands is evident because the three separate ligand-based oxidations are well resolved. Two irreversible ligand reduction steps, each consisting of three unresolved components related to each of the three ligands, were also observed. The CoII/CoIII couple in (5) was observed as a slow irreversible redox process. The different electronic spectra for each of the redox states of (5) are discussed.

Probing the role of a conserved salt bridge in the intramolecular electron transfer kinetics of human sulfite oxidase

Sulfite oxidase (SO) is a vital metabolic enzyme that catalyzes the oxidation of toxic sulfite to sulfate. The proposed mechanism of this molybdenum cofactor dependent enzyme involves two one-electron intramolecular electron transfer (IET) steps from the molybdenum center to the iron of the b 5-type heme and two one-electron intermolecular electron transfer steps from the heme to cytochrome c. This work focuses on how the electrostatic interaction between two conserved amino acid residues, R472 and D342, in human SO (hSO) affects catalysis. The hSO variants R472M, R472Q, R472K, R472D, and D342K were created to probe the effect of the position of the salt bridge charges, along with the interaction between these two residues. With the exception of R472K, these variants all showed a significant decrease in their IET rate constants, k et, relative to wild-type hSO, indicating that the salt bridge between residues 472 and 342 is important for rapid IET. Surprisingly, however, except for R472K and R472D, all of the variants show k cat values higher than their corresponding k et values. The turnover number for R472D is about the same as k et, which suggests that the change in this variant is rate-limiting in catalysis. Direct spectroelectrochemical determination of the Fe(III/II) reduction potentials of the heme and calculation of the Mo(VI/V) potentials revealed that all of the variants affected the redox potentials of both metal centers, probably due to changes in their environments. Thus, the position of the positive charge of R472 and that of the negative charge of D342 are both important in hSO, and changing either the position or the nature of these charges perturbs IET and catalysis.

Electrochemical illumination of thienyl and ferrocenyl chromium(0) Fischer carbene complexes

A series of ferrocenyl and thienyl mono- and biscarbene chromium(0) complexes 1-6 were synthesised. The complexes were characterised both spectroscopically and electrochemically, and the single crystal X-ray structure of 3 was determined. Electrochemical measurements in CH2Cl2 revealed that the carbene double bond of 1-6 is reduced to an anion radical, (-)Cr-C˙ at formal reduction potentials <-1.7 V vs. FcH/FcH(+). A computational study on 1, 3 and 4 (B3LYP/def2-SVP level) is consistent with electrochemical results in showing that electrochemically generated chromium(I) species may be further electrochemically irreversibly oxidised to chromium(II) at Epa > 0.95 V. The reactivity towards follow-up chemical reactions of the anodically produced Cr(II) species is much higher than the reactivity of the cathodically produced radical anions as the latter was still observably reoxidised to the parent Cr=C species at fast scan rates. The ferrocenyl group is oxidised electrochemically reversibly to ferrocenium at larger potentials than the electrochemically reversible oxidation of the Cr(0) centre to Cr(I). That all redox centres in 1-6 are involved in one-electron transfer steps was confirmed by comparing the ferrocenyl voltammetric wave with those of the other redox centres in linear sweep voltammetric experiments. The ferrocenyl group was electrochemically shown to stabilise the Cr=C centre almost as much as the NHBu, and much more than the ethoxy and thienyl groups.

A rechargeable room-temperature sodium superoxide (NaO2) battery

In the search for room-temperature batteries with high energy densities, rechargeable metal-air (more precisely metal-oxygen) batteries are considered as particularly attractive owing to the simplicity of the underlying cell reaction at first glance. Atmospheric oxygen is used to form oxides during discharging, which-ideally-decompose reversibly during charging. Much work has been focused on aprotic Li-O(2) cells (mostly with carbonate-based electrolytes and Li(2)O(2) as a potential discharge product), where large overpotentials are observed and a complex cell chemistry is found. In fact, recent studies evidence that Li-O(2) cells suffer from irreversible electrolyte decomposition during cycling. Here we report on a Na-O(2) cell reversibly discharging/charging at very low overpotentials (< 200 mV) and current densities as high as 0.2 mA cm(-2) using a pure carbon cathode without an added catalyst. Crystalline sodium superoxide (NaO(2)) forms in a one-electron transfer step as a solid discharge product. This work demonstrates that substitution of lithium by sodium may offer an unexpected route towards rechargeable metal-air batteries.

Single turnover studies of oxidative halophenol dehalogenation by horseradish peroxidase reveal a mechanism involving two consecutive one electron steps: Toward a functional halophenol bioremediation catalyst

Horseradish peroxidase (HRP) catalyzes the oxidative para-dechlorination of the environmental pollutant/carcinogen 2,4,6-trichlorophenol (2,4,6-TCP). A possible mechanism for this reaction is a direct oxygen atom transfer from HRP compound I (HRP I) to trichlorophenol to generate 2,6-dichloro 1,4-benzoquinone, a two-electron transfer process. An alternative mechanism involves two consecutive one-electron transfer steps in which HRP I is reduced to compound II (HRP II) and then to the ferric enzyme as first proposed by Wiese et al. [F.W. Wiese, H.C. Chang, R.V. Lloyd, J.P. Freeman, V.M. Samokyszyn, Arch. Environ. Contam. Toxicol. 34 (1998) 217–222]. To probe the mechanism of oxidative halophenol dehalogenation, the reactions between 2,4,6-TCP and HRP compounds I or II have been investigated under single turnover conditions (i.e., without excess H2O2) using rapid scan stopped-flow spectroscopy. Addition of 2,4,6-TCP to HRP I leads rapidly to HRP II and then more slowly to the ferric resting state, consistent with a mechanism involving two consecutive one-electron oxidations of the substrate via a phenoxy radical intermediate. HRP II can also directly dechlorinate 2,4,6-TCP as judged by rapid scan stopped-flow and mass spectrometry. This observation is particularly significant since HRP II can only carry out one-electron oxidations. A more detailed understanding of the mechanism of oxidative halophenol dehalogenation will facilitate the use of HRP as a halophenol bioremediation catalyst.

Electronic Structures of the [V(tbpy)3]z (z = 3+, 2+, 0, 1−) Electron Transfer Series

The electron transfer series of complexes [V((t)bpy)(3)](z) (z = 3+, 2+, 0, 1-) has been synthesized and spectroscopically characterized with the exception of the monocationic species. Magnetic susceptibility measurements (4-290 K) establish an S = 1 ground state for [V((t)bpy)(3)](3+), S = (3)/(2) for [V((t)bpy)(3)](2+), S = (1)/(2) for [V((t)bpy)(3)], and an S = 0 ground state for [V((t)bpy)(3)](1-). The electrochemistry of this series recorded in tetrahydrofuran solution exhibits four reversible one-electron transfer steps. Electronic absorption, X-band electron paramagnetic resonance (EPR), and V K-edge X-ray absorption (XAS) spectra were recorded. All complexes have been studied computationally with density functional theory (DFT) using the B3LYP functional. It is unequivocally shown that the electronic structure of complexes is best described as [V(III)((t)bpy(0))(3)](3+), [V(II)((t)bpy(0))(3)](2+), [V(II)((t)bpy(•))(2)((t)bpy(0))](0), and [V(II)((t)bpy(•))(3)](1-), where ((t)bpy(0)) represents the neutral form of the ligand and ((t)bpy(•))(1-) is the one-electron reduced mononanionic radical form. In the neutral and monoanionic members, containing two and three ((t)bpy(•))(1-) ligands, respectively, the ligand spins are strongly antiferromagnetically coupled to the spins of the central V(II) ion (d(3); S = (3)/(2)) affording the observed ground states given above.

Fully Delocalized (Ethynyl)(vinyl)phenylene Bridged Triruthenium Complexes in up to Five Different Oxidation States

Triruthenium [(dppe)(2)Ru{-C≡C-1,4-C(6)H(2)-2,5-R(2)-CH═CH-RuCl(CO)(P(i)Pr(3))(2)}(2)](n+) (4a, R = H; 4b, R = OMe) containing unsymmetrical (ethynyl)(vinyl)phenylene bridging ligands and displaying five well-separated redox states (n = 0-4) are compared to their bis(alkynyl)ruthenium precursors (dppe)(2)Ru{-C≡C-1,4-C(6)H(2)-2,5-R(2)-C≡CR'} (2a,b: R' = TMS; 3a,b: R' = H) and their symmetrically substituted bimetallic congeners, complexes {Cl(dppe)(2)Ru}(2){μ-C≡C-1,4-C(6)H(2)-2,5-R(2)-C≡C} (A(a), R = H; A(b), R = OMe) and {RuCl(CO)(P(i)Pr(3))(2)}(2){μ-CH═CH-1,4-C(6)H(2)-2,5-R(2)-CH═CH} (V(a), R = H; V(b), R = OMe) as well as the mixed (ethynyl)(vinyl)phenylene bridged [Cl(dppe)(2)Ru-C≡C-1,4-C(6)H(4)-CH═CH-RuCl(CO)(P(i)Pr(3))(2)] (M(a)). Successive one-electron transfer steps were studied by means of cyclic voltammetry, EPR and UV-vis-NIR-IR spectroelectrochemistry. These studies show that the first oxidation mainly involves the central bis(alkynyl) ruthenium moiety with only limited effects on the appended vinyl ruthenium moieties. The second to fourth oxidations (n = 2, 3, 4) involve the entire carbon-rich conjugated path of the molecule with an increased charge uniformly distributed between the two arms of the molecules, including the terminal vinyl ruthenium sites. In order to assess the charge distribution, we judiciously use (13)CO labeled analogues to distinguish stretching vibrations due to the acetylide triple bonds and the intense and charge-sensitive Ru(CO) IR probe in different oxidation states. The comparison between complex pairs 4a,b(n+) (n = 0-3), A(a,b)(n+) and V(a,b)(n+) (n = 0-2) serves to elucidate the effect of the methoxy donor substituents on the redox and spectroscopic properties of these systems in their various oxidation states and on the metal/ligand contributions to their frontier orbitals.