Consecutive One-electron Transfers Research Articles

New Mobilization Pathway of Antimonite: Thiolation and Oxidation by Dissimilatory Metal-Reducing Bacteria via Elemental Sulfur Respiration.

Antimony (Sb) mobilization is widely explored with dissimilatory metal-reducing bacteria (DMRB) via microbial iron(III)-reduction. Here, our study found a previously unknown pathway whereby DMRB release adsorbed antimonite (SbIII-O) from goethite via elemental sulfur (S0) respiratory reduction under mild alkaline conditions. We incubated SbIII-O-loaded goethite with Shewanella oneidensis MR-1 in the presence of S0 at pH 8.5. The incubation results showed that MR-1 reduced S0 instead of goethite, and biogenic sulfide induced the formation of thioantimonite (SbIII-S). SbIII-S was then oxidized by S0 to mobile thioantimonate (SbV-S), resulting in over fourfold greater Sb release to water compared with the abiotic control. SbIV-S was identified as the intermediate during the oxidation process by Fourier transform ion cyclotron resonance mass spectrometry and electron spin resonance analysis. The existence of SbIV-S reveals that the oxidation of SbIII-S to SbV-S follows a two-step consecutive one-electron transfer from Sb to S atoms. SbV-S then links with SbIII-S by sharing S atoms and inhibits SbIII-S polymerization and SbIII2S3 precipitation like a "capping agent". This study clarifies the thiolation and oxidation pathway of SbIII-O to SbV-S by S0 respiration and expands the role of DMRB in the fate of Sb.

Impedance of copper electrode in slightly acid Cu(II)-glycine solutions

Electrochemical impedance spectroscopy was used to study the kinetics of reduction of Cu (II) in weakly acidic (pH 4 and 5) solutions of Cu (II) containing glycine (0.04 or 0.1 M) as a complexing agent. A rather slow equilibration was observed at pH 4 and glycine concentration cL = 0.04 M. With increasing pH or cL, the total concentration of intermediate Cu(I) decreases. At the same time, impedance increases over the entire range of frequencies applied. An equivalent circuit with two parallel sub-circuits, each containing the charge transfer resistance and Warburg impedance in series, was used to quantify the impedance spectra. The exchange current densities of two consecutive one-electron transfers were estimated. Their average values are ~1 µA cm–2 and ~0.1 mA cm–2.

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.

Nitrite Reduction to Nitrous Oxide and Ammonia by TiO2 Electrons in a Colloid Solution via Consecutive One-Electron Transfer Reactions.

The mechanism of nitrite reduction by excess electrons on TiO2 nanoparticles (eTiO2(-)) was studied under anaerobic conditions. TiO2 was loaded with up to 75 electrons per particle, induced by γ-irradiation of acidic TiO2 colloid solutions containing 2-propanol. Time-resolved kinetics and material analysis were performed, mostly at 1.66 g L(-1) TiO2. At relatively low nitrite concentrations (R = [eTiO2(-)]o/[nitrite]o > 1.5), eTiO2(-) decays via two consecutive processes; at higher concentrations, only one decay step is observed. The stoichiometric ratio Δ[eTiO2(-)]/[nitrite]o of the faster process is about 2. This process involves the one-electron reduction of nitrite, forming the nitrite radical (k1 = (2.0 ± 0.2) × 10(6) M(-1) s(-1)), which further reacts with eTiO2(-) (k2) in competition with its dehydration to nitric oxide (NO) (k3). The ratios k2/k3 = (3.0 ± 0.5) × 10(3) M(-1) and k2 > 1 × 10(6) M(-1) s(-1) were derived from kinetic simulations and product analysis. The major product of this process is NO. The slower stage of the kinetics involves the reduction of NO by eTiO2(-), and the detailed mechanism of this process has been discussed in our earlier publication. The results reported in this study suggest that several intermediates, including NO and NH2OH, are adsorbed on the titanium nanoparticles and give rise to inverse dependency of the respective reaction rates on the TiO2 concentration. It is demonstrated that the reduction of nitrite by eTiO2(-) yields mainly N2O and NH3 via consecutive one-electron transfer reactions.

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.

Reduction of Quinones by NADH Catalyzed by Organoiridium Complexes

Reduction of Quinones by NADH Catalyzed by Organoiridium Complexes

Ferricytochrome c Directly Oxidizes Aminoacetone to Methylglyoxal, a Catabolite Accumulated in Carbonyl Stress

Age-related diseases are associated with increased production of reactive oxygen and carbonyl species such as methylglyoxal. Aminoacetone, a putative threonine catabolite, is reportedly known to undergo metal-catalyzed oxidation to methylglyoxal, NH4 + ion, and H2O2 coupled with (i) permeabilization of rat liver mitochondria, and (ii) apoptosis of insulin-producing cells. Oxidation of aminoacetone to methylglyoxal is now shown to be accelerated by ferricytochrome c, a reaction initiated by one-electron reduction of ferricytochrome c by aminoacetone without amino acid modifications. The participation of O2 •− and HO• radical intermediates is demonstrated by the inhibitory effect of added superoxide dismutase and Electron Paramagnetic Resonance spin-trapping experiments with 5,5′-dimethyl-1-pyrroline-N-oxide. We hypothesize that two consecutive one-electron transfers from aminoacetone (E0 values = −0.51 and −1.0 V) to ferricytochrome c (E0 = 0.26 V) may lead to aminoacetone enoyl radical and, subsequently, imine aminoacetone, whose hydrolysis yields methylglyoxal and NH4 + ion. In the presence of oxygen, aminoacetone enoyl and O2 •− radicals propagate aminoacetone oxidation to methylglyoxal and H2O2. These data endorse the hypothesis that aminoacetone, putatively accumulated in diabetes, may directly reduce ferricyt c yielding methylglyoxal and free radicals, thereby triggering redox imbalance and adverse mitochondrial responses.

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.

EPR Spectroscopic Studies of the Fe–S Clusters in the O2-Tolerant [NiFe]-Hydrogenase Hyd-1 from Escherichia coli and Characterization of the Unique [4Fe–3S] Cluster by HYSCORE

The unusual [4Fe-3S] cluster proximal to the active site plays a crucial role in allowing a class of [NiFe]-hydrogenases to function in the presence of O(2) through its unique ability to undergo two rapid, consecutive one-electron transfers. This property helps to neutralize reactive oxygen species. Mechanistic details and the role of the medial and distal clusters remain unresolved. To probe the Fe-S relay, continuous wave and pulse electron paramagnetic resonance (EPR) studies were conducted on the O(2)-tolerant hydrogenase from Escherichia coli (Hyd-1) and three variants with point mutations at the proximal and/or medial clusters. Reduction potentials of the proximal ([4Fe-3S](5+/4+/3+)) and medial ([3Fe-4S](+/0)) clusters were determined by potentiometry. The medial [3Fe-4S](+/0) reduction potential is exceptionally high, implicating a mechanistic role in O(2)-tolerance. Numerous experiments establish that the distal cluster has a ground state S > 1/2 in all three variants and indicate that this is also the case for native Hyd-1. Concurrent with the Hyd-1 crystal structure, EPR data for the 'superoxidized' P242C variant, in which the medial cluster is 'magnetically silenced', reveal two conformations of the proximal [4Fe-3S](5+) cluster, and X-band HYSCORE spectroscopy shows two (14)N hyperfine couplings attributed to one conformer. The largest, A((14)N) = [11.5,11.5,16.0] ± 1.5 MHz, characterizes the unusual bond between one Fe (Fe(4)) and the backbone amide-N of cysteine-20. The second, A((14)N) = [2.8,4.6,3.5] ± 0.3 MHz, is assigned to N(C19). The (14)N hyperfine couplings are conclusive evidence that Fe(4) is a valence-localized Fe(3+) in the superoxidized state, whose formation permits an additional electron to be transferred rapidly back to the active site during O(2) attack.

Mechanistic studies on the intramolecular electron transfer in an adduct species of the oxo-centred trinuclear iron(III) cation and l-ascorbic acid in aqueous solution

The kinetics of the intra-molecular electron transfer of an adduct of l-ascorbic acid and the [Fe 3 III O(CH3COO)6(H2O)3]+ cation in aqueous acetate buffer was studied spectrophotometrically, over the ranges 2.55 ≤ pH ≤ 3.74, 20.0 ≤ θ ≤ 35.0 °C, at an ionic strength of 0.50 and 1.0 mol dm−3 (NaClO4). The reaction of l-ascorbic acid and the complex cation involves the rapid formation of an adduct species followed by a slower reduction in the iron centres through consecutive one-electron transfer processes. The final product of the reaction is aqueous iron(II) in acetate buffer. The proposed mechanism involves the triaqua and diaqua-hydroxo species of the complex cation, both of which form adducts with l-ascorbic acid. At 25 °C, the equilibrium constant for the adduct formation was found to be 86 ± 15 and 5.8 ± 0.2 dm3 mol−1 for the triaqua and diaqua-hydroxo species, respectively. The kinetic parameters derived from the rate expression have been found to be: k 0 = (1.12 ± 0.02) × 10−2 s−1 for the combined spontaneous decomposition and k 1 = (4.47 ± 0.06) × 10−2 s−1 (ΔH 1 ‡ = 51.0 ± 2.3 kJ mol−1, ΔS 1 ‡ = −100 ± 8 J K−1 mol−1), k 2 = (4.79 ± 0.38) × 10−1 s−1 (ΔH 2 ‡ = 76.5 ± 0.8 kJ mol−1, ΔS 2 ‡ = 6 ± 3 J K−1 mol−1) for the triaqua and diaqua-hydoxo species, respectively.

Euphorbia Peroxidase Catalyzes Thiocyanate Oxidation in Two Different Ways, the Distal Calcium Ion Playing an Essential Role

The oxidation of the pseudohalide thiocyanate (SCN(-)) by Euphorbia peroxidase, in the presence or absence of added calcium, is investigated. After incubation of the native enzyme with hydrogen peroxide, the formation of Compound I occurs and serves to catalyze the thiocyanate oxidation pathways. The addition of a stoichiometric amount of SCN(-) to Compound I leads to the native enzyme spectrum; this process clearly occurs via two electron transfers from pseudohalide to Compound I. In the presence of 10 mM calcium ions, the addition of a stoichiometric amount of SCN(-) to Compound I leads to the formation of Compound II that returns to the native enzyme after addition of a successive stoichiometric amount of SCN(-), indicating that the oxidation occurs via two consecutive one-electron transfer steps. Moreover, different reaction products can be detected when the enzyme-hydrogen peroxide-thiocyanate reaction is performed in the absence or presence of 10 mM Ca(2+) ions. The formation of hypothiocyanous acid is easy demonstrated in the absence of added calcium, whereas in the presence of this ion, CN(-) is formed as a reaction product that leads to the formation of an inactive species identified as the peroxidase-CN(-) complex. Thus, although monomeric, Euphorbia peroxidase is an allosteric enzyme, finely tuned by Ca(2+) ions. These ions either can enhance the catalytic efficiency of the enzyme toward some substrates or can regulate the ability of the enzyme to exploit different metabolic pathways toward the same substrate.

The effect of tert-butanol adsorption on the two-step electroreduction of Zn 2+ and its dependence on the NaClO 4 concentration

The influence of water molecules on the adsorption of organic substances and kinetics of electroreduction of inorganic depolarizers is very often overlooked. The electroreduction of Zn 2+ is a typical example of a reaction controlled by both diffusion and charge transfer. This process in 1.0, 0.5 and 0.1 M NaClO 4 solutions at a mercury electrode in the presence of tert-butanol is expected to involve two consecutive one-electron transfer steps in the overall reaction. Solutions of tert-butanol were prepared to cover the concentration range from 0.01 to 0.3 M. Measurements were performed using an impedance method for a wide range of both the potential and frequency. The difference between the potential of the anodic and cathodic peaks, which was obtained from the cyclic voltammetry method, increases with an increase in the concentration of both tert-butanol and NaClO 4. From the dependences of ln ⁡ k f t = f ( E ) , the true standard rate constants k s t and the constants k s 1 t and k s 2 1 characterizing the stage of the first electron transfer and the stage of the second electron exchange, respectively, were determined. The obtained results indicate that the inhibiting properties of tert-butanol are the weakest in 0.1 M NaClO 4. In addition the stage of the second electron transfer is less sensitive to the inhibiting effect of tert-butanol than the stage of the first electron exchange.

Oxidation States and CO Ligand Exchange Kinetics in a Self‐Assembled Monolayer of a Triruthenium Cluster Studied by In Situ Infrared Spectroscopy

Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer (SAM) of an oxo-centered triruthenium cluster [Ru(3)(mu3-O)(mu-CH3COO)6(CO)(L1)(L2)] (L1 = [(NC5H4)CH2NHC(O)(CH2)10S-]2, L2 = 4-methylpyridine) have been extensively investigated on the surface of a gold electrode in aqueous and nonaqueous solutions. The SAM exhibits three consecutive one-electron transfers and four oxidation states, which have been characterized by electrochemistry, in situ infrared spectroscopy, and in situ sum frequency generation (SFG) vibrational spectroscopy measurements. The original electron-localized state of the Ru cluster center was changed to electron delocalization states by oxidation or reduction of the central Ru ions. These changes are revealed by the IR absorptions of the CO ligand and the bridging acetate ligands of the triruthenium cluster in the SAM. The IR absorptions of the two kinds of ligands are strongly dependent on the oxidation state of the Ru cluster center. One-electron oxidation of the central Ru ion in the SAM triggers a CO ligand liberation process. Solvent molecules may then occupy the CO site to result in a CO-free SAM. One-electron reduction of this CO-free SAM in a CO-saturated solution leads to re-coordination of the CO ligand into the SAM. Both processes can be precisely controlled by tuning the electrode potential. The kinetics of the CO exchange cycle in the SAM, including liberation and coordination, has been investigated by in situ IR and SFG measurements for the first time. The CO exchange cycle is significantly dependent on the temperature. The reaction rate greatly decreases with decreasing solution temperature, which is an important factor in the CO ligand exchange process. The activation energies of both CO liberation and coordination have been evaluated from the reaction rate constants obtained at various temperatures.

Isoprenoid biosynthesis in chloroplasts via the methylerythritol phosphate pathway: the (E)-4-hydroxy-3-methylbut-2-enyl diphosphate synthase (GcpE) from Arabidopsis thaliana is a [4Fe?4S] protein

The mevalonate-independent methylerythritol phosphate pathway is widespread in bacteria. It is also present in the chloroplasts of all phototrophic organisms. Whereas the first steps, are rather well known, GcpE and LytB, the enzymes catalyzing the last two steps have been much less investigated. 2-C-Methyl-D-erythritol 2,4-cyclodiphosphate is transformed by GcpE into 4-hydroxy-3-methylbut-2-enyl diphosphate, which is converted by LytB into isopentenyl diphosphate or dimethylallyl diphosphate. Only the bacterial GcpE and LytB enzymes have been investigated to some extent, but nothing is known about the corresponding plant enzymes. In this contribution, the prosthetic group of GcpE from the plant Arabidopsis thaliana and the bacterium Escherichia coli has been fully characterized by Mossbauer spectroscopy after reconstitution with (57)FeCl(3), Na(2)S and dithiothreitol. It corresponds to a [4Fe-4S] cluster, suggesting that both plant and bacterial enzymes catalyze the reduction of 2-C-methyl-D-erythritol 2,4-cyclodiphosphate into (E)-4-hydroxy-3-methylbut-2-enyl diphosphate via two consecutive one-electron transfers. In contrast to the bacterial enzyme, which utilizes NADPH/flavodoxin/flavodoxin reductase as a reducing shuttle system, the plant enzyme could not use this reduction system. Enzymatic activity was only detected in the presence of the 5-deazaflavin semiquinone radical.

Electrochemical studies on cells M/YSZ/Pt (M=Pt, Pt–Ga 2O 3) in NO, O 2, N 2 gas mixtures

The cathodic process of the Pt(Ga 2O 3)/YSZ electrode in O 2, N 2 gas mixtures with and without NO is studied by electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) in a wide range of gas composition at temperature range 450–800 °C in order to investigate its electrocatalytic activity for the electrochemical reduction of oxygen and NO, respectively. The results of the impedance spectra and j c– η c polarisation curves suggest different cathodic mechanisms at two temperature regions. For temperatures <650 °C, the cathodic O 2 reduction occurs in two consecutive one-electron transfer steps. At ϑ>650 °C, O 2 reduction follows a two-electron transfer step. The electrocatalytic activity of the Pt/YSZ electrode is enhanced due to presence of NO in the gas mixture leading to a decrease of the apparent activation energy and an increase of the exchange current density. The admixture of Ga 2O 3 to Pt/YSZ shows an inhibitor effect on the cathodic O 2 reduction at low temperature. Aside from this, it has decreased the electrocatalytic efficiency of the Pt/YSZ electrode for cathodic NO reduction.

Sequence of steps in the pitting of aluminum by chloride ions

Corrosion pit initiation in chloride solutions is given by an electrode kinetic model which takes into account adsorption of chloride ions on the oxide surface, penetration of chloride ions through the oxide film, and localized dissolution of aluminum at the metal/oxide interface in consecutive one-electron transfer reactions. A previous model has been extended here to consider that penetration of chloride ions can occur by oxide film dissolution as well as by migration through oxygen vacancies. Pit initiation occurs by chloride-assisted localized dissolution at the oxide/metal interface. The electrode kinetic model leads to a mathematical expression which shows that the critical pitting potential is a linear function of the logarithm of the chloride concentration (at constant pH), in agreement with experiment. The model also predicts that the critical pitting potential is independent of pH (at constant chloride concentration), also in agreement with experiment. Corrosion pit propagation leads to formation of blisters beneath the oxide film due to localized reactions which produce an acidic localized environment. The blisters subsequently rupture due to the formation of hydrogen gas in the occluded corrosion cell. Calculation of the local pH within a blister from the calculated hydrogen pressure within the blister gives pH values in the range 0.85 to 2.3.

Optically and thermally induced electron transfer pathways in hexakis[4-(N,N-diarylamino)phenyl]benzene derivatives.

The optically and thermally induced electron transfer pathways of highly symmetrical (D(3)) hexaarylbenzene systems with six triarylamine redox sites have been investigated. Owing to slightly different local redox potentials, the radical trication could be selectively generated by electrochemical methods. This trication shows a strong intervalence charge-transfer band in the near infrared (NIR) that was measured by spectroelectrochemistry and analysed using multi-dimensional Mulliken-Hush theory. Quantum chemical AM1 CI calculations indicate that there is no optically induced concerted three-electron transfer that transforms the ground state into a state in which all three positively charged triarylamine moieties change place with their neutral neighbours. The potential energy surface of the ground state was constructed by using quadratic potentials. From this potential surface it is apparent that there is also no thermally allowed concerted three-electron transfer pathway. Instead, three consecutive one-electron transfer steps are necessary for this process.

The metal-catalyzed oxidation of methionine in peptides by Fenton systems involves two consecutive one-electron oxidation processes

The one-electron oxidation of methionine (Met) plays an important role in the redox reactions of Met in peptides and proteins under conditions of oxidative stress, e.g., during the metal-catalyzed oxidation of β-amyloid peptide (βA). However, little information is available with regard to mechanisms and product formation during the metal-catalyzed oxidation of Met. Here, we demonstrate that two-electron oxidation of Met in Fenton reactions, carried out aerobically by [Fe II(EDTA)] 2− and H 2O 2 (EDTA = ethylenediaminetetra acetate) is the consequence of two consecutive one-electron transfer reactions carried out by either free or complexed hydroxyl radicals, followed by the reaction of an intermediary sulfur-nitrogen bonded radical cation (sulfuranyl radical) with O 2. The model peptide Met-Met represents an ideal substrate for these investigations as its one-electron oxidation, followed by reaction with molecular oxygen, produces unique intermediates, azasulfonium diastereomers, which can be chemically isolated before hydrolysis to sulfoxide occurs.

Multidimensional Electron Transfer Pathways in a Tetrahedral Tetrakis{4-[N,N-di(4-methoxyphenyl)amino]phenyl}Phosphonium Salt: One-Step vs Two-Step Mechanism

Different electron-transfer pathways have been investigated in a three-dimensional redox system: tetrakis{4-[N,N-di(4-methoxyphenyl)amino]phenyl}phosphonium tetrafluoroborate 1+BF4- which comprises of four triarylamine redox centers arranged in a pseudo-tetrahedral geometry. Using a UV/Vis/NIR spectroelectrochemical setup we generated the mixed-valence species 12+, 13+, and 14+ and measured the UV/Vis/NIR spectra. These spectra were analyzed and interpreted according to a multi-dimensional Marcus−Hush approach. The analysis revealed that both the photoexcited as well as the thermal ET in 13+ are forbidden as a concerted two-electron transfer but allowed as two consecutive one-electron transfer steps.

Cathodic processes in the ligand-deficient Cu/Cu(II) ethylenediamine system

A plateau of a limiting current observed on voltammograms in the ligand-deficient Cu/Cu(II) ethylenediamine system is a prewave resulting from abrupt changes in the solution composition at the electrode surface. Voltammograms may be described quantitatively by equations of formal kinetics accounting for two consecutive one-electron transfers provided that the process of Cu 2++e −→Cu + is a rate-determining step. Cu 2O developed under open-circuit conditions exhibits properties typical of an adsorbed layer. The exchange current density of Cu(II) reduction decreases with the thickness of such layers. The irreversible reduction of Cu 2O occurs at sufficiently high cathodic overvoltages, resulting in a significant activation of the electrode surface.