Chemical redox

MetaHacarborane dicarboUide complexes are generaUy more robust than the corresponding cyclopentadiene complexes. The bis-dicarboUide sandwich complexes of general formula [M(C2Bg)H )2] , where M is Fe ", Co ", and exhibit great thermal, chemical, redox, and radiolytic stabUity. 

The final remark of this section concerns the polaronic transition of m-LPPP around 1.9 eV, where we can observe P2 with its vibronic replica P3 at 2.1 eV. In Figure 9-20 we show this polaronic absorption in m-LPPP as detected by photoin-duced absorption (a), chaige-induced absorption in conventional light-emitting devices (b), and chemical redox-reaction (c). Only under pholoexcilation, which creates both neutral and charged species, the triplet signal at 1.3 eV is also observed. 

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

Certainly, the same arguments apply for chemical redox catalysis , but as discussed above, thinner films may be effective in this case. Hence, it will be reasonable to work with modified electrodes having a large effective area instead of thick films, i.e. three-dimensional, porous or fibrous electrodes. The notorious problem with current/potential distribution in such electrodes may be overcome by the potential bias given by selective redox catalysts. Some approaches in this direction are described in the next section. 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

Various methods have been used to determine the redox potentials (Table XI). Very commonly, EPR-monitored chemical redox titration is performed, which can be used to measure the redox potential not only in isolated complexes but also in membrane preparations. In general, there is good agreement between redox potentials determined in membranes, isolated complexes, or isolated Rieske proteins or fragments the only exception is the water-soluble Rieske fragment from spinach bef complex where differences of more than 50 mV have been observed by the same group but using different methods (31). 

The hydrogen transfer reaction (HTR), a chemical redox process in which a substrate is reduced by an hydrogen donor, is generally catalysed by an organometallic complex [72]. Isopropanol is often used for this purpose since it can also act as the reaction solvent. Moreover the oxidation product, acetone, is easily removed from the reaction media (Scheme 14). The use of chiral ligands in the catalyst complex affords enantioselective ketone reductions [73, 74]. 

Electrochemistry is the coupling of a chemical redox process with electron flow through a wire. The process represented in Figure 19-7 is electrochemical because the redox reaction releases electrons that flow through an external wire as an electrical current. On the other hand. Figure 19-5 shows a redox process that is not electrochemical, because direct electron transfer cannot generate an electrical current through a wire. 

Elimination Procedures. Chemical redox-induced elimination was performed on precursor thin films by exposure to bromine or iodine vapor or by immersion of films in hexane solutions of these halides. 

The general scheme of Figure 11 is for both electrochemical and chemical redox elimination. If we consider the case of oxidative elimination with bromine, generation of the necessary radical cation... 

The general relationship for the activation energy including both the electrode reaction and the chemical redox reaction as derived by Marcus in the form... 

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

The reduction of protons is one of the most fundamental chemical redox reactions. Transition metal-catalyzed proton reduction was reviewed in 1992.6 The search for molecular electrocatalysts for this reaction is important for dihydrogen production, and also for the electrosynthesis of metal hydride complexes that are active intermediates in a number of electrocatalytic systems. 

In the case of either eq. 2 or eq. 3, even though excited state energy has been converted into stored chemical redox energy, the storage is temporary because of back electron transfer between the separated redox products, e.g., eq. 4. 

To learn that mediators are redox species used to effect redox chemistry on biological species that are electroactive, yet inert at most conventional electrodes the charge employed in electro-converting the mediator is the same as would have been used in converting the analyte if it was electroactive. To appreciate that mediators can be electro-reduced or electro-oxidized, yet will effect a chemical redox reaction with the analyte. 

Mediators may allow for coulometry to be carried out when an analyte is not itself electroactive. Such mediators are often aromatic organic molecules which can interconvert reversibly between several oxidation states. The mediator is itself electromodiiied at an electrode, and then effects a chemical redox charge to the analyte in the same solution. Coulometry is then possible, provided that the transfer of charge from the mediator to the analyte is 100% efficient. 

In some cases, hidden adsorption is responsible for the differences between electrode and purely chemical redox reactions. The aromatic derivatives of divalent sulfur on reduction at the mercury-dropping electrode do not show any adsorption waves within the corresponding polarogam. Thus, the reduction... 

The ligands 369 react with [RuCl2(dmso)4] to yield [RuCl2(dmso)2(369-A, 0)], characterized W spectroscopic and electrochemical methods. Complexes in the families [Ru"(bpy)(370)2] and [Ru" (aca( (370)2] have been reported. The complexes [Ru(bpy)(370)2] undergo a reversible Ru"/Ru" oxidation followed by an irreversible Ru /Ru process the bpy-centered one-electron reduction is also observed. Chemical oxidation of the complexes [Ru(bpy)(370)2] gives [Ru(bpy)(370)2] (isolated as the iodides), the electronic and ESR spectroscopic properties of which have been described. The crystal structure of [Ru(acac)(371)2] has been established, and the electrochemical and chemical redox reactions of [Ru(acac)(370)2] and [Ru(acac)(371)2] generate Ru" and Ru species that have been characterized by spectroscopic and electrochemical techniques. ... 

A rich variety of reagents and methods have been applied to generate radical ions. As illustrated above, the first methods were chemical redox reactions. Radical anions have long been generated via reduction by alkali metals. Because of the high reduction potentials of these metals, the method is widely applicable, and the reductions are essentially irreversible. 

In certain solids such as titanium dioxide or cadmium sulfide, the energy of the band gap corresponds to that of light (visible, ultraviolet, or infrared), with the result that the solid, when illuminated, may become electrically conducting or acquire potent chemical redox characteristics because of the promotion of electrons to the conduction band (which is normally unoccupied). These properties have obvious practical significance and are considered at length in Chapter 19. 

Let us now take a brief look at some important redox reactions of organic pollutants that may occur abiotically in the environment. We first note that only a few functional groups are oxidized or reduced abiotically. This contrasts with biologically mediated redox processes by which organic pollutants may be completely mineralized to C02, HzO and so on. Table 14.1 gives some examples of functional groups that may be involved in chemical redox reactions. We discuss some of these reactions in detail later. In Table 14.1 only overall reactions are indicated, and the species that act as a sink or source of the electrons (i.e., the oxidants or reductants, respectively) are not specified. Hence, Table 14.1 gives no information about the actual reaction mechanism that may consist of several reaction steps. 

Electrochemical regeneration of expensive but highly selective chemical redox agents (indirect electrolysis). This is very attractive, if the application of such reagents in stoichiometric amounts is excluded because of economical and ecological reasons. 

The carbon cycle is illustrated in Fig. 5.2. All living systems require an external source of energy, either in the form of chemical bond energy, as chemical (redox) potential or as some form of electromagnetic radiation usually in or near the visible light region. 

Somewhat less attention has been directed toward characterizing chemical redox reactions which can be stimulated by the initial surface photoexcitation and most of these have been directed at a... 

In chemical redox reactions, organic ion radicals are formed at the final stage of donor-acceptor (D-A) interactions ... 

Once separated, this pair can induce chemical redox transformation with the species adsorbed on the surface. It is assumed that surface hydroxide ions act as hole traps by producing OH radicals ... 

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