Redox ions, electron transfer

Redox systems with inorganic and organic components lodonium ions Electron-transfer agents, e.g., alkali metals, alkah-aromatic complexes, alkali metal ketyls Activated transition metal oxides... 

Proton transfer is a more demanding case than simple electrochemical electron transfer (ECET)90 redox processes. It involves coupled ion-electron transfer or electrochemical ion transfer (ECU), recently described90 as a process in which partial loss of the solvation sphere... 

Therefore, the co-ordinating properties (intrinsic Lewis acidity) and the redox properties (electron transfer) of the metal ion are important for the first step, and for the transformation of the adsorbed hydrocarbon into the nitrile rather than to carbon oxides and HCN. It is also worth noting that when a conventional alkene ammoxidation catalyst is used, such as Bi/Mo/O, the performance is much worse (yield 4.5% at 450°C). 

Peroxidases are heme-iron proteins involved in oxidative stress control. The mechanisms of this class of enzymes involve rather complex redox and electron transfer processes. These include alterations of spin state and ligands to the active-site heme as well as a novel role for Ca ions in the process as discussed by Moura and colleagues (Chapter 6). The topology and mechanism of the electron transfer process can be studied in detail using dynamic NMR data and molecular modeUng tools as illustrated by Pettigrew and co-workers (Chapter 7). The action of these types of peroxidases can be considered of crucial importance for the redox state regulation of the cell. 

For redox and electron-transfer properties of [Os(bipy)j] + and [Os(phen)3] + see p. 539. Osmium(IIJ). Oxidation of [Os(bipy)3] + with chlorine yields the violet [Os(bipy)3] ion," "" while the red [Os(phen)3] is similarly prepared by oxidation of [Os(phen)3] +.Resolution of [Os(phen3)f + has been achieved by oxidation of Ae corresponding [Os(phen)3] species. Spectroscopic properties. The electronic absorption spectra of [Os(bipy)3] " and of [Os(phen)3] " " have been measured, as have their circular dichroism spectra " "" and also Ae spectra of [Os(4,4 -Me2bipy)3] " The lifetimes of the charge transfer (MLCT) excited states of... 

In our simple model, the expression in A2.4.135 corresponds to the activation energy for a redox process in which only the interaction between the central ion and the ligands in the primary solvation shell is considered, and this only in the fonn of the totally synnnetrical vibration. In reality, the rate of the electron transfer reaction is also infiuenced by the motion of molecules in the outer solvation shell, as well as by other... 

Electron transfer can be established experimentally in reactions involving only ions in solution. Inert electrodes, made from platinum, are used to transfer electrons to and from the ions. The apparatus used is shown in Figure 4.3. the redox reaction being considered... 

Polymerization Initiator. Some unsaturated monomers can be polymerized through the aid of free radicals generated, as transient intermediates, in the course of a redox reaction. The electron-transfer step during the redox process causes the scission of an intermediate to produce an active free radical. The ceric ion, Ce" ", is a strong one-electron oxidizing agent that can readily initiate the redox polymerization of, for example, vinyl monomers in aqueous media at near ambient temperatures (40). The reaction scheme is... 

Photochromism Based on Redox Reactions. Although the exact mechanism of the reversible electron transfer is often not defined, several viologen salts (pyridinium ions) exhibit a photochromic response to uv radiation in the crystalline state or in a polar polymeric matrix, for example,... 

FIGURE 18.19 The structures and redox states of the nicotinamide coenzymes. Hydride ion (H , a proton with two electrons) transfers to NAD to produce NADH. 

The Creutz-Taube anion, [(NH3)5Ru- N(CH=CH)2N Ru(NH3)5] + displays more obvious redox properties, yielding both 4+ and 6- - species, and much interest has focused on the extent to which the pyrazine bridge facilitates electron transfer. A variety of spectroscopic studies supports the view that low-energy electron tunnelling across the bridge delocalizes the charge, making the 5- - ion symmetrical. Other complexes, such as the anion [(CN)5Ru (/z-CN)Ru (CN)5] , are asymmetric... 

A voltaic cell produces electrical energy through spontaneous redox chemical reactions. When zinc metal is placed in a solution of copper sulfate, an electron transfer takes place between the zinc metal and copper ions. The driving force for the reaction is the greater attraction of the copper ions for electrons ... 

The presupposition is that parallel electrochemical reactions (i.e., ion or electron transfer) occur across the phase boundary, if the measured ions and interfering ions are both present in the solution. A redox process in which electrons pass the phase boundary is also considered an interfering electrochemical reaction. 

In this reaction, copper metal plates out on the surface of the zinc. The blue color of the aqueous Cu2+ ion fades as it is replaced by the colorless aqueous Zn2+ ion (Figure 18.1). Clearly, this redox reaction is spontaneous it involves electron transfer from a Zn atom to a Cu2+ ion. 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

It lias also been suggested that photoexcited benzoyl peroxide is somewhat more susceptible to induced decomposition processes involving electron transfer than the ground state molecule. Rosenthal et c//.15 reported on redox reactions with certain salts (including benzoate ion) and neutral molecules (e.g. alcohols). 

A further important feature of HMPA is its stabilizing effect on the Redox potential of [Fe(CO)4]2 by ion solvation. In less polar solvents, electron-transfer reactions take place and [Fe(CO)4]2 is oxidized to [HFe3(CO)iThis redox reaction is suppressed in HMPA. 

It was mentioned earlier (Sec. 8.6) that for iodo-de-diazoniation no catalyst is necessary because the redox potential of the iodide ion (E° = 1.3 V) is sufficient for an electron transfer to the arenediazonium ion. The reaction was actually observed by Griess (1864 c). Four iodo-de-diazoniation procedures are described in Organic Syntheses. For the syntheses of iodobenzene and 4-iodophenol (Lucas and Kennedy, 1943, and Daines and Eberly, 1943, respectively) KI is used in equimolar quantity and in 1.2 molar excess. However, for 2-bromoiodobenzene and for 1,3,4-triiodo-5-nitrobenzene (replacement of a diazonio group in the 4-position by iodine), up to... 

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

The beauty of bromide-mediated oxidations is that they combine mechanistic complexity with practical simplicity and, hence, utility. They involve an intricate array of electron transfer steps in which bromine atoms function as go-betweens in transfering the oxidizing power of peroxidic intermediates, via redox metal ions, to the substrate. Because the finer mechanistic details of these elegant processes have often not been fully appreciated we feel that their full synthetic potential has not yet been realized. Hence, we envision further practical applications in the future. 

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

As for the salt formation and single-electron transfer, thermodynamics for simple redox processes may be applied to predict their selectivity. As a first approximation, a cation with red lower and higher than 0.2 V would give a salt and a radical pair, respectively, when combined with [2 ]. In practice, the cations which were found to give salts with [2 ] have red values more negative than —0.8 V. On the other hand, quantitative single-electron transfer has been observed from [2 ] to the heptaphenyltropylium ion which is relatively unstable p/fR+ —0.54 in methanol (Battiste and Barton, 1968) and E ed —0.30 V vs. Ag/Ag in acetonitrile (Kitagawa et al., 1992). 

Redox reactions are more complicated than precipitation or proton transfer reactions because the electrons transferred in redox chemishy do not appear in the balanced chemical equation. Instead, they are hidden among the starting materials and products. However, we can keep track of electrons by writing two half-reactions that describe the oxidation and the reduction separately. A half-reaction is a balanced chemical equation that includes electrons and describes either the oxidation or reduction but not both. Thus, a half-reaction describes half of a redox reaction. Here are the half-reactions for the redox reaction of magnesium and hydronium ions ... 

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

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