Electron transfer metalation

For a reaction with one elementary step (e.g., electron transfer), conformational mobility at the active site (electron transfer metal atoms... 

Willner and coworkers have extended this approach to electron relay systems where core-based materials facilitate the electron transfer from redox enzymes in the bulk solution to the electrode.56 Enzymes usually lack direct electrical communication with electrodes due to the fact that the active centers of enzymes are surrounded by a thick insulating protein shell that blocks electron transfer. Metallic NPs act as electron mediators or wires that enhance electrical communication between enzyme and electrode due to their inherent conductive properties.47 Bridging redox enzymes with electrodes by electron relay systems provides enzyme electrode hybrid systems that have bioelectronic applications, such as biosensors and biofuel cell elements.57... 

The heart of interfacial electrochemical kinetics is electron transfer—metal to solution and solution to metal. The electron is a particle, the movement and properties... 

It is specifically noted that this active-metal supported coupling makes cyclic olefins of otherwise unfavorable ring sizes (n = 8-12) available without problems (cf. Figure 1). Reactions following eq. (1) thus belong to the modem synthetic methodology. From a mechanistic point of view, electron transfer (metal to substrate) is of key importance (e. g., pinacolate-titanium intermediates). 

Keywords Electron transfer Metal complex photocatalyst Photocatalytic CO2 reduction Semiconductor photocatalyst Supramolecular chemistry... 

Both low and high oxidation states of nickel porphyrins are easily accessible in many nonaqueous solvents and the exact site of electron transfer (metal vs ring) upon reduction or oxidation has been the topic of numerous studies [7]. Bocian and coworkers [325], as well as Connich and Macor [326], showed that a Ni(II) porphyrin tt-cation radical can be converted to a Ni(III) porphyrin in binding solvents such as Py, THF or MeCN. The type of Ni(II) porphyrin tt-cation radical, that is, aiu or a2u, wiU depend on the type of porphyrin macrocycle anda Ni(II) jr-cation... 

Notably absent from this list are any questions about basic mechanisms of electron transfer, metal site geometry, control of redox potentials, spin-spin coupling, interpretation of spectral properties, and so forth. 

Metals of changing valency influence oxidation rates by complex formation. The ligands are either substrate or enzyme protein molecules or both. Electron distribution is altered as well in the center as in the ligands, providing us with a number of catalysts of graded reactivity. As a consequence of one electron transfer, metal ions of changing valency may initiate chain reactions, whereby the rate of the oxidative process is greatly increased. 

Metal to ceramic (oxide) adhesion is very important to the microelectronics industry. An electron transfer model by Burlitch and co-workers [75] shows the importance of electron donating capability in enhancing adhesion. Their calculations are able to explain the enhancement in adhesion when a NiPt layer is added to a Pt-NiO interface. 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

As outlined above, electron transfer through the passive film can also be cmcial for passivation and thus for the corrosion behaviour of a metal. Therefore, interest has grown in studies of the electronic properties of passive films. Many passive films are of a semiconductive nature [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102 and 1031 and therefore can be investigated with teclmiques borrowed from semiconductor electrochemistry—most typically photoelectrochemistry and capacitance measurements of the Mott-Schottky type [104]. Generally it is found that many passive films cannot be described as ideal but rather as amorjDhous or highly defective semiconductors which often exlribit doping levels close to degeneracy [105]. 

Much of tills chapter concerns ET reactions in solution. However, gas phase ET processes are well known too. See figure C3.2.1. The Tiarjioon mechanism by which halogens oxidize alkali metals is fundamentally an electron transfer reaction [2]. One might guess, from tliis simple reaction, some of tlie stmctural parameters tliat control ET rates relative electron affinities of reactants, reactant separation distance, bond lengtli changes upon oxidation/reduction, vibrational frequencies, etc. 

Using the electron transfer definition, many more reactions can be identified as redox (reduction-oxidation) reactions. An example is the displacement of a metal from its salt by a more reactive metal. Consider the reaction between zinc and a solution of copper(If) sulphate, which can be represented by the equation... 

Group I metals—sodium is the one usually employed—in liquid ammonia as the solvent convert alkynes to trans alkenes The reaction proceeds by a four step sequence in which electron transfer and proton transfer steps alternate... 

The mechanism by which the Birch reduction of benzene takes place (Figure 118) IS analogous to the mechanism for the metal-ammonia reduction of alkynes It involves a sequence of four steps m which steps 1 and 3 are single electron transfers from the metal and steps 2 and 4 are proton transfers from the alcohol... 

Hydroperoxides are more widely used as initiators in low temperature appHcations (at or below room temperature) where transition-metal (M) salts are employed as activators. The activation reaction involves electron-transfer (redox) mechanisms ... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Intense sodium D-line emission results from excited sodium atoms produced in a highly exothermic step (175). Many gas-phase reactions of the alkafl metals are chemiluminescent, in part because their low ioni2ation potentials favor electron transfer to produce intermediate charge-transfer complexes such as [Ck Na 2] (1 )- There appears to be an analogy with solution-phase electron-transfer chemiluminescence in such reactions. 

Reactions involving the peroxodisulfate ion are usually slow at ca 20°C. The peroxodisulfate ion decomposes into free radicals, which are initiators for numerous chain reactions. These radicals act either thermally or by electron transfer with transition-metal ions or reducing agents (79). 

Hydroperoxides are decomposed readily by multivalent metal ions, ie, Cu, Co, Fe, V, Mn, Sn, Pb, etc, by an oxidation-reduction or electron-transfer process. Depending on the metal and its valence state, metallic cations either donate or accept electrons when reacting with hydroperoxides (45). Either one... 

With most transition metals, eg, Cu, Co, and Mn, both valence states react with hydroperoxides via one electron transfer (eqs. 11 andl2). Thus, a small amount of transition-metal ion can decompose a large amount of hydroperoxide and, consequendy, inadvertent contamination of hydroperoxides with traces of transition-metal impurities should be avoided. 

In the reaction of lead tetraacetate with 1,3- or 1,4-dihydtopetoxides (10) to produce cychc monoperoxides there are two electron transfers to the metal (eq. 14). 

As with other hydroperoxides, hydroxyaLkyl hydroperoxides are decomposed by transition-metal ions in an electron-transfer process. This is tme even for those hydroxyaLkyl hydroperoxides that only exist in equiUbrium. For example, those hydroperoxides from cycHc ketones (R, R = alkylene) form an oxygen-centered radical initially which then undergoes ring-opening -scission forming an intermediate carboxyalkyl radical (124) ... 

Peroxyesters decompose by an electron-transfer process catalyzed by transition metals (44,168,213) (eq. 34). This reaction has been used synthetically to bond an acyloxy group to appropriate coreactive substrates (eq. 35). 

This difference is a measure of the free-energy driving force for the development reaction. If the development mechanism is treated as an electrode reaction such that the developing silver center functions as an electrode, then the electron-transfer step is first order in the concentration of D and first order in the surface area of the developing silver center (280) (Fig. 13). Phenomenologically, the rate of formation of metallic silver is given in equation 17,... 

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