Bonded electron transfer sphere mechanism

Two types of redox catalysts are used in indirect electrolyses [3] (1) pure, outer-sphere, or non-bonded electron transfer agents, and (2) redox reagents that undergo a homogeneous chemical reaction that is intimately combined with a redox step. This may be called inner-sphere electron transfer or bonded electron transfer mechanism. 

Redox processes between metal complexes are divided into outer-sphere processes and inner-sphere processes that involve a ligand common to both coordination spheres. The distinction is fundamentally between reactions in which electron transfer takes place from one primary bond system to another (outer-sphere mechanism) and those in which electron transfer takes place within a primary bond system (inner-sphere mechanism) (Taube, 1970). 

Inner Sphere Mechanism, also known as Bonded Electron Transfer... 

Inner sphere mechanism, also known as bonded electron transfer or bridge mechanism... 

Electron transfer between metal ions contained in complexes can occur in two different ways, depending on the nature of the metal complexes that are present. If the complexes are inert, electron transfer occurring faster than the substitution processes must occur without breaking the bond between the metal and ligand. Such electron transfers are said to take place by an outer sphere mechanism. Thus, each metal ion remains attached to its original ligands and the electron is transferred through the coordination spheres of the metal ions. 

Most of the kinetic models predict that the sulfite ion radical is easily oxidized by 02 and/or the oxidized form of the catalyst, but this species was rarely considered as a potential oxidant. In a recent pulse radiolysis study, the oxidation of Ni(II and I) and Cu(II and I) macrocyclic complexes by SO was studied under anaerobic conditions (117). In the reactions with Ni(I) and Cu(I) complexes intermediates could not be detected, and the electron transfer was interpreted in terms of a simple outer-sphere mechanism. In contrast, time resolved spectra confirmed the formation of intermediates with a ligand-radical nature in the reactions of the M(II) ions. The formation of a product with a sulfonated macrocycle and another with an additional double bond in the macrocycle were isolated in the reaction with [NiCR]2+. These results may require the refinement of the kinetic model proposed by Lepentsiotis for the [NiCR]2+ SO/ 02 system (116). 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

Figure 13.18 S-adenosyl methionine (SAM), a source of 5 -deoxyadenosyl radicals. SAM binds to the subsite iron (in blue) of the reduced [4Fe-4S] cluster via its a-aminocarboxylate group. The 5 -deoxyadenosine radical is formed by electron transfer which occurs either (a) by outer-sphere mechanism or (b) by p-sulfide alkylation followed by homolytic cleavage of the 5 -S-CH2Ado bond. In both cases, methionine is released. (From Fontecave et al., 2004. Copyright 2004, with permission from Elsevier.)...

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

The reduction of arylalkyl halides of the triphenylmethyl type by electro-chemically generated outer sphere single electron donors offers an example of a sequencing of the bond-breaking and electron-transfer steps different from what has been described before. The cleavage of the halide ion then precedes electron transfer which thus involves the carbocation, a mechanism reminiscent of 8, 1 reactions (Andrieux et ai, 1984b) as shown in (102). The... 

All these observations point to the occurrence of a 8 2 rather than an outer sphere, dissociative electron-transfer mechanism in cases where steric constraints at the carbon or metal reacting centres are not too severe. It is, however, worth examining two other mechanistic possibilities. One of these is an electrocatalytic process of the Sg -type that would involve the following reaction sequence. If, in the reaction of the electron donor (nucleophile), the bonded interactions in the transition state are vanishingly small, the alkyl radical is formed together with the oxidized form of the electron donor, D . Cage coupling (144) may then occur, if their mutual affinity is... 

An inner-sphere electron reduction has been proposed as a possible mechanism for the Fe(II)-induced decomposition of 1,2,4-trioxolanes (ozonides) (75) and (76). Benzoic acid was found to be the major product. The nucleophilic Ee(II) species attack the ozonide from the less hindered side of the electrophilic 0-0 a orbital to generate exclusively the Ee(III) oxy-complexed radical (inner-sphere electron transfer). After selective scission of the C-C bond, the resulting carbon-centred radical produced the observed product. The substituent effect determine the regioselective generation of one of the two possible Fe(III)-complexed oxy radicals. The bond scission shown will occur if R is bulkier than R. ... 

The solution of the riddle posed by Kornblum s dark Sj l reaction is as follows. The nucleophile does work as a single electron-transfer initiator of the chain process. However, the mechanism of initiation does not consist of a mere outer-sphere electron transfer from the nucleophile to form the anion-radical of the substrate. Rather, it involves a dissociative process in which electron transfer and bond breaking are concerted (Costentin and Saveant 2000). Scheme b at the beginning of Section 7.8 illustrates the concerted mechanism. 

The rate law for the oxidation of [Ru(NH3)5(FlL)] + (HE = isonicotinamide) by I2 in acidic solution contains two terms, one depending on P2] and one depending on [I3 ] and [Ru complex]. An outer-sphere electron-transfer mechanism is proposed for each term. Reduction of [Ru (NFl3)5L] + (TIL = nicotinamide or isonicotinamide) to [Ru (NH3)5L]+ is accompanied by an isomerization from the amide-bonded L to pyridine-bonded FIL. Bromine oxidation of... 

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