Bonded electron transfer

Occasionally, the successful application of the Marcus expressions (5.35) and (5.37) to a reaction can support its designation as outer-sphere. The reduction of a series of substituted benzenediazonium salts by Fe(CN)5 and (Me5cp)2pe conforms to the simple Marcus expression and represents supporting evidence for the formulation of these reactions as outer sphere (or non-bonded electron transfer in organic systems)... 

ArNJ -I- Fe —> ArNj -I- Fe rather than inner sphere (or bonded electron transfer) ... 

TAZUKE ET AL. Polymer-Bonded Electron-Transfer Sensitizers... 

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. 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

During the course of an electron transfer event, the electron can be transported from one substrate to another through one of two pathways an outer-sphere (non-bonded) electron transfer or an inner-sphere (bonded) process. In an outer-sphere electron transfer, the coordination sphere of the donor and acceptor involved in the reaction remain intact and there is very little or, at most, a weak interaction between the components in the transition state. Conversely, in an inner-sphere electron transfer, the donor and acceptor interact through a ligand or atom and there is strong interaction between the components in the transition state. 

Kinetic schemes with an initial non-bonded electron-transfer step 106... 

Before we proceed to the quantitative aspects of Marcus theory, let us briefly look at an MO description of non-bonded electron transfer (36) between two species each with an even number of electrons. The electron is transferred from the highest occupied MO (HOMO) of the reductant (R) to the lowest... 

Symmetry rules for non-bonded electron transfer reactions... 

Thus the Marcus theory gives rise to a free energy relationship of a type similar to those commonly used in physical organic chemistry. It can be transformed into other relationships (see below) which can easily be subjected to experimental tests. Foremost among these are the remarkably simple relationships that were developed (Marcus, 1963) for what have been denoted cross reactions. All non-bonded electron-transfer processes between two different species can actually be formulated as cross reactions of two self-exchange reactions. Thus the cross reaction of (59) and (60) is (61), and, neglecting a small electrostatic effect, the relationship between kn, k22 and kl2... 

KINETIC SCHEMES WITH AN INITIAL NON-BONDED ELECTRON-TRANSFER STEP... 

Balzani (75) should be the preferred choice in the treatment of non-bonded electron-transfer processes. 

Summarizing, the Marcus model for non-bonded electron-transfer mechanisms provides several types of predictions that can be tested experimentally ... 

Ballardini et al., 1978. It should be noted that quenching of triplet methylene blue by aliphatic amines has quite different characteristics (Kayser and Young, 1976) and cannot be reconciled with a non-bonded electron-transfer mechanism Bock el al., 1979a aa Meisel, 1975... 

Concluding, the Marcus theory and related treatments can be applied to organic systems with some confidence and should be useful as one tool among others to distinguish non-bonded electron transfer mechanisms from other... 

It was suggested at one time that the chlorination of polymethyl-substituted aromatic hydrocarbons might proceed by a non-bonded electron-transfer mechanism (for arguments for and against, see Kochi, 1975 Baciocchi and Illuminati, 1975 Hart et al., 1977). The low value of E° for the C12/C12 couple, 0.6 V (Malone and Endicott, 1972), makes this suggestion rather unlikely (E° of, e.g. hexamethylbenzene is 1.85 V). 

Radical cations act both as electrophiles and one-electron oxidants toward nucleophiles (Eberson, 1975 Bard et al, 1976 Eberson et al., 1978a,b Evans and Blount, 1978) as shown in (6), and it is therefore important to find out which factors govern the competitition between these reaction modes. Evans and Blount (1978) measured rate constants and products for a number of [9,10-diphenylanthracene)+ /nucleophile reactions and found that iodide, rhodanide, bromide and cyanide undergo oxidation, whereas nucleophiles that are more difficult to oxidize form a C—Nu bond directly. Entry no. 13 of Table 15 shows non-bonded electron transfer to be feasible for these ions, and the reactions of [perylene]+ with iodide, rhodanide and bromide (entry no. 14) presumably can be classified in the same way. The reaction with chloride ion... 

Under this heading we can easily include dozens of redox reagents and literally thousands of individual reactions, but, as before, we shall limit ourselves to suspected or postulated non-bonded electron-transfer steps. The reagents are mostly oxidants, such as Co(III), Mn(III), Ag(II), Ir(IV), Ce(IV), and Fe(III), and the substrates mostly of a type that does not make ligand attachment to the metal ion possible. Again, accurate or sometimes even approximate E° data are not always available for the systems studied, so that one has to rely on data pertaining to aqueous solution for estimates of rate constants in non-aqueous systems. 

Hexachloroiridate ion, IrClJ-, is a complex inert to substitution and is known to undergo outer-sphere electron transfer with other inorganic species (cf. Cecil and Littler, 1968). Some of its reactions have been treated in Tables 12 and 14 and shown to be of the non-bonded electron-transfer type. Its reaction with various alkylmetals has been thoroughly studied, and some results are shown in Table 16 (entries nos. 14 and 15). Except for sterically hindered tetralkyltins the Marcus theory makes incorrect predictions for these reactions, and non-bonded electron transfer does not appear to be feasible. 

Tl(III) trifluoroacetate, sometimes with the requirement that boron trifluoride etherate should be present, causes fast dehydrodimerization of many aromatic compounds to give biaryls and/or diphenylmethanes in competition with thallation (McKillop et al., 1980). This system thus seems to be a typical exponent of (96). Table 16, entry no. 16, presents a crude estimate of the possibility of non-bonded electron transfer between Tl(III) trifluoroacetate and naphthalene, showing that it indeed appears to be feasible. Note however that both observed and estimated rate constants are based upon rather uncertain... 

We have already discussed several cases of fast Fe(III) oxidations which occur by a non-bonded electron-transfer mechanism (Tables 13 and 14). One case of a relatively slow reaction, involving the substitution-inert hexacyanoferrate(III) ion, is shown in Table 14 (entry no. 17) and clearly demonstrates the electron-transfer oxidizing properties of this species with respect to easily oxidized aliphatic amines. Whether the same mechanism holds for compounds more resistant to oxidation, such as methylnaphthalenes (Andrulis et al., 1966) remains to be seen (the estimated rate constant at 25°C is ca. 10-7 M l s-1). Generally, hexacyanoferrate(III) seems to be a good non-bonded electron-transfer reagent (for a review, see Rotermund, 1975). 

Pb(IV) oxidations mostly seem to proceed via aryllead intermediates, but it has been suggested that a non-bonded electron-transfer mechanism might operate in TFA (Norman et al., 1973) where methyl-substituted benzenoid compounds... 

Entries nos. 1 and 2 deal with a very common type of oxidant in organic chemistry, the so-called high-potential quinones (for a review, see Becker, 1974) which are normally considered to act as hydride-transfer reagents. Entry no. 1 is, however, unique in the sense that all substrates contain aromatic C—H bonds only, the strength of which precludes the operation of a hydride-transfer mechanism. Consequently, we see almost ideal electron-transfer behaviour, provided that E° (DDQH+/DDQH ) in TFA is set equal to 0.87 V. This value is entirely in line with those reported for other media (Becker, 1974). As we go to entry no. 2, where the substrate is difficult to oxidize and has at least one weak C—H bond, electron transfer is not feasible and hydride transfer takes place. The same holds for DDQ oxidation of substituted toluenes (Eberson et al., 1979). 

The influence of structural and other factors upon the rates of non-bonded electron-transfer reactions can be summarized in the following way ... 

In systems in which the donor and acceptor centers are in direct contact with each other or connected by a conducting bridge (conjugated bonds), electron transfer rates are very fast (kET = 10"13 -10 12 s 1). The transition occurs markedly slower when the donor-acceptor mutual orientation is not favorable for positive orbital overlap and, therefore, the electron coupling V is small. 

---