Electron alkylmetals

For a particular iron(III) oxidant, the rate constant (log kpe) for electron transfer is strongly correlated with the ionization potential Ip of the various alkylmetal donors in Figure 4 (left) (6). The same correlation extends to the oxidation of alkyl radicals, as shown in Figure 4 (right) (2). [The cause of the bend (curvature) in the correlation is described in a subsequent section.] Similarly, for a particular alkylmetal donor, the rate constant (log kpe) for electron transfer in eq 1 varies linearly with the standard reduction potentials E° of the series of iron(III) complexes FeL33+, with L = substituted phenanthroline ligands (6). 

Figure 4. Correlation of the ionization potentials of alkylmetal donors with the electron-transfer rate constant (log kFe) for Fe(phen)s3+ (%), Fe(bpy)s3+ (O), and Fe(Cl-phen)s3+ ((D), (left). The figure on the right is the same as the left figure for Fe(phen)s3+ except for the inclusion of electron-transfer rates for some alkyl radicals as identified, (Note the expanded scale,)...

Figure 16. Relationship between the activation jree energy and the driving force for electron transfer for alkylmetals to TCNE (left) and IrCl6z (right) according to Marcus Equation 4.

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. 

Organometallic Complexes as Electron Donors or Acceptors 621 2.2.1 Organometallic Donors Alkylmetals... 

Metallocenes are useful electron donors as judged by their low (vertical) ionization potentials in the gas phase and oxidation potentials in solution (see Table 2). In fact, the electron-rich (19 e ) cobaltocene with an oxidation potential of E°ox = -0.9 V relative to the SCE [45] is commonly employed as a very powerful reducing agent in solution. Unlike the alkylmetals (vide supra), the HOMOs of metallocenes reside at the metal center [46] which accounts for two effects (i) Removal of an electron from the HOMO requires minimal reorganization energy which explains the facile oxidative conversion from metallocene to metallocenium. (ii) The metal-carbon bonding orbitals are little affected by the redox process, and thus the resulting metallocenium ions are very stable and can be isolated as salts. 

The electron donicity of alkylmetal complexes is strongly enhanced when a negative charge is introduced. Thus, polyalkylmetallates such as borates or aurates exhibit oxidation (peak) potentials much lower than neutral polyalkylmetal complexes (compare Tables 1 and 5). An extreme case represents dimethylaurate(I) with an oxidation (peak) potential of 0.1 V (relative to the SCE) [64] which is more than 1 V lower than that of the isoelectronic dimethylmercury(II) complex (Eqx = 1 46 V relative to the SCE) [42]. Similarly, the oxidation potential of tetramethylborate (Eotl = 0.58 V relative to the SCE) [65] is substantially lower than that of the isoelectronic neopentane which exceeds 3 V [66]. 

Figure 19. Correlation of the activation energy with the free energy (ACet) for electron transfer from alkylmetals (as...

Thus, the reported product yields so far are moderate to good (usually lower than 50%). This disadvantage was circumvented by the use of dienes or acetylenes as described later, since these substrates afford structurally or electronically more stable intermediates, such as allyl- or alkenylmetals, compared with alkylmetals, which not only... 

The reaction of a C-H bond at the carbon atom p to the metal in an alkylmetal complex leads to a facile elimination of an alkene, that is, P-hydride eUmination [Eq. (6.80)], which is the main decomposition pathway for metal alkyls. Requirements are a two-electron vacant site at the metal and a near-coplanar arrangement of the M-C-C-H moiety to bring the P-H close to the metal. ... 

Furthermore, alkyl- and aryl-metals of the third and fifth main group react very readily with other electron-donors or -acceptors, respectively thus alkylmetals of the third group (electron acceptors) react with those of the fifth group (electron donors) to form addition compounds, for example, (CH3)3B As(CH3)3. A detailed review of this aspect has been published by Stone.20... 

Protonolysis of electron-rich alkylmetals may proceed via initial electrophilic attack at metal, i.e. hydride complex formation. Different from the case of the halogenolysis, the subsequent C-H bond formation occurred via internal reductive elimination with overall retention of configuration (Eq. 8.24) [129]. 

Ferrocene is a viable electron donor by virtue of its vertical ionization potential of only 6.86 eV in the gas phase [37] and its oxidation potential of only + 0.41 vs SCE [39] in CH3CN solution. Unlike the alkylmetal donors, the one-electron oxidation product, ferricenium cation, is stable, and various salts of it can be isolated. This stability arises from the metal-centered nature of the HOMO (e2g in 05 symmetry) [40] which minimizes the effect of electron removal from the metal-carbon bonding orbital. Indeed, ferrocene and related metallocenes undergo multiple redox reactions without disruption of the sandwich structure [41]. 

Since the fate of the redox pair must be back electron-transfer (in the absence of a diversionary reaction), interest centers on chemical reactions fast enough to obviate the back electron transfer process in Scheme V. Clearly, instability in the reduced acceptor or oxidized donor can promote efficient photoreaction. For example, oxidized alkylmetal donors are unstable, and the charge-transfer photolysis of R4M (M = Sn and Pb) in molecular complexes with TCNE is a convenient source of alkyl radicals [206] ... 

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