Redox alkylmetals

Alkylmetals (MR) have long been added to the reactor to increase the polymer yield of Cr/silica catalysts. They can be strong reducing agents or scavengers to remove the usual redox by-products (scheme 3). The induction time of Cr(VI)/silica is usually eliminated but the maximum activity is not greatly affected. 

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. 

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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