Metallocomplex Cation-Radicals

There is a large volume of literature describing one-electron oxidation of metallocomplexes. Such abundance is caused by the chemical nature of metallocomplexes in which the metallic center readily transforms into a higher state of oxidation. Kochi (1986) and Kaim (1987) have covered many of these problems in their reviews. Astruc (1995) has done a very important generalization in the field. 

One of the most important intricacies in the redox chemistry of organyl metallocomplexes is that both a metal and a ligand can be involved in oxidation. The following examples illustrate both the possibilities and manifest the corresponding consequences in chemical reactivity of the complexes. 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

Reversible one-electron oxidation of ferrocene (Fc) and its derivatives toward cation-radicals (the so-called ferrocenium cations) is a well-known reaction. The cation-radical center is localized at the iron atom. According to photoelectronic spectra and voltammetry data, such localization is 

Ratera et al. (2003) discovered valence tautomerism in the ferrocene connected through the ethylenic bond with perchlorotriphenylmethyl radical. As ascertained by Moessbauer spectroscopy, this species in the solid state exhibited a thermally induced intramolecular electron transfer resulting in the formation of ferrocenium and perchlorotriphenylmethyl anion moieties. The authors used the initial species in its trans form. If the cis form would be available, the possibility of rotation around the ethylenic bond would be interesting to disclose. According to the authors, the interconversion of the cation-radical and anion centers proceeds gradually. At ambient temperature, equilibrium composition of the tautomers is achieved. This peculiarity is important with respect to potential technical applications. 

---

The difference between the two reactions of Scheme 2.9 may also be considered in terms of the complete electron transfer in both cases. If the a-nitrostilbene anion-radical and metallocomplex cation-radical are formed as short-lived intermediates, then the dimerization of the former becomes doubtful. The dimerization under electrochemical conditions may be a result of increased concentration of reactive anion-radicals near the electrode. This concentration is simply much higher in the electrochemical reaction because all of the stuff is being formed at the electrode, and therefore, there is more dimerization. Such a difference between electrode and chemical reactions should be kept in mind. In special experiments, only 2% of the anion-radical of a-nitrostilbene were prepared after interruption of controlled-potential electrolysis at a platinum gauze electrode. The kept potential was just past the cathodic peak. The electrolysis was performed in the well-stirred solution of trani -a-nitrostilbene in AN. Both processes developed in this case, namely, trans-to-cis conversion and dimerization (Kraiya et al. 2004). The partial electrolysis of a-nitrostilbene resulted in redox-catalyzed equilibration of the neutral isomers. 

One-electron reduction of metalloorganic complexes or coordination between a metal and an anion-radical ligand may expand an electron shell of the central metal atom. Sometimes, anion-radical metallocomplexes contrast in this regard with the cation-radical ones. Thus, the same metal-loporphyrins form cation-radicals with charges and unpaired electrons on ligands (Shinomura et al. 1981) and anion-radicals with charges and unpaired electrons on metals (Lexa et al. 1989). 

Vinyl polymerization using metallocomplexes commonly proceeds by a radical pathway and rarely involves an ionic mechanism. For instance, metal chelates in combination with promoters (usually halogenated hydrocarbons) are known as initiators of homo- and copolymerization of vinylacetate. Similar polymer-bound systems are also known [3]. The polymerization mechanism is not well understood, but it is believed to be not exclusively radical or cationic (as polymerization proceeds in water). The macrochelate of Cu with a polymeric ether of acetoacetic acid effectively catalyzes acrylonitrile polymerization. Meanwhile, this monomer is used as an indicator for the radical mechanism of polymerization. Mixed-ligand manganese complexes bound to carboxylated (co)polymers have been used for emulsion polymerization of a series of vinyl monomers. Macromolecular complexes of Cu(N03)2 and Fe(N03)3 with diaminocellulose in combination with CCI4 are active in polymerization of MMA, etc. 

---