Paramagnetic rings anion radicals

These examples demonstrate the well-known process of polymerization initiated by anion-radicals. Our next consideration is devoted to an unusual case of initiation. Intercalation of fullerenes by metals results in the formation of fullerene-metal derivatives. Paramagnetic metallofullerenes (anion-radicals) are the fullerenes doped with endohedral metal. According to calculations and structural studies, LaCs2, for example, contains La in the center of one hexagonal ring of the fuller-ene cage (Akasaka et al. 2000, Nishibori et al. 2000, Nomura et al. 1995). Intrafullerene electron transfer in metallofullerenes is possible (Okazaki et al. 2001). 

Silacyclobut-l-yl, 1-methylsilacyclobut-l-yl, and 3-methylsilacyclobut-l-yl radicals were prepared by 7-irradiation of the corresponding silanes in adamantane matrices at 77 K. Electron paramagnetic resonance (EPR) studies show that the radicals have a nonplanar ring <1988JOM(341)273>. The reduction of phenylsilacyclobutanes with potassium gave anion radicals, and their EPR spectra were recorded at — 75 °C (Table 5) <1983ZOB1315>. 

Voltammetric data for ester reductions are available for several aromatic esters [51-54], and in particular cyclic voltammetry shows clearly that in the absence of proton donors reversible formation of anion radical occurs [51]. In dimethylfonnamide (DMF) solution the peak potential for reduction of methyl benzoate is —2.29 V (versus SCE) for comparison dimethyl terephthalate reduces at —1.68 V and phthalic anhydride at —1.25 V [4]. Half-wave potentials for reduction of aromatic carboxylate esters in an unbuffered solution of pH 7.2 are linearly correlated with cr values [51] electron-withdrawing substituents in the ring or alkoxy group shift Ei/o toward less negative potentials. Generally, esters seem to be more easily reducible than the parent carboxylic acids. Anion radicals of methyl, ethyl, and isopropyl benzoate have been detected by electron paramagnetic resonance (epr) spectroscopy upon cathodic reduction of these esters in acetonitrile-tetrapro-pylammonium perchlorate [52]. The anion radicals of several anhydrides, including phthalic anhydride, have similarly been studied [55]. 

In addition to substitution of the carbonyl groups, changes in the cyclopentadienyl ring have also been extensively studied. The use of pentamethylcyclopentadiene led to formation of some of the first organometallic multiply bonded complexes as discussed in Section 4.8. The tris(pyrazolyl)borate ion (Tp , see Tris(pyrazolyl)borates) is a Cp analog." It reacts with Mo(CO)6 to yield the TpMo(CO)3 anion. Oxidation yields the paramagnetic radical that shows no tendency to form a single metal-metal bond. Decarbonylation yields a triply bonded stracture. These reactions are summarized in equation (16). 

According to the mechanism, the extent of CO2 reduction (and the consequent kind of products) is determined by the reduction ability of the anionic form of the complex and the stability of the intermediate products . On the other hand, the difference in the ability to produce methane between the amino and the butyl Cu phthalocyanine indicates the strong dependence of the reduction on the substituents in the phthalocyanine ring. Then the mechanism is based on the strong reducing behavior of the reduced form of CuPc. Then in the first stage, the reduced complex can transform CO2 in the radical cation and some amount of this radical becomes HCOOH whereas another coordinates with the Cu center (paramagnetic). After various steps, the coordinated radical anion terms into CO. If the adduct is sufficiently stable, the consecutive reduction steps permits the transfor-... 

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