Ferricenium, reaction with

These observations for Fe(Cp)2 in the presence of the polymer-bound Cr complex are consistent with Fe(Cp)2+, generated electrochemically, undergoing a reaction with Cr(CN-[P])c resulting in the chemical reduction of Fe(Cp)2+ and oxidation of the Cr species. Therefore, when the cathodic part of the Fe(Cp)2 /Fe(Cp)2+ wave is scanned, little ferricenium ion remains to be reduced electrochemically. As a result, the ferrocene molecule has effected the transfer of electrons from the polymer to the electrode. 

Electrophilic aromatic substitution of 2,3-dimethyl-4-methoxyani]ine by reaction with the tricarbonyliron-coordinated cyclohexadienylium salt generates the aryl-substituted tricarbonyliron-cyclohexadiene complex. Treatment of this complex with very active manganese dioxide results in oxidative cyclization and aromatization with concomitant demetallation to afford directly 4-deoxycarbazomycin B, a degradation product of the antibiotic carbazomycin B [32]. Using ferricenium hexafluorophos-... 

In a sensitive, indirect method Se(IV, VI) is reduced by Cr(II) to hydrogen selenide which, on passing in a stream of nitrogen through an alkaline solution of Fe(CN)6 ", reduces the latter to Fe(CN)6 [Se(-II) — Se(IV)]. The ferrocyanide formed reacts with the 1,10-phenanthroline complex of Fe(III) to give an equivalent amount of the complex Fe(phen)3. The absorbance of this complex is measured at 508 nm (e = 6.8-10 ) [49]. The molar absorptivity increases to e = 1.4-10 (Xmax = 535), if bathophenanthroline is used instead of 1,10-phenanthroline. In another indirect method Se(lV) oxidizes ferrocene to the ferricenium ion which is oxidized to Fe(III), then reduced to Fe(II), to be determined finally by the colour reaction with 1,10-phenanthroline (e = 4.2-10 ) [50]. 

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

The data for the ferricenium half-reaction deserve several comments. The Michaelis-Menten kinetics obtained in the UV-vis experiment supports the formation of the GO-ferricenium intermediates postulated in Scheme 5. The ratio k2i)/K which corresponds to the bimolecular interaction of GO(red) with the ferricenium ion, equals ca. 1 x 105 M-1 s 1 (79) and this must be compared with the observed rate constants of 1.4 x 105 M-1 s 1 found for ferrocene using the electrochemical technique under similar conditions (87). 

The reaction takes place if R1 is tert-Bu and R2 is —C6H4N(CH3)2-p. If R1 is still tert-Bu but R2 is Fc (ferrocenyl), no cleavage occurs (Schmittel et al. 2001). The ferrocenyl-con-taining cation radical is apparently a persistent ferricenium type of species. Intramolecular activation of the scission with ferrocene as a redox unit is not operative. 

In contrast to pentamethylpyrrole, octamethyl-l,l -dipyrrole 903 is stable to air for some days. The reaction of the dipyrrole with common chemical oxidants such as ferricenium (fc ) or silver salts leads to complex mixtures of unidentified products. Oxidation with I2 in acetonitrile results in a dipyrrolium iodide [903H" ]l3 -1/212. Treatment of the dipyrrole 903 with NOiBFJ (CH2CI2, rt, 12 h) gives the radical cation salt 904 (Scheme 174) <2000J(P2)353>. The primarily formed radical-cation 904 slowly abstracts a hydrogen atom, possibly from the solvent or the supporting electrolyte, to yield protonated dipyrrole 905. 

One very unusual reaction involving the /xs-ij -RCCRCR" ligand is shown in Scheme 13. Oxidation of the Cp Ru 3 (H)4 -HCCRCR) by treatment with ferricenium ion... 

The efficiency of electron-transfer reduction of Cgo can be expressed by the selfexchange rates between Coo and the radical anion (Ceo ), which is the most fundamental property of electron-transfer reactions in solution. In fact, an electrochemical study on Ceo has indicated that the electron transfer of Ceo is fast, as one would expect for a large spherical reactant. This conclusion is based on the electroreduction kinetics of Ceo in a benzonitrile solution of tetrabutylammonium perchlorate at ultramicroelectrodes by applying the ac admittance technique [29]. The reported standard rate constant for the electroreduction of Ceo (0.3 cm s ) is comparable with that known for the ferricenium ion (0.2 cm s l) [22], whereas the self-exchange rate constant of ferrocene in acetonitrile is reported as 5.3 x 10 s , far smaller than the diffusion limit [30, 31]. 

The rate constant ( ex) of the electron-exchange reaction between r-BuCeo and t-BuCeo was determined as 1.9 x 10 M s at 298 K by analyzing the linewidth variations of the ESR spectra [43], Although Ceo is perturbed by the t-Bu addend, the much larger value, compared with the value for the ferrocene/ferricenium exchange (5.3 x 10 s ), corroborates the high efficiency of electron transfer... 

The very characteristic oxidation reactions of these substances probably occur through removal of one or more of these nonbonding electrons. The magnetic susceptibility of the ferricenium ion (2.49 BM) 52) is in accord with the E2g state for this ion, as is a recent Mossbauer determination of the sign of the electric field gradient in ferrocene 9) and the observed decrease in quadrupole splitting in the Mossbauer spectrum of ferrocene on oxidation to the ferricenium ion 69). 

EPh3 (E = P, As, Sb, Bi) was also coordinated to a molybdenum carbonyl. This ionic complex was formed by treatment of the [CpMo(CO)3]2 dimer with ferricenium tetrafluoroborate in the presence of EPh3 (E = P, As, Sb, Bi) and Ph2P(CH2)PPh2, where the Mo—Mo bond of the dimer is cleaved in a photochemical reaction . Similarly, the highly reactive tributyl bismuth can be coordinated to group 6 metal carbonyls in a photochemical reaction (equation 38) . 

One very unusual reaction involving the /u-a-jj -RCCRCR" ligand is shown in Scheme 13. Oxidation of the Cp Ru 3(H)4(/u.3-j7 -HCCRCR) by treatment with ferricenium ion gives the cationic complex with a closed C3 ring, which may be considered to be a jj -cyclopropenyl. This is a rare example of a C-C reductive elimination initiated by a two-electron oxidation. ... 

Photochemical studies of iron-group metallocene substrates have received much attention, e.g. quenching of excited uranyl ion luminescence, formation of charge-transfer complexes with TCNE, redox reactions of octamethyl-ferrocene and of carboxylate anions of the types FcCOj and Fc(X)COr (X = saturated or unsaturated alkyl chain). Enzyme-catalysed one-electron oxidation of ferrocene derivatives has also been studied. Ferrocene-ferricenium cation redox systems have been incorporated into electrochemical and photo-electrochemical cells, and have found use for catalysis of photo-addition of MeOH to Bu NCO. Inter-valence electron-transfer properties of partially oxidized Fc(C C)nFc ( = 0—2), meso-tetraferrocenylporphyrin, and poly(vinylferrocene) have been assessed. 

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