Ferricenium ions

Ferricenium ion oxidation of acetoin in acidic aqueous ethanol (50 % v/v) involves rate-determining enolisation , producing a zero-order decay of oxidant. The oxidation rate approximates to that of deuteration under similar conditions. 

A recently characterized class of dehydrogenases are the quinoproteins which contain a pyrroloquinolene quinone prosthetic group and do not require a separate co-factor Electron transfer mediators such as phenazine ethosulphate 2,6-dichloroindophenol and ferricenium ions have been used to recycle the quinoprotein the reduce mediator is detected amperometrically. 

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. 

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 temperature dependence of kET(CR3> revealed only a moderate change (2.6-3.0 sec ) upon varying the temperature between 163 and 203 K [47]. The longest lifetime of the resulting charge-separated state (i.e., ferricenium ion Ceo radical anion pair) in frozen benzonitrile (PhCN) is determined as 0.38 sec [47], which is more than one order of magnitude louger than any other intramolecular... 

Tarr and Wiles,15 attempting to explain the fact that acylferrocenes are mildly photolytically unstable in all hydroxylic solvents, postulate Eq. (10) leading to the unstable, substituted ferricenium ion. 

Here GO(ox) and GO(red) are the oxidized (native) and reduced forms of the enzyme, respectively. The effect of dioxygen is neglected and the transfer of the first electron from reduced FADH2 at ferricenium ion is considered as the rate-limiting step for simplicity. Equation (30) describes the steady-state rate of the enzymatic ferricenium fading. 

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). 

Attempts to correlate the activity of ferricenium ions toward GO(red) with their redox potentials are disappointing. Clear-cut dependencies have been established neither in the earlier (72), nor in the recent studies (76). The investigation of over 50 substituted ferrocene derivatives has confirmed that the rate of mediation with GO depends on a number of conflicting factors (76). The redox potentials varied in the range from -0.20 (HOFc) to +0.39 V (Me3NCH2Fc+ 4) vs. SCE the rate constants calculated as... 

Fig. 5. Lowering of the rate constant for oxidation of reduced GO by ferricenium ions ki2 with elongation of the alkyl radical of H(CH2) Fc + (pH 7.0 0.1 M phosphate 25 °C). From Ref. (90).

Consequently, the oxidation of GO(red) by ferricenium ions is very sensitive to their bulkiness. 

HRP-catalyzed steady-state oxidation of ferrocenes by H2O2 is fun to study by UV-vis spectroscopy because ferricenium ions generated are the only absorbing species at 500-700 nm (Fig. 3). A problem, actually solved by using micellar solutions, is the limited solubility of ferrocenes in water. The kinetics of oxidation of n-alkylferrocenes (alkyl = H, Me, Et, Bu and CsHn) (119) and later of larger variety of ferrocenes shown in Chart 1 (120) via Eq. (37) has been studied in detail in micellar systems of Triton X-100, , and SDS, mostly at pH 6.0 and 25 °C. Ferrocenes with longer alkyl radicals are oxidized immeasurably slow. 

According to Scheme 27, the rate of formation of ferricenium ions is given by Eq. 28, where the catalytic rate constant (/ cat) corresponds to 2A et- The other cofacial dicobalt porphyrins [Co2(DPA), Co2(DPB), and Co2(DPD)] (DPA = bisporphyrin with anthrocene spacer, DPB = bisporphyrin with biphenylene spacer, DPD = bisporphpyrin with dibenzofuron spacer) also catalyze the reduction of O2 by Fe(CsH4Me)2, but the amount of [Fe(C5H4Me)2] formed is <4 equiv of O2 (168). Thus, the clean four-electron reduction of O2 by [Fe-(C5H4Me)2]... 

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 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). 

The most elegant and effective form of this sensor uses ferricenium ion [FeII (Cp)2 in place of O2 as the election acceptor.3 The electrolysis current of the sensor, which is proportional to the FelllfCp) concentration, decreases as the... 

Thus, any oxidase-substrate reaction that consumes O2 (ferricenium ion) in a well-defined process is amendable to the development of electrochemical sensors and assays for substrates and enzyme activity. 

Superoxide ion is an effective reducing agent of transition-metal complexes examples include copper(II),4 50 manganese(in),5I,52 iron(III).53,54 It also reduces ferricenium ion, MnIV202(o-phen)44- -, CoPI(o-phen)33+, and IrIVci62- by one-electron processes.55... 

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]. 

Reports on the electronic spectra of substituted ferrocenes are quite frequent 39,140,190,191,224,314,354,358,399,400,529,578), and. even the effect of pressure on the spectra of ferrocene and ferricenium ion has been reported 531>. With all this information available the ferrocenes should attract the interest of more photochemists in the future. 

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... 

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