Ferricenium

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. 

Ferricyanide has been employed as an electron acceptor for lactate in place of NAD Alternatively, diaphorase can be used in conjunction with ferricyanide Bindschedler s Green ferricenium or dichlorophenylindophenol to catalyze the oxidation of NADH while providing a more easily monitored species than NADH itself. 

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. 

FIG. 2 Cyclic voltammogram of the ferricenium transfer across the water-DCE interface at lOmVs. The electrochemical cell featured a similar arrangement to Fig. 1(b), but the organic phase contained 2mM of ferrocene. Heterogeneous oxidation of Fc occurred in the presence of 0.2mM CUSO4 in the aqueous phase. Supporting electrolytes were lOmM 02804 and lOmM BTPPATPBCl. The transfer of the standard tetramethylammonium (TMA+) under the same condition is also superimposed. 

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. 

We can see that ferrocene is ideally suited to this application from model compounds. Using Cr(CNBun)5 as our model for the polymer-based Cr complex (21), we can estimate that the eq for reaction between ferricenium and Cr(CN [P])6, leading to removal of one electron, is approximately 10. It is also thermodynamically favorable [Ken = 1012] for a second electron to be removed to give the bound Cr(II) species. 

In most cases oxidation of uncharged borabenzene complexes produces cations which can only be observed electrochemically. The iron compounds 62 and 63 may serve as an example. Oxidation is fully reversible in rigorously dried CH2C12 but irreversible in more basic solvents such as THF and acetonitrile (62). Preparative oxidation with Ce(IV) salts cleanly produces monosubstituted ferricenium cations 64 (Scheme 10) (66). In contrast to the above mentioned boranediyl extrusions, the substituent at boron is retained here in the newly formed cyclopentadienyl ring. 

Metallocenes. — With the exception of the 4d6 system, ruthenocene, Ru(Cp)2, all the metallocenes for which adequate electronic spectra are available belong to the first transition series. For the 3d series metallocenes (and 1,1 dimethylmetallocenes) are known as neutral, Af(Cp)2, species for the elements from vanadium to nickel inclusive, whilst the cationic, M(Cp)2+, systems are found for chromium, iron, cobalt, and nickel, these being described generically as metallicenium salts. A number of substituted ferricenium species have also been reported and studied, including the 1,1 dimethyl derivative, but few spectro-... 

In principle it should be possible to determine the anisotropy of the paramagnetic susceptibility for some systems of pseudo-axial symmetry, but apart from the citation (74) of a private communication regarding the ferricenium cation, no experimental data are available for metal sandwich complexes. Such measurements should however be possible for at least some metallocenes and mixed sandwich complexes since these are found to crystallise in either a monoclinic (Fe(Cp)2 (6)) or orthorhombic (Ru(Cp)2, (Cp)V(Ch),... 

As before the data may be interpreted using the theory outlined above, but again understanding the listed k values as 11 23 and the results for the ferricenium cation and a number of substituted species are given in Table 19. 

There are though a number of other reasons for believing that the results of Prins et al. do correspond to those for the Fe(Cp)2+ system. Thus, it is hard to understand the wide range of g values found for the various substituted ferricenium complexes if these are all to be ascribed to Fe3+, and in addition the distortion parameters deduced by Prins et al. are very much of the same order of magnitude as those determined for the carborane and... 

Scheme VI. Representation of the interface energetics for intrinsic a-Si H at short circuit, dark equilibrium with ferricenium/ferrocene in EtOH, electrolyte solution (left) and under illumination with 632.8-nm light with a load in series in the external circuit (right). The diagrams are adapted from data in Reference 65 for intrinsic a-Si H (1-4-fi thick) on stainless steel first coated with heavily n-doped a-Si H (200-A thick) to ensure an ohmic contact near the bottom of the conduction band. In typical experiments Eredox = 0.4 V vs. SCE.

The concentrations of the two forms, P and Q, of the cosubstrate may be regarded as approximately uniform within the film containing the enzyme molecules, the attaching chains, and the ferrocene and ferricenium moieties. Under such conditions, the current may be expressed by... 

The iron-mediated synthesis of 2-oxygenated carbazole alkaloids is limited and provides only a moderate yield (11%) for the oxidative cyclization to 2-methoxy-3-methylcarbazole using iodine in pyridine as the reagent [90]. Ferricenium hexafluorophosphate is the superior reagent for the iron-mediated arylamine cyclization leading to 3-oxygenated carbazoles (Scheme 12). Electrophilic substitution of the arylamines 16 with the complex salt 6a leads to the iron complexes 17. Oxidative cyclization of the complexes 17 with an excess of ferricenium hexafluorophosphate in the presence of sodium carbonate affords... 

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... 

Alternatively, using the iron-mediated arylamine cyclization, a short access to carazostatin (247), albeit in lower yield, was achieved. The oxidation of the complex 714 with ferricenium hexafluorophosphate in the presence of sodium carbonate... 

An alternative method for the oxidative cyclization of the arylamine-substituted tricarbonyl(r -cyclohexa-l,3-diene)iron complex (725) is the iron-mediated arylamine cyclization. Using ferricenium hexafluorophosphate in the presence of sodium carbonate provided hyellazole (245) directly, along with the complex 727, which was also converted to the natural product (599,600) (Scheme 5.71). 

Removal of an electron from neutral ferrocene can yield a large number of different ferricenium salts. This is a dramatic visual transformation, due to the vivid blue color of the ferricenium cation in contrast to the pale orange of ferrocene solutions. The cation displays two maxima 250 mp (<= = 12,000) and 617 mp (e = 340).18... 

Since the spectrum of the ferricenium cation is so distinctively different from that of ferrocene in halogenated solvents, no ion must be formed as yet and the 307 mu band must be due to a predissociative species probably close to that depicted as 3. 

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