Ferrocenium ion

In a recent continuation of the work on dediazoniation of 2-(2 -propenyloxy)ben-zenediazonium salts (10.55, Z = 0, n= 1, R=H) in the presence of ferrocene, Beckwith et al. (1992) found that 3-ferrocenylmethyl-2,3-dihydrobenzofuran (10.65) is formed. The results are consistent with a mechanism involving electron transfer and dediazoniation followed by homolytic attack on the ferrocenium ion. This investigation resolved a long-lasting dispute regarding the heterolytic or homolytic character of the formation of arylferrocenes from arenediazonium ions (for literature since 1955 see Beckwith et al., 1992, references 1-7). 

Slightly removed from this in rigor is the use of a substituent to make a pure exchange into a net chemical reaction. No isotopic label is then needed. For example, the first reliable estimate of the rate constant for the exchange of ferrocenium ions and ferrocene was made on the basis of kinetic data for processes such as... 

We have investigated the ferrocene/ferrocenium ion exchange to determine the effects of different solvents on electron-transfer rates. There is probably only a very small work term and very little internal rearrangement in this system. Thus the rates should reflect mostly the solvent reorganization about the reactants, the outer-sphere effect. We measured the exchange rates in a number of different solvents and did not find the dependence on the macroscopic dielectric constants predicted by the simple model [Yang, E. S. Chan, M.-S. Wahl, A. C. J. Phys. Chem. 1980, 84, 3094]. Very little difference was found for different solvents, indicating either that the formalism is incorrect or that the microscopic values of the dielectric constants are not the same as the macroscopic ones. 

As we will discuss, such a symmetric profile is typical of an electron removal which does not lead to important structural changes. In fact, the 17-electron ferrocenium ion, [Fe(C5H5)2] +, generated upon oxidation, is a stable species which substantially maintains the original molecular frame (but for the fact that, because of the electron removal, the iron-carbon bonds are slightly weakened and hence elongated by about 0.04 A with respect to the neutral parent see Chapter 4, Section 1.1). 

The e (antibonding), a (nonbonding) and e 2 (bonding) are assumed to be the frontier orbitals for metallocenes. The non-bonding nature of the a orbitals, the HOMO, is in agreement with the observation that for ferrocene (18 valence electrons terminal electronic configuration e2a f) the removal of one electron to form the ferrocenium ion (17 valence electrons) does not substantially destabilize the molecular frame. 

Ferrocene gives rise to an anodic process, which in controlled potential coulometry involves one electron per molecule. As a consequence of the exhaustive oxidation the original yellow solution turns blue-green, a colour typical of the iron-centred ferrocenium ion (/Uax = 620 nm). The voltammogram of the final solution is complementary to that shown in Figure 2. 

As far as the AEP values are concerned, it cannot be ruled out that the molecular rearrangements that occur on passing from ferrocene to ferrocenium ion might also play a role, even if minimal, in their departure from the value of 59 mV, or in lowering the degree of electrochemical reversibility of the process. 

In this connection, in order to judge the level of these molecular rearrangements, the solid state X-ray structures of ferrocene and ferrocenium ion could be compared. Unfortunately, the molecular disorder caused by the rotation of the cyclopentadienyl rings in ferrocene means that the comparison procedure is far from simple and, in fact, the first results were interpreted in terms of a staggered conformation of the two cyclopentadienyl rings. It is now believed that the eclipsed conformation is the more stable (with a rotation angle of about 10°).2 However, as the rotational barrier is notably low (about 4 kJ mol-1), the conformation that one observes is probably that imposed by crystal packing forces. 

A similar situation occurred for the ferrocenium ion. Until 1983, on the basis of the available structural data, it was thought that the cyclopentadienyl rings assumed an eclipsed conformation. However, more recent data demonstrate that it assumes a staggered conformation, even if once again the crystal packing forces are considered to be the determining factor.3... 

Table 3 Comparison between the bond lengths (rounded off to the second decimal figure) offerrocene and ferrocenium ion...

To conclude the matter of monoferrocene molecules we consider those cases in which the cyclopentadienyl substituents form bridges between the two rings. Such derivatives are named ferrocenophanes. The best known ferrocenophanes are those which contain saturated (tri- or tetra-methylene) carbon chains. Like ferrocenes, these molecules also exhibit reversible one-electron oxidation to the corresponding ferrocenium ions. 

Figure 14 Schematic representation of the electronic effects in mixed-valent species according to the Robin-Day classification. Here the charge has been assumed to be positive as in ferrocenium ions...

In support of the electrochemical evidence, the molecular structure of the biferrocenium ion in [( -CsF Fe -CsfLOf -CsfLOFe -CsHs)] [I3] shows that both pairs of cyclopentadienyl rings have an eclipsed conformation, Figure 16. Furthermore, the mean Fe-Cp(Centroid) distance is equivalent in both the ferrocenyl units and equal to 1.68 A. Speculatively, this value is intermediate between the values previously observed for ferrocene and ferrocenium ions, thus supporting charge delocalization between the two centres.27... 

It is useful to note that in contrast to the elongation of the M-C bonds observed on passing from ferrocene to ferrocenium ion, in the case of the cobaltocene/cobaltocenium transition, because of the removal of... 

The bridge effect was scrutinized in the range of diferrocenyl derivatives, especially of those that are applicable in catalysis and material science (Atkinson et al. 2004). One-electron oxidation of these derivatives also proceeds easily, reversibly, and gives rise to cation-radicals (ferrocenium ions). In contrast to the cation-radical of ferrocenylacrylonitrile, the hole transfers through conjugated systems were proven for the bis(ferrocenyl)acetylene cation-radical (Masuda and Shimizu 2006), the bis(ferrocenyl) ethylene cation-radical (Delgado-Pena et al. 1983), and the cation-radical of bis(fulvaleneiron) (LeVanda et al. 1976). These structures are presented in Scheme 1.30. 

The complexes [Ru(NH3)5(26)] and [Ru(NH3)5(27)] + have been prepared and isolated as PFg salts. Oxidation by ferrocenium ion occurs with the redox change being centered on Ru. The solvent has a significant effect on the difference between Pi/2°(Fe3+/Fe2+) and Pi/2°(Ru +/Ru2+) and there is a linear relationship between this A and the Gutmarm solvent donor number. ... 

Both in acetonitrile and in other non-aqueous solvents, a major problem arises in terms of the manner in which the potential values are reported by various investigators. Koepp, Wendt, and Strehlow [6] noted that hydrogen ion is the poorest reference material on which to base nonaqueous potentials because of the extreme differences in its solvation in various solvents. On the basis of an investigation of the solvent dependence of 18 redox couples, these investigators concluded that ferrocene/ferrocenium ion (i.e. bis(cyclopentadienyl)iron(III/II), abbreviated as Fc+ /Fc°) and/or cobal-tocene/cobalticenium ion represented optimal potential reference materials for nonaqueous studies. On the basis of their minimal charge (+1, 0) and their symmetry (treated as though they were roughly spherical), the potentials of these two redox couples are presumed to be relatively independent of solvent properties. 

Vesicles can be formed not only by simply adding a surfactant (eventually with a cosurfactant) but also upon sonification. As shown by Gokel and coworkers [32], their reversible collapse can be controled chemically by influencing the polarity of head groups. For instance, cholestanyl ferrocenylmethyl ether 92 does not form any aggregates but its corresponding ferrocenium salt obtained by oxidation afforded vesicles upon sonification. However, the aggregates collapsed when ferrocenium ion was reduced to its normal... 

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

Some redox couples of organometallic complexes are used as potential references. In particular, the ferrocenium ion/ferrocene (Fc+/Fc) and bis(biphenyl)chromium(I)/ (0) (BCr+/BCr) couples have been recommended by IUPAC as the potential reference in each individual solvent (Section 6.1.3) [11]. Furthermore, these couples are often used as solvent-independent potential references for comparing the potentials in different solvents [21]. The oxidized and reduced forms of each couple have similar structures and large sizes. Moreover, the positive charge in the oxidized form is surrounded by bulky ligands. Thus, the potentials of these redox couples are expected to be fairly free of the effects of solvents and reactive impurities. However, these couples do have some problems. One problem is that in aqueous solutions Fc+ in water behaves somewhat differently to in other solvents [29] the solubility of BCr+BPhF is insufficient in aqueous solutions, although it increases somewhat at higher temperatures (>45°C) [22]. The other problem is that the potentials of these couples are influenced to some extent by solvent permittivity this was discussed in 8 of Chapter 2. The influence of solvent permittivity can be removed by... 

An acid catalyzed double Schiff-base condensation of 5 with o-phenylenediamine, 6, then gives the cyclic non-aromatic sp3 texaphyrin ring 7, a macrocyclic product that can be viewed as being an expanded porphyrinogen [24], The sp3 form can be oxidized with ferrocenium ion to give the corresponding aromatic derivative, namely the metal-free sp2 form 8 (Scheme 2) [25],... 

The formation of a 1 4 complex 2(13)4 (13 ferrocene) was confirmed both by NMR spectroscopy and crystallographic analysis (Figure 8.13) [29]. The oxidation potential of ferrocene to ferrocenium ion [Fe(III)] in the cage was anodically shifted by 73 mV as compared to that in water. Thus, the cationic cage 2 suppresses the oxidation of ferrocene to ferrocenium ion. 

Reaction of 66 with FeBr2 forms the cluster [Fe5S4(CO)i2]2- (68). One-electron oxidation of 68 by ferrocenium ion or [Fe(phen)3]3+ results in quantitative conversion to the monoanion analog [Fe5S4(CO)12]" (69). The... 

Extension from atriad (Fc—ZnP—Ceo) to atetrad (Fc—ZnP—H2P—C60) results in remarkable elongation of the final CS state (44). The multistep ET processes afford the final CS state, Fc —ZnP—H2P—Ceo which is detected as the transient absorption spectrum obtained by nanosecond laser flash photolysis [Fig. 8(rz)] (44). The Ceo fingerprint ( 1000 nm) NIR band is clearly seen, whereas the weak absorption features of the ferrocenium ion prevents its direct detection. The quantum yield of the CS state was determined to be 0.24 (44). The relatively low quantum yields results from the competition of ET from ZnP to H2P versus the BET from Ceo to H2P to give the triplet excited state ( H2P ... 

The previous examples rehed on a radical termination. Cationic termination could also be used, after suitable oxidation of the final radical (Scheme 75) [211]. Thus, ester 252 was treated with base and ethylchloro-formate to give anion 253, which was oxidized to malonyl radical 254 by the ferrocenium ion. Cyclization/oxidation gave cation 256, which yielded 93% of malonate 257 by loss of the TMS group. This adduct was further elaborated upon and led to a cyclopentanoid monoterpene, dihydronepetal-actone. 

---