Metallocenes oxidation potentials

Owing to its stability, solubility, and highly reproducible oxidation behavior, ferrocene has long been used as an electrochemical standard in nonaqueous solvents. Not surprisingly, the electron-donor or -acceptor properties of ring substituents in ferrocenes and other metallocenes have been repeatedly evaluated with electrochemical techniques. Measurements have been obtained using polarography,150 cyclic voltammetry (CV),151 chronopotentiometry,152 photoelectron spectroscopy, 53 and Fourier transform ion cyclotron resonance mass spectrometry.154 Extensive compilations of such data are available.155 156 Historically, variations of oxidation potentials have been discussed almost solely in terms of the... 

The importance of solvent effects can be extracted from correlations of half-wave potentials determined by CV or polarographic methods with other measures of oxidizability, especially gas-phase free energies of ionization and ionization potentials. Within a series of unsubstituted metallocenes that differ only by the metal, the oxidation potentials of the compounds in solution and their gas-phase free energies of ionization vary directly (Fig. 17),154, 60 indicating that the effect of solvent is consistent from one metallocene to another. 

In substituted metallocenes, however, oxidation potentials do not always correlate well with gas-phase ionization energies. For example, although there is a positive trend between the irreversible oxidation potentials of a series of substituted ruthenocenes and their gas-phase energies of ionization, the quantitative correlation between the two is poor (Fig. 18).161 The weak correlation is believed to reflect changes in solvation that accompany the addition of substituents. 

Fig. 17. Correlation of the gas-phase free energies of ionization and oxidation potentials for unsubstituted metallocenes. The vlP for all compounds are from Ref. 154 the sources of the Ex and values are as follows [CoCp2] and [CrCp J. Refs. 198. 199 [NiCps] and [FeCpi]. Refs. 198. 200 [OsCp2l. Refs. 201. 202.

It is well known that ferrocene and related metallocene compounds undergo reversible oxidation by a variety of chemical oxidants. The particular ease with which ferrocene undergoes oxidation to its ferricinium cation allows one to determine the effect of a series of substituents on the oxidation potential of the ferrocene system. Thus a sensitive probe is available for determining the electronic effect a substituent has on the iron heteroatom and a device is made available by which one might elucidate the mechanism by which such an effect is transmitted from the substituent to the iron atom in the ferrocene system. It is unfortunate that lack of suitable derivatiyes has precluded similar studies in systems containing other transition metal atoms. 

Metallocenes are useful electron donors as judged by their low (vertical) ionization potentials in the gas phase and oxidation potentials in solution (see Table 2). In fact, the electron-rich (19 e ) cobaltocene with an oxidation potential of E°ox = -0.9 V relative to the SCE [45] is commonly employed as a very powerful reducing agent in solution. Unlike the alkylmetals (vide supra), the HOMOs of metallocenes reside at the metal center [46] which accounts for two effects (i) Removal of an electron from the HOMO requires minimal reorganization energy which explains the facile oxidative conversion from metallocene to metallocenium. (ii) The metal-carbon bonding orbitals are little affected by the redox process, and thus the resulting metallocenium ions are very stable and can be isolated as salts. 

Table 2. Vertical ionization potentials [IP] and oxidation potentials ( °ox) of metallocenes and (arene) metal sandwich complexes.

The formal replacement of a CH group in a ferrocene by an sp2 phosphorus atom is associated with an anodic oxidation potential shift of about 185 mV. This value is essentially independent of the substitution pattern of the metallocene [33]. This electronic destabilization of the ferricinium cation upon incorporation of the phosphorus atom probably reflects the high n electronegativity of 2-coordinate sp2 hybridized phosphorus which has been observed in MCD stud-... 

Ferrocene is a viable electron donor by virtue of its vertical ionization potential of only 6.86 eV in the gas phase [37] and its oxidation potential of only + 0.41 vs SCE [39] in CH3CN solution. Unlike the alkylmetal donors, the one-electron oxidation product, ferricenium cation, is stable, and various salts of it can be isolated. This stability arises from the metal-centered nature of the HOMO (e2g in 05 symmetry) [40] which minimizes the effect of electron removal from the metal-carbon bonding orbital. Indeed, ferrocene and related metallocenes undergo multiple redox reactions without disruption of the sandwich structure [41]. 

Table III Ionization Potentials and Oxidation Potentials of Metallocenes and Other Sandwich Donors.

As far as inorganic salts are concerned, they are normally introduced by blending their molten state with the CNTs or by sublimation. Many inorganic salts have been used with most of the transition metals and alkali/alkaline earth metals, with halides being the most typical anions [92], together with hydroxides [93]. The tubes can also be doped with individual metals, their oxides and with organometallic species such as metallocenes (Fig. 3.16) [94]. Fabrication of these materials is driven by potential applications in nanoelectronics. 

Because of their reversible electrochemical properties, ferrocene [biscyclopentadie-nyl-iron(II), FeCp2 and cobaltocenium [biscyclopentadienyl-cobalt(III), CoC p2 1 I are the most common electroactive units used to functionalize dendrimers. Both metallocene residues are stable, 18-electron systems, which differ on the charge of their most accessible oxidation states zero for ferrocene and + 1 for cobaltocenium. Ferrocene undergoes electrochemically reversible one-electron oxidation to the positively charged ferrocenium form, whereas cobaltocenium exhibits electrochemically reversible one-electron reduction to produce the neutral cobaltocene. Both electrochemical processes take place at accessible potentials in ferrocene- and cobaltocenium-containing compounds. 

Various oxidation and reduction reactions of substituted metallocenes have already been discussed. A large number of substituted metallocenes have been oxidized chronopotentiometrically at a platinum foil in acetonitrile solution (39, 46). Electron-withdrawing substituents decrease the ease of oxidation, while electron-donating substituents increase the ease of oxidation with respect to the parent metallocenes. A plot of chronopotentiometric quarter-wave potentials, El, vs. Hammett para-sigma constants shows a definite linear relationship. The Ei s for ruthenocene and osmocene indicate these metallocenes are more difficultly oxidized than ferrocene, in agreement with earlier qualitative observations (18). 

The ultimate aim of the project was to see if rectification (asymmetric current-voltage responses) or gating could be achieved by use of the redox-active metallocene moieties. Not only did channel-like function prove difficult to establish but the potentials required to oxidize the Fe(II) center for example (ca. 720 mV), were beyond the limits of the experimental setup. Indeed, membrane stability at very high voltages (>200 mV) is questionable.41 With the small amounts of data obtained (Figure 14) it is difficult to substantiate whether any rectification is realized. 

Of specific potential are several processes which presently experience (or even surpass) the pilot-plant stage vinyl acetate from syngas, precursors of polymers such as polycarbonate and polyurethanes via reductive or oxidative carbonylation, methyl mediacrylates and adipic acid through alternative routes, polypropene and COCs (cf. Section 4.1.14) by means of metallocenes (cf. Section 2.3.1.5) - new routes have been opened in all these cases. The last-named example emphasizes in an almost classical way the principle of tailor-making novel, optimized, homogeneous catalysts. Chapter 3 should again be consulted for details. 

Since, for secrecy reasons, information on new processes and the state of their development is not always published, or only after long delays, the classification applied or recent developments may be misleading. For example, the potential of phase-transfer catalyzed processes may already be more important than the present literature indicates. The same statement could apply for areas such as amidocarbonylation, the synthesis of fine chemicals by means of metallocenes, the reductive/oxidative carbonylation of aromatic amines or nitro derivatives, Heck coupling using palladacycles and heterocyclic carbene complexes, catalytic McMurry coupling, or other proposed methods. Recent developments must therefore leave open the stage of development reached, perhaps signaling that at the time of publication no commercialized, licensable process is yet known to the scientific community. 

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