Ferrocenium

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... 

Ferrocenium hexafluorophosphate (48) and catecholboronbromide (49) (Figure 3.6) are efficient catalysts that have been tested in the cycloadditions of cyclic and acyclic dienes with a variety of dienophiles [48]. Catalyst 48 is less active than 49, but is less corrosive. 

In 90% aqueous DMSO. Measured vs. Ag/Ag" " in acetonitrile rev reversible, irrev irreversible. "Kinoshita et al. (1994). In DMSO-water (molar ratio 85 15). In DMSO. The potential determined vs. ferrocene/ferrocenium was converted to the value vs. Ag/Ag by adding -(-0.083 V (Komatsu etal., 1982a). 

All bonding or nonbonding orbitals are filled resulting in a stable diamagnetic 18-electron complex. Single-electron oxidation to a ferrocenium cation provides a 17-electron species, in which one electron is unpaired. 

As mentioned above, ferrocene is amenable to electrophilic substitution reactions and acts like a typical activated electron-rich aromatic system such as anisole, with the limitation that the electrophile must not be a strong oxidizing agent, which would lead to the formation of ferrocenium cations instead. Formation of the CT-complex intermediate 2 usually occurs by exo-attack of the electrophile (from the direction remote to the Fe center. Fig. 3) [14], but in certain cases can also proceed by precoordination of the electrophile to the Fe center (endo attack) [15]. 

To examine if the higher catalytic activity and selectivity of 47a as compared to the COP-X system 46 is mainly caused by the pentaphenyl ferrocenium or by the imidazoline moiety, oxazoline 53-Cl was prepared in diastereomerically pure form starting from carboxylic acid 51 and (5)-valinol via oxazoline 52 (Fig. 27) [73]. 

The steric environment of COP-X 46 and 47a around the catalytic palladium site mainly differs in a Ph (47a) and an i-Pr group (46) next to the coordinating N-site and the type and distance of the spectator ligand. While the distance of the two sandwich ligands differs only slightly between COP and 47a (3.4 A vs. 3.3 A), oxidation of the ferrocene to a ferrocenium species is expected to shorten this distance further. Overall, the steric hindrance to access the Pd-center is more distinct for 47a. These steric effects are capable to explain the higher ee obtained with 47a. 

From an electronic point of view, the differences are more pronounced since the overall charge of the ligand (1 for COP, 0 for 47a after oxidation to the ferrocenium species) changes. As Pd(II) most likely acts as a carbophUic Lewis acid coordinating to an olefin, the lower electron density in 47a is suitable to explain the higher reactivity found for this complex. 

Noviandri I, Brown KN, Fleming DS, Gulyas PT, Lay PA, Masters AF, Phillips L (1999) The decamethylferrocenium/decamethylferrocene redox couple a superior redox standard to the ferrocenium/ferrocene redox couple for studying solvent effects on the thermodynamics of electron transfer. J Phys Chem B 103 6713-6722... 

Treatment of Na2[IrCL] with H2bpb (101) and H2bpc (102) gives Na[Ir(bpb)Cl2] and Na[Ir(bpc)Cl2], respectively.166 Both complexes undergo reversible, one-electron, ligand-based oxidations at +0.21 V and +0.31 V vs. ferrocenium/ferrocene, respectively, in CH3CN. 

The dark red Ir11 binuclear complex [Ir(CO)(P(/i-tolyl)3)(mnt)]2, formed via one-electron oxidation of [Ir(CO)(P(/ -tolyl)3(mnt)] by ferrocenium, contains chelating mnt ligands bridging the Ir—Ir bond.639 The X-ray crystal structure shows square-pyramidal geometry at each Ir, and an Ir—Ir bond length of 2.706(2) A. 

The structure of [Ir(cod)(dppf)]PF6 shows approximately square-planar geometry at Ir, and the cp rings of the dppf ligand are close to parallel and staggered.592 The systems [Ir(cod)(LL)]C104, where LL = dppf, l-diphenylphosphino-2-(7V,7V-dimethylamino)methyl ferrocene and 1,6-diferrocene-2,5-diazahexane, catalytically trimerize PC=CH to 1,3,5-triphenylbenzene.593 The electrochemistry of [Ir(dppf)2]BPh4 shows two one-electron reductions at —1.560 V and -1.755 V vs. ferrocenium/ ferrocene.753... 

The 1,4,7-trithiacyclononane ligand, [9]aneS3, zinc complex was synthesized to compare with the electrochemistry of related complexes and showed an irreversible oxidation and an irreversible reduction at +1.30 V and —1.77 V vs. ferrocene/ferrocenium, and the X-ray crystal structure of the bis macrocycle zinc complex was reported.5 0,720... 

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