Ferrocenium cations

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]. 

The coupling reaction is very fast because unstable cyclopentadienols resulting from the decomposition of ferrocenium cations can be trapped, in good yields (80%), by triazoline diones [73]. The cycloadduct is obtained with totally regio and stereo selectivities (Scheme 51). 

Reversible one-electron oxidation of ferrocene (Fc) and its derivatives toward cation-radicals (the so-called ferrocenium cations) is a well-known reaction. The cation-radical center is localized at the iron atom. According to photoelectronic spectra and voltammetry data, such localization is... 

Similarly, the fluorescence intensity of the 1,4-disubstituted azine with ferrocene and pyrene units (17) can be reversibly modulated by sequential redox reactions of ferrocene moiety. In the neutral state, compound 17 displays weak fluorescence owing to the electron transfer from the ferrocenyl group to the excited pyrene unit or by energy transfer from the excited pyrene unit to the ferrocenyl unit. Oxidation of the ferrocenyl unit, however, leads to remarkable fluorescence enhancement. This is because the ferrocenium cation shows weak electron donating ability and also the corresponding spectral overlap becomes small.27... 

Oxidative homo-coupling of enolates from acyl oxazolidinones to give the corresponding dimers can be achieved in the presence of oxidants. Titanium and ytterbium enolates of 252 were coupled in the presence of a chiral diol or chiral bisoxazoline in the presence of ferrocenium cation 254 (Scheme 63) [166]. The amount of the meso dimer varied with the chiral ligand with a maximum of 5 1. TADDOL 172 performed best providing a 76% ee for the meso product. Ytterbium enolate gave a low ee of 34% with the same ligand. 

The cyclic voltammetry of 1,2,3,4,5-pentachloro- and of decachloroferrocene have been studied in acetonitrile. A plot of redox potential E(f) vs. op (ap is the Hammett para coefficient for the chloro substituent) shows that the neutral molecules are stabilized with respect to the corresponding ferrocenium cations by 0.16 to 0.12 V per Cl260. 

Oxidation of ferrocene to the ferrocenium cation almost always results in the collapse of the Q.S.182 This effect has been related to the redistribution of electron density in the d-orbitals upon oxidation, which leads to a more symmetrical electric field gradient. Consequently, observed Q.S.s... 

Much larger iodine catenates are obtained when metal iodides are present the reaction of Til with (MesS) resulted in a number of species from which (Mc3 8)3126 has been structurally characterized, and shown to be of [15] and [17] anions with intercalated iodine molecules. A mixture is identified in [Me3PNHPMe3]4(l28), the linear [13] , the Z-shaped [Ig] , and the zigzag chains of In [Cp2Fe]3[l29], the most iodine-rich polyiodide, the ferrocenium cations, are made to interact in the cavities of an anionic three-dimensional network cage structure described by the formula [ (l5 )i/2 l2 (li2 )i/2T2 -l2]. ... 

Potentially bidentate ligands, such as the formamidinates [RNCR NR], the monoanion derived from 2,6-bis(phenylamino)pyridine and 7-methyl-l,8-naphthyridin-2-one, and others, formed complexes with a paddle wheel structural motif. Figure 5(d), and some of these complexes could be oxidized by ferrocenium cations to give mixed-valent species. Generally, such oxidation led to a slight lengthening of the metal-metal bond. 

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