Electronic effects in ferrocene

In substituted ferrocenes, the substituent affects the reactivity of the substituted ring, the relative reactivity of the two rings and hence the reactivity of the molecule as a whole. Thus the ease of oxidation of ferrocene is reduced by electron withdrawing, —I, substituents and, in add media, the relative ease of oxidation is alltylferrocenes ferrocene acylferrocene. A striking example is ferrocenylcarboxaldehyde which resists oxidation by permanganate under conditions which will oxidize benzaldehyde to benzoic acid [285]. 

Acylation experiments show that disubstitution occurs heteroannularly indicating that as expected —I substitutents relatively deactivate the substituted ring. In those cases where small yields of homoannularly di-acetylated products are isolated, the acyl substituents enter the 1,2-positions of the r-cyclopentadienyl ring rather than the 1,3-podtions. Thus in the monoacetylated ring the 3-position is deactivated relative to the 2-position. The transmission of electronic effects from one ring to the 

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We like to conclude the present section with an example which points out the role played by the electronic effects of ferrocene ligands in stabilizing uncommon oxidation states in metal complexes rather than their electrochemical properties. Figure 10 shows the molecular structure of the Ir(I) monocation [Ir(dppf)2]+ (dppf= l,l -bis(diphenylphosphi-no)ferrocene)7 and its electrochemical behaviour in thf solution.8... 

Two distinct approaches to the elucidation of homoannular electronic effects within the transition metal-complexed aromatic ring have been utilized (1) relative site reactivities and (2) NMR spectroscopy. Because of the relative availability of many ferrocene derivatives considerable attention has been devoted to this system. In several instances results for the ferrocene system have provided a basis for the analysis of electronic effects in related organometallic Tr-complexes. 

The first indication of an electronic effect in the ring migration studies was demonstrated when l,T-diethylferrocene was observed to undergo ligand exchange with mesitylene much more readily than did ferrocene... 

Fedin et al. (183) have also undertaken a study of the transmission of electronic effects in substituted ferrocenes. Their results were divided into two... 

Herrera-Alvarez, C., Gomez-Benitez, V., Red6n, R. et al. (2004) [l,l -Bis (diphenylphosphino)ferrocene]palladium(ll) complexes with fluorinated benzenethiolate ligands examination of the electronic effects in the sohd state, solution and in the Pd-catalyzed Heck reaction with the catalytic system [Pd(dppf)(SRF)2]. J. Organomet. Chem., 689, 2464-72. 

Effects of spacer groups on the formation and properties of the mixed-valence states of conjugated ferrocene dimers have been extensively studied by both electrochemical and spectroscopic methods. It should be noted that a characteristic feature in the electronic spectra of ferrocene dimers with conjugated spacer groups is the appearance of metal-to-ligand charge transfer (MLCT) bands in the neutral form as well as IT bands in the mixed-valence state. The dimer Fc — CH=CH — Fc... 

Next let us examine a QM/MM model system of complex 3 where the steric influence of the phenyl substituents and of the ferrocene is accounted for but the electronic effects have been largely eliminated (model B). In other words, the peripheral groups have been delegated to the MM region, while keeping the molecular system used for electronic structure calculation identical to that in model A. 

It exhibits a single oxidation process (Ea = + 0.59 V, vs. SCE) affected by some adsorption problems. These adsorption phenomena, which typically affect the electrochemical response of these derivatives, sometimes make it difficult to ascertain by controlled potential coulometry the effective number of electrons involved in the oxidation step. In this case, the (approximate) number of electrons involved per molecule of dendrimer, nd, can be roughly calculated by comparing the cyclic voltammetric responses of the dendrimer with that of the ferrocene monomer using the following empirical equation.27,40... 

Analogous droplet-size dependence has been observed for electron transfer between ferrocene and hexacyanoferrate(III) across a droplet/water interface with the droplet radius of <5 /an, as described in Section III [80]. In this system, FeCp-X+ transfer is coupled with the electron transfer process and the physical properties of the droplet have been suggested to vary with r. However, droplet size effects on surface capillary waves analogous to those in the MT process may also govern the electron transfer process in the FeCp-X/Fe(IIl) system. 

The principle aim of the reported studies was to model structures, conformational equilibria, and fluxionality. Parameters for the model involving interactionless dummy atoms were fitted to infrared spectra and allowed for the structures of metallocenes (M = Fe(H), Ru(II), Os(II), V(U), Cr(II), Cofll), Co(ni), Fe(III), Ni(II)) and analogues with substituted cyclopentadienyl rings (Fig. 13.3) to be accurately reproduced 981. The preferred conformation and the calculated barrier for cyclopentadienyl ring rotation in ferrocene were also found to agree well with the experimentally determined data (Table 13.1). This is not surprising since the relevant experimental data were used in the parameterization procedure. However, the parameters were shown to be self-consistent and transferable (except for the torsional parameters which are dependent on the metal center). An important conclusion was that the preference for an eclipsed conformation of metallocenes is the result of electronic effects. Van der Waals and electrostatic terms were similar for the eclipsed and staggered conformation and the van der Waals interactions were attractive 981. It is important to note, however, that these conclusions are to some extent dependent on the parameterization scheme, and particularly on the parameters used for the nonbonded interactions. 

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... 

Continuing advances in the theoretical treatment of metallocenes and substituted derivatives can be anticipated. Despite the historical difficulties with molecular orbital calculations on ferrocene, the problems are now recognized to stem from failure to account for electron correlation effects. New approaches to addressing this issue, coupled with inevitable increases in computing power, should make more metallocenes with substituted groups systems amenable to accurate calculation. More realistic predictions of donor ability, and thus better estimates of their effects in nonlinear optical (NLO) systems, will be possible. 

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