Ferrocene electronic effects

Most of the data concerning ferrocene electronic effects correlate rather well, f errocene emerges as a system which homoannularly allows electron withdrawal to take place principally from the 2-position and secondarily... 

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. 

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... 

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. 

The ferrocene group has been introduced for its special steric and electronic properties. It has a cylindrical shape and the potential to block a substantial portion of the metal s coordination sphere making stereo- and regio-control possible in catalytic applications [166, 172-175]. Similarly, the ease with which ferrocene can be oxidised to the corresponding cation [176,177] makes it not only a standard in electrochemistry, but can have advantageous electronic effects as a substituent for carbenes as well [166]. 

Since all aromatic organometallic 7T-complex systems possess several possible sites for which substituent effects could be analyzed, it seems best that the various sites be studied independently of one another. One particular site, the substituted ring, has been shown to be amenable to such analysis. The electronic effects on the substituted ring in ferrocenes, i.e., homoannular electronic effects, have recently been the subject of an extensive review (75). For this reason discussion of ferrocene systems in this section will be abbreviated and attention will be focused upon recent advances in related metallocene and other aromatic organometallic systems. 

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 study of relative site reactivities for aromatic electrophilic substitution reactions has provided useful information for assessing homoannular electronic effects within the ferrocene system. Ferrocenes of the type (I)... 

More recently, UV data for a related system, substituted cobalticinium salts (77) have been tabulated. A similarity in the positions of the absorption bands in this system and the analogous substituted ferrocenes was observed. A decrease in the absorbance of the band at 260-275 m/x with both donating and withdrawing groups was the only electronic effect of note. 

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... 

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. 

To examine the idea of a long-range transmittance of electronic effects to the iron atom, several ortho-, meta-, and para-substituted phenyl-ferrocenes were prepared and their oxidation potentials studied potentio-metricly by oxidation with dichromate 44). Little and associates concluded from the o-phenylferrocenes that steric effects are relatively unimportant and do not much influence the potential. Likewise, Mason and Rosenblum 47) observed good correlation between potentials for a series of -phenyl-ferrocenes and Op constants. 

It has also been observed that the regression lines for T- and 2-substituted methylferrocenes, obtained from the plot of the chemical shift values of the -CHj protons vs. electronic effect through the ferrocene nucleus. The ratio of the slopes (in percentage terms) gave a value of 28%, which agreed reasonably well with the value obtained by Butter and Beachell (22%) (11). 

Electronic effects of diphosphine ligands have been reported by Unruh, Casey and Van Leeuwen [16-18]. Unruh studied the hydroformylation of 1-hexene using substituted ferrocene derived diphosphines Fe(C5H4PR2)2 (Fig. 6.4). 

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

The dimeric complex 39 exhibits individual Fe(III)/ Fe(II) redox couples at +0.47 and +0.60 V v. SCE in CH2CI2. The averaged value (+0.54 V) is close to that of the corresponding monomeric complex 38 (+0.51 V). These results are explained by the electronic effect as reported in the literature. The trimer 40 exhibits two redox couples at +0.51 and +0.67 V in a similar condition. The positive peak suggests the presence of significant electronic interaction between two ferrocene units (center and C-terminus). This result indicates the possibility of constructing an expanded conjugation system of polymeric ferrocene amino acid and the development of the conductive devices by partial oxidation. 

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. 

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