Ferrocene staggered

Molecular orbital diagram and interactions in ferrocene (staggered conformation). 

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

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... 

In this connection, in order to judge the level of these molecular rearrangements, the solid state X-ray structures of ferrocene and ferrocenium ion could be compared. Unfortunately, the molecular disorder caused by the rotation of the cyclopentadienyl rings in ferrocene means that the comparison procedure is far from simple and, in fact, the first results were interpreted in terms of a staggered conformation of the two cyclopentadienyl rings. It is now believed that the eclipsed conformation is the more stable (with a rotation angle of about 10°).2 However, as the rotational barrier is notably low (about 4 kJ mol-1), the conformation that one observes is probably that imposed by crystal packing forces. 

One can see that in the neutral derivative the pentamethylcyclopenta-dienyl rings assume a staggered conformation. Conversely, in the monocation they are disposed in an eclipsed conformation.5 A comparison between the bonding distances of decamethylferrocene and the decamethylferrocenium ion (already reported in Chapter 2, Table 1) reveals that the rotation of the rings following the electron removal is accompanied by a lengthening of the Fe-C distance by about 0.05 A. This is approximately equal to that measured in the case of ferrocene. 

Once again the disposition of the cyclopentadienyl rings is staggered. The metal-carbon bond length in cobaltocene (19 valence electrons, terminal electronic configuration a[2e x) is greater than in ferrocene... 

Concerned with the electron diffraction structure, the rotation of the cyclopentadienyl rings appears even faster than in ferrocene, hence their mutual disposition is difficult to establish. An X-ray diffraction study at 101 K has however shown that at this temperature the conformation is staggered.94 The increment in the Ni-C distance (2.18 A) compared to the Co-C distance (2.10 A) in cobaltocene reflects once again the increased population of the antibonding e" orbital (nickelocene has 20 valence electrons and a terminal electronic configuration e a e"2). 

X Experimentally it is not clear whether the staggered or eclipsed form is the more stable they are very close in energy. Most derivatives of ferrocene show the eclipsed conformation in the solid state, but there is evidence that ferrocene itself is staggered. 

The energy difference between the staggered (Dw) and eclipsed Dm,) rotomers of ferrocene is apparently very small (si Real/mol), with the latter being perhaps the more stable [cf. R. K. Bohn and A. Haaland, J. Organomet. Chem., 5, 470 (1966)). The two symmetries are equally suitable and convenient for a discussion of bonding. Since most of the research literature has used Z)w, we make the same choice here. 

A molecule may possess higher order rotational axes. Consider the eclipsed form of the molecule ferrocene (Fig. 3.8a). which has a Cs axis through the iron atom and perpendicular to the cyclopentadienyl rings. Now consider the staggered form of... 

Fig. 15.34 Molecular structure of staggered decahenzy[ferrocene. (From Schumann, H. Jamah, C. Kohn. R. D. Loebel. J. Dietrich. A. J. Orgummet. Chen. UM9.365. 137-ISO...

Fig. 3.8 Molecules containing live-fold rotulion.il axes (a) edipsed ferrocene, aile and lop view (b) staggered ferrocene, side and lop view. Each molecule has five C> axes, only one of whrti is shown. Upon rotation about (he C. axis, (he atoms interchange I - 1. etc.

Many molecules have more than one C axis. For example, staggered ferrocene has five C, axes, one of which lies in the plane of the paper. Eclipsed ferrocene also has five C, axes, though they are different from the ones in tbe staggered conformcr (Fig. 3.8) In those cases in which more than one rotational axis is present, the one of highest order is termed the principal axis and is usually the s axis. Planes that contain the principal axis are termed vertical planes, crr, and a minor plane perpendicular to the principal axis is called a horizontal plane, aFor example, borazine (Fig. 3.2c) has three vertical planes (one is shown) and one horizontal plane (the plane of the molecule). 

To probe further the bonding descriptions of the transition metal metallocenes it is instructive to assemble a back of the envelope MO scheme. The available structural evidence (249, 280) indicates that the energies of the staggered (DSd) and eclipsed conformations are rather close. Thus, in the solid state, ferrocene adopts the DSd conformation (106), while the eclipsed arrange-... 

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. 

For ferrocene the symmetry of the space group requires the two five membered rings to have staggered conformation. In ruthenocene and osmocene the rings adopt the eclipsed conformation. 

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