Staggered rings, ferrocene

Fig. 5-3. Electron diffration of ferrocene. A Superposition of a section from the electron density function for ferrocene giving a composite picture of the staggered rings sandwiching the iron atom. B A section of the electron density function in the plane of one of the cyclopentadiene rings. Reprinted with permission from J. D. Dunitz, L. E. Orgel, and A. Rich, Acta Cryst. 9 374 (1956).

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). 

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... 

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. 

The basic qualitative features of the bonding in ferrocene are well understood, and will serve to illustrate the basic principles for all (t7-C H )M bonding. The discussion of bonding does not depend critically on whether the preferred rotational orientation of the rings (see Fig. 16-30) in an (i7-C5H5)2M compound is staggered (Did) or eclipsed (DSh) in any event, the barriers to ring rotation in all types of arene-metal complex are very low, ca. 10-20 kJ mol-1. 

The first dibora[2] ferrocenophane (94) was prepared from l,L-dilithioferrocene and l,2-dichlorobis(dimefhylamino)di-borane. A dynamic process due to motion of the cyclopenfa-dienyl rings between staggered and eclipsed conformations was revealed by low-temperature NMR spectroscopy. A number of l,3-dibora[3]ferrocenophanes with B-E-B bridges (95 E=0, S, Se, Te, NR) were reported. Moreover, Wagner used diborylated ferrocenes for the synthesis of doubly bridged switchable ania-metallocenes (96) and (97), in which the bridges are created and... 

The molecular and crystal structure of (81) is simpler than that of ferrocene as only one polymorph featuring eclipsed conformation of the cyclopentadienyl rings, has been found at ambient and low (100 K) temperature. The larger metal-carbon distance (2.186 A in (81) vs. 2.03 or 2.06 A in ferrocene) corresponds to the larger metal covalent radius see Covalent Radii) and may also be responsible for the fact that an eclipsed conformation is found for the solid-state structure of decamethylruthenocene (82), as opposed to decamethylferrocene where more closely spaced methyl groups impose the staggered Dsd conformation. 

Figure 7-51 shows the molecular structure of dichloro[l,l -bis(diphenylphos-phino)ferrocene]palladium(ii) [51, 155], The palladium atom possesses a square planar coordination. The cyclopentadienyl rings of the ferrocene group are nearly staggered and tilted from the parallel disposition by 6.2° [51, 155]. 

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