Cyclopentadienyl equivalents

One of the most successful approaches towards alternative ligand sets for lanthanide complexes is the use of bulky heteroallylic ligands. These ligands have been shown to behave as steric cyclopentadienyl equivalents [24], i.e. their cone angle is very similar to that of C5H5 or C5Me5. Scheme 1 shows the heteroallylic anions which have been successfully employed. 

The other isolable molecular compoimds of uncommon divalent rare earths have been imtil now mostly organometallic complexes featuring cyclopentadienyl ligands or cyclopentadienyl "equivalents" such as pyrazolylborates. The vast majority of characterised compounds contain Tm", the less reactive of the uncommon divalent rare earths (which unfortunately is also the most expensive). Many other stable compounds seem within reach, given that a good steric protection is achieved, such as for instance amides or alkoxides. Also, it may be possible to synthesise more heteroleptic complexes such as LR°I with an open coordination sphere, which could lead to a very rich new chemistry. A pending question finally involves the possibility of finding yet other complexes of rare earths in new divalent states (for instance Y " "). 

Problem 15.6 Draw the five resonance structures of the cyclopentadienyl anion. Are all carbon-carbon bonds equivalent How many absorption lines would you expect to see in the lH NMR and, 3C NMR spectra of the anion ... 

More than twenty years ago, Nesmeyanov s group showed that chlorine can be substituted by a variety of nucleophiles in FeCp(r 6-PhCl)+ [83, 84]. Indeed the chlorine substituent in the chlorobenzene (even) ligand is 1000 times more reactive than when it is located on the cyclopentadienyl (odd) ligand [85]. The FeCp+ is a good withdrawing group which is equivalent to two nitro groups in terms of activation. The reactions proceed under ambient conditions with primary or secondary amines and have been extended to other substituted chloroarene complexes [86, 87] Eq. (22), Table 2. 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

Of the 24 lines expected for coupling between V (I =7/2) and two equivalent C (I = V ) nuclei, 18 were resolved in the epr spectrum of VO(S CNEt2)2, showing that the C(2s) orbital can also participate in transannular interactions (61). [Cp2V(S2CNEt2)BF4] and dithiophos-phate have been used in an extension of studies on the C4V oxovana-dium(IV) chelates to yield C2V bis(cyclopentadienyl) complexes. 

It is strong evidence for Hiickel s rule that 59 and 60 are not aromatic while the cyclopropenyl cation (55) and the cyclopentadienyl anion (39) are, since simple resonance theory predicts no difference between 59 and 55 or 60 and 39 (the same number of equivalent canonical forms can be drawn for 59 as for 55 and for 60 as for 39). 

The reaction of 2equiv. of a pyrrole-substituted cyclopentadiene with Zn[N(SiMe3)2]2 (Scheme 19) afforded the dicyclopentadienylzinc complex 23.52 A solid-state structure of this compound was not obtained, but room-temperature 1H NMR spectroscopic studies showed two equivalent cyclopentadienyl groups, whose signals broadened on cooling. 

The lithium salt of a substituted cyclopentadienyl anion has been used in reaction with phosphorus trichloride for carbon-phosphorus bond formation.70 The resultant simple displacement product ultimately undergoes dimerization and loss of four (from the dimer) equivalents of HC1 (Equation 4.25). 

An interesting catalytic ruthenium system, Ru(7/5-C5Ar4OH)(CO)2H based on substituted cyclopentadienyl ligands was discovered by Shvo and coworkers [95— 98]. This operates in a similar fashion to the Noyori system of Scheme 3.12, but transfers hydride from the ruthenium and proton from the hydroxyl group on the ring in an outer-sphere hydrogenation mechanism. The source of hydrogen can be H2 or formic acid. Casey and coworkers have recently shown, on the basis of kinetic isotope effects, that the transfer of H+ and TT equivalents to the ketone for the Shvo system and the Noyori system (Scheme 3.12) is a concerted process [99, 100]. 

The complex (C4Hg)Cp2Ta- MdirtV,I 5)3 (38), whose cationic part is isoelectronic with the neutral Zr and Hf complexes (Section IV.A.l), has been prepared by the reaction of complex 37 (R = R1 = R2 = H) with two equivalents of NaCp, followed by abstraction of the cr-bound cyclopentadienyl ligand (Scheme 9)76. Bonding of the butadiene ligand in 38 in the s-trans conformation was determined by X-ray diffraction analysis. 

In support of the electrochemical evidence, the molecular structure of the biferrocenium ion in [( -CsF Fe -CsfLOf -CsfLOFe -CsHs)] [I3] shows that both pairs of cyclopentadienyl rings have an eclipsed conformation, Figure 16. Furthermore, the mean Fe-Cp(Centroid) distance is equivalent in both the ferrocenyl units and equal to 1.68 A. Speculatively, this value is intermediate between the values previously observed for ferrocene and ferrocenium ions, thus supporting charge delocalization between the two centres.27... 

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