Steric 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 " "). 

For a bulky substituted cyclopentadienyl group, such as CsMes and CsMeaR (R = Et, Pr, SiMes), tris(cyclopentadienyl) lanthanide complexes cannot be prepared via the above metathesis reaction because of the steric hindrance. The reaction of anhydrous LnCls with three equivalents of alkali metal pentamethylcyclopentadienyl in THF (tetrahydrofuran) led to the THF ring-opened product (Equation 8.4) [8]. 

A wide variety of metallocenes containing different ligands based on indenyl (Ind) or fiuorenyl (El) derivatives, which are isoelectronic and equivalent to cyclopentadienyl (Cp) ligands, have since been synthesized. The Cp ligands in metallocenes stabilize and control the steric environment around to the metallic center, which is the active site for olefin coordination. 

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