Lanthanide pentamethylcyclopentadienyl

The highly syndiospecific-living polymerization of methyl methacrylate has been initiated by the neutral bis(pentamethylcyclopentadienyl)lanthanide-alkyl or -hydride complexes [215,216]. The plausible reaction mechanism is shown in Scheme XI. 

Mono(organo)nickel compounds, via oxidative addition, 8, 44 Monoorganotin hydroxides, preparation, 3, 850 Monoorganotin oxides, preparation, 3, 850 Mono(pentamethylcyclopentadienyl) actinide(IV) compounds, reactions, 4, 207 Mono(pentamethylcyclopentadienyl) lanthanide(III) compounds, synthesis and characteristics, 4, 66 Mono(pentamethylcyclopentadienyl) uranium(IV) sulfido complex, synthesis, 4, 207-208 Mono(phenoxy-aldehyde) trichlorides, with Zr(IV),... 

The ytterbium(III) product 14o was isolated as large colorless crystals in 33% yield. A comparable pentamethylcyclopentadienyl derivative, (C5Me5)2Yb(S2CNEt2) was described by Andersen et al. in 1982 [52]. The bent [PhC(NSiMe3)2]2Yb unit in 14o structurally resembles the bent lanthanide metallocenes containing (C5Me5)2Ln moieties (Fig. 5). 

A recent development in this field involves the combination of benzamidi-nate ligands with cyclopentadienyl or cyclooctatetraenyl ligands in the coordination sphere of lanthanide ions. The first lanthanide complexes containing both benzamidinate and pentamethylcyclopentadienyl bonded to yttrium was reported by Teuben et al. Yttrium aryloxides as well as anhydrous yttrium trichloride can be used as starting materials in these preparations (Eq. 19, Scheme 5) [27] ... 

The organometallic chemistry of scandium is generally similar to that of the later lanthanides. It thus forms a cyclopentadienyl ScCps that has mixed mono- and pentahapto-coordination like LuCps. An anionic methyl [Li(tmed)]3[M(CH3)6] is formed by scandium, as by the lanthanides. However, there are often subtle differences that should be borne in mind. The pentamethylcyclopentadienyl compound [ScCp 2Me] is a monomer but the lutetium compound is an asymmetric dimer [Cp 2Lu(/u.-Me)LuCp 2Me]. Similarly, whilst triphenylscandium is obtained as a bis(thf) adduct, [ScPh3(thf)2], which has a TBPY structure with axial thf molecules, the later lanthanides form octahedral [LnPh3(thf)3]. Triphenylscandium and the phenyls of the later lanthanides are made by different routes. 

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]. 

The first tris(pentamethylcyclopentadienyl) lanthanide complex was isolated accidentally from the reaction of a divalent samarium complex (CsMes )2Sm with cyclooctatetraene [9]. Following this discovery, two more convenient methods were developed for the preparation of the sterically crowded complexes (C5Mes)3Ln [10],... 

Another route to lanthanide dialkyls is a metathesis reaction. Lanthanide dihalides supported by a bulky monoanionic ancillary ligand, such as pentamethylcyclopentadienyl and related derivatives [4], P-diketiminato and guanidinato groups [49, 50], and so on, are generally used as the starting materials. A variety of lanthanide dialkyl compounds, including methyl compounds have been prepared and structurally characterized via successive metathesis reactions (Figure 8.11). 

It is worth noting that the sterically crowded tris(pentamethylcyclopentadienyl) lanthanide complexes (C5Mes)3Ln have similar reductive reactivity to the divalent samarium complex. This phenomenon has been termed sterically induced reduction (Section 8.2.1.2). 

Bi(pentamethylcyclopentadienyl) lanthanide hydride and methyl complexes [(C5Me5)2Ln(p.-R)]2 (R = H, CH3) were the first to be reported as excellent initiators for the highly... 

Hou, Z.M., Zhang, Y, Tardif, O. etal. (2001) (Pentamethylcyclopentadienyl)samarium(II) alkyl complex with the neutral CsMesK ligand a precursor to the first dihydrido lanthanide(III) complex and a precatalyst for hydrosilylation of olefins. Journal of the American Chemical Society, 123, 9216. 

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