Tetramethylaluminates, rare-earth

Scheme 21 Synthesis of half-sandwich rare-earth bis(tetramethylaluminate) complexes from homoleptic tris(tetramethylaluminate) rare-earth precursors...

X-ray structure analysis revealed a 7-coordinate rare-earth metal center with two asymmetrically / -coordinating tetramethylaluminate ligands, an asymmetrically / -coordinating siloxide ligand and one methyl group of a trimethylaluminum donor molecule (Fig. 28). Such heteroleptic complexes can be regarded as molecular models of covalently bonded alkylated silica surface species. Moreover, isoprene was polymerized in the presence of 1-3 equivalents of diethylaluminum chloride, with highest activities observed for (Cl) (Ln) ratios of 2 1 (Table 12) (Fischbach et al., 2006, personal communication) [150]. 

The high mobility of the methyl ligands of Ln/Al organometallics in solution was also evidenced for heterobridged trimethylaluminum adduct complexes (Scheme 75). As for the homobridged tetramethylaluminate complexes, this fluxional process may be suppressed at lower temperature and/or in the presence of sterically demanding ligand environments and smaller rare-earth metal centers. 

Rare-earth tetramethylaluminate complexes have been identified as key intermediates in Ziegler-Natta-type olefin polymerizatiOTi and may be readily prepared by reaction of rare-earth precursors with organoaluminum reagents [112]. The first homoleptic Ln(AlR4)3 species (Ln = Y, Nd R = Me) were reported by Evans et al. in 1995 [114], The synthesis of these heterobimetallic... 

Half-Sandwich Rare-Earth Tetramethylaluminate Precursors... 

Various half-sandwich rare-earth bis(tetramethylaluminate) complexes have been readily synthesized by methane elimination reactions between Ln(AlMe4)3 derivatives and appropriate HCp ligands (Scheme 21, A) [119-121]. Reaction of homoleptic tetramethylaluminate derivatives with phospholide salts also allowed access to the corresponding half-sandwich bis(tetramethylaluminate) complexes through a salt-metathesis route (Scheme 21, B) [122]. 

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