Dialkylaluminum complexes

Chart 11 Coordination modes in lanthanide dialkylaluminum complexes... 

The reaction of di- u-alkyl-bis(cyclopentadienyl) rare earth dialkylaluminum complexes with equimolar amounts of pyridine in toluene at room temperature gives dimeric dicyclopentadienyl rare earth methyl complexes, which are isolated in about 80% yield as air-sensitive crystalline solids. They are stable for short periods up to 150°C, soluble in CH2CI2, hot toluene or benzene, partly soluble in cold toluene or benzene, but insoluble in saturated hydrocarbons (Holton et al., 1976a and 1979c). The similar reaction of the corresponding scandium derivative with pyridine or tetrahydrofuran does not give the bridging dimer, but a monomer with coordinated pyridine or tetrahydrofuran ... 

The first well-defined monomeric aluminum complex featuring a methanediide ligand stabilized by hypervalent phosphoms was reported by CaveU in 1999 [103]. The reaction, analogous to this of Robinson et al., was performed in toluene at 100°C with the more bulky ligand 6. It afforded a spirocyclic bis-dialkylaluminum complex, [(6)(AlMe2)2l. An intermediate was observed during the course of the reaction and was synthesized quantitatively at room temperature using only 1 equiv. of trimethylaluminum in toluene. 

Bis(amidinate) ligands derived from bulky terphenyl systems were prepared and used for the preparation of dialkylaluminum bis(amidinate) complexes In a... 

This hypothesis is made quite probable by the isolation of the complex between gallium chloride and diethylaluminum chloride. Moreover, inclusion of potassium chloride in the reaction mixture to complex with the dialkylaluminum halide and free the gallium chloride raises the yield to 90% of triethylgallium (142). 

The first metallocene catalysts were biscyclopentadienyl titanium complexes and dialkylaluminum chloride. These catalysts were not stable and produced very low yields. However, they were the first catalyst systems to produce copolymers of polypropylene and 1-butene with very high comonomer uniformity, due to the fact that they had only one type of active site. 

Dimeric association has been observed in some more complex compounds such as various dialkylaluminum chelates derived from hydroxy carbonyl ligands (e.g. lactate, tropolonate, 2-acetyl-4-chlorophenolate) [89] or in Al4Me6(/t3-0)2(dpa) [dpa = deprotonated di-2-pyridylamine] [90] or in the cyclic chelate aminoalkoxy derivatives 40a (Al-0 1.847 and 1.938 A) and 40b (Al-0 1.843 and 1.927 A) [911. 

Interestingly, solid-state structure investigations on methyl thiosalicylate dialkylaluminum compounds uncovered close intermolecular S - C(ti) contacts (with an average S - C distance of 3.382 A significantly below the sum of the corresponding van der Waals radii [76, 77]) between the Al-S thiolate units and the ester component (28, Fig. 9) that can effectively compete with the putative sulfur-aluminum hypercoordinate bond (27, Fig. 9) [141]. The latter results provide the first evidence for the competition of intermolecular n Ti interactions, involving the thiolate sulfur atom and the electrophilic ester carbon atom, with the hypercoordinate bond in metal complexes it opens up an interesting area for further studies. 

SCHEME 7 Initiation by charge transfer complexes derived from dialkylaluminum halides in conjimction with electron acceptors. 

Among the multinuclear and mononuclear active species evaluated for lactone polymerization, those effective for the polymerization of P-lactones implying an O-acyl scission (Scheme 9.16a) are aluminum and dialkylaluminum alkoxides, yttrium alkoxides, zinc alkoxides, aluminoxanes, zincoxanes, bimetallic g-oxoalkoxides and aluminum porphyrins of the (TPP)AIOR type [50]. However, aluminum porphyrins such as (TPP)AICI or (TPP)A10C(0)R and aluminum SchifFs base complexes such as (Sal)AlCl (where Sal = salcenato group) are able to promote lactone polymerization by O-alkyl scission (Scheme 9.16b) [50]. 

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