Lanthanide Metallocene Reactivity with Dialkyl Aluminum Chlorides: Modeling Reactions Used to Generate Isoprene Polymerization Catalysts

The well-defined coordination environment of trivalent [(C 5 Me 5 ) 2 Ln] + complexes has been used to examine the reaction chemistry of the lanthanide carboxylate and R 2 AlCl (R = Me, Et, i Bu) components used in the preparation of lanthanide-based diene polymerization catalysts. Each of the R 2 AlCl reagents can replace a carboxylate ligand with chloride in reactions with [(C 5 Me 5 ) 2 Sm(O 2 CC 6 H 5 )] 2, but instead of forming a simple chloride complex like [(C 5 Me 5 ) 2 SmCl] 3, bimetallic lanthanide aluminum dichloro complexes (C 5 Me 5 ) 2 Sm(μ-Cl) 2 AlR 2 are generated by ligand redistribution. These bis(chloride)-bridged complexes are also readily formed from the divalent precursor (C 5 Me 5 ) 2 Sm(THF) 2 and R 2 AlCl. However, the analogous reaction between (C 5 Me 5 ) 2 Sm(THF) 2 and Et 3 Al gives (C 5 Me 5 ) 2 Sm(THF)(μ-η 2 -Et)AlEt 3, which contains the first Ln(III)−(η 2 -Et) linkage, a coordination mode that differentiates Et from Me. To determine if mixed mono-chloride/alkyl-bridged (C 5 Me 5 ) 2 Ln(μ-Cl)(μ-R)AlR 2 complexes can be isolated, (C 5 Me 5 ) 2 Y(μ-Cl)YCl(C 5 Me 5 ) 2 was reacted with R 3 Al. These reactions form [(C 5 Me 5 ) 2 Y(μ-Cl)(μ-R)AlR 2 ] x complexes, but again there is a differentiation on the basis of R: the Me complex is a dimer and the others are monomers. (C 5 Me 5 ) 2 Y(μ-Cl) 2 AlR 2 complexes were similarly prepared for comparison with the mixed ligand species and for additional Me, Et, and i Bu comparisons.