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

The well-defined coordination environment of trivalent [(C5Me5)2Ln]+ complexes has been
used to examine the reaction chemistry of the lanthanide carboxylate and R2AlCl (R = Me,
Et, iBu) components used in the preparation of lanthanide-based diene polymerization
catalysts. Each of the R2AlCl reagents can replace a carboxylate ligand with chloride in
reactions with [(C5Me5)2Sm(O2CC6H5)]2, but instead of forming a simple chloride complex
like [(C5Me5)2SmCl]3, bimetallic lanthanide aluminum dichloro complexes (C5Me5)2Sm(μ-Cl)2AlR2 are generated by ligand redistribution. These bis(chloride)-bridged complexes
are also readily formed from the divalent precursor (C5Me5)2Sm(THF)2 and R2AlCl. However,
the analogous reaction between (C5Me5)2Sm(THF)2 and Et3Al gives (C5Me5)2Sm(THF)(μ-η2-Et)AlEt3, which contains the first Ln(III)−(η2-Et) linkage, a coordination mode that
differentiates Et from Me. To determine if mixed mono-chloride/alkyl-bridged (C5Me5)2Ln(μ-Cl)(μ-R)AlR2 complexes can be isolated, (C5Me5)2Y(μ-Cl)YCl(C5Me5)2 was reacted with
R3Al. These reactions form [(C5Me5)2Y(μ-Cl)(μ-R)AlR2]x complexes, but again there is a
differentiation on the basis of R:  the Me complex is a dimer and the others are monomers.
(C5Me5)2Y(μ-Cl)2AlR2 complexes were similarly prepared for comparison with the mixed ligand
species and for additional Me, Et, and iBu comparisons.