Homoleptic rare-earth metal(III) tetramethylaluminates: Structural chemistry, reactivity, and performance in isoprene polymerization

The complexes [Ln(AlMe4)(3)] (Ln=Y, La, Ce, Pr, Nd, Sm, Ho, Lu) have been synthesized by an amide elimination route and the structures of [Lu(mu-Me)(2)AlMe2)(3)], [Sm{(mu-Me)(2)-AlMe2)(3)], [Pr{(mu-Me)(2)AlMe}(3)], and [La{(mu-Me)(2)AlMe2}(2){(mu-Me)(3)AlMe}] determined by X-ray crystallography. These structures reveal a distinct Ln (3+) cation size-dependency. A comprehensive insight into the intrinsic properties and solution coordination phenomena of [Ln(AlMe4)(3)] complexes has been gained from extended dynamic H-1 and C-13 NMR spectroscopic studies, as well as ID Y-89, 2D H-1/Y-89, and Al-27 NMR spectroscopic investigations. [Ce-(AlMe4)(3)] and [Pr(AlMe4)(3)] have been used as alkyl precursors for the synthesis of heterobimetallic alkylated rare-earth metal complexes. Both carboxylate and siloxide ligands can be introduced by methane elimination reactions that give the heterobimetallic complexes [Ln{(O(2)CA(iPr))(2)(rho-AlMe2)](2)(-) (AlMe4)(C6H14)(n)] and [Ln{OSi-(OtBu)(3)}(AlMe3)(AlMe4)2], respectively. [Pr[OSi(OtBu)(3)}(AlMe3)(AlMe4)(2)] has been characterized by X-ray structure analysis. All of the cerium and praseodymium complexes are used as precatalysts in the stereospecific polymerization of isoprene (1-3 equivalents of Et2AlCl as co-catalyst) and compared to the corresponding neodymium-based initiators reported previously. The superior catalytic performance of the homoleptic complexes leads to quantitative yields of high-cis-1,4-polyisoprene (>98%) in almost all of the polymerization experiments. In the case of the binary catalyst mixtures derived from carboxylate or siloxide precatalysts quantitative formation of polyisoprene is only observed for n(Ln) :n(Cl)=1:2. The influence of the metal size is illustrated for the heterobimetallic lanthanum, cerium, praseodymium, neodymium, and gadolinium carboxylate complexes, and the highest activities are observed for praseodymium as a metal center in the presence of one equivalent of Et2AlCl.