Homoleptic Rare-Earth Metal(III) Tetramethylaluminates: Structural Chemistry, Reactivity, and Performance in Isoprene Polymerization

• Published in: Chemistry : a European journal Vol. 13; no. 31; pp. 8784 - 8800
• Main Authors: Zimmermann, Melanie, Frøystein, Nils Åge, Fischbach, Andreas, Sirsch, Peter, Dietrich, H. Martin, Törnroos, Karl W., Herdtweck, Eberhardt, Anwander, Reiner
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01-01-2007; WILEY‐VCH Verlag
• Subjects: aluminum; homogeneous catalysis; lanthanides; NMR spectroscopy; polymerization
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200700534

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{(μ‐Me)2AlMe2}3], [Sm{(μ‐Me)2AlMe2}3], [Pr{(μ‐Me)2AlMe2}3], and [La{(μ‐Me)2AlMe2}2{(μ‐Me)3AlMe}] determined by X‐ray crystallography. These structures reveal a distinct Ln3+ 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 1H and 13C NMR spectroscopic studies, as well as 1D 89Y, 2D 1H/89Y, and 27Al 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{(O2CAriPr)2(μ‐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 nLn:nCl=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.
Ionicity overcomes covalency: The monolanthanide complexes [Ln(AlMe4)3] reveal distinct Ln3+ size‐dependent coordination modes of the tetramethylaluminate ligands in the solid state (see picture)—a highly fluxional coordination scenario that is also confirmed by dynamic 1H/13C and metal‐centered 27Al/89Y NMR spectroscopy.