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

摘要

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)2AlMe2}3],[Sm{(mu-Me)2-AlMe2}3],[Pr{(mu-Me)2AlMe2}3],and [La{(mu-Me)2AlMe2}2{(mu-Me)3AlMe}] 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 ~1H and ~(13)C NMR spectroscopic studies,as well as 1D ~(89)Y,2D ~1H/~(89)Y,and ~(27)Al 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 carboxyl-ate and siloxide ligands can be introduced by methane elimination reactions that give the heterobimetallic complexes [Ln{(O2CAr~(iPr))2(mu-AlMe2)}2-(AlMe4)(C_6H_(14))_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 neodymi- um-based initiators reported previously.The superior catalytic performance of the homoleptic complexes leads to quantitative yields of high-cis-1,4-poly-isoprene(>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.