Structure−Reactivity Relationships in Rare-Earth Metal Carboxylate-Based Binary Ziegler-Type Catalysts

The organoaluminum-mediated alkylation of tailor-made rare-earth metal carboxylate complexes was studied, and implications of the degree of Ln alkylation and organoaluminum-chloride-mediated cation formation for 1,3-diene polymerization were investigated. Highly substituted rare-earth metal benzoate complexes {Ln(O2CC6H2Me3-2,4,6)3} n (Ln = Y, La, Nd), {Ln(O2CC6H2 iPr3-2,4,6)3} n (Ln = Y, La, Nd, Gd, Lu), {Ln(O2CC6H2 tBu3-2,4,6)3(THF)} n (Ln = Y, La), {Ln(O2CC6H3Ph2-2,6)3(THF)} n (Ln = Y, La), and {Ln(O2CC6H3Mes2-2,6)3(THF)} n (Ln = Y, La) were obtained quantitatively according to the silylamide route from Ln[N(SiMe3)2]3 and alkyl(aryl)-substituted benzoic acids. Such oligomeric carboxylate complexes are insoluble in aliphatic and aromatic solvents, but could be crystallized from donor solvents such as THF, DMSO, and pyridine. X-ray crystallographic analyses indicated the formation of monomeric [Nd(O2CC6H2Me3-2,4,6)3(DMSO)3] and dimeric [La(O2CC6H2Me3-2,4,6)2(μ-O2CC6H2Me3-2,4,6)(DMSO)2]2 depending on the metal ion size. Depending on the steric demand of the benzoate ligands, mono- and bis(tetraalkylaluminate) complexes [Me2Al(O2CC6H2 iPr3-2,4,6)2]2Ln[(μ-Me)2AlMe2] and {Ln(O2CC6H2 tBu3-2,4,6)[(μ-Me)2AlMe2]2}2, respectively, could be identified as major product components from the reaction with excess AlR3 (R = Me, Et), by means of 1H NMR spectroscopy and X-ray structure analysis. When activated with Et2AlCl, the resulting binary Ziegler-type catalysts efficiently polymerized isoprene (>99% cis-1,4), the polymerization performance depending on the metal center (Nd > Gd > La) and the degree of alkylation (“Ln(AlMe4)2” > “Ln(AlMe4)”). Equimolar reaction of [Me2Al(O2CC6H2 iPr3-2,4,6)2]2Ln[(μ-Me)2AlMe2] with R2AlCl (R = Me, Et) quantitatively produced [Me2Al(O2CC6H2 iPr3-2,4,6)]2, proposing “Me2LnCl” as the polymerization-initiating species. Homoleptic Ln(AlMe4)3 was spotted as a crucial reaction intermediate and was used for the high-yield synthesis of the various alkylated carboxylate complexes according to a novel “tetraalkylaluminate” route.