Aminopyridinato-Ligand-Stabilized Lanthanoid Complexes: Synthesis, Reactivity, Ethylene and Isoprene Polymerization
The aim of the present thesis was the synthesis and complete characterization of aminopyridinato-ligand-stabilized complexes of the lanthanoids. The lanthanoid complexes were synthesized by amine or alkane elimination. The thus obtained compounds were investigated in regard to their properties as precatalysts for the polymerization of ethylene or isoprene und their ability to form hydrido complexes or cations. Previous investigations carried out in our group have shown that aminopyridinate-stabilized organoyttrium cations exhibit very high activity in the polymerization of ethylene in the presence of aluminium alkyl compounds. This work showed that the thereby used precursor [Ap*Y(CH2SiMe3)2(thf)] can selectively react with phenylsilane or hydrogen to a novel trinuclear lanthanoid alkyl hydrido cluster. The corresponding lutetium derivative reacts analogous to the yttrium compound. The lanthanoid alkyl hydrido clusters were characterized by X-ray structure analyses, and the presence of the hydrid ligands were clearly proved by 1H NMR spectroscopy. Dialkyl complexes, stabilized by aminopyridinato ligands, react with anilinium borate to yield organolanthanoid cations after alkane elimination. They were isolated and characterized as thf adducts. The dibenzyl complexes of scandium and erbium were characterized by single crystal structure analyses. The dialkyl complexes of scandium are selective and active catalysts for the 3,4-selective polymerization of isoprene after activation with borates. We could even obtain isotactically enriched 3,4-polyisoprene through appropriate choice of the polymerization conditions (cocatalyst, polymerization temperature). The aminopyridinate-stabilized diamide of scandium can polymerize isoprene in the presence of anilinium borate and trialkylaluminium compounds, to obtain a polymer with a high cis-1,4-content. The dialkyl compounds of yttrium, erbium and lutetium are also suitable precatalysts for the initiation of the polymerization of isoprene. Although the 3,4-polyisoprene content decreased with an increased ionic radius of the trivalent lanthanoid, the cis-1,4-content increased. Addition of aluminium alkyl compounds leads to drastical changes of the microstructure of the obtained polymer which depends on the sterical demand of the alkyl ligand of the aluminium compound and the polymerization temperature. The synthesized aminopyridinato-ligand-stabilized bis(trimethylsilylmethyl) complexes are also suitable precatalysts (with exception of the ytterbium compound) for the polymerization of ethylene in the presence of ammonium borates and aluminium alkyl compounds. In contrast to the scandium derivative, the erbium, lutetium and yttrium compounds show characteristics of a CCTP catalyst. The activity is significantly dependent on the size of the lanthanoid ion, the highest activity was observed for the organoerbium cation. Because of the extreme air and moisture sensitivity as well as the thermal instability of the used trialkyl lanthanoid complexes, we searched for an alternative starting material. Hence, the triamide complexes of the composition [Ln{N(SiHMe2)2}3(thf)x] (x = 1, 2) proved to be suitable starting materials due to their facile synthesis and thermal stability. The reaction of these triamides with the bulky aminopyridines, used in this work, lead to the monosubstituted aminopyridinate-complexes after amine elimination. These are not suitable starting materials for the generation of catalysts for the coordinative chain transfer polymerization. NMR investigations of the reactivity with triethylaluminium and diisobutylaluminium revealed a fast and irreversible transfer of the aminopyridinato-ligand from the lanthanoid metal to the aluminium atom. This ligand transfer precludes the use of these amide complexes as suitable precursors for the CCTP, because of their deactivation during the alkylation step.
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