Rare Earth Metal Alkyl Complexes with Methyl-Substituted Triazacyclononane-amide Ligands:  Ligand Variation and Ethylene Polymerization Catalysis

A series of rare earth metal dialkyl complexes was prepared with the general formula [Me 2 TACN-(B)-NR]M(CH 2 SiMe 3 ) 2 (TACN = 1,4,7-triazacyclononane, B = (CH 2 ) 2, SiMe 2; R = t Bu, sec Bu, n Bu; M = Sc, Y, Nd, La). For M = Sc, mixed monoalkyl−monochloro complexes were also accessible. Selected examples of these complexes were structurally characterized and show the metal in a distorted octahedral environment. With Lewis or Brønsted acid activators the dialkyl compounds can be converted to the corresponding monoalkyl cations, which were characterized by NMR spectroscopy. Comparative testing in catalytic ethylene polymerization showed that the catalyst activity is most strongly influenced by the metal ionic radius, but that variations in the ligand backbone and substitution pattern do influence other factors, such as polymer molecular weight and catalyst stability. Catalysts with the intermediately sized rare earth metal yttrium generally showed the highest activity, but some of these catalysts produce polyethylene with broad molecular weight distributions, suggesting multisite behavior. Hypothetically this could be caused by intermolecular ligand scrambling processes. Evidence that these may occur was found in the isolation of the “half-flyover” bimetallic yttrium complex {[η 3:η 1 -[Me 2 TACN(CH 2 ) 2 N t Bu]Y(CH 2 SiMe 3 )}{η 3:μ-η 1 -[Me 2 TACN(CH 2 ) 2 N t Bu]Y(CH 2 SiMe 3 ) 3 .