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- N-Ligand Stabilized Lanthanide Complexes (2009)
- A series of lanthanide complexes stabilized by N-ligands has been synthesized. Most of these complexes have been structurally characterized. The overall results emphasize the importance of the steric bulk of the applied ligands to stabilize various lanthanide complexes with a distinct reactivity. To highlight and compare the steric bulk of an aminopyridinato with those of amidinate ligands mononuclear seven coordinated complexes of lanthanum were synthesized by salt elimination route. X-ray crystal structure analyses were carried out to compare the steric demand of the two amido ligands. A similar overall primary coordination site bulkiness for both ligands and distinct differences regarding this bulkiness for different directions were observed. A better shielding of the second coordination sphere was observed for the aminopyridinate. Based on their steric demand mono(aminopyridinato) organoyttrium complexes were selectively synthesized in very good yields by alkane elimination from trialkylyttrium complexes. The corresponding yttrium cations were accessible by abstracting one of the two alkyls using ammonium borates. Based on the appropriate steric bulk of the used aminopyridinato ligand these yttrium cations show very high ethylene polymerization activity at 80 °C in the presence of small amounts of aluminium alkyls. During these polymerizations a reversible polyethylene chain transfer between the organoyttrium cation and aluminium compounds was observed. The chain transfer catalyst system described here is able to produce relatively long chain (up to 4000 g mol-1) Al-terminated polyethylene with a molecular weight distribution < 1.1. Instead of salt elimination or alkane elimination, aminopyridinato lanthanide complexes are accessible even under solventless conditions at elevated temperatures. The direct reaction between ytterbium metal and bulky aminopyridines was an effective way to synthesize true homoleptic monomeric aminopyridinato complexes of ytterbium. A systematic steric variation leads to bis- or tris(aminopyridinato)ytterbium complexes. The divalent ytterbium complexes show interesting intermolecular agostic interactions. Such agostic interactions do not persist if salt metathesis reactions are carried out in THF, since coordination of THF blocks the vacant site responsible for such interactions. A further increase in the steric bulk of the applied ligands leads to mixed amido/ iodo complexes in the salt metathesis reaction. The attempted reduction of these mixed amido/ iodo rare earth metal complexes using KC8 led to the formation of bis(aminopyridinato) complexes which have been characterized by X-ray diffraction studies, NMR spectroscopic investigations and elemental analyses. Most likely reduction took place followed by disproportionation and the formation of bis(aminopyridinates). Due to enhanced reactivity and, in particular, the rarity of cyclopentadienyl free rare earth metal hydrido complexes we became interested to synthesize bis(aminopyridinato)lanthanide hydrido complexes. Slight variation in the steric bulk enabled us to selectively synthesize the corresponding bis(aminopyridinato)lanthanide halide precursors. Due to the specific steric “pressure” the same coordination number was observed for La and Sc despite the large difference in their ionic radii. Since the most common synthetic route to the hydrido complexes is sigma-bond metathesis reaction of parent alkyl complex with phenyl silane, we synthesized bis(aminopyridinato)lanthanide alkyl complexes. Corresponding hydrides generated by reaction of alkyl complexes with PhSiH3 undergo a very fast intramolecular metallation reaction at room temperature. The intramolecular C-H activation is highly dependent on the size of the used lanthanides. For larger lanthanides the rate of decomposition of the parent alkyl is fast enough that it precludes the isolation of stable alkyl complexes. However gradual decrease of the metal atom size enables the isolation of stable alkyl complexes which then may undergo intramolecular C-H activation via a transient hydride species at reasonable rates at room temperature.