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Amine Elimination Reactions between Homoleptic Silylamide Lanthanide Complexes and an Isopropylidene-Bridged Cyclopentadiene−Fluorene System

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posted on 19.06.2002, 00:00 authored by Aswini K. Dash, Abbas Razavi, André Mortreux, Christian W. Lehmann, Jean-François Carpentier
This contribution describes the amine elimination process as an alternative synthetic route to traditional salt metathesis for introducing the isopropylidene-bridged unsymmetrical ligand C5H5-CMe2-C13H9 (CpH-CMe2-FluH) onto group III-metal centers (Y, La, Nd) to give in turn the neutral, ate-complex-free ansa-lanthanidocenes. The reactions of homoleptic Ln[N(SiMe3)2]3 (Ln = Y (1), La (2), Nd (3)) with CpH-CMe2-FluH (4) in THF under mild conditions lead to the formation of ansa-complexes (η55-Cp-CMe2-Flu)Ln(η5-Cp-CMe2-FluH) (Ln = Y (8), La (12), Nd (13)) in 70−84% isolated yields (based on 4). These reactions proceed via the rapidly formed bis(amido)lanthanide intermediates (η5-Cp-CMe2-FluH)Ln[N(SiMe3)2]2 (Ln = Y (5), La (9)), which undergo readily disproportionation/ligand redistribution reactions at 5−23 °C to give either a mono(amido)lanthanide complex (η5-Cp-CMe2-FluH)2Ln[N(SiMe3)2] (Ln = Y (6)) or another species assumed to be the binuclear complex (η5-Cp-CMe2-FluH)2Ln[μ-N(SiMe3)2]2Ln[N(SiMe3)2]2 (Ln = La (10)), respectively. Complexes 6 and 10 undergo an intramolecular amine elimination reaction under THF reflux to yield the corresponding ansa-complexes 8 and 12, respectively. The reversibility of the process has been investigated in the yttrium case:  complex 8 converts back to 6 in the presence of (SiMe3)2NH in toluene at 90 °C with 50% conversion after 12 h. The effect of a noncoordinating apolar solvent on the reaction outcome of tris(amido) complexes 13 with 4 has been also studied using toluene, in which the low solubility presumably shifts the disproportionation equilibria and leads to the isolation of another class of compounds Ln(η5-Cp-CMe2-FluH)3 (Ln = Y (7), La (11)) in reasonable yields. Compounds 512 have been characterized in solution by 1D and 2D NMR techniques (1H, 13C, 1H−1H COSY, and 1H−13C HETCOR), and the solid state structures of 6 and of the mono(THF) adducts of ansa-lanthanidocenes 12 and 13 have been established by X-ray diffraction studies. The latter ansa-complexes feature very narrow Cp(centroid)−Ln−Flu(centroid) bite angles (Ln = La, 103.67(1)°; Ln = Nd, 105.08(1)°).