Amine elimination reactions between homoleptic silylamide lanthanide complexes and an isopropylidene-bridged cyclopentadiene-fluorene system

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 (eta(5),eta(5) -Cp-CMe2-Flu)Ln(eta(5)-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 (eta(5)-Cp-CMe2- FluH)Ln[N(SiMe3)(2)](2) (Ln = Y (5), La (9)), which undergo readily disproportionation/ligand redistribution reactions at 5-23 degreesC to give either a mono(amido)lanthanide complex (eta(5)-Cp-CMe2-FluH)(2)Ln[N(SiMe3)(2)] (Ln = Y (6)) or another species assumed to be the binuclear complex (eta(5)-Cp-CMe2- FluH)(2)Ln[mu-N(SiMe3)(2)](2)Ln[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)(2)NH in toluene at 90 degreesC with 50% conversion after 12 h. The effect of a noncoordinating apolar solvent on the reaction outcome of tris(amido) complexes 1-3 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(eta(5)-Cp- CMe2-FluH)(3) (Ln = Y (7), La (11)) in reasonable yields. Compounds 5-12 have been characterized in solution by 1D and 2D NMR techniques (H-1, C-13, H-1-H-1 COSY, and H-1-C-13 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)degrees; Ln = Nd, 105.08(1)degrees)