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 C 5 H 5 -CMe 2 -C 13 H 9 (CpH-CMe 2 -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(SiMe 3 ) 2 ] 3 (Ln = Y ( 1 ), La ( 2 ), Nd ( 3 )) with CpH-CMe 2 -FluH ( 4 ) in THF under mild conditions lead to the formation of ansa -complexes (η 5,η 5 -Cp-CMe 2 -Flu)Ln(η 5 -Cp-CMe 2 -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 (η 5 -Cp-CMe 2 -FluH)Ln[N(SiMe 3 ) 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 (η 5 -Cp-CMe 2 -FluH) 2 Ln[N(SiMe 3 ) 2 ] (Ln = Y ( 6 )) or another species assumed to be the binuclear complex (η 5 -Cp-CMe 2 -FluH) 2 Ln[μ-N(SiMe 3 ) 2 ] 2 Ln[N(SiMe 3 ) 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 (SiMe 3 ) 2 NH 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 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(η 5 -Cp-CMe 2 -FluH) 3 (Ln = Y ( 7 ), La ( 11 )) in reasonable yields. Compounds 5 − 12 have been characterized in solution by 1D and 2D NMR techniques ( 1 H, 13 C, 1 H− 1 H COSY, and 1 H− 13 C 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)°).