β-SiH agostic bonding in sterically crowded lanthanidocene silylamide complexes

The synthesis as well as the spectroscopic and structural characterization of mononuclear metallocene complexes of the trivalent rare earth elements yttrium and lanthanum is described. Lanthanidocene silylamide complexes were obtained ate-complex free and in high yields according to a silylamine elimination reaction from complexes Ln[N(SiHMe2)2]3(THF)2 (Ln=Y, La) and highly substituted cyclopentadiene derivatives (CH3)5C5H, (CH3)4C5H2 and (C6H5)4C5H2. Deprotonation of tetraphenylcyclopentadiene was accomplished only for the lanthanum derivative indicating steric constraints due to the size of the metal cation. IR and 1H-NMR spectroscopy reveal the presence of asymmetrically bonded silylamide ligands featuring a strong agostic interaction between the electron-deficient metal centers and the SiH moiety of the bis(dimethylsilyl)amide ligand: SiH stretching vibrations as low as 1827 cm−1 are indicative of a distinct weakening of the SiH bonding. X-ray structure analyses of complexes [(CH3)4C5H]2YN(SiHMe2)2, [(CH3)5C5]2YN(SiHMe2)2 and [(C6H5)4C5H]2LaN(SiHMe2)2 show that the structural features of the agostically bonded bis(dimethylsilyl)amide moiety depend on the steric crowding of the ancillary cyclopentadienyl ligand: Y⋯Si and Y⋯H contacts as close as 3.0506(7) and 2.40(3) Å, respectively, are detected, forcing Ln–N–Si angles as low as 99.7(1)°.