Ancillary ligand effects on organo-f-element reactivity. ansa-metallocenes with bridge-tethered donors

The synthesis and reactivity of ansa-metallocenes based upon the new ligand system R(Me)SiCp″₂^2- (Cp″ = C₅Me₄ and R = (CH₂)_nOMe, n = 3-5) is reported. The tethered donor functionality, which is appended via varying chain lengths to the silicon bridge, is introduced to examine possible intramolecular reactivity modifications. The key step of the ligand synthesis is the hydrosilylative fusion of the donor functionality R to the silicon center. The complexes [R(Me)SiCp″₂]LnCH(SiMe₃)₂ (Ln = Y, Sm) were synthesized to investigate the influence of the tethered ether group on the reactivity and catalytic properties. Catalytic reactions in which the substrate is a relatively weak donor, such as in olefin hydrogenation, exhibit depressed turnover frequencies, presumably reflecting competition between the substrate and donor functionality for the empty coordination site at the metal center. In contrast, for amino-alkene hydroamination/cyclization in which a strong amine donor is the substrate, catalytic activity is significantly enhanced while diastereoselectivity is essentially unchanged. The molecular structure of the precatalyst [MeO(CH₂)₅(Me)SiCp″₂]-YCH(SiMC ₃)₂ has been determined by X-ray diffraction.