An Unusual Organoyttrium Alkyl Complex Containing a [C5HMe3(η3-CH2)-C5H4N-κ]- Ligand and an Elusive Cyclopentadienide-Based Scandium Tuck-Over Zwitterion Obtained by C-H Bond Activation

The acid-base reaction between Y(CH2SiMe3)(3)(thf)(2) and the pyridyl-functionalized cyclopentadienyl (Cp) ligand C5Me4H-C5H4N (1 equiv) at 0 degrees C afforded a mixture of two products: (eta(5):kappa-C5Me4-C5H4N)Y-(CH2SiMe3) (2)(thf) (1a) and (eta(5):kappa-C5Me4-C5H4N)(2)Y-CH2SiMe3 (1b), in a 5: 2 ratio. Addition of the same ligand (2 equiv) to Y(CH2SiMe3)(3)(thf)(2), however, generated 1b together with the novel complex 1c, the first well defined yttrium mono(alkyl) complex (eta(5):kappa-C5Me4-C5H4N)[C5HMe3(eta(3)-CH2)-C5H4N-kappa] Y(CH2SiMe3) containing a rare k/eta(3)-allylic coordination mode in which the C-H bond activation occurs unexpectedly with the allylic methyl group rather than conventionally on Cp ring. If the central metal was changed to lutetium, the equimolar reaction between Lu(CH2SiMe3)(3)(thf)(2) and C5Me4H-C5H4N exclusively afforded the bis(alkyl) product (eta(5):kappa C5Me4-C5H4N)Lu(CH2SiMe3)(2) (thf) (2a). Similarly, the reaction between the ligand (2 equiv) and Lu(CH2SiMe3)(3)(thf)(2) gave the mono(alkyl) complex (eta(5):kappa-C5Me4-C5H4N)(2)LuCH2SiMe3 (2b), in which no ligand redistribution was observed. Strikingly, treatment of Sc(CH2SiMe3)(3)-(thf) 2 with C5Me4H-C5H4N in either 1:1 or 1:2 ratio at 0 degrees C generated the first cyclopentadienide-based scandium zwitterionic "tuck-over" complex 3, (eta(5):kappa-C5Me4H-C5H4N)Sc(thf)[eta-eta(5):eta(1):kappa-C5Me3(CH2)-C5H4N] Sc(CH2SiMe3)(3). In the zwitterion, the dianionic ligand [C5Me3(CH2)-C5H4N](2-) binds both to Scl(3+) and to Sc2(3+), in eta(5) and eta(1)/k modes. In addition, the reaction chemistry, the molecular structures, and the mechanism are also discussed in detail.