Multinuclear magnetic resonance investigation of the fluxionality of CH(SiMe3)2 groups in organolanthanide complexes

Abstract

A series of related complexes, [La(C₅Me₅){CH(SiMe₃)₂}₂]1, [M(C₅Me₅)₂{CH(SiMe₃)₂}][M = La 2 or Lu 3], [Lu{CH(SiMe₃)₂}₃(µ-Cl)K]4 and the homoleptic tris-alkyls [M{CH(SiMe₃)₂}₃](M = Lu 5, La 6 or Al 7), have been prepared to investigate the factors influencing the fluxionality of their CH(SiMe₃)₂ groups. Complexes 1–7 have been studied by the complementary techniques of variable-temperature solution-and solid-state ¹³C NMR spectroscopy. Differences are observed that can only be metal dependent. Variable-temperature very fast magic-angle spinning solid-state ¹H NMR spectra of complexes 2, 3 and 5–7 have also been recorded. Sequences of relative mobility of the CH(SiMe₃)₂ groups in the solid state can be formulated: complex 2 > 1 > 3 and complex 5 > 6 > 7. In complex 4 a preferential orientation of the CH(SiMe₃)₂ ligands is caused by KCl co-ordination. Collation of M–C_γ and M–Si_β distances for the structurally characterized M–CH(SiMe₃)₂ and M–N(SiMe₃)₂ complexes results in the mean distances falling in a narrow range, M–CH(SiMe₃)₂ M′–C_γ 1.867(0.11), M′–Si_β 2.244(0.085)Å, M–N(SiMe₃)₂ M′–C_γ 1.973(0.11), M′–Si_β 2.205(0.056)Å. Co-ordination of a β-Si–C σ bond would lead to just such a consistency of data, given the absence of other geometric constraints. It is suggested that in electrophilic complexes, containing a CH(SiMe₃)₂ or N(SiMe₃)₂ ligand, the metal may be stabilized by a β-Si–Me–M rather than a γ-C–H–M interaction.