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–Cγ and M–Siβ 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′–Cγ 1.867(0.11), M′–Siβ 2.244(0.085)Å, M–N(SiMe3)2 M′–Cγ 1.973(0.11), M′–Siβ 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.