Stabilization of reactive rare earth alkyl complexes through mechanistic studies

Abstract

Rare earth tris(alkyl) complexes such as M(CH₂SiMe₃)₃(sol)_n are widely used as precursors for many compounds and as homogeneous catalysts for alkene polymerization and alkane functionalization. However, the thermal instability of those most conveniently made from the commercially available lithium salt of the neosilyl anion, LiCH₂SiMe₃, Li(r), restricts their utility. We present a new range of synthetically useful, more kinetically stable rare earth neosilyl solvates, derived from a full kinetic study of the various possible decomposition mechanisms of 7 known and 12 new solvated rare earth neosilyl complexes M(CH₂SiMe₃)₃(sol)_n M = Sc(III), Y(III), Lu(III), Sm(III), and sol = THF; TMEDA; DMPE; diglyme ((CH₃)₂(OCH₂CH₂)₂O, G₂), triglyme ((CH₃)₂(OCH₂CH₂)₃O, G₃). Surprisingly, simply using higher-denticity donors to sterically disfavor neosilyl γ-H elimination is not effective. While Sc(r)₃((CH₃)₂(OCH₂CH₂)₂O) has a half-life, t_1/2, of 258.1 h, six times longer than for Sc(r)₃(C₄H₈O)₂ (t_1/2 = 43 h), Lu(r)₃((CH₃)₂(OCH₂CH₂)₂O) and Y(r)₃((CH₃)₂(OCH₂CH₂)₂O) do not show the expected, analogous increased t_1/2. This is because new decomposition pathways appear for poorly fitting donors. Finally, kinetic studies demonstrate the impact of small, and increasing amounts of LiCl on the kinetics of the reactivity of the smaller alkyls Y(r)₃(THF)₂ and Lu(r)₃(THF)₂; molecules used in hydrocarbon chemistry and catalysis for fifty years. A new route to pure Y(r)₃(THF)₂, which avoids the traditional use of Li(r), is presented.