Half-sandwich scandium methylidenes†

Abstract

AlMe₃ sticks, GaMe₃ quits. A series of Lewis acid stabilized scandium methylidenes with commercially available cyclopentadienyl ligands Cp^R (Cp^R = C₅Me₅ (Cp*), C₅Me₄SiMe₃ (Cp′)) were synthesized. The-salt metathesis reaction of new half-sandwich dichloride precursors Cp^RScCl₂(μ-Cl)Li(thf)₃ with LiAlMe₄ and AlMe₃ at ambient temperature yielded [Cp^RSc(AlMe₄)Cl]₂. [Cp′Sc(AlMe₄)Cl]₂ was further methylated at ambient temperature to yield Cp′Sc(AlMe₄)Me. At 70 °C, the reaction of Cp^RScCl₂(μ-Cl)Li(thf)₃ with LiAlMe₄ and AlMe₃ led to the formation of Lewis acid stabilized Sc/Al₂ methylidenes Cp^RSc(CH₂)(AlMe₃)₂. The new mixed Sc/Al/Ga methylidene Cp′Sc(CH₂)(AlMe₃)(GaMe₃) was obtained from the reaction of Cp′Sc(AlMe₄)Me with GaMe₃. When heated, complex Cp′Sc(CH₂)(AlMe₃)(GaMe₃) converted into the Sc/Al methylidene [Cp′Sc(CH₂)₂AlMe]₃via release of the comparatively weak Lewis acid GaMe₃ and methane. The core of trimeric [Cp′Sc(CH₂)₂AlMe]₃ can be described as a triscandacyclohexane {Sc(CH₂)}₃ stabilized by a trialacyclohexane {Al(CH₂)}₃via Pearson hard/hard matching. Complexes Cp^RSc(CH₂)(AlMe₃)₂ and [Cp′Sc(CH₂)₂AlMe]₃ differ in rigidity, thermal stability and reactivities toward ketones and Lewis bases. The isolated methylidenes were analyzed by ¹H, ¹³C{¹H}, ⁴⁵Sc, and variable temperature ¹H NMR spectroscopy, SC-XRD, IR spectroscopy, and elemental analysis. Complexes Cp^RSc(CH₂)(AlMe₃)₂ feature pronounced Sc⋯HC α-agostic interactions. The reaction of Cp^RScCl₂(μ-Cl)Li(thf)₃ with LiAlMe₄ and AlMe₃ was investigated via in situ¹H and ⁴⁵Sc NMR spectroscopy.