Half-Sandwich Scandium Methylidenes

Abstract

AlMe3 sticks, GaMe3 quits. A series of Lewis acid stabilized scandium methylidenes with commercially available cyclopentadienyl ligands CpR (CpR = C5Me5 (Cp*), C5Me4SiMe3 (Cp’)) were synthesized. The-salt metathesis reaction of new half-sandwich dichloride precursors CpRScCl2(µ−Cl)Li(thf)3 with LiAlMe4 and AlMe3 at ambient temperature yielded [CpRSc(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 CpRScCl2(µ−Cl)Li(thf)3 with LiAlMe4 and AlMe3 led to the formation of Lewis acid stabilized Sc/Al2 methylidenes CpRSc(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 CpRSc(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, 45Sc, and variable temperature 1H NMR spectroscopy, SC-XRD, IR spectroscopy, and elemental analysis. Complexes CpRSc(CH2)(AlMe3)2 feature pronounced Sc---HC α-agostic interactions. The reaction of CpRScCl2(µ−Cl)Li(thf)3 with LiAlMe4 and AlMe3 was investigated viain situ1H and 45Sc NMR spectroscopy.