A new family of monocyclopentadienyl organoscandium bis-alkyls supported by a bulky trialkylphosphine oxide ancillary

Treatment of the oligomeric compound [Cp*ScCl2](n) (Cp* = C5Me5) with tri-tert-butylphosphine oxide in THF leads to the monocyclopentadienyl scandium dichloride [Cp*(t-Bu3P=O)ScCl2] as a monomeric, THF-free solid in 72% yield. This compound can be alkylated with MeLi to give the corresponding dimethyl derivative, [Cp*(t-Bu3P=O)ScMe2], 1. The crystal structures of these two compounds have been done and reveal a three-legged piano stool geometry about the scandium center. The coordinated phosphine oxide ligand exchanges with free donor in both compounds via an associative mechanism, as indicated by the activation parameters (DeltaH(c) = 6.4-8.4 kcal mol(-1-); DeltaS(c) = -22.5 to -26.6 eu). Thus, the Cp*(t-Bu3P=O) ligand set provides a stable platform for organoscandium chemistry. This was demonstrated by the activation of 1 by treatment with B(C6F5)(3) to form the fully characterized contact ion pair [Cp*(t-Bu3P=O)ScMe](+)[MeB(C6F5)(3)](-), 2, which is an active ethylene polymerization catalyst.