Coping With Extreme Lewis Acidity: Strategies for the Synthesis of Stable, Mononuclear Organometallic Derivatives of Scandium

The evolution of organoscandium chemisty in our research group over the past ten years is described. The synthesis of well-defined, mononuclear organoscandium compounds is challenging due to the propensity of the highly Lewis acidic scandium center to induce oligomerization with formation of {Sc-(μ-R)-Sc} bridges. By using two bulky pentamethylcyclopentadienyl (Cp*) ligands, thermally stable, monomeric hydride and hydrocarbyl derivatives of the type Cp*₂Sc-R (R = H, alkyl, aryl, alkyl) have been obtained. Due primarily to their extreme Lewis acidity, these formally 14-electron compounds react readily with a variety of C-H bonds (σ bond metathesis) as well as with olefins, nitriles and CO (insertion reactions). In addition, Cp*₂Sc-CH₂CH₂CH₃ was found to be a "living" ethylene-polymerization catalyst. Desiring to extend the latter reaction to α olefin substrates, further synthetic and reactivity studies with less sterically encumbered scandocene derivatives, e. g. {(η⁵-C₅Me₄)2SiMe₂} Sc-R and {(η⁵-C₅H₃CMe₃)₂SiMe₂}Sc-R, were undertaken. New types of catalytic reactions such as the dimerization of α olefins, the cyclization of α,ω-dienes, and skeletal rearrangements for 1,3-dienes, were uncovered. More recently, by employing a new [SiMe₂]-linked cyclopentadienyl-amido ligand system, scandium hydride-catalyzed α olefin polymerization has been realized. 1. Introduction 2. Bis(pentamethylcyclopentadienyl)scandium Hydride, Alkyl and Aryl Derivatives 3. Synthesis and Reactivity of Less Sterically Crowded Scandocene Hydrides and Alkyls 4. Synthesis and Characterization of Scandocene Hydrides and Alkyls with Linked Cyclopentadienyl-Amide Ligands 5. Conclusions.