Chemistry of Cationic Dicyclopentadienyl Group 4 Metal-Alky I Complexes

This chapter discusses the chemistry of cationic dicyclopentadienyl group 4 metal–alkyl complexes. Neutral, d0 group 4 metal alkyl complexes of general form Cp2 M(R)2 (R = H, hydrocarbyl) or Cp2 M(R)(X) (X = anionic, two–electron donor) comprise the most extensively studied class of group 4 organometallics (I). For many years, it has been suspected that cationic d0 metal–alkyl species Cp2M(R)+ are involved in metallocene-based Ziegler–Natta olefin polymerization catalyst systems of the general type Cp2MX2/AIRnX3–n. Several key advances in the early 1980s generated renewed interest in the proposal that Cp2M(R)+ ions are active species in the soluble catalyst systems. A simple but key point in the development of this chemistry was the realization that noncoordinating, nonreactive counterions, such as BPh4– are required for the isolation of stable salts. In 1976, Kaminsky reported the isolation of [(C5H5)2Zr(CH2C(H)– (AIEt2)2)][C5H5] from the reaction of (C5H5)4Zr and AIEt3. Cationic Cp2M(R)(L)+ complexes exhibit a variety of interesting structural features, which result from the high Lewis acidity of the cationic d0 metal centers. The tetrahydrofuran (THF) ligands in these complexes are labile and generally undergo rapid (nuclear magnetic resonance (NMR) time scale) exchange with free THF by dissociative or associative mechanisms at ambient temperature. Cationic, d0 Cp2M(R)(L)+ and base-free Cp2M(R)+ complexes are easily prepared from readily available Cp2M(R)2 compounds. There is strong evidence that Cp2M(R)+ ions are the active species in Cp2MX2-based Ziegler–Natta olefin polymerization catalysts.