Synthesis, characterization and structures of zirconocene complexes of sterically demanding pentaphenylcyclopentadienyl and tetraphenyl-m-tolyl cyclopentadienyl ligands

The synthesis, characterization and catalytic activity of zirconium complexes containing bulky pentaarylcyclopentadienyl ligands are reported. The monocyclopentadienyl 12-electron complex, trichloropentaphenylcyclopentadienylzirconium(IV), C5Ph5ZrCl3 (I), has an unusual monomeric structure in the solid state and has short Zr–ligand distances suggesting that the complex has significant Lewis acidity. The phenyl rings adopt a propeller arrangement in the solid state; VT 1H-NMR measurements of the analogous trichloro(tetraphenyl-m-tolylcyclopentadienyl)zirconium(IV) (II) indicate that there is rapid rotational motion of the phenyl rings. The Lewis acidity of this complex is manifested in its ability to catalyze the [4+2] cycloadditions of acrolein and methyl acrylate to cyclopentadiene. The sandwich complex, C5Ph5CpZrCl2 (III), has more typical Zr–ligand distances, but has a more parallel arrangement of the two cyclopentadienyl rings and does not display the reactivity shown by I and II.