Zwitterionic Metallocenes Derived from rac and meso-Ethylenebisindenyl Zirconocene Olefin Complexes and Pentafluorophenyl-Substituted Boranes

Trimethylphosphine-stabilized ethylene complexes 1 of the ethylenebisindenyl-supported zirconocene fragment are prepared via magnesium reduction of rac-(EBI)ZrCl₂ in the presence of ethylene and PMe₃. When the reaction is halted after 6 h, good yields of a mixture containing rac-(EBI)Zr(η²-CH₂=CH₂)PMe₃, rac-1, and the racemic diastereomer of the zirconacyclopentane derivative (EBI)Zr(η²-CH₂CH₂CH₂CH ₂), rac-2, are obtained. This mixture may be converted to pure rac-1 if treated with excess PMe₃. If the magnesium reduction of rac-(EBI)ZrCl₂ is left for 3 days in the presence of magnesium chloride, complete epimerization to meso-1 is observed. Thus, both diastereomers of 1 are available. Compounds meso-1 and rac-2 have been characterized crystallographically. The coordinated ethylene ligands in compounds 1 are susceptible to electrophilic attack by the pentafluorophenyl-substituted boranes HB(C₆F₅)₂ and B(C₆F₅)₃, forming zwitterionic metallocene products. For reactions involving HB(C₆F₅)₂, the products meso-3 and rac-3 are characterized by a strong zirconium-hydrido borate interaction, as well as a weak Zr-Cβ bonding. In addition to complete solution characterization, the structure of meso-3 was determined crystallographically. Upon treatment of rac-1 with B(C₆F₅)₃, a more charge-separated zwitterion, rac-4, was formed. In addition to retaining its PMe₃ ligand, the complex is stabilized by a strong β-CH₂ interaction, as determined by X-ray crystallography and NMR spectroscopy. Structural comparisons between the compounds reported herein and previous examples in metallocene chemistry are presented.