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(eta(2)-CH2=CH2)PMe3, rac-1, and the racemic diastereomer of the zirconacyclopentane derivative (EBI)Zr(eta(2)-CH2CH2CH2CH2), 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 berate interaction, as well as a weak Zr-C-beta 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 beta-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.