Lwm. Lee et al., Zwitterionic metallocenes derived from rac and meso-ethylenebisindenyl zirconocene olefin complexes and pentafluorophenyl-substituted boranes, ORGANOMETAL, 18(19), 1999, pp. 3904-3912
Trimethylphosphine-stabilized ethylene complexes 1 of the ethylenebisindeny
l-supported zirconocene fragment are prepared via magnesium reduction of ra
c-(EBI)ZrCl2 in the presence of ethylene and PMe3. When the reaction is hal
ted after 6 h, good yields of a mixture containing rac-(EBI)Zr(eta(2)-CH2=C
H2)PMe3, rac-1, and the racemic diastereomer of the zirconacyclopentane der
ivative (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 r
eduction of rac-(EBI)ZrCl2 is left for 3 days in the presence of magnesium
chloride, complete epimerization to meso-1 is observed. Thus, both diastere
omers of 1 are available. Compounds meso-1 and rac-2 have been characterize
d crystallographically. The coordinated ethylene ligands in compounds 1 are
susceptible to electrophilic attack by the pentafluorophenyl-substituted b
oranes HB(C6F5)(2) and B(C6F5)(3), forming zwitterionic metallocene product
s. For reactions involving HB(C6F5)(2), the products meso-3 and rac-3 are c
haracterized 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 treatmen
t 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 a
nd NMR spectroscopy. Structural comparisons between the compounds reported
herein and previous examples in metallocene chemistry are presented.