Synthesis, Characterization and Reactivity of Formal 20 Electron Zirconocene-Pentafulvene Complexes

The reaction of low-valent
zirconocene reagents formed either from
the Negishi reagent Cp2Zr­(n-butyl)2 or from the reduction of Cp2ZrCl2 with
Mg with various pentafulvenes yielded the first zirconocene–pentafulvene
complexes, Cp2ZrFv (Fv = 6,6-di­(aryl), 6,6′-adamantylidene,
and 6-tert-butylfulvene), which are formal 20-electron
complexes. In addition to NMR spectroscopic analysis and DFT calculations,
three complexes were characterized by X-ray crystallography, showing
a η5:η1 dianionic binding mode of
the fulvene to the metal. The obtained complexes react with carbonyl
compounds (aldehydes and ketones) to unexpectedly afford double-insertion
products, as shown by NMR spectroscopy and X-ray studies. In the case
of aldehydes these diolate complexes were obtained as single diastereoisomers.
The reaction of Cp2ZrFv with internal alkynes did not result
in the formation of insertion products but release of the coordinated
fulvene and reductive dimerization of the alkynes to the corresponding
zirconacyclopentadienes.