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.