Zirconium Sandwich Complexes with η⁹ Indenyl Ligands: Well-Defined Precursors for Zirconocene-Mediated Coupling Reactions

A family of isolable, well-defined bis-indenyl zirconium sandwich complexes, (η5-C9H5-1,3-R2)(η9-C9H5-1,3-R2)Zr (R = silyl, alkyl), have been prepared by either alkane reductive elimination or alkali metal reduction of a suitable zirconium(IV) dihalide precursor. Crystallographic characterization of two of these derivatives, R = SiMe2CMe3 and CHMe2, reveals unprecedented η9 coordination of one of the indenyl ligands. Variable-temperature and EXSY NMR studies establish that the η5 and η9 rings are rapidly interconverting in solution. The sandwich complexes serve as effective sources of low-valent zirconium reacting rapidly with both olefins and alkynes at ambient temperature. In contrast to bis-cyclopentadienyl chemistry, the olefin adducts of the bis-indenyl zirconium sandwiches undergo preferential C−H activation to yield the corresponding allyl hydride compounds, although reaction with excess olefin proceeds through the η2-olefin adduct, forming the corresponding zirconacyclopentane.