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-R ₂)-(η⁹-C₉H₅-1,3-R ₂)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.