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, (eta(5)-C9H5-1,3-R-2)-(eta(9)-C9H5-1,3-R-2)Zr (R = silyl, alkyl), have been prepared by either alkane reductive elimination of alkali metal reduction of a suitable zirconium(IV) dihalide precursor. Crystallographic characterization oil two of these derivatives, R = SiMe2CMe3 and CHMe2, reveals unprecedented eta(2) coordination of one of the indanyl ligands. Variable-temperature and EXSY NMR studies establish that the eta(5) and eta(2) 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 eta(2)-olefin adduct, forming the corresponding zirconacyclopentane.