Carbon−Oxygen Bond Cleavage with η⁹,η⁵-Bis(indenyl)zirconium Sandwich Complexes

Treatment of the η9,η5-bis(indenyl)zirconium sandwich complex, (η9-C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr, with dialkyl ethers such as diethyl ether, CH3OR (R  Et, nBu, tBu), nBu2O, or iPr2O resulted in facile CO bond scission furnishing an η5,η5-bis(indenyl)zirconium alkoxy hydride complex and free olefin. In cases where ethylene is formed, trapping by the zirconocene sandwich yields a rare example of a crystallographically characterized, base-free η5,η5-bis(indenyl)zirconium ethylene complex. Observation of normal, primary kinetic isotope effects in combination with rate studies and the stability of various model compounds support a mechanism involving rate-determining CH activation to yield an η5,η5-bis(indenyl)zirconium alkyl hydride intermediate followed by rapid β-alkoxide elimination. For isolable η6,η5-bis(indenyl)zirconium THF compounds, thermolysis at 85 °C also resulted in CO bond cleavage to yield the corresponding zirconacycle. Both mechanistic and computational studies again support a pathway involving haptotropic rearrangement to η5,η5-bis(indenyl)zirconium intermediates that promote rate-determining CH activation and ultimately CO bond scission.