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-(SiMe ₃)₂)(η⁵-C₉H₅-1,3- (SiMe₃)₂)Zr, with dialkyl ethers such as diethyl ether, CH₃OR (R = Et, ⁿBu, ^tBu), ⁿBu ₂O, 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.