Synthesis of Bis(indenyl)zirconium Dihydrides and Subsequent Rearrangement to η⁵,η³-4,5-Dihydroindenediyl Ligands:  Evidence for Intermediates during the Hydrogenation to Tetrahydroindenyl Derivatives

Exposure of η⁹,η⁵-bis(indenyl)zirconium sandwich complexes to 4 atm of H₂ resulted in facile oxidative addition to furnish the corresponding zirconocene dihydrides, (η⁵-C₉H₅-1,3-R₂)₂ZrH₂ (R = SiMe₃, SiMe₂Ph, CHMe₂). Continued hydrogenation completed conversion to the tetrahydroindenyl derivatives, (η⁵-C₉H₉-1,3-R₂)₂ZrH₂. Deuterium labeling studies established that dihydrogen (dideuterium) addition to the benzo rings is intramolecular and stereospecific, occurring solely from the endo face of the ligand, proximal to the zirconium. In the absence of dihydrogen, the bis(indenyl)zirconium dihydrides rearranged to new zirconium monohydride complexes containing an unusual η⁵,η³-4,5-dihydroindenediyl ligand, arising from metal-to-benzo ring hydrogen transfer. Mechanistic studies, including a normal, primary kinetic isotope effect measured at 23 °C, are consistent with a pathway involving regio- and stereoselective insertion of a benzo CC bond into a zirconium hydride. The stereochemistry of the insertion reaction, and hence the η⁵,η³-4,5-dihydroindenediyl product, is influenced by the presence of donor ligands and controlled by the preferred conformation of the indenyl rings. Exposure of the zirconium hydrides containing the η⁵,η³-4,5-dihydroindenediyl rings to 1 atm of dihydrogen afforded the tetrahydroindenyl zirconium dihydride complexes, establishing the intermediacy of this unusual coordination environment during benzo ring hydrogenation.