Ligand-induced haptotropic rearrangements in bis(indenyl)zirconium sandwich complexes

Addition of principally σ-donating ligands such as THF, chelating diethers, or 1,2-bis(dimethyl)-phosphinoethane to η⁹, η⁵-bis(indenyl)zirconium sandwich complexes, (η⁹- C₉H₅-1,3-R₂)(η⁵-C ₉H₅-1,3-R₂)Zr (R = alkyl or silyl), induces haptotropic rearrangement to afford (η⁶-C₉H ₅-1,3-R₂)(η⁵-C₉H ₅-1,3-R₂)ZrL adducts. Examples where L = THF and DME have been characterized by X-ray diffraction and revealed significant buckling of the η⁶ benzo ring, consistent with reduction of the arene, and highlight the importance of the zirconium(IV) canonical form. For the THF-induced haptotropic rearrangements, the thermodynamic driving force for ring migration has been measured as a function of indenyl substituent and demonstrates silylated sandwiches favor THF coordination and the η⁶,η⁵ bonding motif over their alkylated counterparts. In the case of chelating diethers, measurement of the corresponding equilibrium constants establish more stable η⁶, η⁵ adducts with five- over four-membered chelates and with smaller oxygen and carbon backbone substituents. Kinetic studies on both THF and DME addition to (η⁹-C₉H₅-1,3-(SiMe ₃)₂)(η⁵-C₉H₅-1,3- (SiMe₃)₂)-Zr established a first-order dependence on the incoming ligand, consistent with a mechanism involving direct attack of the incoming nucleophile on the η⁹,η⁵ sandwich. These results further highlight the ability of the indenyl ligand to smoothly adjust hapticity to meet the electronic requirements of the metal center.