Received March 30, 2005

Abstract:

Addition of principally -donating ligands such as THF, chelating diethers, or 1,2-bis(dimethyl)phosphinoethane to ⁹,⁵-bis(indenyl)zirconium sandwich complexes, (⁹-C₉H₅-1,3-R₂)(⁵-C₉H₅-1,3-R₂)Zr (R = alkyl or silyl), induces haptotropic rearrangement to afford (⁶-C₉H₅-1,3-R₂)(⁵-C₉H₅-1,3-R₂)ZrL adducts. Examples where L = THF and DME have been characterized by X-ray diffraction and revealed significant buckling of the ⁶ 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 ⁶,⁵ bonding motif over their alkylated counterparts. In the case of chelating diethers, measurement of the corresponding equilibrium constants establish more stable ⁶,⁵ adducts with five- over four-membered chelates and with smaller oxygen and carbon backbone substituents. Kinetic studies on both THF and DME addition to (⁹-C₉H₅-1,3-(SiMe₃)₂)(⁵-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 ⁹,⁵ sandwich. These results further highlight the ability of the indenyl ligand to smoothly adjust hapticity to meet the electronic requirements of the metal center.

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