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-C9H5-1,3-R2)Zr
(R = alkyl or silyl), induces haptotropic rearrangement to afford (η6-C9H5-1,3-R2)(η5-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 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-(SiMe3)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.