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

Addition of principally a-donating ligands such as THF, chelating diethers, or 1,2-bis(dimethyl)-phosphinoethane to eta(9),eta(5)-bis(indenyl)zirconium sandwich complexes, (eta(9)-C9H5-1,3-R-2)(eta(5)-C9H5-1,3-R-2)Zr (R = alkyl or silyl), induces haptotropic rearrangement to afford (eta(6)-C9H5-1,3-R-2)(eta(5)-C9H5-1,3-R-2)ZrL adducts. Examples where L = THF and DME have been characterized by X-ray diffraction and revealed significant buckling of the 716 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 eta(6),eta(5) bonding motif over their alkylated counterparts. In the case of chelating diethers, measurement of the corresponding equilibrium constants establish more stable eta(6), eta(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 (eta(9)-C9H5-1,3-(SiMe3)(2))(eta(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 eta(9),eta(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.