Bis(indenyl)hafnium Chemistry: Ligand-Induced Haptotropic Rearrangement and Fundamental Reactivity Studies at a Reduced Hafnium Center

The chemistry of reduced bis(indenyl)hafnium compounds has been explored. Sodium amalgam reduction of (η⁵-C₉H₅-1,3-(SiMe₃)₂)₂HfCl₂ was studied in several different solvents. While reduction in toluene resulted in decomposition, performing the reaction in the presence of THF or 1,2-dimethoxyethane (DME) yielded η⁶,η⁵-bis(indenyl)hafnium solvent (THF or DME) complexes arising from ligand-induced haptotropic rearrangement. Using less coordinating solvents such as diethyl ether or 2,5-dimethyl THF furnished a rare example of a structurally characterized hafnocene(III) complex, (η⁵-C₉H₅-1,3-(SiMe₃)₂)₂HfCl. An unusual sodium chloride complex of η⁶,η⁵-bis(indenyl)hafnium, [(η⁶-C₉H₅-1,3-(SiMe₃)₂)(η⁵-C₉H₅-1,3,-(SiMe₃)₂)HfCl]₂Na[Na(DME)₃], was also isolated and crystallographically characterized as a byproduct from synthesis of the DME compound. The reactivity of the η⁶,η⁵-bis(indenyl)hafnium THF compound as an isolable source of Hf(II) was also evaluated. Addition of H₂ or N,N-dimethylaminopyridine (DMAP) resulted in facile oxidative addition, while C−O cleavage of THF was observed upon mild heating. Preparation of the corresponding hafnocene dicarbonyl derivatives allowed comparison of the electronic properties of the new complexes as compared to their zirconocene congeners.