Ancillary ligand effects on fundamental transformations in metallocene catalyzed olefin polymerization

The preparation of a series of unlinked and ansa-zirconocene dihydride complexes from hydrogenation of the corresponding dimethyl complexes is described. In general, sterically demanding ligands promote formation of monomeric dihydride complexes. The ansa-zirconocene dihydride, meso-[Me₂Si(η5-C₅H ₃-3-CMe₃)₂ZrH₂]₂, has been characterized by X-ray diffraction. The monomeric, ansa-zirconocene, rac-Me₂Si(η5-C₅H₂-2-SiMe ₃-4-CMe₃)₂ZrH₂ (BpZrH₂) undergoes thermal reductive elimination of dihydrogen forming, BpZr(μ ₂-N₂)ZrBp which displays a side-on coordination of the dinitrogen fragment.; Rates of olefin insertion and β-hydrogen elimination have been measured for a series of zirconocene and hafnocene dihydride and alkyl hydride complexes. In both cases, increased cyclopentadienyl substitution slows the rate of insertion or elimination, although the former is more sensitive to steric perturbations. From these studies, the transition state for olefin insertion/β-hydrogen elimination has been established. Equilibration of zirconocene isobutyl hydride complexes with the corresponding normal butyl hydrides has allowed for determination of the relative ground state energies of the two alkyl hydride metallocenes. These data in combination with the activation barriers for β-hydrogen elimination have allowed for delineation of ground and transition state effects in these processes. Likewise, ancillary ligand effects on the rate of alkyl isomerization have also been examined with a series of isotopically labeled zirconocene alkyl complexes. In general, increasing the substitution on the cyclopentadienyl rings has little effect on the rates of isomerization.; The preparation of ansa-tantalocene olefin-hydride complexes is described. These complexes serve as models for the transition state for olefin insertion in the corresponding group IV metallocene olefin polymerization catalysts. Preparation of the C₁-symmetric ethylene hydride complex, Me₂Si(η5-C₅H ₄)(η5-C₅H₃-3-CHMe₂)Ta(η 2-C₂H₂)(H) affords predominantly (∼95 %) of one isomer, where the ethylene ligand is coordinated on the side of the wedge away from the isopropyl substituent. Similar results have been obtained with related tantalocene propylene-hydride and styrene-hydride complexes, the latter being characterized by X-ray diffraction. Additionally, several singly and doubly bridged tantalocene trimethyl complexes have been prepared and characterized by X-ray diffraction.