Synthesis, Stability, and Reactivity of High Oxidation State Pentamethylcyclopentadienyl Acetamidinate β-Hydride- or β-Methide-Bearing Alkyl Complexes of Zirconium, Titanium, and Tantalum

Title of Document: Synthesis, Stability, and Reactivity of High Oxidation State Pentamethylcyclopentadienyl Acetamidinate β-Hydrideor β-Methide-Bearing Alkyl Complexes of Zirconium, Titanium, and Tantalum Albert Epshteyn, Doctor of Philosophy, 2006 Directed By: Professor Lawrence R. Sita Department of Chemistry and Biochemsitry University of Maryland, College Park, MD 20740 In the course of study into the stability and reactivity of early transition metal (TM) complexes, it has been further confirmed that the pentamethylcyclopentadienyl acetamidinate ligand set is particularly apt for the stabilization of early TM-alkyl species. The first examples of a solid state structure of cyclobutylmethyl complexes were obtained, after pentamethylcyclopentadienylacetamidinatezirconium (Cp*ZA) cyclobutylmethyl compounds were prepared via hydrozirconation of methylenecyclobutane. The ring opening of neutral and cationic cyclobutylmethyl Cp*ZAs and the conformation of the product pendant olefin pentenyl cation were studied via selective neuclear Overhouser effect (nOe) confirming that the pendant olefin was coordinated to the cationic Zr center. To further study the properties of pendant olefin complexes, a methyl butenyl Cp*ZA complex was prepared, and was observed to convert to a Cp*ZA butadiene complex, presumably via a β-H abstraction by the metal-based methyl group. The reactivity of the first known non-base stabilized η-styrene zirconium complex, supported by the Cp* acetamidinate ligands, was studied with olefins, 1,3butadienes, and 2-butyne. It was shown that the initial products of α-olefin insertion into the η-styrene complex are saturated zirconacyclopentanes that can then undergo a dehydrogenation producing H2 and a Cp*ZA butadiene complex. The reaction of the η-styrene Cp*ZA with 2-butyne was shown to produce a Cp*ZA butadiene complex via a sequential butyne insertion followed by a formal 1,3-hydride shift, and the reaction with 1,3-butadienes yielded Cp*ZA butadiene complexes with loss of styrene. The chemistry of pentamethylcyclopentadienyl acetamidinate titanium (Cp*TiA) was pursued, and the reaction of the Cp*TiA dichloride with 2 equiv. of MeLi yielded the dimethyl Ti(IV) complex, while the reaction with 2 equiv. of EtLi yielded the Ti(III) ethyl complex. Studies were carried out with Cp* acetamidinate compounds of Ta (Cp*TaAs). The reactions of Cp*TaA trichloride with EtLi, BuLi, BuLi, NpLi produced a series of new Ta(V), Ta(IV), and Ta(III) complexes. The Ta(IV) dichloride was also accessed via a Na/Hg reduction allowing further access to Ta(IV) alkyl compounds.The thermal decomposition of the Ta(IV) diisobutyl yielded a Ta(IV) TMM complex and a diamagnetic Ta(IV) dimeric dihydride. Synthesis, Stability, and Reactivity of High-OxidationState Pentamethylcyclopentadienyl Acetamidinate β-Hydrideor β-Methide-Bearing Alkyl Complexes of Zirconium, Titanium, and Tantalum