Synthesis, Stability, and Reactivity of High-Oxidation-State Pentamethylcyclopentadienyl Acetamidinate Beta-Hydride- or Beta-Methide-Bearing Alkyl Complexes of Zirconium, Titanium, and Tantalum
Sita, Lawrence R.
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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 beta-H abstraction by the metal-based methyl group. The reactivity of the first known non-base stabilized n2-styrene zirconium complex, supported by the Cp* acetamidinate ligands, was studied with olefins, 1,3-butadienes, and 2-butyne. It was shown that the initial products of α-olefin insertion into the n2-styrene complex are saturated zirconacyclopentanes that can then undergo a dehydrogenation producing H<sub>2</sub> and a Cp*ZA butadiene complex. The reaction of the n2-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 pentamethylcyclopentadienylacetamidinatetitanium (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, <sup>n</sup>BuLi, <sup>i</sup>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.