Authors: Pamela J. Shapiro
Source: CaltechTHESIS (California Institute of Technology) (article, 2019-11-05)
Field: Chemistry — Organic Chemistry

Part I. Scandium Hydride and Alkyl Complexes with a Linked Monocyclopentadienyl-Amido Ligand-Framework: Single Component Catalysts for the Polymerization of α-Olefins. Part II. Hydrazido(1-) and 2,2-Dimethylhydrazido(1-) Derivatives of Permethylscandocene. Preparation and Structural Characterization of their Products from Reactions with Acetonitrile Various monocyclopentadienyl scandium amido derivatives with the general formulas {(η5-C5Me4)SiMe2(η1-NCMe3}ScR, (Cp*SiNR)ScR, and {(η5-C5H3CMe3)SiMe2(η1-NCMe3}ScR, (tBuCpSiNR)ScR, have been prepared. [(Cp*SiNR)(PMe3)ScH]2 is prepared from the hydrogenation of (Cp*SiNR)ScCH(SiMe3)2 in the presence of PMe3. The scandium hydride effects the multiple insertion of α-olefins in a controlled fashion to form low molecular weight polymers. β-H elimination appears to be a principal chain-transfer pathway in this catalyst system. The stoichiometric reaction between [(Cp*SINR)(PMe3)ScH]2 and two equivalents of ethylene produces the ethylene bridged dimer [(Cp*SiNR)(PMe3)Sc]2(µ, η2, η2-C2H4) and an equivalent of ethane. The stoichiometric reaction between the scandium hydride and either propene or butene affords the PMe3-free, alkyl-bridged scandium dimers [(Cp*SiNR)Sc]2(µ-CH2CH2CH3)2 and [(Cp*SiNR)Sc]2(µ-CH2CH2CH2CH3)2. The absence of coordinating phosphine makes these complexes more active olefin polymerization catalysts. The complexes (Cp*SiNR)(PMe3)ScCH2CH(CH)3CH2CH3 and (Cp*SiNR)(PMe3)ScCH(C6H5)CH2CH2CH2(C6H5) have also been prepared and spectroscopically characterized. Kinetic analysis of the rate dependence of 1-pentene polymerization by [(Cp*SiNR)(PMe3)ScH]2 on added PMe3 reveals a phosphine dissociation pre-equilibrium to form a PMe3-free active intermediate. The monomeric nature of this intermediate is revealed by the kinetic analysis of 1-pentene polymerization by [(Cp*SiNR)Sc]2(µ-CH2CH2CH3)2 as a function of scandium concentration. (Cp*SiNR)ScR was found to be &gt;99% head-to-tail regioselectfve in the polymerization of propene. Preliminary 13C NMR data indicate lower regioselectivity in propene polymerization by (tBuCpSiNR)ScR. A slight syndiotactic preference is observed in the stereochemistry of the polypropene produced with [(Cp*SiNR)Sc]2(µ-CH2CH2CH3)2. By contrast, [(tBuCpSiNR)ScMe]x displays no stereocontrol, producing an atactic polymer. A rare example of an unsubstituted hydrazido(1-) complex, Cp*2ScNHNH2 (Cp* = (η5-C5Me5)), prepared by reaction of one equivalent of anhydrous hydrazine with Cp*2ScCH3, reacts with acetonitrile to form [chemical formula; see abstract in scanned thesis for details]. The crystal structure of [chemical formula; see abstract in scanned thesis for details] reveals a five-membered, nearly planar [chemical formula; see abstract in scanned thesis for details] ring. The results of a labeling study using 15N=CCH3 are consistent with a mechanism involving insertion of acetonitrile into the Sc-N bond of Cp*2ScNHNH2, followed by tautomerization to form [chemical formula; see abstract in scanned thesis for details]. The closely related compounds Cp*2ScNHNMe2 and [chemical formula; see abstract in scanned thesis for details] have also been prepared, and the structure of the latter compound has been determined. Chemistry ; Hydride ; Scandium ; Chain transfer ; Polymerization ; Phosphine ; Polymer chemistry ; Stoichiometry ; Propene ; Catalysis ; Inorganic chemistry ; Organic chemistry ; Polymer ; Radical polymerization ; Hydrogen Organometallic Complex Synthesis and Catalysis ; Asymmetric Hydrogenation and Catalysis ; Synthesis and characterization of novel inorganic/organometallic compounds