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Synthesis and Investigations into the Reactivity of Electron Deficient Organoscandium Complexes

Citation

Thompson, Mark Edward (1986) Synthesis and Investigations into the Reactivity of Electron Deficient Organoscandium Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/wcaf-jd79. https://resolver.caltech.edu/CaltechTHESIS:10252019-155031892

Abstract

A new class of coordinatively unsaturated, monomeric scandium complexes, (Cp*2ScR (Cp* = η5-C5(CH3)5; R = H, alkyl, aryl, halide) have been prepared. Cp*2ScCl is prepared by the reaction of ScCl3(THF)3 with LiCp*, and Cp*2ScR (R = CH3, C6H5, C6H4CH3, CH2C6H5) by the reaction of Cp*2ScCl with the appropriate organoalkali reagent. Cp*2ScR complexes react readily with H2 to give RH and Cp*2ScH. The hydride ligand exchanges rapidly with hydrogen gas and inserts olefins to give alkyl complexes (e.g. Cp*2ScCH2CH3). Cp*2ScH reacts with allene to give Cp*2Sc(η3-CH2CH=CH2). Cp*2ScR and Cp*2ScH react with pyridine to give Cp*2Sc(C,N-η2-C5H4N). The crystal structure of this complex was determined and is reported herein.

Spectroscopic data for Cp*2ScCH3 and Cp*2ScCH2CH3 and crystallographic data for the former indicate that the methyl ligand is bound to scandium in a conventional manner, while the ethyl ligand may participate in an agostic interaction.

The reactions of scandium alkyl, aryl and hydride complexes were investigated. H/D exchange between H2, arenes and the 1° and 2° C-H bonds of alkanes is catalyzed by Cp*2ScH. In C6H6 solution Cp*2ScH and Cp*2ScC6H5 are in equilibrium, ΔH° = 6.7 ± 0.3 kcal/mole and ΔS° = 1.5 ± 0.1 e.u.. Thus in this system a scandium-hydride bond is 1.5 ± 0.4 kcal/mole stronger than a scandium-phenyl bond. Cp*2ScCH3 reacts with a wide range of hydrocarbons (RH) by C-H bond activation to give CH4 and Cp*2ScR (RH = 13CH4, arenes, styrenes, propyne). From the reactions of Cp*2ScCH3 with styrenes, the activation parameters (ΔH = 11.5-12.6 kcal/mole, ΔS = -34 to -38 e.u.) for these C-H activation reactions were determined. A deuterium isotope effect of 2.9 is observed for the intermolecular activation of C-H in the reaction of Cp*2ScCH3 with benzene. Very small differences in the rates of vinylic C-H bond activation for CH2=CHC6H4X-para (X = CF3, OCH3), and the aryl C-H bonds of C6H5X (X = CF3, H, CH3, N(CH3)2), as well as the positional nonselectivity for the activation of the meta and para C-H bonds of toluene indicate that the scandium center does not interact substantially with the π-system of these substrates in the transition states for these reactions. Thus for these sterically encumbered organoscandium compounds, sp2 C-H bond activation occurs without formation of a π-complex. A general mechanism for these C-H and H-H activation reactions is proposed, and is termed "σ-bond metathesis".

The reactions of Cp*2ScR complexes {R = hydride, alkyl, aryl) with small olefins and alkynes were examined. The hydride, methyl and benzyl complexes function as ethylene polymerization catalysts, while Cp*2ScC6H5 does not react. Cp*2ScH and Cp*2ScCH3 react stoichiometrically with propene by a series of insertion and vinylic C-H activation reaction. The final scandium product in both cases is trans-Cp*2ScCH=CHCH3. The scandium allyl complex, Cp*2Sc(η3-CH2CH=CH2), is not observed and is not a reaction intermediate.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Bercaw, John E.
Thesis Committee:
  • Collins, Terrence J. (chair)
  • Bercaw, John E.
  • Grubbs, Robert H.
  • Chan, Sunney I.
Defense Date:19 August 1985
Funders:
Funding AgencyGrant Number
Atlantic Richfield FoundationUNSPECIFIED
Record Number:CaltechTHESIS:10252019-155031892
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:10252019-155031892
DOI:10.7907/wcaf-jd79
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11866
Collection:CaltechTHESIS
Deposited By: Melissa Ray
Deposited On:28 Oct 2019 19:21
Last Modified:16 Apr 2021 23:21

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