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.

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