Synthesis and Investigations into the Reactivity of Electron Deficient Organoscandium Complexes

Author: Thompson, Mark Edward

Year: 1986

Degree: Dissertation (Ph.D.)

Advisor: Bercaw, John E.

Committee Members: Collins, Terrence J.; Bercaw, John E.; Grubbs, Robert H.; Chan, Sunney I.

Option: Chemistry

DOI: 10.7907/wcaf-jd79

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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