Date of Award
Doctor of Philosophy
James H. Espenson
Methylrhenium dioxide (MDO), prepared in aqueous solution from the reaction of methylrhenium trioxide (MTO) and hypophosphorous acid, reacts with oxo donor compounds by oxygen-atom abstraction at record-high rates. These include inorganic oxoanions (e.g. ClO4-), organic oxo substrates (e.g. sulfoxides), and few metal oxides (e.g. VO2+). The mechanism of oxygen-atom transfer from oxygen donors (X=O) to MDO involves nucleophilic attack of the substrate at the oxophilic rhenium forming an adduct prior to oxo transfer. The kinetic data have been related to some extent to the element-oxygen force constants;MTO reacts with hydrogen peroxide to form the catalytically active species CH3ReO2([eta]2- O2), A, and CH3ReO([eta]2- O2)2(H2O), B. The catalytic reactions of these rhenium peroxides with a large family of phosphines have been investigated. Systematic changes in the substituents on phosphorous were made to vary the nucleophilicity and cone angle of the phosphine. The kinetic data support a mechanism that allows nucleophilic attack of the substrate at the rhenium peroxides. MTO also catalyzes the oxidation of [beta]-diketones by H2O2 to give cleavage products, carboxylic acids. The kinetics of the initial oxidation which features epoxidation of the enol form, the majority species, have been investigated for a group of cyclic [beta]-diketones. Its rate responds to substituents in the "normal" manner: electron-donating groups accelerate the reaction. In contrast, the subsequent oxidation steps controlled by O-insertion into a C-C bond involve A and B as nucleophiles rather than their "normal" electrophilic behavior;In the absence of a substrate, the MTO- H2O2 catalyst undergoes decomposition to afford methanol and perrhenate. The deactivation kinetics feature complex dependences on hydrogen peroxide and pH. The catalyst is most stable at high acid and peroxide concentrations. Methanol and perrhenate are formed from the action of HO2- on MTO; the mechanism of deactivation involves methyl migration to a peroxo bound oxygen. The diperoxo complex, B, on the other hand, is stable towards decomposition to methanol and perrhenate; instead it evolves molecular oxygen and regenerates the starting MTO.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Mahdi Muhammad Abu-Omar
Abu-Omar, Mahdi Muhammad, "Oxo and peroxo ligand transfer reactions involving methylrhenium trioxide " (1996). Retrospective Theses and Dissertations. 11356.