Reactions at nitrogenous ligands on oxidizing group 8 metal centers

Strongly oxidizing transition metal species are important as reagents and catalysts in a number of industrial and biological processes, but much is unknown about their modes of reactivity. This dissertation describes research designed to better understand the reactions of strongly oxidizing species—primarily Os(IV) anilides—with the goal of developing fundamental knowledge of the factors that guide selective oxidations by late-metal complexes.; Reaction of Os(IV) anilides with secondary amines (R₂NH) gives materials with derivatized anilido ligands in ca. 30% yield. These products are the result of nucleophilic aromatic substitution of hydrogen at the para-position of the anilide aryl ring. The remainder of osmium starting material is reduced by one electron and one proton (or one H·) to form Os(III) aniline analogues. The reactions are thus a novel example of umpolung-coordination of an electron-rich anilide fragment to an electron-poor and oxidizing Os(IV) center inverts the typical reactivity. A mechanism that resembles organic nucleophilic aromatic substitution reactions is postulated. Extensions of this chemistry to ruthenium analogues are described. Further work is shown O₂ to oxidize select Os(III) amines to Os(IV) amido species in a base-catalyzed process. The likely mechanism of this unusual reaction involves initial deprotonation of the amine complex and subsequent single-electron transfer to O₂.; Hydrogen-atom transfer (HAT) reactions are ubiquitous in biological systems and represent simple one-electron oxidation processes. To better understand these reactions, degenerate H· self-exchange reactions between Os(IV) anilide and Os(III) amine complexes have been examined. Concerted H· exchange between these complexes is exceptionally slow as compared to related reactions. Osmium analogues that differ by only one proton or one electron have been prepared and two proton transfer and two electron transfer self-exchange rates related to H· self-exchange were measured. This is the first system where all five relevant rates have been determined. Individual proton and electron transfer rates are significantly faster (10 6–108)—no correlation is observed with H· self-exchange. The slow, concerted transfer of H · is ascribed to large work terms for formation of the precursor complex, as described by Marcus theory. Cross-reactions with sterically-hindered nitroxyl radical species and anilides were used probe these issues.