Reactions of Late Transition Metal Complexes with Molecular Oxygen

Abstract

Limited natural resources, high energy consumption, economic considerations, and environmental concerns demand that we develop new technologies for the sustainable production of chemicals and fuels. New methods that combine the selective activation of C-H bonds of hydrocarbons with oxidation by a green oxidant such as molecular oxygen would represent huge advances toward this goal. The spectacular selectivity of transition metals in cleaving C-H bonds offers the potential for the direct use of hydrocarbons in the production of value-added organics such as alcohols. However, the use of oxygen, which is abundant, environmentally benign, and inexpensive (particularly from air), has proven challenging, and more expensive and less green oxidants are often employed in transition-metal-catalyzed reactions. Advances in the use of oxygen as an oxidant in transition-metal-catalyzed transformations of hydrocarbons will require a better understanding of how oxygen reacts with transition metal alkyl and hydride complexes. For alkane oxidations, researchers will need to comprehend and predict how metals that have shown particularly high activity and selectivity in C-H bond activation (e.g. Pt, Pd, Rh, Ir) will react with oxygen. In this Account, we present our studies of reactions of late metal alkyls and hydrides with molecular oxygen, emphasizing the mechanistic insights that have emerged from this work. Our studies have unraveled some of the general mechanistic features of how molecular oxygen inserts into late metal hydride and alkyl bonds along with a nascent understanding of the scope and limitations of these reactions. We present examples of the formation of metal hydroperoxide species M-OOH by insertion of dioxygen into Pt(IV)-H and Pd(II)-H bonds and show evidence that these reactions proceed by radical chain and hydrogen abstraction pathways, respectively. Comparisons with recent reports of insertion of oxygen into other Pd(II)-H complexes, and also into Ir(III)-H and Rh(III)-H complexes, point to potentially general mechanisms for this type of reaction. Additionally, we observed oxygen-promoted C-H and H-H reductive elimination reactions from five-coordinate Ir(III) alkyl hydride and dihydride complexes, respectively. Further, when Pd(II)Me(2) and Pt(II)Me(2) complexes were exposed to oxygen, insertion processes generated M-OOMe complexes. Mechanistic studies for these reactions are consistent with radical chain homolytic substitution pathways involving five-coordinate M(III) intermediates. Due to the remarkable ability of Pt(II) and Pd(II) to activate the C-H bonds of hydrocarbons (RH) and form M-R species, this reactivity is especially exciting for the development of partial alkane-oxidation processes that utilize molecular oxygen. Our understanding of how late transition metal alkyls and hydrides react with molecular oxygen is growing rapidly and will soon approach our knowledge of how other small molecules such as olefins and carbon monoxide react with these species. Just as advances in understanding olefin and CO insertion reactions have shaped important industrial processes, key insight into oxygen insertion should lead to significant gains in sustainable commercial selective oxidation catalysis.