Probing the electronic structure of dioxygen as a ligand : using x-ray absorption spectroscopy to quantify backbonding

The search for more economical and environmentally friendly oxidation catalysts for organic functional group transformations is currently one of the most prevalent research areas in chemistry. The coordination and activation of dioxygen by transition metal complexes holds particular promise and has thus been thoroughly investigated. Although work has predominantly focused on synthesis of new transition metal dioxygen complexes, determining how the dioxygen ligand interacts with transition metals is also particularly critical. The nature of the M-O2 bond should have important implications on both the chemical and structural properties of the complexes. This thesis focuses on developing and exploring the spectroscopic characteristics of a series of newly reported M-O2 with highly unusual bonding. Such complexes, best described as singlet dioxygen adducts, represent a new class of metal dioxygen complexes with characteristics that are very different from typical metal-superoxo and peroxo complexes. The electronic properties of a variety of rhodium and ruthenium complexes were explored utilizing a combination of synchrotron-based X-ray Absorption Spectroscopy (XAS) techniques in conjunction with Density Functional Theory (DFT) calculations. To quantitatively investigate the complexes, a new fitting methodology was developed, and is described herein. For several of the rhodium dioxygen complexes, the Rh L2,3-edge data provided evidence that no formal oxidation occurred at the metal center upon dioxygen coordination. The data extracted from the experimental spectra provided the first quantitative π-backbonding information for second row transition metal complexes known thus far. Both Rh K-edge XAS and DFT results corroborate the findings from the L-edge spectra. A set of complementary ruthenium complexes, thought to have similar M-O2 binding characteristics, were studied in an analogous manner. Ultimately, this thesis provides the first example of utilizing second row transition metal L2,3-edge XAS data to experimentally determine π-backbonding. Although the research described herein focuses on π-backbonding between dioxygen and transition metals, it provides a basis for applying similar experimental strategies for investigating π-backbonding interactions in other systems of catalytic interest.Science, Faculty ofChemistry, Department ofGraduat