Experimentally measuring how ligand modifications affect metal-ligand bonding and electronic structure is an important goal with relevance to diverse fields such as transition metal catalysis and f-element separations. X-ray absorption spectroscopy (XAS) is an excellent technique for investigating structure/function relationships in metal complexes because it can be used to quantify variations in covalent metal-ligand bonding and electronic structure. Here I describe a series of XAS investigations aimed at elucidating how ligand and structural changes affect chemical bonding and properties in transition metal and uranium complexes. The synthesis and characterization of several new classes of f-element complexes are also discussed.
Diphosphines are very important ligands in homogeneous catalysis because they can be used to tune reaction rates, the electronic properties of transition metals, and the stereochemistry of catalytic products. P K-edge XAS studies on solid Ni and Pd diphosphine compounds have shown that M-P covalency is not exclusively dependent on the P-M-P angle (i.e. bite angle), which changes as a function of differing linker groups on the diphosphine backbone. Building on these studies, I show how changes in diphosphine bite angle influence Pd-P covalency in solution and when phenyl substituents attached to phosphorus are replaced with alkyl substituents. This work required the development of an improved solid energy calibration standard for routine energy referencing of P K-edge XAS spectra. I discuss the limitations of P K-edge XAS energy standards used previously and propose tetraphenylphosphonium bromide as a new energy calibration standard for future P K-edge XAS work.
The use of nuclear power has resulted in critical challenges surrounding the long-term storage and remediation of nuclear waste. Advanced nuclear fuel cycles can address the scientific challenges of nuclear waste, but require the difficult separation of minor actinides and lanthanides. A multi-donor ligand containing a thioether appendage was prepared to determine if it would bind differently to lanthanide and actinide metals, thereby resulting in metal complexes that could be separated due to differing solubilities. In a related study, a new class of homoleptic lanthanide and actinide borohydride complexes called phosphinodiboranates were prepared. I discuss how the differing solution structures of f-element phosphinodiboranates may offer potential for f-element separations.
A series of polyoxovanadate alkoxide clusters was synthesized to investigate the underlying electronic properties that make them useful in catalysis and small-molecule activation. The clusters can access four discrete pseudo-reversible one-electron transfer reactions. V K-edge XAS studies were performed to elucidate variations in electronic structure and bonding in the clusters as a function of redox events, and the results indicate that covalency plays a significant role in the one-electron transfer reactions. These results inspired the synthesis and characterization of an actinide polyoxovanadate compound. Activation and functionalization of the oxo bond in UO22+ is important for understanding the fundamental chemistry of uranium and for developing metal separation processes. A novel uranyl polyoxovanadate compound was studied with V K-edge XAS to determine if covalent interactions between the uranium and vanadium metal centers exist (similar to those observed for the polyoxovanadate alkoxide clusters), and if such interactions could be exploited for activation of the oxo bond in the uranyl dication. Results indicate, however, that the U-O bond remains inactivated.