Synthesis, Characterization, and Reactivity Studies of Cerium-Ligand Multiple Bonds and Organometallics and Comparisons with Thorium, Uranium, and Group IV Congeners

With support from the Chemical Synthesis program in the Chemistry Division, Professor Eric Schelter and his research group at the University of Pennsylvania are investigating the organometallic chemistry of cerium, thorium, uranium, and Group IV metal compounds, especially complexes of those elements that include metal-ligand multiple bonds. Cerium is a rare-earth metal, but it exhibits properties similar both to other rare-earth metals and to the transition and actinide metals that surround it in the periodic table. This project aims at comparing the behavior of cerium with these surrounding metals in order to determine the factors that determine the unique properties of cerium. A better understanding of cerium chemistry is expected to contribute to the design new applications for cerium in catalytic, materials and chemical biology applications. The results of these studies will also likely contribute to a greater understanding of rare earth metals that are critical for renewable energy and defense applications. The project will provide student training in the handling and manipulation of the rare earths and contribute to the maintenance of a trained work force in this important area. Hands-on research projects for high school students are also envisioned to provide early exposure to science. The overall goal of this project is to synthesize new metal-ligand multiply bonded organometallic compounds of cerium, uranium, thorium, and Group IV metals. The spectroscopic properties and reactivity patterns across these metals will be compared. The central hypothesis is that the properties of isostructural, tetravalent metal complexes will inform on bonding differences and 4f-/5d-covalency across this broad spectrum of elements. Organometallic complexes and metal-ligand multiple bonds of cerium, uranium, thorium, and group IV elements will be synthesized. Their spectroscopic properties will be studied by multi-nuclear NMR and X-ray absorption spectroscopy. Computational methods will be applied to model the spectroscopic properties and to understand the relative covalent contributions to bonding. Isostructural complexes that undergo equilibrium reactions will also be used to observe dynamic differences in bonding across the elements of cerium, thorium and uranium, and extending to the group IV elements. The fundamental knowledge developed from the work is expected to contribute new and unique examples of organometallic compounds and associated spectroscopic information that will broaden understanding of chemical bonding. Rare earth element-containing materials are used in the energy and defense industries and there are currently no substitutes. Advancing cerium/rare earth chemistry will contribute to improving beneficiation and separation of rare earths, and in so doing, contribute to national security. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.