Fullerene Cages as Molecular Nanocontainers to Stabilize Unprecedented Uranium Clusters and Bonding Motifs

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Professors Luis Echegoyen and Skye Fortier of the University of Texas at El Paso are supported by the Macromolecular, Supramolecular and Nanochemistry Program of the Division of Chemistry to prepare, purify and characterize novel uranium-based endohedral fullerenes. The goal is to explore the fundamental bonding properties of uranium within the fullerene cage with other atoms such as carbon, sulfur, and oxygen. Fullerenes are chemical compounds composed exclusively of many carbon atoms that are bonded together to form spherical or pseudo-spherical closed structures, that in some cases resemble nano-sized soccer balls. Fullerenes are essential components in materials applications such as in solar cells and photovoltaics. The hollow cavities inside these fullerenes offer a unique environment to study isolated chemical systems with unusual structures and bonding motifs as well as interesting electronic properties. The entrapment of metals or metal clusters inside these endohedral fullerenes provides a stable nanocontainer environment. This enables the study of unique and unusual uranium clusters that will allow for a better fundamental and practical understanding of uranium electronic properties in particular and the actinide series of elements in general. The project provides an entryway into an essentially new branch of fullerene and actinide science. A group of students from the University of Texas at El Paso, which is a minority serving institution, are involved in the project. Given the collaborative nature of the project, they are also exposed to an international research experience. In addition, several learning modules, targeting middle school, mainly Hispanic science students, are developed. This new, one month, 20-25 student community outreach program is intended to stimulate their interest in science while developing critical thinking and problem-solving skills. In this project, a new series of uranium containing endohedral fullerenes is synthesized, purified and isolated. It is expected that these carbon caged nanocontainers will allow access to unusual uranium clusters and provide a better fundamental understanding of actinide bonding and chemistry. Initial results on the isolation and structural characterization of a non-IPR (Isolated Pentagon Rule) uranium endohedral, U@C80, provide incontrovertible support for the uniqueness of these molecules and the feasibility of the approach. The synthetic methodology is being optimized using different reactor gases, while soot extraction conditions are being tested to maximize the yield. The isolated compounds are being studied through a full battery of techniques that includes X-ray crystallographic analysis, cyclic voltammetry, electronic and vibrational spectroscopy, SQUID magnetic analysis, and theoretical modeling to fully elucidate their structures and properties. Both national and international collaborations are being leveraged to accomplish these critical tasks. This work provides an entryway into an essentially new branch of fullerene and actinide science and is expected to yield fundamental knowledge and new discoveries on both fronts.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.