Leslie J. Murray - US grants

In this project funded by the Chemical Synthesis program of the Chemistry Division, Professor Leslie Murray of the University of Florida is developing molecular systems that will mirror and improve our understanding of the reactivity of metal clusters found in numerous enzymes in biology. The biological metal clusters perform reactions ranging from the conversion of greenhouse gases (e.g., nitrous oxide or carbon dioxide) into benign products (e.g., water and nitrogen) or fuels (e.g., carbohydrates). Therefore, this project ultimately aims to develop bioinspired approaches to performing efficiently these types of reactions, all of which are of significant societal importance. This project also includes outreach to K-12 students.

The central hypothesis of this project is that high spin late 3d metal ions can react cooperatively in clusters to activate strong bonds and subsequently perform heteroatom transfer at relatively low energetic cost. This project focuses on using macrobicycles as ligands to support and template triiron clusters. These ligands provides steric protection for the three metal centers to afford low coordinate metal centers, which limits the formation of metal-metal bonds or strong interactions in the clusters and, simultaneously, templates an internal cavity for possible substrate binding. Upon chemical reduction, these triiron clusters can complex and activate dinitrogen by these compounds upon chemical reduction. Mechanistic studies of this reduction will involve changing reaction outcomes by introducing structural changes as well as examining the reactivity of intermediates with exogenous substrates (e.g., X-H bond activation). Reaction intermediates and products will be characterized by physical methods including X-ray crystallography, EPR spectroscopy, Mössbauer spectroscopy and X-ray absorption spectroscopy. An outreach program couples established university-based chemistry department programs with K-12 institutions that serve local students from predominantly underrepresented groups. The broader impacts of this work include the potential societal benefits from a fundamental understanding of how biological systems catalyze challenging reactions which relate to the development of new molecular approaches geared toward energy independence.

Title: Ligand effects on reactivity of hydride-decorated and reduced multi-iron compounds Abstract Metal cluster cofactors provide substrates with many potential orientations to bind and subsequently undergo chemical transformations. This is particularly true for cluster cofactors that activate small molecule substrates. The focus of this proposal is on the chemistry of the iron-molybdenum cofactor in the molybdenum-dependent nitrogenases, which catalyzes the eight electron and eight proton reduction of dinitrogen and two protons to generate two equivalents of ammonia and one of dihydrogen. The current mechanism proposed for the conversion of N2 to NH3 by this enzyme uses concepts that are common to numerous other metal cofactors, such as the protonation of bridging sulfide donors, the use of metal hydrides to store reducing equivalents, and the potential to coordinate the hydrides and dinitrogen in either terminal or bridging modes. How the iron-molybdenum cofactor binds hydride donors and dinitrogen, as well as intermediates during the catalytic reaction, are fundamental aspects of the mechanism but remain unclear. This is the knowledge gap that this proposal addresses. This proposal will accomplish this goal by using synthetic clusters in which substrates (hydrides and dinitrogen) can bind in either bridging or terminal coordination modes, which mirrors the coordinative flexibility possible for these substrates on the iron-molybdenum cofactor. As part of this inquiry, this proposal will dissect how number, identity, and connectivity of bridging ligands modulate substrate coordination. The results generated in this proposal have broader implications for biochemical reactions, and specifically, shed light on the principles that govern biological catalysis of multi-electron multi-proton redox reactions (e.g., water oxidation in photosynthesis, dioxygen reduction in respiration).

With funding from the Chemical Synthesis Program in the Chemistry Division, Leslie Murray of the Department of Chemistry at the University of Florida will investigate how iron compounds are able to facilitate the formation of new carbon-carbon bonds between carbon monoxide molecules. Carbon monoxide is a feedstock used by chemical industry to prepare many important products. The motivation for this study is to understand the details of a process (called the Fischer-Tropsch (FT) synthesis) that converts carbon monoxide into lubrication oils and fuels. While the FT synthesis has been known for nearly 100 years, some important details of how it functions are still unclear. In particular, the details of how two carbon monoxide molecules link together in the presence of iron atoms has not yet been established. This project will prepare a series of compounds that contain several iron atoms and that are capable of coupling carbon monoxide molecules. The detail of how this coupling occurs will be determined using both chemical and physical studies. In addition to the scientific objective, this project includes training of graduate and undergraduate students and outreach to K-12 students, including those from underrepresented minority groups. To validate the outreach activities, evaluation methods will be developed to determine the best practices associated with a K-6 outreach program.

The goal of this project is to investigate the mechanistic details for the conversion of CO and dihydrogen to hydrocarbons by the FT process. Such mechanistic studies can both provide fundamental information on the FT process and point to new chemical pathways toward CO-derived C1 and C2 ligands. The project targets the C–C bond forming step in FT synthesis, which is not well understood, and centers on the deoxygenative and non-deoxygenative homocoupling of CO using a diiron siloxycarbyne compound. Specifically, CO bond scission and C–C bond formation steps will be evaluated. Starting with Fe2(COSiR3)(CO)L and related compounds the intermediates that form during C-O cleavage will be determined, their reactivity studied, and the mechanism of the subsequent generation of Fe2(CCO)L determined. In addition, the effect of functionalization of CO derived species will be evaluated. Once again using Fe2(COSiR3)(CO), the routes for functionalizing CO to generate C–X bonds will be targeted. In particular, the transfer the CCO fragment from Fe2(CCO)L to organic substrates, coupling of CO with the siloxycarbyne ligand in Fe2(COSiMe3)(CO)L, and the ability of diiron siloxycarbyne and diiron ketenylidene species to transfer the COSiMe3 and C2O ligands to exogenous substrates will be explored.

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.