This award in the Chemical Synthesis (SYN) program in the Division of Chemistry at NSF supports work by Professor William J. Evans at the University of California, Irvine, to carry out fundamental studies on redox reactions utilizing the unique properties of the lanthanide and actinide metals. These reduction/oxidation reactions constitute one of the two most basic types of chemical reactions and influence our lives in countless ways. Since the lanthanide and actinide elements represent extremes in the periodic table, they provide an excellent set of metals for expanding the limits of redox reactivity. Activation of small molecules like dinitrogen and nitric oxide will be studied since the redox chemistry of these molecules is critical to making the fertilizer that provides the food for the world and nitric oxide redox chemistry is also crucial to understanding topics as diverse as smog and vasodilation. The project will explore methods to access "virtual" oxidation states of metals that provide redox reactivity even though they have never been isolated. The ligands attached to metals will also be used to generate redox reactivity and develop molecules that can effect multi-electron redox processes. The latter are critical to reactions like solar water splitting that have the potential to provide clean energy without increasing carbon dioxide levels in the atmosphere.
The pervasive nature of redox chemistry means that this research can have a broad impact on science and technology. At the same time, students will be trained in the chemistry of the lanthanide metals, which are a valuable national resource with numerous technological applications, and the actinide metals, whose chemistry is critical to efficient radioactive waste disposal.
Fundamental aspects of the rare earth metals were explored in this project to provide a basis for improving chemistry of critical importance to our lives. The rare earth metals have unique properties that make them the best metals for applications like energy-efficient magnets (neodymium/dysprosium) and lighting (europium/terbium), MRI (gadolinium), and catalytic converters in automobiles (cerium). They are so useful in energy-efficient devices that there is now a global shortage of rare earth metals. The special chemistry of these metals has been used to expand one of the two most fundamental types of chemical reactions, the reduction/oxidation "redox" reactions that pervade our life e.g., in metabolism, batteries, corrosion. We have found new ways of doing the reduction part of redox chemistry. This is surprising since redox reactions have been studied for over 100 years. Specifically, we have found new ways to reduce dinitrogen, N2, the relatively inert gas that is used to make the fertilizer needed to grow the crops that feed the world. Dinitrogen reduction is done on such a large scale in the Haber process that it uses 1-2% of the world’s total energy supply. Nature can also reduce dinitrogen without using a high temperature process, but scientists have been unable to reproduce this low energy process despite intense study around the world. We have discovered a new reduced form of dinitrogen, the (N2)3– ion, that has gone undetected in the decades of research on dinitrogen reduction. The special properties of the rare earth metals allowed this radical trianion to be isolated for the first time. The existence of this ion will cause a re-evaluation of how scientists try to mimic biological dinitrogen reduction. The reactivity of the new (N2)3– ion allowed the isolation of the (NO)2– ion for the first time. This is a new reduced form of nitric oxide, a gas that is important in industry, in biological systems as a vasodilator, and in air pollution. This discovery will cause the re-examination of the NO chemistry in these broad areas. The isolation of the (N2)3– ion has also had impact in an area completely different from reduction or dinitrogen chemistry. This ion is quite effective in coupling two magnetic rare earth ions to provide the best single molecule magnets (SMMs) in the world. SMMs are the smallest magnets possible and have tremendous potential in improving high-density information storage and making smaller, more energy-efficient systems in every device that uses magnetism, e.g., in motors. SMMs have been known for 20 years but only operate at very low temperatures. The SMMs based on the (N2)3– ion demonstrate a new approach to this heavily-studied area and have quadrupled the operating temperature. This is a good demonstration of how research in one area of science can lead to unexpected breakthroughs in completely different areas. The reduction studies have also led to the first example of powerful light-driven reduction using the rare earth metals. Traditionally, it was well established that the rare earth metals have no useful photochemistry. This project has shown that the difficult reduction of dinitrogen can be done photochemically using the appropriate rare earth compounds synthesized for the first time in our laboratory. This could provide a basis to use light and particularly sunlight to reduce dinitrogen instead of the heat used in the Haber process. Finally, this project has led to the identification of six new oxidation states in the periodic table. One of the fundamental characteristics of any metal is the extent to which it loses electrons to form charged species in different formal oxidation states. This ionization can occur in the gas phase to form short-lived species in a wide range of oxidation states, but the number of oxidation states available for any metal in molecules in solution is smaller. Chemists have tested the limits of oxidation states of all the elements for over 100 years and the boundaries of oxidation states are well established. It was therefore quite surprising to discover the first examples of Ho2+, Er2+, Tb2+, Gd2+, Pr2+, and Lu2+. Decades of previous data had suggested that these ions could not be isolated. The new ions will force the rare earth community to re-evaluate the chemistry of these metals. The Ln2+ ions will also provide exciting new opportunities to explore reduction chemistry. In addition, preliminary data suggest that these ions have electron configurations that differ from traditional Ln2+ ions. One consequence of these new electron configurations could be magnetic moments higher than any other metals. Hence, the new ions may have important implications in magnetism.
