Abstract of NSF 0808909: University of Vermont, William Geiger (PI), supported by the Analytical and Surface Science Program of the National Science Foundation
Intellectual Summary: Electrochemistry is one of the most versatile tools available to the practicing chemist. Two of the most important areas of applications are in analysis and catalysis. The first of these deals with the identification and quantification of molecules and is critically important to a wide range of needs addressing environmental, biological, and medical problems. The proposed work seeks to develop a new family of analytical 'sensors' based on the idea of labeling the analytical target with a 'marker' that can be turned on and off by the equivalent of an electrical switch. The new marker is derived from an easily prepared manganese complex which can be reversibly switched in an electrolyte medium previously developed at the University of Vermont (UVM). The other major area of inquiry is intended to develop lower-energy and greener ways to utilize simple olefins, which are oil-based organic building blocks, in the preparation of desirable organic compounds that will be of general use to synthetic chemists. A highly efficient electrochemical method will be developed and evaluated with the goal of replacing energy-wasting thermal and photochemical methods by an organometallic rhenium complex which is capable of electro-catalyzing the desired reactions.
Broader Impact: In terms of the research results, a successful outcome to the proposed work is expected to broaden the impact of organometallic electrochemistry as a problem-solving method among chemists and biochemists. It is also important for chemists to ask how their efforts might contribute to the daunting task of increasing the efficiency of energy usage in this country and in our world. There are only four important general energy sources at our disposal: nuclear, heat, light, and electricity. Our goal has been, and continues to be, the training of electrochemists to develop advances in the last of these four areas. Graduate students trained in electrochemistry at UVM are employed in a variety of academic, industrial, and government research positions. Because it is important to expand these opportunities to underrepresented groups, this project will offer summer training projects to financially disadvantaged high school students through Project Seed (overseen by the American Chemical Society) and initiate a pilot project which offers a summer research opportunity at UVM to underrepresented undergraduate students from a city-based college having a large minority enrollment. Thanks to their experience in a research group composed of highly-motivated graduate students and postdocs, the high school and visiting undergraduate students will be more aware of their opportunities in pre- and post-graduate higher education, thereby increasing the probability that they will become scientific professionals.
Project Outcome Report for General Public Grant: NSF CHE 08-08909: New Organometallic Radical-Cation Based Homogeneous and Heterogeneous Reactions Grantee: University of Vermont; William E. Geiger, principal investigator Background Chemists are constantly looking for new, more efficient, ways to transform one molecule into another or to make a given molecule perform a new function. Every chemical reaction requires energy, in some form, to make it go. The three generally available sources of energy (putting aside nuclear energy) rely on heat, light, or electricity. The research group at the University of Vermont specializes in exploring how electricity influences the chemical reactions of molecules. The molecules studied are either completely organic or have organic fragments that are linked (bonded) to a metal such as iron or manganese. The metal-containing molecules are called organo-metallic compounds. Our experiments apply small voltages (3 V or less) to an electrode immersed in a solution that contains the test molecule. By precise control of the applied voltage, the test molecule is induced to donate an electron to the electrode. After this "electron transfer", the molecule finds itself in possession of an odd number of electrons, which is a generally unstable molecular state. In fact, such molecules are commonly referred to as "radicals", befitting their inclination to do chemical reactions that are not allowed by the molecular parents from which they were derived. Both nature and the laboratory have found ways to make good use of this "radical reactivity" to carry out desirable reactions as diverse as the human respiratory process and the efficient production of plastics. Since an electron is negatively-charged, its removal from a molecule causes the radical to be positively charged and the resulting derivative is called a "radical cation". In developing well-ordered reactions of radical cations, one must provide a benign medium in which they are generated. Having done that, one can then add chemical reagents in a controlled way to achieve the desired reactions. The major goal of the research conducted in the present NSF grant and in its immediate predecessor (NSF CHE 04-11703) has been to find a superior, more benign, way to generate reactive radical cations. That has been achieved, providing an improved way to make use of electrical energies to carry out chemical reactions and to use electro-chemistry to analyze molecules within chemical and biological environments. Technical Aspects The main problem in building a radical cation "tool kit" having systematically varied organic and metal ‘parts’ has been the undesired tendency of the cations to react with the anions present in the medium being used to create the cations. The Vermont group showed that simply using a large organic anion in the electrolyte salt stabilizes the radical cations.The reason this works is that the radical cations are inherently less reactive towards large anions than the small anions used previously. Radical cations generated in the presence of the large anion [B(C6F5)4]- therefore have a much longer life and can be characterized and reacted as desired. Consider the example of cyclopentadienyl manganese tricarbonyl (1). Nicknamed cymantrene, this compound has been investigated many times over about a forty year period with the goal of removing one electron to give a stable cymantrene radical cation, 1+. Always, however, 1+ decomposed within seconds of its electro-chemical production, until our ploy of anion replacement. When the electron-transfer of 1 is carried out in a [B(C6F5)4]- - containing medium, 1+ is stable. It was characterized by various kinds of spectroscopy and reacted in a controlled way to produce new products that are important to those carrying out research in organo-metallic chemistry. There are two more broadly important aspects of the findings on cymantrene. First, it establishes how a simple change of the salt medium (often called the electrolyte) allows production of long-lived organo-metallic radical cations. This opens new possibilities for the use of these radical cations in a plethora of chemical reactions. A second factor arises from the very strong response of metal-carbonyl compounds to infrared radiation. By "tagging" a molecule with a manganese carbonyl group, the position of the compound can be monitored by infrared (IR) spectroscopy. The target may be a biological sample, even a particular cell, in which the compound finds residence. Its reaction with the cell could be monitored by IR imaging. It has not been possible previously to monitor both the position and the electron count of spectroscopically-tagged biological systems. Summary An innovative electro-chemical approach to chemical reactivity has been developed. The new method is based on the use of electrolyte salts that contain charge-delocalized anions. These anions stabilize molecular radical cations against decomposition and allow for their controlled reactions. New possibilities now exist for the application of organo-metallic complexes in energy-efficient chemical synthesis, chemical analysis, and molecular and biological imaging. Prepared by W.E.G., 9/5/12
