The Research award in the Inorganic, Bioinorganic and Organometallic Chemisry program supports work by Professor Marcetta Darensbourg at the Texas A&M University in College Station, TX, to carry out fundamental studies on small molecule models of enzyme active sites responsible for controlling hydrogen metabolism in nature. In this research, synthetic analogues of the unique metal-organic fragments found in hydrogenase enzymes are prepared and assayed for their ability to mimic the hydrogen-producing and hydrogen-consuming catalysis found in organisms that subsist on a micro-hydrogen economy. The atom-atom connections, the geometrical structure, and spectroscopic footprints of these natural catalysts are reproduced in the small molecules. The intricate and facilitating role of the surrounding protein matrix is approached by imbedding the new molecules in polymer beads and cyclodextrins. By this approach we develop understanding of the way Nature has engineered hydrogen processing ability into base metal constructs of iron and nickel, whereas commercial fuel cells must rely on expensive and resource-limited platinum.
Students at all levels (graduates and undergraduates) are excited by the chemistry of fuel cells and interest in hydrogen as an energy carrier is high. In the process of opening young minds to the needed research in alternative energy sources and challenges, inspiration from nature leads into the literature of biology, biochemistry, biophysics, as well as synthetic inorganic chemistry. Our culturally diverse research group works as a team in projects to encourage interest in science in grammar school children; undergraduates from historically black colleges and universities and from South Texas, largely Hispanic colleges, join the research group to help in the projects through summer research.
A home-run from this work would result in hydrogen processing catalysts feasible enough to serve as a molecular fuel cell electrode, replacing platinum as a fuel cell electrode, and to make energy-efficient fuel cell technology feasible for a broader range of applications.
Synthetic analogues of the iron- and nickel-containing enzyme active sites (EAS) of hydrogen-processing biocatalysts known as Hydrogenases are designed with a goal of using abundant metals to perform the same catalytic function as does the expensive and resource-limited platinum metal catalyst in modern hydrogen fuel cells. Inspired by the natural composition of the unique diiron catalytic unit that exists in the EAS of the [FeFe]-H2ase, and with use of fundamental concepts of electronic and steric donor properties of ligands well-established in organometallic chemistry, almost precise structural models of the illusive "rotated" structure displayed in the as-isolated, mixed-valent FeIIFeI state of the enzyme have been reproduced. Through local and international collaborations, resulting in two major publications of several years in preparation, in depth analyses of the electronic structures of the models by Mössbauer, EPR (ENDOR), and, especially fruitful, computational chemistry, has provided rationale for the occurrence of the deceptively simple "rotated" structure (an isomeric form of a truly simple organoiron compound), and permitted resolution of the delicate balance of steric and electronic features that account for the shift from the metal-metal bonded, butterfly structure of the FeIFeI organometallic to the SP/inverted-SP, with stabilizing bridging carbonyl, isomeric form that is in our mixed-valent FeIIFeI synthetic analogue and represents the resting state of the enzyme, Figure 1. Such insight has prompted broader design of advanced biomimetics with ability to serve as hydrogen-evolving electrocatalysts. Through recent progress in understanding of the biosynthetic paths to the hydrogenase EAS’s, Scheme 1, we recognize that synthetic chemists are on the right track towards recognizing the consequence of nature’s use of 1) a redox active pendent iron-sulfur cluster as a buffer for oxidation state changes at the diiron catalytic site; 2) the necessity for a overhanging base to assist in proton delivery to reduced iron; 3) bridging thiolates to maintain close Fe-Fe distance; and 4) diatomic ligands to stabilize both reduced and oxidized iron. With the last criterion in mind, the use of nitric oxide as a non-innocent, redox-buffering ligand to iron has filtered into our diiron hydrogenase enzyme active site biomimetic studies because of their prospects to delocalize charge and lower over potential for e−/H+ coupled processes. Foundational studies of the Fe(NO)2 unit, as surrogate for the Fe(CO)(CN)2 unit in [NiFe]H2ase, or the Fe(CO)2(CN) unit in [FeFe]H2ase, are establishing our group as experts in the dinitrosyl iron complex area, DNICs. The remarkable stability of the Fe(NO)2 unit is responsible for notable chemical features such as their service as a platform for imidazole conversion to N-heterocyclic carbenes, Scheme 2. Its ready conversion between oxidized and reduced forms has been demonstrated in a study of carbon monoxide-induced elimination of disulfide from thiolates bound within DNICs, Scheme 3. We have used ambidentate thiocyanate and cyanate binding in DNICs to probe the "softness" of the dinitrosyl iron unit. [M225] Preliminary synthetic studies of the use of (N2S2)M (M = NiII or Fe(NO)) as metallodithiolate ligands to the DNI unit are recognized as a novel class of dissymmetric models of hydrogenase active site models whose dissymmetry is readily analyzed for defining molecular features of hydrogenase structure/function activity. [M226] The broader impact of this work addresses the role of hydrogen as an energy vector--in energy storage applications, and the need for sustainable catalysts in both the production and utilization of H2. The sustainable energy-related research area impacts society as a whole and represents cross-disciplinary and collaborative training opportunities for students. It impresses coworkers of the need to have a broad scientific knowledge base in chemistry, biology, spectroscopy, theory, etc., and to be ever alert for the universal language of science, chemistry in particular, for the solving of problems. The M.Y.D. group has been a leader in the nature-inspired catalyst development and participates in national and international Solar Fuels, Hydrogenase, and Bioinorganic conferences. Coworkers within the group are diverse and encouraged to be advocates for science training. They contribute to our annual Chemistry Open House, the Expanding Your Horizons program for 6th grade girls, and other outreach programs, such as NOBCChE. The TAMU Research Experience for Undergraduates summer program is augmented in our lab to build a group of 3 to 4 undergraduates who are mentored in research. Synthetic advances filter rapidly into undergraduate education. As a group project we are developing an inorganic synthesis laboratory experiment based on the EPR and IR-active [N2S2(V≡O)] and [N2S2Fe(NO)] stable complexes, to be distributed to fellow inorganic professors for further testing of suitability and eventual publication in the J. Chem. Education. For references (M###), please see PI's publications at www.chem.tamu.edu/rgroup/marcetta/pubs.html.
