This CAREER award by the Inorganic, Bioinorganic and Organometallic Chemistry Program supports work by Professor Ferman Chavez at Oakland University to develop small molecule models for the active sites of oxalate degrading manganese enzymes oxalate oxidase and oxalate decarboxylase. The reactivity of these models towards oxalate will be studied in detail. As an outcome of the proposed investigations, he expects to help clarify mechanisms involved in catalysis for these enzymes as well as provide small molecules with enhanced oxidative activity. The proposed educational component will involve the initiation of a high school outreach program. Experiments and demonstrations related to Professor Chavez's research will be developed to teach basic chemistry concepts to high school students as part of their curriculum. These activities will then be tested in high school classrooms settings in the southeastern Michigan area. Professor Chavez will conduct workshops aimed at high school chemistry teachers featuring newly created experiments and demonstrations. A laboratory workbook and website containing this information will be published.

Project Report

The goal of this project was to make small molecule models for the metal binding sites of oxalate-degrading manganese enzymes (oxalate oxidase and oxalate decarboxylase). These synthetic models were designed to help understand the steps involved for these enzymes. The goal for the educational component of this grant was to implement a high school outreach program and to teach advanced chemistry concepts to high school students as part of their curriculum. For the oxalate oxidase study, we synthesized several functional model compounds that were capable of catalytically degrading oxalate under ambient conditions. [MnLCl] (L = 1-benzyl-4-acetato-1,4,7-triazacyclononane) is our best structural model. The MnII/III redox couple for this compound in methanol (E1/2 = 649 mV vs Ag/AgCl) becomes negatively shifted (E1/2 = 390 mV) in the presence of oxalate. When [MnLCl] is reacted with excess oxalate, two equivalents of carbon dioxide per oxalate are seen. This is identical to what is seen for oxalate oxidase. In addition, a peroxide containing species and carbonate product are generated after the oxalate is consumed. The requirement of light for this reaction has implications for how the enzyme might work. We have also been able to reproduce the reactivity of oxalate decarboxylase when oxygen is absent. A related project focused on the synthesis and aqueous solution behavior of [FeLCl2]. In water, several antiferromagnetically coupled binuclear species were identified. Investigations into new ligand systems that are related to the oxalate oxidase project have yielded many results. In our efforts to expand the triazamacrocyclic ring size from a 9-membered ring as seen in L (1,4,7-triazacyclononane) to an 11-membered ring (1,4,8-triazacycloundecane, tacud), we discovered that very little work had been done on this ligand. Studying the coordination chemistry of tacud has led to many new lines of investigation. A binuclear nickel(II) [(Ni2(tacud)2(mu-H2O)(mu-Cl)2] was prepared containing two Ni(II) ions bridged by two chloro and one aqua ligand resulting in a short Ni···Ni distance of 3.0187(10) Å. Variable temperature SQUID magnetic susceptometry found mild ferromagnetism (J = +2.28 cm-1) in this compound. Additionally, two binuclear Mn(II) and Fe(II) complexes featuring tacud were generated. The presence of weak ferromagnetic interactions is seen in the iron(II) complex while weak antiferromagnetic coupling in the manganese(II) complex is observed. A systematic study of iron(II) bis-triazamacrocycle complexes has resulted in compounds being identified to possess spin-crossover behavior. Spin-crossover compounds have potential in the development of molecular based electronics, data storage, sensory devices, magnetic switches, and display devices. To more accurately model the histidine group found in oxalate oxidase, we studied tris(imidazoloyl)phosphine as a tris-histidine model ligand. This study led to the synthesis of a manganese compound coordinated to tris(1-ethyl-4-isopropyl-imidazolyl)phosphine (T1Et4iPrIP). Oxalate was degraded by this compound as well under ambient conditions. Several projects grew out of this study. The preparation of a series of cobalt (II) and nickel (II) complexes supported by T1Et4iPrIP have also been carried out. Results indicate that T1Et4iPrIP can bind cobalt in both a tridentate and bidentate fashion while for nickel, only the tridentate mode is observed. Further studies suggest that the cobalt (II) complexes in solution display low energy barriers allowing for facile interconversions between tridentate and bidentate coordination modes. Nitrosyl adducts of [Fe(T1Et4iPrIP(OTf)2] have been characterized as well and serve as good models His3Fe-containing enzymes. Our main contribution in physical-analytical chemistry is the development of a new method to identify and quantify carbon dioxide at low levels. We have applied an NMR method towards the detection of hydrogen peroxide in our reaction mixture. We have also developed an IR, NMR, and HPLC method for the detection of oxalate in a reaction mixture. The preparation of small molecule catalysts to detect and/or degrade oxalate in the environment will assist in the environmental management of this compound which can be detrimental to human health. The Broader Impacts component of this project resulted in the implementation of a high school outreach program. Educational materials have also been prepared for High school teachers to help them teach advanced chemistry concepts to students. The development of human resources was a significant aspect of this grant. Students at all levels were engaged in this research and were trained in multidisciplinary methodology. During the course of this work, 2 postdocs, 10 graduate students, 22 undergraduate students, 2 laboratory technicians, and 9 high school students were trained. 51% of these students were female and two were from minority groups.

National Science Foundation (NSF)
Division of Chemistry (CHE)
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Timothy E. Patten
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Oakland University
United States
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