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In the first year of implementation time, effort and resources have been devoted to the project described in the original PRF proposal. Our project targets a new class of fused N-heterocyclic carbene ligands with a rigid bidentate architecture, ligands that we designed as supports for homo- and heterometallic complexes. The ligands incorporate redox active moieties. The reversible electrochemical oxidation and reduction of the redox moiety will provide a handle for additional fine-tuning of the carbene center and will offer the possibility to control the stoichiometric and catalytic reactivity of the corresponding transition metal complexes.  Understanding the chemistry of these redox-active ligands will enable us to evaluate their potential role in applications, such as redox-switchable catalysts, that could allow for on-off switching of catalyst activity. In addition, this could provide a strategy for the separation of homogeneous catalysts from the products of a reaction. The chemical or electrochemical change in the oxidation state of the redox moiety would result in drastic changes in polarity and, as a result, the possibility to exert high control over the solubility of the catalyst. Ideally, the catalyst would be precipitated by electrodeposition and recovered for reuse, thus increasing cost-efficiency and reducing contamination of the reaction products with heavy-metal impurities. The underlying objective is to elucidate and learn to exploit the interrelationship between molecular architecture, electronic structure and chemical reactivity of these novel compounds.

The proposed synthesis of 2 by Buchwald-Hartwig amination did not lead to satisfactory results. However, the synthesis of 2 (R = neopentyl Np) was achieved by an alternate route as outlined in Scheme 1 using a sequence of reactions involving nitration, reduction of nitro groups, double acylation with pivaloyl chloride, reduction with LiAlH₄ and cyclization with HC(OEt)₃. The reaction of 2 (R = Np) with HBr was too slow. The reaction of 2 with BBr₃ led to the desired salt 3. However, the purification and full characterization of 3 was impeded by low solubility in most organic solvents.

Compound 3 (R = Np) is very soluble in DMSO (but not too stable in DMSO, possible due to oxidation) and sparingly soluble in acetone and acetonitrile. The reaction of 3 with silver oxide oxidized the redox-switchable moiety, but did not produce the desired carbene silver complex (most likely due to steric hindrance). Preliminary cyclic voltammetry experiments were performed on 3 and showed that 3 has two electroactive sites (one reversible DE_p = 0.046 V, the other quasi-reversible DE_p = 0.3795 V). Figure 1 shows the cyclic voltammograms of 2 mM solution of 3 at a glassy carbon electrode, in buffered solutions (pH 3.5) with a decrease in the scan window from 1.250 to 0.850 V. Cyclic voltammograms of 2 mM solution of 3 at various pHs (same ionic strength) (pH = 2.5, 3.5, 4.8, 5.5, 8.0) are shown in Figure 2. The reduction potentials are shifting to more negative values with increase pH. Currently we are working on the synthesis of other salts of type 3 with different substituents on the two nitrogen atoms in order to improve solubility and to decrease steric crowding around the carbene center.

Fig.1.                                                                                                                  Fig. 2

This PRF grant allowed five undergraduate students to participate in research, both as independent studies for credit during Fall and Spring semester and as paid stipends over summer. Two of them have graduated in spring 2010. Uyen Do was accepted in dental school and Gerhard Kummerow is currently preparing his application for graduate school in chemistry.  Chris Ghattas, Mahatab Chowdhury and Gary Richoux will continue to work on this project for another year. Due to his academic achievements and his involvement in undergraduate research, Chris was awarded one of the National Science Foundation scholarships available in our college and was the recipient of the Merck award for achievements in organic chemistry in Spring 2010. The research infrastructure in my lab received a boost by the acquisition of a new dry-box. This dry-box is mainly used for carbenes' chemistry. All five students were trained to use it.  After an initial training period, the students were actively involved in the preparation and characterization of the desired compounds. We plan on presenting the initial results of our work at the Southeast Regional Meeting of the American Chemical Society in New Orleans, LA and the ACS National Meeting in Spring 2010.  Our combined efforts led to progress of the original project and paved the way to exciting new avenues of research that will be highlighted in the next year's report. Finally, this award strengthened my tenure and promotion portfolio and it was one of the contributing factors for my receipt of  the Kennesaw State University Foundation Prize for Publication this Fall.