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Heme/copper and heme/non-heme diiron model systems for O2 reduction

Final Report Summary - HEME/(CU-FE)-O2-RED (Heme/copper and heme/non-heme diiron model systems for O2 reduction)

During the next century, one of the main issues the scientific community will have to face is replacing the use of oil-based energy sources with cheap, environmentally safe technologies. After millions of years of evolution, nature has solved the energetic issue in living beings by two complementary processes, photosynthesis and cellular respiration, employing abundant elements such as C, H, N, O and S (in the form of amino acids) in combination with available first-row metals such as Fe, Cu and Mn. Both processes occur under mild conditions (room temperature) and generate benign subproducts (water, dioxygen and carbon dioxide).

Photosynthesis is a process occurring in plants where solar energy is fixed in the form of C-H and C-C bonds, using water as a proton and electron source and carbon dioxide as carbon source. This process is coupled with the generation of dioxygen during the last step occurring in Photosystem II. Living organisms, including plants, carry out cellular respiration, a process occurring in cytochrome c oxidase (CcO) that couples dioxygen reduction to water to oxidize molecules containing C-H and C-C, generating energy with concomitant formation of water and carbon dioxide, which can be subsequently reused during the photosynthetic process, closing the energy cycle.

Several research labs are pursuing the development of new energy sources inspired on photosynthesis. The main idea is to combine water, light and a catalyst to generate molecular hydrogen, an approach also described as a water-splitting photochemical cell. The fuel produced in these cells can be directly used (combustion) but it usually leads to low efficiencies (20-25%). However, we can develop a fuel cell, based on cellular respiration, to make use of hydrocarbons, hydrogen peroxide and/or hydrogen gas to generate energy with higher efficiency (50-60%). To do so, there is a necessity to improve the catalysts for fuel cells in terms of cost, robustness and overpotential used in the 4H+/4e- dioxygen reduction.

The first two years of the project called “Heme/(Cu-Fe)-O2-red”, (outgoing phase) were carried out under the mentorship of Prof. Kenneth D. Karlin in The Johns Hopkins University (Baltimore, U.S). The main objectives of this phase were based on the study of heme-peroxo-copper systems (inspired in active center of CcO) for a better understanding of the dioxygen reduction process occurring in the natural system CcO. The fellow developed a new simple methodology to generate a wide range of these chemical entities, the results being recently published in a prestigious peer-reviewed journal (JACS 2015, 137, p1032). This new method led for the first time to generate heme-peroxo-copper intermediates with monodentate ligands such as imidazoles (the actual ligands found in the natural system CcO).

The last year of the project (return phase) was executed in the University of Girona under the mentorship of Prof. Miquel Costas. Taking advantadge of his research groups expertise, we complemented the research explored during outgoing phase studying the reactivity of the different heme-peroxo-copper complexes towards a series of proton and electron sources. We established, for the first time in this type of heme-peroxo-copper complex, that addition of weak acids (acidic phenols) and electron sources (substituted ferrocenes) were able to trigger the O-O reduction of the heme-peroxide-copper imidazolyl intermediates. The results found are especially relevant because they provide relevant information about the reaction pathway of the dioxygen reduction occurring in the natural system CcO. Moreover, it also led to rationalize the development of new cheap, efficient and environmentally benign catalytic systems for dioxygen reduction.

Execution of the project called “Heme/(Cu-Fe)-O2-red” has had a deep impact on the career of the fellow. He has the acquired skills in a wide range of cutting-edge experimental and computational techniques. He also has been involved in other research projects, collaborating and mentoring PhD students, which has led to the publication of several articles in peer-review journals (8). This fellowship has allowed the researcher to attend to international conferences and workshops, leading to establish new collaborations with other research groups that will benefit both outgoing and return host institutions.

Finally, implementation of the project has had and will have a positive socio-economic impact at different levels. The results presented herein serve to enhance the competitiveness of the European Union in the area of the environmentally friendly and sustainable energy sources by means of finding alternatives to the oil-based technologies. Moreover, this project will contribute to increase the competitiveness of the European Union towards the current leadership of the United States in the field of green energy sources, where millions of dollars are currently invested. The research line presented has can be further explored by the returning host organization (University of Girona) which will potentially lead to the generation of energy based in environmentally benign sources from which the European Community will benefit.