By Xuemei Yang
A top challenge faced by the current generation of scientists is the need for a clean, sustainable energy source. Although hydrogen is a promising potential energy for future, the sustainability of producing hydrogen from renewable sources is a concern. To address the need of platinum for electrodes in electrolysis, scientists and researchers turn their attention to available first row transition metals, especially Ni and Fe complexes, which are biomimetics of natural Hydrogenase active sites. However, most of the these biomimetic H2 production catalysts are oxygen sensitive, as in the natural Hydrogenases, which results in extra cost in H2 production. To explore oxygen tolerance of the catalysts, my research focus is on a biomimetic of the active sites of [NiFe]- enzymes: [NiFeS]-H2ase and [NiFeSe]-H2ase (Figure 1), and explore on how they react with oxygen, and how the oxygen damage might be repaired. My research project can be divided into four parts: the biomimetic synthesis, oxygen reaction, mechanism study and exploration on how to improve catalysts’ oxygen tolerance. In the last year (2018), I finished the model synthesis and oxygen reaction parts of my project (manuscripts accepted by Chemical Science and J. Am. Chem. Soc). In summary, I have established a biomimetic study for S/Se oxygenation in Ni(μ-EPh)(μ-SN2)Fe, (E = S or Se; SN2= Me-diazacycloheptane- CH2CH2S) ; Fe = (η5 -C5H5)FeII(CO)) complexes related to the oxygen-damaged active sites of [NiFeS]/[NiFeSe]-H2ases. Mono- and di- oxygenates (major and minor species, respectively) of the chalcogens result from exposure of the heterobimetallics to O2; one was isolated and structurally characterized to have Ni-O-SePh-Fe-S connectivity within a 5-membered ring (Figure 2). The treatment with O-abstraction agents such as P(o-tolyl)3 or PMe3 remediated the O damage. This is the first example of an “in-situ” biomimetic study of oxygenation of complexes which related to the oxygen-damaged active sites of [NiFeS]/[NiFeSe]-H2ases. This study provides molecular-level evidence for higher oxygen tolerance of [NiFeSe]-H2ases and gives new direction for H2 production catalysts’ synthesis which are based on the active site structures of Hydrogenases. After the first two steps, my next steps are the mechanism exploration and design on novel oxygen tolerant catalyst for H2 production. I believe my research will provide fundamental molecular level study for future H2O electrolyzers and H2 fuel cell catalysts. This project not only has application for new energy exploration, which is satisfying on its own, but also benefits my personal intellectual life. It is like a little hole for me to look into how the atoms rearrange and make new connections– and how nature controls these tiny molecules.
What I would like to do, and enjoy doing, is digging the hole bigger and bigger. For me, this specific PhD program project is an excellent way to explore the new things in nature. In addition, the synthetic work gives me the joy as a “god” for the molecules. On the way to explore more, and to be a female chemist, I truly appreciate this project. It gives me the professional training, basic skills and a broad vision for my future academic life.