Bioinspired Inorganic Chemistry Group
Fixation of atmospheric Nitrogen to Ammonia
Fixation of atmospheric nitrogen to ammonia is an essential biochemical process in nature(1). Amazingly, nitrogenase enzymes are capable to reduce nitrogen to ammonia under an ambient conditions and the process provides the mostly fixed nitrogen sources in nature. Since the discovery of this novel class of enzymes(2), many efforts were given to know the structure and mechanism of the enzyme(3). Recent spectroscopic, biochemical and structural data suggests that iron is the most likely site for nitrogen binding and reduction, in the MoFe–nitrogenases(4). The reaction involves the hydrolysis of at least 16 mol equiv of ATP per molecule of N2(4). Kinetic study reveals that iron is the only transition metal essential for the nitrogenase activity.
Active site structure of MoFe-nitrogenase
To date, several models have been developed to know the mechanism of the novel enzyme. So far only few examples are known where little catalytic turnover has been achieved with molybdenum complexes. In 2003, Schrock et al reported the catalytic reduction of nitrogen by a molybdenum complex supported by a ‘tren’ derived bulky tetradentate ligand(5). Recently, Nishibyashi et al have developed Mo-catalysts supported by PNP-type pincer ligands(6). However, no iron-catalysts are known for the nitrogen reduction under an ambient conditions; although couple of stoichiometric nitrogen reductions have been reported by Peters, NIshibyashi, Holland and Chirik et al (7,8). Although, it has been established that iron is the most important element for the biological nitrogen fixation reaction. The most difficult point faced in the reported models is to cleave the strong nitrogen–nitrogen bond. Putting more electron density upon iron by reducing its oxidation state is one option, which will eventually affect the cleavage of nitrogen-nitrogen bond more easily. An alternative strategy is to weaken the N–N bond strength via the formation of nitrogen bridged binuclear complexes. Thus to achieve such a challenging goal, our plan is to design nitrogen/phosphorous rich ligands to support binuclear complexes. Iron complexes supported by those ligands will be synthesized and characterized. The reaction of those complexes will be studied with nitrogen in the presence of proton and electron sources. Major emphasis will be given to develop suitable models for catalytic nitrogen to ammonia conversion and characterization of metastable reaction intermediates involved during the reduction of nitrogen, by various spectroscopic techniques.
References
(1) Burgess, B. K. Chem. Rev. 1990, 90, 1377−1406.
(2) Kim, J.; Rees, D. C. Science 1992, 257, 1677−1682
(3) Crossland, J. L.; Tyler, D. R. Coord. Chem. Rev. 2010, 254, 1883−1894.
(4) Hoffman, B. M.; Dean, D. R.; Seefeldt, L. C. Acc. Chem. Res. 2009, 42, 609-619.
(5) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76-78.
(6) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2010, 3, 120-125.
(7) McWilliams, S. F.; Holland, P. L. Acc. Chem. Res. 2015, 48, 2059-2065.
(8) Chirik, P. J. Acc. Chem. Res. 2015, 48, 1687-1695.
Water Splitting by Homogeneous Electrocatalysts
Minimization of global warming is an immediate challenge to our present society. The main reasons for increasing greenhouse gases in the atmosphere are because of the increasing consumption of fossil fuels (coal, gases, diesel, petrol, etc.). According to the literature worldwide energy consumption was 15 TW in 2008, which is expected to be double by 2050(1). Moreover, the fossil fuel supply is assumed to be depleted in 50-150 years(2). Thus, for the existence of the modern life and concerning the human health, finding an alternative energy source is the most important topic of research to the scientific community. At present only few percentage of total annual energy consumption comes from the renewable energy sources like solar panel, wind, tide, geothermal heat, etc. In nature, couple of processes are known where storage of energy takes place in the form of chemical compounds. Hydrogenase (present in prokaryotes and eukaryotes) is an enzyme produces hydrogen from water(3), nitrogenase enzymes (present in some bacterial species) converts atmospheric nitrogen to ammonia(4). Finally, the natural photosystem II produces molecular oxygen by splitting water(5). My research interest here is to develop artificial photosynthesis systems by the use of first-row transition metal ion complexes.
Oxidation of water to oxygen and hydrogen
Structure of Oxygen evolving Mn4CaO5 Cluster
The natural photosynthesis occurs in green plants uses sunlight as the energy source to split water into molecular oxygen, electrons and protons. The produced electrons can be utilized to reduce proton to hydrogen, which is an alternative energy source and friendly towards the environment. However, the total oxidation of water into hydrogen is a very complicated process and can be subdivided into two half reactions: (i) oxidation of water into oxygen, protons & electrons and (ii) reduction of protons into hydrogen. In order to mimic these reactions synthetically, the first step is the formation of oxygen according to equation (i). However, the oxidation of water is a thermodynamically as well as kinetically demanding process and is one of the main reasons that inhibiting to develop the artificial photosynthesis systems for practical use. The biological water oxidation occurs in photosystem II is catalyzed by a Mn4CaO5 cluster; where each of the Mn and Ca atoms are connected by two bridging oxygen atoms in a ‘cubane’ like arrangement (5,6). Many strategies have been taken to split water artificially; includes the development of homogeneous water oxidation catalysts (WOC) based on transition metal complexes, polyoxometallates, transition metal oxide materials, etc(7). The strategy of using suitable catalysts is to reduce the over potential of the water splitting. Our aim is to develop homogeneous elecrocatalysts based on earth abundant metal sources to study the artificial photosynthesis reactions, with the ultimate aim to make artificial leaf.
References
(1) Du, P.; Eisenberg, R. Energy Environ. Sci. 2012, 5, 6012-6021.
(2) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729-15735.
(3) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245-2274.
(4) van der Ham, C. J. M.; Koper, M. T. M.; Hetterscheid, D. G. H. Chem. Soc. Rev. 2014, 43, 5183-5191.
(5) Umena, Y.; Kawakami, K.; Shen, J.-R.; Kamiya, N. Nature 2011, 473, 55-60.
(6) Loll, B.; Kern, J.; Saenger, W.; Zouni, A.; Biesiadka, J. Nature 2005, 438, 1040-1044.
(7) Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Chem. Rev. 2015, 115, 12974–13005.