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14. Hermann Staudinger Lecture mit Nobelpreisträger Richard R. Schrock

Richard Schrock

How to reduce dinitrogen catalytically to ammonia with protons and electrons

Nobelpreisträger Richard R. Schrock, Nobelpreis in Chemie 2005

"How to reduce dinitrogen catalytically to ammonia with protons and electrons" // Podcast available
Wann 10.06.2013
von 19:15 bis 20:30
Wo Chemie Hörsaal, Albertstr. 21, 79104 Freiburg
Kontakttelefon +49(0)761 203-97407
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How to reduce dinitrogen catalytically to ammonia with protons and electrons

Since the middle of the 19th century it has been known that leguminous plants (alfalfa, clover, peas, beans, etc.) have the ability to "fix" atmospheric nitrogen (N2, 78% of our atmosphere) to give ammonia (NH3), which can then be assimilated by plants. This process is necessary for all life and is estimated to be carried out on a scale of >108 tons per year on earth. Reduction of N2 is carried out by an enzyme in bacteria in the soil, the most efficient of which contains iron and molybdenum. The process is arguably the most complex catalytic reaction in biology. Hundreds of man years over a period of forty years were expended in efforts to reduce nitrogen to ammonia outside the enzyme before it finally was achieved with a molybdenum catalyst in 2003. The mechanism of this remarkable reaction will be discussed in some detail.

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Report on the 14th Hermann Staudinger Lecture with Richard R. Schrock on June 10th, 2013

How to reduce dinitrogen catalytically to ammonia with protons and electrons

For the 14th Hermann Staudinger Lecture FRIAS invited Nobel Prize Laureate Professor Richard R. Schrock from the Massachusetts Institute of Technology, Cambridge, USA to give an insight in the exciting journey towards catalytic nitrogen reduction to ammonia with protons and electrons outside biological systems.

Richard Schrock was awarded the Nobel Prize in Chemistry 2005 together with Yves Chauvin and Robert H. Grubbs "for the development of the metathesis method in organic synthesis". Their discoveries have had great effects on academic research, the development of new drugs and other biologically active compounds, polymeric materials and industrial syntheses.

Dinitrogen reduction, so-called nitrogen fixation, is essential for all forms of life because nitrogen is required to biosynthesize basic building blocks of plants and animals, e.g. nucleotides for DNA and amino acids for proteins. Therefore nitrogen fixation is essential for agriculture and the manufacture of fertilizer.

Richard Schrock started his lecture giving retrospect on the elucidation of nitrogen reduction since the 19th century.

After the discovery of nitrogen fixation by enzymes (nitrogenases) in bacteria intense effort has been directed towards understanding the mechanism of biological ammonia syntheses. Already in 1939 D. I. Arnon and P.R. Stout formulated criteria for so-called essential elements (amongst others molybdenum) which are chemical elements that are absolutely needed by plants for their growth and development. The scientific interest in biological nitrogen reduction is motivated by the unusual structure of the active site of the enzyme, which consists of a Fe7MoS9 cluster. Only recent findings by O. Einsle, University of Freiburg, revealed that indeed molybdenum is the substrate binding and reduction site.

In 1898 Adolph Frank and Nikodem Caro developed a first technical process to gain ammonia from nitrogen. Calcium carbide reacts with nitrogen to calcium cyanamide which is then transformed with water to ammonia.

Catalytic chemical nitrogen fixation at room temperature and under atmospheric pressure has been an ongoing scientific endeavor for over 40 years. First reports on metal complexes containing titanium, vanadium, molybdenum, tungsten, chrome, ruthenium, etc. catalyzing nitrogen reduction were published in the 1960s (Shur et al.). These findings and the knowledge of the presence of molybdenum in the active site of nitrogenases inspired for further investigations focusing on molybdenum as central metal by Hidai et al. in 1969. In the following years several attempts have been made to improve catalytic ammonia production using various metal atoms as catalytic centres without a real breakthrough until Schrock and Yandulov published in 2002 a really elegant system using a molybdenum based catalyst with space filling ligands. “Nature knows molybdenum is best,” commented Schrock his discovery. They were able to synthesize and structurally characterize eight proposed intermediates in the Chatt cycle.

This cycle originally describes the hypothetical mechanism for ammonia production from dinitrogen in nitrogenase consisting of a series of stepwise protonations and reductions. Each intermediate was proven competent to reduce dinitrogen catalytically. Then, in 2003 they reported the catalytic production of ammonia using a molybdenum compound, a proton source, and a strong reducing agent. The cycle proposed by Chatt involved a series of Mo(0) to Mo(III) intermediates. In his talk Schrock showed a reaction cycle that requires Mo(III) to Mo(VI) oxidation but despite that is essentially identical to the Chatt cycle. Schrock revealed the crucial impact of the ligands, the rate of addition of the reducing agent (decamethylchromocene) and the proton source on the turnover rates and the yield of ammonia. The reaction is mainly limited by ligand loss and inhibition of the catalyst by irreversibly bound compounds such as hydrogen. In 2011 another nitrogen reduction system using molybdenum was discovered by Nishibayashi et al. but is still mechanistically not fully secured.

In his very in-depth lecture in the crowded chemistry lecture hall (approximately 250 participants), Richard Schrock did not only impress many students but also many scientists from the Chemistry Department.

(Katrin Brandt)