Catalytic reduction of Dinitrogen to Ammonia

Reduction of dinitrogen at standard conditions (room temperature and 1 atm) is one of the greatest challenges in chemistry. In nature, dinitrogen fixation is done by nitrogenases, whereas the best known and most studied is the FeMo nitrogenase. Nitrogenases produce approximately 108 tons of ammonia per year; this is comparable with the world production of ammonia performed in the Haber-Bosch process, where nitrogen and hydrogen are used as reactants at high temperatures (350 - 550 C) and high pressure (150 - 350 atm). At present, only the laboratory of the Nobel Prize laureate Professor R. R. Schrock is able to reduce dinitrogen catalytically to ammonia only consuming electrons and protons. The reaction is performed at a single molybdenum center, whereas bulky triamidoamine ligands are used to protect the reaction center in order to prevent side reactions, which lead to a deactivation of the catalyst. The proposed research project is dealing with the synthesis of new triamidoamine ligands containing diphenylamino or diphenylpyrrolyl groups and their application towards the catalytic reduction of dinitrogen at a single molybdenum center. Therefore, a series of 3,5-bis(diphenylamino)phenyl derivatives should be synthesized, deriving from 2,4,6- as well as 3,5-substituted 1-iodobenze derivates. Hereby, the electronic behavior, as well as the sterical protection of the metal center can be influenced using electron withdrawing groups (EWG) or electron donating groups (EDG) and bulky groups to substitute the 1-iodobenzes. In addition, 2,5-diphenylpyrrole derivatives are available and again, fine-tuning of the ligand can be done by using EWG, EDG and bulky substituents. The final molybdenum catalyst, which contains either 3,5-bis(diphenylamino)phenyl or 2,5-diphenylpyrrole triamidoamine ligands will be tested towards the catalytic reduction of dinitrogen to ammonia. Hereby, the amount of the produced ammonia will be determined using the indophenol methode. In addition, a further focus of the proposed work is the isolation and characterization of intermediates of the desired catalytic cycle, especially those which have not yet been fully characterized within the Schrock group. With the compounds under investigation, it could be possible to stabilize such intermediates, which could be isolated and characterized. For characterization and studying of the desired products, the Schrock laboratory and the M.I.T provide modern chemical and spectroscopic methods, including NMR, IR, Raman, MS, ESR, UV/vis, X- ray diffraction, electrochemical methods and stop flow kinetic techniques.