Catalytic Ammonia Synthesis with Transition-Metal Complexes: Recent Developments

The structure and reactivity of various catalytic model systems of nitrogenase are reviewed. First the SCHROCK cycle is discussed as an example of a nitrogen-fixing system the mechanism of which has been elucidated both experimentally and theoretically. Then a series of mononuclear iron complexes supported tetradentate ligands of the type EP3 (E=Si, C, B) is discussed. These systems both serve as highly active catalysts for the synthesis of ammonia from N2 and provide important insights into the role of the ligand positioned in trans-position to coordinated N2. In addition, new cobalt, iron, ruthenium and osmium complexes showing catalytic conversion of N2 to ammonia are discussed. As an alternative to the mononuclear systems, the synthesis and catalytic activity of dinuclear Mo(0)dinitrogen complexes supported by terminal pincer ligands are reviewed. The available mechanistic scenarios disagree on the question whether the complex dissociates during the catalytic cycle or not. Furthermore, a new dinuclear iron system is reviewed that is active in synthetic nitrogen fixation and exhibits a further activity increase under irradiation. The implications of the results obtained on all of these systems with respect to the reactivity of nitrogenase are discussed.

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