Nitrogen Fixation Catalyzed by Transition Metal Complexes: Recent Developments

Abstract

The structures and reactivity of various catalytic model systems of nitrogenase are reviewed. Firstly, the Schrock cycle is discussed as an example of a nitrogen-fixing system for which the mechanism has been elucidated both experimentally and theoretically. Then a series of mononuclear iron complexes based on 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 situated in the position trans to coordinated N2. In addition, new cobalt, iron, ruthenium, and osmium complexes that display catalytic conversion of N2 into ammonia are discussed. As an alternative to the mononuclear systems, the synthesis and catalytic activity of dinuclear Mo0–dinitrogen complexes supported by terminal pincer ligands are reviewed. The available mechanistic scenarios disagree on the question of whether or not the complex dissociates during the catalytic cycle. Furthermore, a new dinuclear iron system that is active in synthetic nitrogen fixation and exhibits a further activity increase under irradiation is reviewed. The implications of the results obtained with all of these systems with respect to the reactivity of nitrogenase are discussed.