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 EP 3 (E = Si, C, B) is discussed. These systems both serve as highly active catalysts for the synthesis of ammonia from N 2 and provide important insights into the role of the ligand situated in the position trans to coordinated N 2 . In addition, new cobalt, iron, ruthenium, and osmium complexes that display catalytic conversion of N 2 into 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 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.