Overviews of the Preparation and Reactivity of Transition Metal-Dinitrogen Complexes

Nitrogen, the fifth most abundant element in the solar system, is the most abundant element in the atmosphere of Earth [1] as well as the fourth most abundant element in cellular biomass [2]. However, it is rather a trace element in the lithosphere of Earth [3]. Thus, utilization of chemically inert gaseous molecular dinitrogen (N2) that exists in the atmosphere of Earth as the primary nitrogen source is inevitable in both biogeography and industry. Indeed, fixation of atmospheric nitrogen can be achieved by the conversion of molecular dinitrogen into ammonia (NH3) containing the most reduced form of nitrogen (−3) that can be a convenient precursor for several nitrogen-containing compounds and has been the most fundamental reaction pathway of the global nitrogen cycle [4, 5]. Industrially, NH3 is one of the 10 largest commodity chemical products and has been produced by the Haber–Bosch process in which atmospheric dinitrogen reacts with gaseous dihydrogen (N2 + 3 H2 → 2 NH3) since the early twentieth century [6–14]. Haber and van Oordt in 1904 first succeeded in the conversion of the mixture of N2 and H2 into NH3 in the presence of transition metal catalyst (Fe or Ni) at a high temperature in a laboratory [15–17]. Later, modification of the reactors and catalysts was achieved, and 90 g of ammonia was shown to be obtained every hour by using an osmium-based catalyst with the total yield of ammonia up to 8 vol% at 550 ∘C and a total pressure of 175 atm of a stoichiometric mixture of dinitrogen and dihydrogen (1 : 3) in an experimental lecture held in Karlsruhe on 18 March 1909 [18–20]. Further modification of the catalysts for industrialization was investigated by Mittasch and coworkers in BASF, leading to the discovery of the combination of iron, K2O, and Al2O3 as one of the most active catalysts by 1910 [6, 21]. The first commercial plant for ammonia synthesis at Oppau began its operation by 1913 in collaboration with Bosch and coworkers at BASF, while the earlier commercial methods to fix atmospheric nitrogen such as Frank–Caro cyanamide process (CaC2 +N2 →CaCN2 +C) and Birkeland–Eyde electric arc process (N2 +O2 → 2 NO) were gradually replaced by the Haber–Bosch ammonia process [6–14]. Typical reaction conditions of the Haber–Bosch process are