Abiological nitrogen fixation describes chemical processes that fix (react with) N<sub>2</sub>, usually with the goal of generating ammonia. The dominant technology for abiological nitrogen fixation is the Haber process, which uses iron-based heterogeneous catalysts and H<sub>2</sub> to convert N<sub>2</sub> to NH<sub>3</sub>. This article focuses on homogeneous (soluble) catalysts for the same or similar conversions.
Dinitrogen fixation is essential for human life. Currently, the industry uses the HaberâÂÂBosch process to convert N<sub>2</sub> and H<sub>2</sub> to NH<sub>3</sub> based on the metal catalysis under very high pressure and temperature conditions. Alternative strategies that realize the transformation from N<sub>2</sub> to NH<sub>3</sub> under mild conditions are a long-lasting goal in chemistry. In the past decades, a number of transition-metal species have been found to bind (and even functionalize) N<sub>2</sub>. The prevalence of transition metals in dinitrogen activation is attributed to the fact that the unoccupied and occupied d orbitals could be both energetically and symmetrically accessible to accept electron density from and back donate to N<sub>2</sub>. Nevertheless, the development of low-valent, low-coordinate main-group elements which mimic the electronic properties of transition metal provides more opportunities to unearth the N<sub>2</sub> activation by main group elements.
Lithium can also react with N<sub>2</sub> at room temperature to give an isolable product Li<sub>3</sub>N. However, it was until recently that the controllable, stepwise N<sub>2</sub> activation by main group element began to thrive, especially for those whose key intermediates were well structurally characterized and even isolated.
An early influential discovery of abiological nitrogen fixation was made by Vol'pin and co-workers in Russia in 1970. Aspects are described in an early review: <blockquote>"using a non-protic Lewis acid, aluminium tribromide, were able to demonstrate the truly catalytic effect of titanium by treating dinitrogen with a mixture of titanium tetrachloride, metallic aluminium, and aluminium tribromide at 50 ðC, either in the absence or in the presence of a solvent, e.g. benzene. As much as 200 mol of ammonia per mol of was obtained after hydrolysis...."</blockquote> These results led to many studies on dinitrogen complexes of titanium and zirconium.
Because Mo and Fe are found at the active site of the most common and most active form of nitrogenase, these metals have been the focus of particular attention for homogeneous catalysis. Most catalytic systems operate according to the following stoichiometry:
The reductive protonation of metal dinitrogen complexes was popularized by Chatt and coworkers, using Mo(N<sub>2</sub>)<sub>2</sub>(dppe)<sub>2</sub> as substrate. Treatment of this complex with acid gave substantial amounts of ammonium. This work revealed the existence of several intermediates, including hydrazido complexes (Mo=N-NH<sub>2</sub>). Catalysis was not demonstrated. Schrock developed a related system based on the amido Mo(III) ocomplex Mo[(HIPTN)<sub>3</sub>N]. With this complex, catalytic nitrogen fixation occurred, albeit with only a few turnovers.
Intense effort has focussed on family of pincer ligand-supported Mo(0)-N<sub>2</sub> complexes. In terms of it donor set, and oxidation state, these pincer complexes are similar to Chatt's complexes. Their advantage is that they catalyze the hydrogenation of dinitrogen. A Mo-PCP (PCP = phosphine-NHC-phosphine) complex exhibits >1000 turnovers when the reducing agent is samarium(II) iodide and the proton source is methanol.
Iron complexes of N<sub>2</sub> are numerous. Derivatives of Fe(0) with C<sub>3</sub>-symmetric ligands catalyze nitrogen fixation.
Photolytic nitrogen splitting is also considered.
Although nitrogen fixation is usually associated with transition metal complexes, a boron-based system has been described. One molecule of dinitrogen is bound by two transient Lewis-base-stabilized borylene species. The resulting dianion was subsequently oxidized to a neutral compound, and reduced using water.
Particular metals can react with nitrogen gas to give nitrides, a process called nitriding. For example, metallic lithium burns in an atmosphere of nitrogen, giving lithium nitride. Hydrolysis of the resulting nitride gives ammonia. In a related process, trimethylsilyl chloride, lithium and nitrogen react in the presence of a catalyst to give tris(trimethylsilyl)amine, which can be further elaborated. Processes that involve oxidising the lithium metal are however of little practical interest, since they are non-catalytic and re-reducing the ion residue is difficult. The hydrogenation of Li<sub>3</sub>N to produce ammonia has seen some exploration since the resulting lithium hydride can be thermally decomposed back to lithium metal.
Some Mo(III) complexes also cleave N<sub>2</sub>:
This and related terminal nitrido complexes have been used to make nitriles.