abiological nitrogen fixation

Abiological nitrogen fixation describes chemical processes that fix (react with) N₂, usually with the goal of generating ammonia. The dominant technology for abiological nitrogen fixation is the Haber process, which uses an iron-based heterogeneous catalysts and H₂ to convert N₂ to NH₃. This article focuses on homogeneous (soluble) catalysts for the same or similar conversions.

Transition metals

Vol'pin and Shur

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:

"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 °C, 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.…"

These results led to many studies on dinitrogen complexes of titanium and zirconium.

Mo- and Fe-based systems

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:
:N₂ + 6H⁺ + 6e⁻ → 2NH₃

The reductive protonation of metal dinitrogen complexes was popularized by Chatt and coworkers, using Mo(N₂)₂(dppe)₂ 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-NH₂). Catalysis was not demonstrated. Schrock developed a related system based on the amido Mo(III) ocomplex Mo HIPTN)₃N 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)-N₂ 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- NHCphosphine complex exhibits >1000 turnovers when the reducing agent is samarium(II) iodide and the proton source is methanol.

Iron complexes of N₂ are numerous. Derivatives of Fe(0) with C₃-symmetric ligands catalyze nitrogen fixation.

Photolytic routes

Photolytic nitrogen splitting is also considered.

p-Block systems

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.

Nitriding

In rare cases, metals 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 involving lithium metal are however of little practical interest since they are non-catalytic and re-reducing the ion residue is difficult.

Some Mo(III) complexes also cleave N₂:
:2Mo(NR₂)₃ + N₂ → 2N≡Mo(NR₂)₃
This and related terminal nitrido complexes have been used to make nitriles.

See also

* Nitrogenase: enzymes used by organisms to fix nitrogen
*Transition metal dinitrogen complex
*Metal nitrido complex

References

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Homogeneous catalysis