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Nitrogen fixation is a process by which molecular nitrogen in the air is converted into ammonia (NH
3
) or related nitrogenous compounds in soil.[1] Atmospheric nitrogen is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation converts N
2
into ammonia, which is metabolized by most organisms.

Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds, such as amino acids and proteins, nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer. It is also, indirectly, relevant to the manufacture of all nitrogen chemical compounds, which includes some explosives, pharmaceuticals, and dyes.

Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria such as Azotobacter and archaea. Some nitrogen-fixing bacteria have symbiotic relationships with plant groups, especially legumes.[2] Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi.[3] It occurs naturally in the air by means of NOx production by lightning.[4][5]

All biological reactions involving the process of nitrogen fixation are catalysed by enzymes called nitrogenases.[6] These enzymes contain iron, often with a second metal, usually molybdenum but sometimes vanadium.

The quest for well-defined intermediates led to the characterization of many transition metal dinitrogen complexes. While few of these well-defined complexes function catalytically, their behavior illuminated likely stages in nitrogen fixation. Fruitful early studies focused on (MN20āˆ’
2)
(dppe)2 (M = Mo, W), which protonates to give intermediates with ligand M=Nāˆ’N
2
. In 1995, a molybdenum(III) amido complex was discovered that cleaved N
2
to give the corresponding molybdenum (VI) nitride.[39] This and related terminal nitrido complexes have been used to make nitriles.[40]

In 2003 a molybdenum amido complex was found to catalyze the reduction of N
2
, albeit with few turnovers.[38][41][42][43] In these systems, like the biological one, hydrogen is provided to the substrate

In 2003 a molybdenum amido complex was found to catalyze the reduction of N
2
, albeit with few turnovers.[38][41][42][43] In these systems, like the biological one, hydrogen is provided to the substrate heterolytically, by means of protons and a strong reducing agent rather than with H
2
.

In 2011, another molybdenum-based system was discovered, but with a diphosphorus pincer ligand.[44] Photolytic nitrogen splitting is also considered.[45][46][47][48][49]

Nitrogen fixation at a p-block element was published in 2018 whereby one molecule of dinitrogen is bound by two transient Lewis-base-stabilized borylene species.[50] The resulting dianion was subsequently oxidized to a neutral compound, and reduced using water.

Photochemical and electrochemical nitrogen reduction

With the help of catalysis and energy provided by electricity and light, NH
3
can be produced directly from nitrogen a

With the help of catalysis and energy provided by electricity and light, NH
3
can be produced directly from nitrogen and water at ambient temperature and pressure.[citation needed]

Research

As of 2019 research was considering alternate means of supplying nitrogen in agriculture. Instead of using fertilizer, researcher

As of 2019 research was considering alternate means of supplying nitrogen in agriculture. Instead of using fertilizer, researchers were considering using different species of bacteria and separately, coating seeds with probiotics that encourage the growth of nitrogen-fixing bacteria.[51]

See also