Abiological nitrogen fixation using homogeneous catalysts
{{Short description|Chemical process that converts nitrogen to ammonia}}
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 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 {{chem|TiCl|4}} was obtained after hydrolysis.…"{{cite journal|last1=Chatt|first1=J.|last2=Leigh|first2=G. J.|title=Nitrogen Fixation|journal=Chem. Soc. Rev.|date=1972|volume=1|page=121|doi=10.1039/cs9720100121}}These results led to many studies on dinitrogen complexes of titanium and zirconium.{{cite journal |last1= Chirik |first1= Paul J. |year= 2010 |title= Group 4 Transition Metal Sandwich Complexes: Still Fresh after Almost 60 Years |journal= Organometallics |volume= 29 |issue= 7|pages= 1500–1517 |doi= 10.1021/om100016p}}
=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 + 6{{nbsp}}H+ + 6{{nbsp}}e− → 2{{nbsp}}NH3
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.{{cite journal | last1 = Schrock | first1 = Richard R. | year = 2006 | title = Reduction of Dinitrogen | journal = PNAS | volume = 103 | issue = 46 | page = 17087 | doi = 10.1073/pnas.0603633103 | pmc = 1859893 | pmid = 17088548 | url = http://www.pnas.org/content/pnas/103/46/17087.full.pdf | bibcode = 2006PNAS..10317087S| doi-access = free }}
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-NHC-phosphine) complex exhibits >1000 turnovers when the reducing agent is samarium(II) iodide and the proton source is methanol.{{Cite journal|last1=Arashiba|first1=Kazuya|last2=Miyake|first2=Yoshihiro|last3=Nishibayashi|first3=Yoshiaki|title=A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of dinitrogen into ammonia|journal=Nature Chemistry|volume=3|issue=2|pages=120–125|doi=10.1038/nchem.906|pmid=21258384|bibcode=2011NatCh...3..120A|year=2011}}
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.{{cite journal | last1 = Rebreyend | first1 = C. | last2 = de Bruin | first2 = B. | year = 2014 | title = Photolytic {{chem|N|2}} Splitting: A Road to Sustainable {{chem|NH|3}} Production? | journal = Angew. Chem. Int. Ed. | volume = 54| issue = 1| pages = 42–44| doi = 10.1002/anie.201409727 | pmid = 25382116}}{{cite journal | last1 = Solari | first1 = E. | last2 = Da Silva | first2 = C. | last3 = Iacono | first3 = B. | last4 = Hesschenbrouck | first4 = J. | last5 = Rizzoli | first5 = C. | last6 = Scopelliti | first6 = R. | last7 = Floriani | first7 = C. | year = 2001 | title = Photochemical Activation of the N≡N Bond in a Dimolybdenum–Dinitrogen Complex: Formation of a Molybdenum Nitride | journal = Angew. Chem. Int. Ed. | volume = 40 | issue = 20| pages = 3907–3909 | doi = 10.1002/1521-3773(20011015)40:20<3907::AID-ANIE3907>3.0.CO;2-# | pmid = 29712125}}{{cite journal | last1 = Huss | first1 = Adam S. | last2 = Curley | first2 = John J. | last3 = Cummins | first3 = Christopher C. | last4 = Blank | first4 = David A. | year = 2013 | title = Relaxation and Dissociation Following Photoexcitation of the (μ-{{chem|N|2}})[Mo(N[t-Bu]Ar)3]2 Dinitrogen Cleavage Intermediate | journal = J. Phys. Chem. B | volume = 117 | issue = 5| pages = 1429–1436 | pmid = 23249096 | doi = 10.1021/jp310122x }}{{cite journal | last1 = Kunkely | first1 = H. | last2 = Vogler | first2 = A. | year = 2010 | title = Photolysis of Aqueous [({{chem|NH|3}})5Os(μ-{{chem|N|2}})Os({{chem|NH|3}})5]5+: Cleavage of Dinitrogen by an Intramolecular Photoredox Reaction | journal = Angew. Chem. Int. Ed. | volume = 49 | issue = 9| pages = 1591–1593 | doi = 10.1002/anie.200905026 | pmid = 20135653}}{{cite journal | last1 = Miyazaki | first1 = T. | last2 = Tanaka | first2 = H. | last3 = Tanabe | first3 = Y. | last4 = Yuki | first4 = M. | last5 = Nakajima | first5 = K. | last6 = Yoshizawa | first6 = K. | last7 = Nishibayashi | first7 = Y. | year = 2014 | title = Cleavage and Formation of Molecular Dinitrogen in a Single System Assisted by Molybdenum Complexes Bearing Ferrocenyldiphosphine | journal = Angew. Chem. Int. Ed. | volume = 53 | issue = 43| pages = 11488–11492 | doi = 10.1002/anie.201405673 | pmid = 25214300}}
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.{{Cite journal|last1=Broere|first1=Daniël L. J.|last2=Holland|first2=Patrick L.|date=2018-02-23|title=Boron compounds tackle dinitrogen|journal=Science|language=en|volume=359|issue=6378|pages=871|doi=10.1126/science.aar7395|issn=0036-8075|pmid=29472470|pmc=6101238|bibcode=2018Sci...359..871B}} The resulting dianion was subsequently oxidized to a neutral compound, and reduced using water.
Nitriding
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.{{cite book|last=Brook|first=Michael A.|title=Silicon in Organic, Organometallic, and Polymer Chemistry|year=2000|publisher=John Wiley & Sons, Inc.|location=New York|pages=193–194}} Processes that involve oxidising the lithium metal are however of little practical interest, since they are non-catalytic and re-reducing the {{chem|Li|+}} ion residue is difficult. The hydrogenation of Li3N to produce ammonia has seen some exploration since the resulting lithium hydride can be thermally decomposed back to lithium metal.{{Cite journal |last1=Goshome |first1=Kiyotaka |last2=Miyaoka |first2=Hiroki |last3=Yamamoto |first3=Hikaru |last4=Ichikawa |first4=Tomoyuki |last5=Ichikawa |first5=Takayuki |last6=Kojima |first6=Yoshitsugu |date=2015 |title=Ammonia Synthesis via Non-Equilibrium Reaction of Lithium Nitride in Hydrogen Flow Condition |url=https://www.jstage.jst.go.jp/article/matertrans/56/3/56_M2014382/_article |journal=Materials Transactions |language=en |volume=56 |issue=3 |pages=410–414 |doi=10.2320/matertrans.M2014382 |issn=1345-9678}}
:2{{nbsp}}Mo(NR2)3 + N2 → 2{{nbsp}}N≡Mo(NR2)3
This and related terminal nitrido complexes have been used to make nitriles.{{cite journal|first1=John J.|last1= Curley |first2=Emma L. |last2=Sceats|first3= Christopher C.|last3= Cummins |title=A Cycle for Organic Nitrile Synthesis via Dinitrogen Cleavage|journal=J. Am. Chem. Soc. |date=2006 |volume=128 |issue=43 |pages=14036–14037 |doi=10.1021/ja066090a|pmid= 17061880 }}
See also
- Nitrogenase: enzymes used by organisms to fix nitrogen
- Transition metal dinitrogen complex
- Metal nitrido complex