Abiological nitrogen fixation using homogeneous catalysts

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.[1]

The conversion of N2 to NH3 abiologically has attracted much attention. Transition metal dinitrogen complexes are well known, some of which assist in scission or hydrogenation of dinitrogen. Some main-group element compounds also react with N2.[2]

Evolution of catalysis

Vol'pin and Shur

An early influential discovery of abiological nitrogen fixation was made by Vol'pin and co-workers in Russia in the 1960's.[3] Aspects are described in an early review:

"[we] 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 TiCl
4 was obtained after hydrolysis...."[4]

These results led to many studies on dinitrogen complexes of titanium and zirconium.[5]

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 H+ + 6 e− → 2 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.[4] Even though catalysis was not demonstrated, Chatt's work revealed the existence of several intermediates, including hydrazido complexes (Mo=N-NH2).

Efforts also focussed on pincer ligand-supported Mo(0)-N2 complexes. These pincer complexes are similar to Chatt's complexes in that they too feature Mo(0). One Mo-PCP (PCP = phosphine-NHC-phosphine) complex catalyzes the hydrogenation of dinitrogen with >1000 turnovers. The reducing agent is samarium(II) iodide and the proton source is methanol.[1][6]

Schrock developed a system unrelated to Chatt's Mo(0) precursors but based on the Mo(III) complex Mo[(HIPTN)3N] where HIPTN is a bulky triamido ligand. This complex catalyzed conversion of dinitrogen to ammonium, albeit with only a few turnovers.[7]

Iron complexes of N2 are numerous. Derivatives of Fe(0) with C3-symmetric ligands catalyze nitrogen fixation.[1][8]

See also

References

  1. ^ a b c d Chalkley, Matthew J.; Drover, Marcus W.; Peters, Jonas C. (2020). "Catalytic N2-to-NH3 (Or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes". Chemical Reviews. 120 (12): 5582–5636. doi:10.1021/acs.chemrev.9b00638. PMC 7493999. PMID 32352271.
  2. ^ Liu, Tong-Tong; Zhai, Dan-Dan; Guan, Bing-Tao; Shi, Zhang-Jie (2022-05-23). "Nitrogen fixation and transformation with main group elements". Chemical Society Reviews. 51 (10): 3846–3861. doi:10.1039/D2CS00041E. ISSN 1460-4744. PMID 35481498. S2CID 248416898.
  3. ^ Vol'Pin, M. E.; Shur, V. B. (1966). "Nitrogen Fixation by Transition Metal Complexes". Nature. 209 (5029): 1236. doi:10.1038/2091236a0.
  4. ^ a b Chatt, J.; Leigh, G. J. (1972). "Nitrogen Fixation". Chem. Soc. Rev. 1: 121. doi:10.1039/cs9720100121.
  5. ^ Chirik, Paul J. (2010). "Group 4 Transition Metal Sandwich Complexes: Still Fresh after Almost 60 Years". Organometallics. 29 (7): 1500–1517. doi:10.1021/om100016p.
  6. ^ Arashiba, Kazuya; Miyake, Yoshihiro; Nishibayashi, Yoshiaki (2011). "A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of dinitrogen into ammonia". Nature Chemistry. 3 (2): 120–125. Bibcode:2011NatCh...3..120A. doi:10.1038/nchem.906. PMID 21258384.
  7. ^ a b Schrock, Richard R. (2006). "Reduction of Dinitrogen" (PDF). PNAS. 103 (46): 17087. Bibcode:2006PNAS..10317087S. doi:10.1073/pnas.0603633103. PMC 1859893. PMID 17088548.
  8. ^ Tanabe, Yoshiaki; Nishibayashi, Yoshiaki (2024). "Catalytic Nitrogen Fixation Using Well-Defined Molecular Catalysts under Ambient or Mild Reaction Conditions". Angewandte Chemie International Edition. 63 (33) e202406404. doi:10.1002/anie.202406404. PMID 38781115.
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