Angeli's salt

Angeli's salt
Names
IUPAC name
Disodium nitroazanidolate
Other names
Sodium α-oxyhyponitrite
Identifiers
3D model (JSmol)
ChemSpider
UNII
  • InChI=1S/N2O3.2Na/c3-1-2(4)5;;/q;2*+1
    Key: XDPWLJKQGUTUKZ-UHFFFAOYSA-N
  • N(=O)[N+](=O)[O-].[Na+].[Na+]
Properties
N2Na2O3
Molar mass 121.991 g·mol−1
Appearance white solid
Very soluble
Acidity (pKa)
  • pKa1(H2N2O3) = 2.5
  • pKa2(H2N2O3) = 9.7 (1 °C)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Angeli's salt, sodium trioxodinitrate, is the inorganic compound with the formula Na2[N2O3]. It contains nitrogen in an unusual reduced state. It is a colorless, water-soluble solid, hence a salt. In research, this salt is used as a source of the metastable nitroxyl (HNO), which is a signalling molecule in nature. It is also known by the name sodium trioxodinitrate(II).

Preparation and properties

As first reported by Angelo Angeli in 1896, the salt is prepared by combining hydroxylamine and an organic nitrate, as a source of nitronium (NO+
2):

NH2OH + RONO2 + 2 NaOR′ → ROH + 2 R′OH + Na2N2O3

The structure of the hydrate has been confirmed by X-ray crystallography. The anion is planar. Starting from the ONN end, the bond distances are 1.35 Å (N–O), 1.26 Å (N–N), 1.31 Å (N–Otrans), and 1.32 Å (N–Ocis). The negative charge is on the oxygen atoms at opposite ends of the molecule. The angles are 112.9° (Osingle–N–N), 118.4° (N–N–Otrans), and 122.5° (N–N–Ocis). This means that the nitrogen–nitrogen bond is a double bond, and that the cis oxygen is slightly repelled by the single oxygen.

Decomposition

Aqueous solutions of trioxodinitrate are unstable below pH 10. The rate of decomposition increases between pH 10 and 8, remains essentially constant between pH 8 and 4, then accelerates again below pH 4.: 3795 

Between pH 8 and 4, the main decomposition mechanism is tautomerization of monoprotonated trioxodinitrate (O−)2N+=N(OH) to O2N−NH(O−), which decomposes reversibly to nitrite (NO−2) and nitroxyl (HNO).: 3798  The resulting nitroxyl dimerizes rapidly to hyponitrous acid (HON=NOH), which dehydrates to nitrous oxide.: 3796 

HN2O−3 ⇌ HNO + NO−2
2HNO → H2N2O2 → H2O + N2O

Below pH 4, the main decomposition mechanism is tautomerization of diprotonated trioxodinitrate (O−)2N+=N+H(OH) to reach the unstable transition state (O−)(H2O+)N+=N(O−). This loses water to yield an excited state of trans-dinitrogen dioxide that, upon de-excitation, fragments to two nitric oxide molecules.: 3799–3800 

H2N2O3 → H2O + 2NO

Reactions

Reaction of Angeli's salt with secondary amines in the presence of a proton source results in extrusion of N2 via isodiazenes as proposed intermediates.[page needed]

References

  1. ↑ https://www.chemspider.com/Chemical-Structure.8549896.html
  2. 1 2 Hope, Hakon; Sequeira, Michael R. (February 1973). "Angeli's salt. Crystal structure of sodium trioxodinitrate(II) monohydrate, Na2N2O3·H2O". Inorganic Chemistry. 12 (2): 286–288. doi:10.1021/ic50120a008.
  3. ↑ Sturrock, Peter E.; Ray, James D.; McDowell, Jean; Hunt, Harold R. (June 1963). "The Dissociation Constants of α-Oxyhyponitrous Acid". Inorganic Chemistry. 2 (3): 649–650. doi:10.1021/ic50007a063.
  4. ↑ Nakagawa, H. (2013). "Controlled release of HNO from chemical donors for biological applications". J. Inorg. Biochem. 118: 187–190. doi:10.1016/j.jinorgbio.2012.10.004. PMID 23140899.
  5. ↑ Angeli, A. (1896). "Sopra la nitroidrossilammina" [On nitrohydroxylamine]. Gazz. Chim. Ital. (in Italian). 26: 17–28. hdl:2027/hvd.cl1hem.
  6. ↑ Hughes, Martin N.; Cammack, Richard (1999). "Synthesis, chemistry, and Applications of Nitroxyl Ion Releasers Sodium Trioxodinitrate or Angeli's Salt and Piloty's Acid". Nitric Oxide, Part C: Biological and Antioxidant Activities. Methods in Enzymology. Vol. 301. pp. 279–287. doi:10.1016/S0076-6879(99)01092-7. ISBN 9780121822026. PMID 9919577.
  7. 1 2 3 4 Dutton, Andrew S.; Fukuto, Jon M.; Houk, K. N. (1 March 2004). "Mechanisms of HNO and NO Production from Angeli's Salt: Density Functional and CBS-QB3 Theory Predictions". Journal of the American Chemical Society. 126 (12): 3795–3800. doi:10.1021/ja0391614.
  8. ↑ DuMond, Jenna F.; King, S. Bruce (May 2011). "The Chemistry of Nitroxyl-Releasing Compounds". Antioxidants & Redox Signaling. 14 (9): 1637–1648. doi:10.1089/ars.2010.3838. PMC 3113415.
  9. ↑ Carey, Francis A.; Sundberg, Richard J. (2007). Advanced Organic Chemistry. Part B: Reactions and Synthesis (5th ed.). New York, NY: Springer. ISBN 9781601195494. OCLC 223941000.