Angeli's salt explained

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.^[1] It is also known by the name sodium trioxodinitrate(II) monohydrate.

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 :^{[2] [3]}

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–O), and 1.32 Å (N–O). The negative charge is on the oxygen atoms at opposite ends of the molecule. The angles are 112.9° (O_single–N–N), 118.4° (N–N–O_trans), and 122.5° (N–N–O_cis). This means that the nitrogen–nitrogen bond is a double bond, and that the cis oxygen is slightly repelled by the single oxygen.^[4]

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.^[5]

Notes and References

1. Nakagawa. H.. Controlled release of HNO from chemical donors for biological applications. J. Inorg. Biochem.. 2013. 118. 187–190. 10.1016/j.jinorgbio.2012.10.004. 23140899.
2. A.. Angeli. Gazz. Chim. Ital.. Sopra la nitroidrossilammina. On nitrohydroxylamine. Italian. 26. 17–28. 1896.
3. Book: Synthesis, chemistry, and Applications of Nitroxyl Ion Releasers Sodium Trioxodinitrate or Angeli's Salt and Piloty's Acid. Hughes. Martin N.. Cammack. Richard. Methods in Enzymology . Nitric Oxide, Part C: Biological and Antioxidant Activities . 1999. 301. 279–287. 10.1016/S0076-6879(99)01092-7. 9919577. 9780121822026 .
4. Hope. Hakon. Sequeira. Michael R.. Angeli's salt. Crystal structure of sodium trioxodinitrate(II) monohydrate, Na₂N₂O₃·H₂O. Inorganic Chemistry. February 1973. 12. 2. 286–288. 10.1021/ic50120a008.
5. Book: Advanced Organic Chemistry. Part B: Reactions and Synthesis. Carey. Francis A.. 2007. Springer. Sundberg. Richard J.. 9781601195494. 5th. New York, NY. 223941000.