Neptunocene Explained

Neptunocene, Np(C₈H₈)₂, is an organoneptunium compound composed of a neptunium atom sandwiched between two cyclooctatetraenide (COT^2-) rings. As a solid it has a dark brown/red colour but it appears yellow when dissolved in chlorocarbons, in which it is sparingly soluble.^[1] ^[2] ^[3] ^[4] ^[5] The compound is quite air-sensitive.

It was one of the first organoneptunium compounds to be synthesised, and is a member of the actinocene family of actinide-based metallocenes.

Structure

The sandwich structure of neptunocene has been determined by single crystal XRD. The COT^2- rings are found to be planar with 8 equivalent C–C bonds of 1.385 Å length, and sit parallel in an eclipsed conformation. The Np–COT distance (to the ring centroid) is 1.909 Å and the individual Np–C distances are 2.630 Å.

Neptunocene assumes a monoclinic crystal structure (P2₁/n space group) which is isomorphous to uranocene and thorocene but not to plutonocene.

Synthesis and properties

Neptunocene was first synthesised in 1970 by reacting neptunium(IV) chloride (NpCl₄) with dipotassium cyclooctatetraenide (K₂(C₈H₈)) in diethyl ether or THF:

NpCl₄ + 2 K₂(C₈H₈) → Np(C₈H₈)₂ + 4 KCl

The same reaction conditions have been routinely reproduced since then for the synthesis of the compound.

The three actinocenes uranocene, neptunocene, and plutonocene share virtually identical chemistry: they do not react in the presence of water or dilute base, but are very air-sensitive, quickly forming oxides. All three are only slightly soluble (up to about 10⁻³ M concentrations) in aromatic or chlorinated solvents such as benzene, toluene, carbon tetrachloride or chloroform.

Notes and References

Karraker. David G.. Stone. John Austin.. Jones. Erwin Rudolph.. Edelstein. Norman.. 1970. Bis(cyclooctatetraenyl)neptunium(IV) and bis(cyclooctatetraenyl)plutonium(IV). Journal of the American Chemical Society. en. 92. 16. 4841–4845. 10.1021/ja00719a014. 0002-7863. 2021-03-22. 2024-05-03. https://web.archive.org/web/20240503013428/https://pubs.acs.org/doi/abs/10.1021/ja00719a014. live.
Book: Greenwood, Norman N.. Chemistry of the Elements. Earnshaw. Alan. Butterworth-Heinemann. 1997. 9780750633659. 2nd. 1278–1280.
Eisenberg. David C.. Streitwieser. Andrew. Kot. Wing K.. 1990. Electron transfer in organouranium and transuranium systems. Inorganic Chemistry. 29. 1. 10–14. 10.1021/ic00326a004. 0020-1669. 2021-03-22. 2024-05-03. https://web.archive.org/web/20240503013432/https://pubs.acs.org/doi/pdf/10.1021/ic00326a004. live.
Ridder. D. J. A. De. Rebizant. J.. Apostolidis. C.. Kanellakopulos. B.. Dornberger. E.. 1996. Bis(cyclooctatetraenyl)neptunium(IV). Acta Crystallographica Section C. en. 52. 3. 597–600. 10.1107/S0108270195013047. 1600-5759. 2021-03-22. 2021-06-12. https://web.archive.org/web/20210612105101/https://onlinelibrary.wiley.com/doi/abs/10.1107/S0108270195013047. live.
Book: Yoshida. Zenko. The Chemistry of the Actinide and Transactinide Elements. Johnson. Stephen G.. Kimura. Takaumi. Krsul. John R.. 2006. Springer. Morss. Lester R.. 3rd. 3. Dordrecht, the Netherlands. 699–812. Neptunium. 10.1007/1-4020-3598-5_6. Yoshida et al.. Edelstein. Norman M.. Fuger. Jean. 2016-08-07. 2018-01-17. https://web.archive.org/web/20180117190715/http://radchem.nevada.edu/classes/rdch710/files/neptunium.pdf. dead.