Neptunocene

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.^{[2][3][4][5][6]} The compound is quite air-sensitive.^[2][3][6]

Neptunocene

Names
IUPAC name Bis(η8-cyclooctatetraene)neptunium[1]
Other names Neptunium cyclooctatetraenide Np(COT)2
Identifiers
CAS Number	154974-81-9 Y
3D model (JSmol)	Interactive image
InChI InChI=1S/2C5H5.Np/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2 Key: FXDJOXAJBKYPFW-UHFFFAOYSA-N
SMILES c1=c[cH-]c=c[cH-]c=c1.[Np+4].c1=c[cH-]c=c[cH-]c=c1
Properties
Chemical formula	C16H16Np
Molar mass	445 g·mol−1
Appearance	dark brown crystals as a solid, yellow in dilute solution
Solubility in water	insoluble, does not react with water
Solubility in chlorocarbons	sparingly soluble (ca. 0.5 g/L)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	radiation hazard, pyrophoric
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

It was one of the first organoneptunium compounds to be synthesised, and is a member of the actinocene family of actinide-based metallocenes.^[3]

Structure

The sandwich structure of neptunocene has been determined by single crystal XRD.^[5] 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.^[5]

Neptunocene assumes a monoclinic crystal structure (P2₁/n space group) which is isomorphous to uranocene and thorocene but not to plutonocene.^[5]

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:^[2]

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.^[4][5]

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.^[2][3][4] 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.^[2][3][5][6]