plutonocene

Plutonocene, Pu(C₈H₈)₂, is an organoplutonium compound composed of a plutonium atom sandwiched between two cyclooctatetraenide (COT^2-) rings. It is a dark red, very air-sensitive solid that is sparingly soluble in toluene and chlorocarbons. Plutonocene is a member of the actinocene family of metallocenes incorporating actinide elements in the +4 oxidation state.

Compared to other actinocenes such as uranocene, plutonocene has been studied to a lesser degree since the 1980s due to the notable radiation hazard posed by the compound. Instead, it has mostly been the subject of theoretical studies relating to the bonding in the molecule.

Structure and bonding

The compound has been structurally characterised by single crystal XRD. The cyclooctatetraenide rings are eclipsed and assume a planar conformation with 8 equivalent C–C bonds of 1.41 Å length; the molecule possesses a centre of inversion at the position occupied by the plutonium atom. The Pu–COT distance (to the ring centroid) is 1.90 Å and the individual Pu–C distances are in the 2.63–2.64 Å range.

Despite the similarity in molecular structures, plutonocene crystals are not isomorphous to other actinocenes, as plutonocene crystallises in the monoclinic ''I''2/''m'' space group whereas thorocene, protactinocene, uranocene and neptunocene all crystallise as monoclinic ''P''2₁/''n.''

Theoretical calculations utilising various computational chemistry methods support the existence of an enhanced covalent character in plutonocene from the interaction of Pu 6''d'' and 5''f'' atomic orbitals with ligand-based π orbitals.

Synthesis

Plutonocene was first synthesized in 1970 form the reaction of tetraethylammonium hexachloroplutonate(IV) ( (C₂H₅)₄sub>2PuCl₆) with dipotassium cyclooctatetraenide (K₂(C₈H₈)) in THF at room temperature:

: (NEt₄)₂PuCl₆ + 2 K₂(C₈H₈) → Pu(C₈H₈)₂ + 2 NEt₄Cl + 4 KCl

This approach is different compared to the synthesis of other actinocenes which usually involves the reaction of the actinide tetrachloride AnCl₄ with K₂(C₈H₈); this is not possible in the case of plutonium, as no stable plutonium(IV) chloride species is known. The reaction also does not work when using the caesium or pyridinium hexachloroplutonate(IV) salts in the place of the tetraethylammonium one.

A more recent synthesis involves 1 e⁻ oxidation of the green crypt)][Pu^III(C₈H₈)₂] salt with Silver iodide, AgI:

: u^III(C₈H₈)₂sup>− + AgI → Pu(C₈H₈)₂ + Ag⁰ + I⁻

The u^III(C₈H₈)₂sup>− anion is obtained ''via'' ligand substitution from K₂(C₈H₈) and other organoplutonium(III) complexes, which can be ultimately derived from reduction of the more common PuO₂ with HBr in THF. Pu^III halides PuCl₃ and PuI₃ have also been used as the plutonium starting material.

Other properties

The product is chemically analogous to uranocene and neptunocene, and they practically exhibit identical chemical reactivity. All three compounds are insensitive to water or dilute aqueous base, but are air-sensitive and react quickly to form oxides. They are only slightly soluble (with saturation concentrations of about 10⁻³ M) in aromatic or chlorinated solvents such as benzene, toluene, carbon tetrachloride or chloroform.

References

{{Plutonium compounds

Plutonium(IV) compounds
Organoactinide compounds
Metallocenes