Sodium Cyclopentadienide

Sodium cyclopentadienide is an organosodium compound with the formula C₅H₅Na. The compound is often abbreviated as NaCp, where Cp⁻ is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are pink owing to traces of oxidized impurities.

Preparation

The first salt of cyclopentadienide to be reported was potassium cyclopentadienide, prepared by Johannes Thiele. In 1901 there was not much interest in the topic.

Sodium cyclopentadienyl is prepared by treating cyclopentadiene with sodium:

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The conversion can be conducted by heating a suspension of molten sodium in dicyclopentadiene.Tarun K. Panda, Michael T. Gamer, Peter W. Roesky "An Improved Synthesis of Sodium and Potassium Cyclopentadienide" Organometallics, 2003, 22, 877–878. In former times, the sodium was provided in the form of "sodium wire" or "sodium sand", a fine dispersion of sodium prepared by melting sodium in refluxing xylene and rapidly stirring. Sodium hydride is a convenient base:
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In early work, Grignard reagents were used as bases. With a p''K''_a of 15, cyclopentadiene can be deprotonated by many reagents.

Sodium cyclopentadienide is commercially available as a solution in THF.

Applications

Sodium cyclopentadienide is a common reagent for the preparation of metallocenes. For example, the preparation of ferrocene and zirconocene dichloride:

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Sodium cyclopentadienide is also used for the preparation of substituted cyclopentadienyl derivatives such as the ester and formyl derivatives:
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These compounds are used to prepare substituted metallocenes such as 1,1'-ferrocenedicarboxylic acid.

Structure

The nature of NaCp depends strongly on its medium and for the purposes of planning syntheses; the reagent is often represented as a salt . Crystalline solvent-free NaCp, which is rarely encountered, is a "polydecker" sandwich complex, consisting of an infinite chain of alternating Na⁺ centers sandwiched between ''μ''- ''η''⁵:''η''⁵-C₅H₅ ligands. As a solution in donor solvents, NaCp is highly solvated, especially at the alkali metal as suggested by the isolability of the adduct Na( tmeda)Cp.

In contrast to alkali metal cyclopentadienides, tetrabutylammonium cyclopentadienide () was found to be supported entirely by ionic bonding and its structure is representative of the structure of the cyclopentadienide anion (, Cp⁻) in the solid state. However, the anion deviates somewhat from a planar, regular pentagon, with C–C bond lengths ranging from 138.0 -140.1 pm and C–C–C bond angles ranging from 107.5-108.8°.

See also

* Lithium cyclopentadienide
* Potassium cyclopentadienyl

References

{{Cyclopentadiene complexes

Organosodium compounds
Cyclopentadienyl complexes
Non-benzenoid aromatic carbocycles