Sodium cyclopentadienide explained

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

Sodium cyclopentadienide is commercially available as a solution in THF. It is prepared by treating cyclopentadiene with sodium:

The conversion can be conducted by heating a suspension of molten sodium in dicyclopentadiene.^[1] 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:^[2]

In early work, Grignard reagents were used as bases. With a pK_a of 15, cyclopentadiene can be deprotonated by many reagents.

Applications

Sodium cyclopentadienide is a common reagent for the preparation of metallocenes. For example, the preparation of ferrocene and zirconocene dichloride:^[3]

Sodium cyclopentadienide is also used for the preparation of substituted cyclopentadienyl derivatives such as the ester and formyl derivatives:^[4]

These compounds are used to prepare substituted metallocenes such as 1,1'-ferrocenedicarboxylic acid.^[5]

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-η⁵:η⁵-C₅H₅ ligands.^[6] 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.^[7]

In contrast to alkali metal cyclopentadienides, tetrabutylammonium cyclopentadienide (Bu₄N⁺C₅H₅⁻) was found to be supported entirely by ionic bonding and its structure is representative of the structure of the cyclopentadienide anion (C₅H₅⁻, 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°.^[8]

See also

• Lithium cyclopentadienide

Notes and References

1. Tarun K. Panda, Michael T. Gamer, Peter W. Roesky "An Improved Synthesis of Sodium and Potassium Cyclopentadienide" Organometallics, 2003, 22, 877–878.
2. Book: Girolami, G. S. . Rauchfuss, T. B. . Angelici, R. J. . Gregory S. Girolami . Robert Angelici . amp . Synthesis and Technique in Inorganic Chemistry . University Science Books: Mill Valley . CA . 1999 . 0935702482.
3. Wilkinson, G. . Birmingham, J. G. . 1954 . Bis-cyclopentadienyl Compounds of Ti, Zr, V, Nb and Ta . . 76 . 17 . 4281–84 . 10.1021/ja01646a008. Geoffrey Wilkinson .
4. Macomber, D. W. . Hart, W. P. . Rausch, M. D. . Functionally Substituted Cyclopentadienyl Metal Compounds. Adv. Organomet. Chem.. 1982. 21. 1–55. 10.1016/S0065-3055(08)60377-9. Advances in Organometallic Chemistry . 9780120311217.
5. 10.1021/om4004972. Large-Scale Preparation of 1,1′-Ferrocenedicarboxylic Acid, a Key Compound for the Synthesis of 1,1′-Disubstituted Ferrocene Derivatives . 2013 . Petrov . Alex R. . Jess . Kristof . Freytag . Matthias . Jones . Peter G. . Tamm . Matthias . Organometallics . 32 . 20 . 5946–5954.
6. Robert E. Dinnebier . Ulrich Behrens . Falk Olbrich . amp . Solid State Structures of Cyclopentadienyllithium, -sodium, and -potassium. Determination by High-Resolution Powder Diffraction . . 1997 . 16 . 17 . 3855–3858 . 10.1021/om9700122.
7. Book: Elschenbroich, C. . Organometallics . 2006 . Wiley-VCH: Weinheim . 978-3-527-29390-2.
8. Reetz. Manfred T.. Hütte. Stephan. Goddard. Richard. 1995-03-01. Tetrabutylammonium Salts of 2-Nitropropane, Cyclopentadiene and 9-Ethylfluorene: Crystal Structures and Use in Anionic Polymerization. Zeitschrift für Naturforschung B. en. 50. 3. 415–422. 10.1515/znb-1995-0316. 45791403 . 1865-7117. free.