As one of the most versatile ligands in an organometallic chemist’s toolkit, cyclopentadienyl (Cp) has yielded complexes with elements from every block of the periodic table (s, p, d, f). In this regard, Cp-transfer agents are highly sought compounds, as they circumvent the need to “crack” dicyclopentadiene, and deprotonate with strong base before effecting the desired metallation. Cyclopentadienyl-transfer agents such as CpLi, CpNa, and CpK are generally very air/moisture-sensitive in their solid form,[1] and do not store well over prolonged periods as the commercially available THF solutions. Cyclopentadienylthallium is a far less air/moisture sensitive Cp-transfer reagent that can be handled in air and conveniently forms insoluble thallium-halide byproducts, easily filtered off after transferring the Cp-ligand. It can easily be synthesized in multigram quantities on the benchtop.[2]
Procedure:[2] To a 500mL one-neck round-bottom flask were added Tl2SO4 (25g, 49.5mmol), NaOH (8.0g, 200mmol), and 250mL of water. To this solution was then added cyclopentadiene (8.2mL), and the reaction stirred overnight. The resulting brown precipitate was isolated by vacuum filtration, where it was washed with 50mL of water, 50mL of methanol, and dried under suction for 2h. The crude product was then purified by vacuum sublimation (90-100degC, 0.001 Torr), yielding CpTl as a light yellow solid (reported yield: 24.4g, 91%).
Notes:
- Cyclopentadiene needs to be “cracked” before use due to its instability, meaning it should be distilled. Cyclopentadiene is the monomer of dicyclopentadiene, both of which exist in a Diels-Alder equilibrium. Although the dimer is favored at room temperature, the retro-Diels-Alder reaction can be effected at high temperature, yielding the monomer which boils at 43degC and is thus easily collected after distillation. Cyclopentadiene has a half-life of approximately 12h at room temperature[3], and thus should be used immediately.
- WARNING: Dicyclopentadiene should never be distilled to dryness due to its propensity to form explosive peroxides in air.[4] Distillation should be carried out under nitrogen/argon. Always remember to leave roughly a third of undistilled material behind.
- Thallium sulfate first reacts with NaOH to form thallium hydroxide, which then acts as the base that deprotonates cyclopentadiene to furnish the product.
- Although CpTl can be handled in air, it slowly decomposes to a brown solid in the presence of air and light. It should be long-term stored in the glovebox freezer or somewhere dark under nitrogen/argon. The material can be purified by sublimation.
- Typical Cp-transfer conditions involve reflux of CpTl and the substrate in THF due to its insolubility in traditional organic solvents.
[1] Dinnebier, R. E.; Behrens, U.; Olbrich, F. Organometallics 1997, 16, 17, 3855-3858.
[2] Nielson, A. J.; Rickard, C. E. F.; Smith, J. M. Cyclopentadienylthallium. Inorg. Synth. 1986, 24, 315-317.
[3] Dickson, R. S.; Dobney, B. J.; Eastwood, F. W. Preparation of cyclopentadiene from its dimer. J. Chem. Educ. 1987, 64, 10, 898.
[4] UC Santa Cruz. List of Peroxide-forming Chemicals