Pentamethylcyclopentadiene
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| Pentamethylcyclopentadiene | |
|---|---|
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| Molecular formula | C10H16 |
| Molar mass | 136.24 g/mol |
| Appearance | Colorless liquid. |
| Odor | Benzene-like odor. |
| CAS number | 4045-44-7 |
| Density | 0.870 g/cm3 |
| Solubility in water | Sparingly soluble |
| Boiling point | 55–60 °C (13 mm Hg) |
| Flash point | 44°C |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H (Me = CH3). 1,2,3,4,5-Pentamethylcyclopentadiene is the organic precursor to the 1,2,3,4,5-pentamethylcyclopentadienyl-ligand (often denoted as Cp* to signify the five methyl groups radiating from the periphery of this ligand as in a five-pointed star).
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[edit] Comparison Cp* with Cp
Cp* is an important ligand in organometallic compounds arising from the binding of the five ring-carbon atoms in C5Me5-, or Cp*-, to metals. Relative to the more common cyclopentadiene (Cp) ligand, pentamethylcyclopentadiene (Cp*) offers certain features that are often advantageous. Being more electron-rich, Cp* is a stronger donor and is less easily removed from the metal. Consequently its complexes exhibit increased thermal stability. Its steric bulk allows the isolation of complexes with fragile ligands. Its bulk also attenuates intermolecular interactions, decreasing the tendency to form polymeric structures. Its complexes also tend to be highly soluble in non-polar solvents.
[edit] Synthesis of 1,2,3,4,5-Pentamethylcyclopentadiene
A traditional route to Cp*H involves the addition of 2-butenyllithium to ethylacetate followed by acid-catalyzed dehydrocyclization:
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- MeCH=C(Li)Me + MeC(O)OEt → (MeCH=C(Me))2C(OLi)Me + LIOEt
- (MeCH=C(Me))2C(OLi)Me + H+ → Cp*H + H2O + Li+
[edit] Synthesis of Cp* complexes
| Cp*-metals and color | |
|---|---|
| Cp*2Fe | yellow |
| Cp*TiCl3 | red |
| [Cp*Fe(CO)2]2 | red-violet |
| [Cp*RhCl2]2 | red |
| Cp*Re(CO)3 | colorless |
| Cp*Mo(CO)2CH3 | orange |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Some representative reactions leading to such Cp*-metal complexes follow:
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- Cp*H + C4H9Li → Cp*Li + C4H10
- Cp*Li + TiCl4 → Cp*TiCl3 + LiCl
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- 2 Cp*H + 2 Fe(CO)5 → [Cp*Fe(CO)2]2 + H2
The Cp* complexes involves the use of hexamethyl Dewar benzene. This method was traditionally used for [Rh(C5Me5)Cl2]2.
[edit] References
R. S. Threlkel, J. E. Bercaw, P. F. Seidler, J. M. Stryker, R. G. Bergman "1,2,3,4,5-Pentamethylcyclopentadiene" Organic Syntheses, Coll. Vol. 8, p.505 (1993); Vol. 65, p.42 (1987)
Fendrick, C. M.; Schertz, L. D.; Mintz, E. A.; Marks, T. J. "Large-Scale Synthesis of 1,2,3,4,5-Pentamethylcyclopentadiene" Inorganic Syntheses, volume 29, pp. 193-197 (1992).
Yamamoto, A. Organotransition Metal Chemistry: Fundamental Concepts and Applications. (1986) p. 105
Overview of Cp* Compounds: Elschenbroich, C. and Salzer, A. Organometallics: a Concise Introduction (1989) p. 47
The First Synthesis of Pentamethylcyclopentadiene: L. de Vries, Journal of Organic Chemistry, volume 25 (1960) p.1838ff.
Initial examples of the synthesis of Cp*-metal complexes: R. B. King, M. B. Bisnette, Journal of Organometallic Chemistry volume, 8 (1967) pp. 287-297.
