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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 114°C
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H (Me = CH3). 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted as Cp* (to signify the five methyl groups radiating from the periphery of this ligand as in a five-pointed star). In contrast to less substituted cyclopentadiene derivatives, Cp*H is not prone to dimerization.



Pentamethylcyclopentadiene is commercially available. It was was first prepared from tiglaldehyde via 1,2,3,4,5-pentamethylcyclopent-2-eneone.[1] Alternatively 2-butenyllithium adds to ethylacetate followed by acid-catalyzed dehydrocyclization:[2][3]

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+

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

Some representative reactions leading to such Cp*-metal complexes follow:

Cp*H + C4H9Li → Cp*Li + C4H10
Cp*Li + TiCl4 → Cp*TiCl3 + LiCl
2 Cp*H + 2 Fe(CO)5 → [Cp*Fe(CO)2]2 + H2

For the related Cp complex, see cyclopentadienyliron dicarbonyl dimer.

An instructive but obsolete route to Cp* complexes involves the use of hexamethyl Dewar benzene. This method was traditionally used for [RhCp*Cl2]2.

Comparison of Cp* with Cp

Cp*H is an important precursor to organometallic compounds arising from the binding of the five ring-carbon atoms in C5Me5-, or Cp*-, to metals.[4] Relative to the more common cyclopentadienyl (Cp) ligand, pentamethylcyclopentadienyl (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.

See also


  1. ^ L. de Vries (1960). "= Preparation of 1,2,3,4,5-Pentamethyl-cyclopentadiene, 1,2,3,4,5,5-Hexamethyl-cyclopentadiene, and 1,2,3,4,5-Pentamethyl-cyclopentadienylcarbinol". J. Org. Chem. 25: 1838. doi:10.1021/jo01080a623.
  2. ^ S. Threlkel, J. E. Bercaw, P. F. Seidler, J. M. Stryker, R. G. Bergman (1993). "1,2,3,4,5-Pentamethylcyclopentadiene". Org. Synth.; Coll. Vol. 8: 505. 
  3. ^ Fendrick, C. M.; Schertz, L. D.; Mintz, E. A.; Marks, T. J. (1992). "Large-Scale Synthesis of 1,2,3,4,5-Pentamethylcyclopentadiene". Inorganic Syntheses 29: 193-198. doi:10.1002/9780470132609.ch47.
  4. ^ 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
  • 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.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pentamethylcyclopentadiene". A list of authors is available in Wikipedia.
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