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Skeletal formula of pentamethylcyclopentadiene
Ball-and-stick model of the pentamethylcyclopentadiene molecule
3D model (JSmol)
ECHA InfoCard 100.021.586
Molar mass 136.24 g/mol
Boiling point 55 to 60 °C (131 to 140 °F; 328 to 333 K) at 13 mmHg (1.7 kPa)
Sparingly soluble
Flash point 114 °C (237 °F; 387 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic dialkene with the formula C5Me5H (Me = CH3).[1] 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted 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 first prepared from tiglaldehyde via 2,3,4,5-tetramethylcyclopent-2-enone.[2] Alternatively 2-butenyllithium adds to ethyl acetate followed by acid-catalyzed dehydrocyclization:[3][4]

2 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+
Sample of pentamethylcyclopentadiene in ampoule.

Organometallic derivatives[edit]

Cp*H is a precursor to organometallic compounds containing the C
ligand, commonly called Cp*.[5]

Cp*–metal complexes
Cp*2Fe yellow
Cp*TiCl3 red
[Cp*Fe(CO)2]2 red-violet
[Cp*RhCl2]2 red
Cp*IrCl22 orange
Cp*Re(CO)3 colorless
Cp*Mo(CO)2CH3 orange

Some representative reactions leading to such Cp*–metal complexes follow:[6]

Cp*H + C4H9Li → Cp*Li + C4H10
Cp*Li + TiCl4 → Cp*TiCl3 + LiCl

Some Cp* complexes are prepared using silyl transfer:

  • Cp*Li + Me3SiCl → Cp*SiMe3 + LiCl
  • Cp*SiMe3 + TiCl4 → Cp*TiCl3 + Me3SiCl

Some Cp* complexes were originally prepared using hexamethyl Dewar benzene as the precursor. This method was traditionally used for [Rh(C5Me5)Cl2]2, but has been discontinued with the increased commercial availability of Cp*H.

2 Cp*H + 2 Fe(CO)5 → [Cp*Fe(CO)2]2 + H2 + CO

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 preparation of the chloro-bridged dimers [Cp*IrCl2]2 and [Cp*RhCl2]2. Such syntheses rely on a hydrohalic acid induced rearrangement of hexamethyl Dewar benzene[7][8] to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either iridium(III) chloride[9] or rhodium(III) chloride.[10]

Synthesis of the iridium(III) dimer [Cp*IrCl2]2 using hexamethyl Dewar benzene.

Comparison of Cp* with Cp[edit]

Complexes of pentamethylcyclopentadienyl differ in several ways from the more common cyclopentadienyl (Cp) derivatives. Being more electron-rich, Cp* is a stronger donor and is less easily displaced from the metal.[citation needed] Its steric bulk stabilizes complexes with fragile ligands. Its bulk also attenuates intermolecular interactions, decreasing the tendency to form polymeric structures. Its complexes also tend to be more soluble in non-polar solvents. The methyl group in Cp* complexes can undergo C–H activation leading to "tuck-in complexes".

See also[edit]


  1. ^ Elschenbroich, C.; Salzer, A. (1989). Organometallics: A Concise Introduction. VCH. p. 47. ISBN 9783527278183. 
  2. ^ De Vries, L. (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 (10): 1838. doi:10.1021/jo01080a623. 
  3. ^ Threlkel, S.; Bercaw, J. E.; Seidler, P. F.; Stryker, J. M.; Bergman, R. G. (1993). "1,2,3,4,5-Pentamethylcyclopentadiene". Organic Syntheses. ; Collective Volume, 8, p. 505 
  4. ^ 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. Inorganic Syntheses. 29: 193–198. doi:10.1002/9780470132609.ch47. ISBN 978-0-470-13260-9. 
  5. ^ Yamamoto, A. (1986). Organotransition Metal Chemistry: Fundamental Concepts and Applications. Wiley-Interscience. p. 105. ISBN 9780471891710. 
  6. ^ King, R. B.; Bisnette, M. B. (1967). "Organometallic chemistry of the transition metals XXI. Some π-pentamethylcyclopentadienyl derivatives of various transition metals". J. Organomet. Chem. 8 (2): 287–297. doi:10.1016/S0022-328X(00)91042-8. 
  7. ^ Paquette, L. A.; Krow, G. R. (1968). "Electrophilic Additions to Hexamethyldewarbenzene". Tetrahedron Lett. 9 (17): 2139–2142. doi:10.1016/S0040-4039(00)89761-0. 
  8. ^ Criegee, R.; Gruner, H. (1968). "Acid-catalyzed Rearrangements of Hexamethyl-prismane and Hexamethyl-Dewar-benzene". Angew. Chem. Int. Ed. Engl. 7 (6): 467–468. doi:10.1002/anie.196804672. 
  9. ^ Kang, J. W.; Mosley, K.; Maitlis, P. M. (1968). "Mechanisms of Reactions of Dewar Hexamethylbenzene with Rhodium and Iridium Chlorides". Chem. Commun. (21): 1304–1305. doi:10.1039/C19680001304. 
  10. ^ Kang, J. W.; Maitlis, P. M. (1968). "Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride". J. Am. Chem. Soc. 90 (12): 3259–3261. doi:10.1021/ja01014a063.