Pentamethylcyclopentadiene

Pentamethylcyclopentadiene

Skeletal formula of pentamethylcyclopentadiene
Ball-and-stick model of the pentamethylcyclopentadiene molecule
Identifiers
CAS Number	4045-44-7 Y
ChemSpider	70069 N
Jmol 3D model	Interactive image
PubChem	77667
InChI InChI=1S/C10H16/c1-6-7(2)9(4)10(5)8(6)3/h6H,1-5H3 N Key: WQIQNKQYEUMPBM-UHFFFAOYSA-N N InChI=1/C10H16/c1-6-7(2)9(4)10(5)8(6)3/h6H,1-5H3 Key: WQIQNKQYEUMPBM-UHFFFAOYAI
SMILES CC1=C(C)C(C)C(C)=C1C
Properties
Chemical formula	C10H16
Molar mass	136.24 g/mol
Boiling point	55 to 60 °C (131 to 140 °F; 328 to 333 K) (13 mm Hg)
Solubility in water	Sparingly soluble
Vapor pressure	{{{value}}}
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox 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.

Synthesis

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

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⁺

Organometallic derivatives

Cp*H is a precursor to organometallic compounds containing the C₅Me₅⁻ (Cp*⁻) ligand.

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:

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 + 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 to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either iridium(III) chloride or rhodium(III) chloride.

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

Comparison of Cp* with Cp

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 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. The methyl group in Cp* complexes can undergo C-H activation leading to "tuck-in complexes".

See also

• Methylcyclopentadiene