Pentamethylcyclopentadiene
Chemical compound
From Wikipedia, the free encyclopedia
1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diene with the formula C5(CH3)5H, often written C5Me5H, where Me is CH3.[3] It is a colorless liquid.[1]
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| Names | |
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| Preferred IUPAC name
1,2,3,4,5-Pentamethylcyclopenta-1,3-diene | |
| Identifiers | |
3D model (JSmol) |
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| ChemSpider | |
| ECHA InfoCard | 100.021.586 |
PubChem CID |
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| UNII | |
CompTox Dashboard (EPA) |
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| Properties | |
| C10H16 | |
| Molar mass | 136.238 g·mol−1 |
| Appearance | Colorless liquid[1] |
| Odor | Mild[1] |
| Density | 0.87 g/cm3[2] |
| Boiling point | 55 to 60 °C (131 to 140 °F; 328 to 333 K) at 13 mmHg (1.7 kPa) |
| Sparingly soluble | |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards |
Flammable |
| GHS labelling: | |
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| Warning | |
| H226 | |
| 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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1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted Cp* (C5Me5) and read as "C P star", the "star" signifying the five methyl groups radiating from the core of the ligand. Thus, the 1,2,3,4,5-pentamethylcyclopentadiene's formula is also written Cp*H. 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 and 2-butenyllithium, via 2,3,4,5-tetramethylcyclopent-2-enone, with a Nazarov cyclization reaction as a key step.[4]
Alternatively, 2-butenyllithium adds to ethyl acetate followed by acid-catalyzed dehydrocyclization:[5][6]
Organometallic derivatives
| Cp*–metal complexes | |
|---|---|
| Cp*2Fe | yellow |
| Cp*TiCl3 | red |
| [Cp*Fe(CO)2]2 | red-violet |
| [Cp*RhCl2]2 | red |
| [Cp*IrCl2]2 | orange |
| Cp*Re(CO)3> | colorless |
| Cp*Mo(CO)2CH3 | orange |
Cp*H is a precursor to organometallic compounds containing the C5Me−5 ligand, commonly called Cp*−.[7] Some representative reactions leading to such Cp*–metal complexes follow:[8] Deprotonation with n-butyllithium:
- Cp*H + C4H9Li → Cp*Li + C4H10
Synthesis of (pentamethylcyclopentadienyl)titanium trichloride:
- Cp*Li + TiCl4 → Cp*TiCl3 + LiCl
Synthesis of (pentamethylcyclopentadienyl)iron dicarbonyl dimer from iron pentacarbonyl:
- 2 Cp*H + 2 Fe(CO)5< → [η5-Cp*Fe(CO)2]2 + H2 + 6 CO
This method is analogous to the route to the related Cp complex, see cyclopentadienyliron dicarbonyl dimer.
Some Cp* complexes are prepared using silyl transfer:
- Cp*Li + Me3SiCl → Cp*SiMe3 + LiCl
- Cp*SiMe3 + TiCl4 → Cp*TiCl3 + Me3SiCl
A now-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, but has been discontinued with the increased commercial availability of Cp*H. Such syntheses rely on a hydrohalic acid induced rearrangement of hexamethyl Dewar benzene[9][10] to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either iridium(III) chloride[11] or rhodium(III) chloride.[12]
The methyl group in Cp* complexes can undergo C–H activation leading to "tuck-in complexes".
Comparison to other Cp ligands
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Complexes of pentamethylcyclopentadienyl differ in several ways from the more common cyclopentadienyl (Cp) derivatives. Being more electron-rich, Cp*− is a stronger donor and dissociation, like ring-slippage, is more difficult with Cp* than with Cp.[13] Its complexes tend to be more soluble in non-polar solvents.
Bulky cyclopentadienyl ligands and 1,2,3-trisubstituted Cp ligands are also known.[14] (Trifluoromethyl)tetramethylcyclopentadienyl (C5Me4CF3) has the steric properties of Cp* and the electronic properties of Cp.[15]