Pentamethylcyclopentadiene
Encyclopedia
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.
Some representative reactions leading to such Cp*-metal complexes follow:
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
.
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
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:- MeCH=C(Li)Me + MeC(O)OEt → (MeCH=C(Me))2C(OLi)Me + LIOEt)2C(OLi)Me + H+ → Cp*H + H2O + Li+
Organometallic derivatives
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.Synthesis of Cp* complexes
| 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 |
Some representative reactions leading to such Cp*-metal complexes follow:
- Cp*H + C4H9LiN-Butyllithiumn-Butyllithium is an organolithium reagent. It is widely used as a polymerization initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene...
→ Cp*Li + C4H10ButaneButane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or... - Cp*Li + TiCl4Titanium tetrachlorideTitanium tetrachloride is the inorganic compound with the formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl4 is an unusual example of a metal halide that is highly volatile...
→ Cp*TiCl3 + LiClLithium chlorideLithium chloride is a chemical compound with the formula LiCl. The salt is a typical ionic compound, although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlorides, such as extraordinary solubility in polar solvents and its hygroscopic...
- 2 Cp*H + 2 Fe(CO)5Iron pentacarbonylIron pentacarbonyl, also known as iron carbonyl, is the compound with formula 5. Under standard conditions Fe5 is a free-flowing, straw-colored liquid with a pungent odour. This compound is a common precursor to diverse iron compounds, including many that are useful in organic synthesis. Fe5 is...
→ [Cp*Fe(CO)2]2 + H2
For the related Cp complex, see cyclopentadienyliron dicarbonyl dimer
Cyclopentadienyliron dicarbonyl dimer
Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula 2Fe24, also abbreviated Cp2Fe24. It is called Fp2 or "fip dimer." It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but...
.
An instructive but obsolete route to Cp* complexes involves the use of hexamethyl Dewar benzene
Dewar benzene
Dewar benzene or bicyclo[2.2.0]hexa-2,5-diene is a bicyclic isomer of benzene with the molecular formula C6H6. The compound is named after James Dewar who included this structure in a list of possible C6H6 structures in 1867....
. This method was traditionally used for preparation of the chloro-bridged dimers [Cp*IrCl2]2
Pentamethylcyclopentadienyl iridium dichloride dimer
Pentamethylcyclopentadienyl iridium dichloride is an organometallic compound with the formula []2, commonly abbreviated [Cp*IrCl2]2 This bright orange air-stable diamagnetic solid is a reagent in organometallic chemistry....
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
Iridium(III) chloride
Iridium chloride is the inorganic compound with the formula IrCl3. This material is relatively rare, but the related hydrate is useful for preparing other iridium compounds. The anhydrous salt is a dark green crystalline solid...
or rhodium(III) chloride
Rhodium(III) chloride
Rhodium chloride refers to inorganic compounds with the formula RhCl3n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt...
.