1,2,3,4,5-Pentamethylcyclopentadiene is a

cyclic Cycle, cycles, or cyclic may refer to: Anthropology and social sciences * Cyclic history, a theory of history * Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr. * Social cycle, various cycles in s ...

diene In organic chemistry, a diene ( ); also diolefin, ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nome ...

with the formula , often written , where Me is . It is a colorless liquid. 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

''1,2,3,4,5-pentamethylcyclopentadienyl'', which is often denoted Cp* () 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 Cyclopentadiene is an organic compound with the chemical formula, formula C5H6. It is often abbreviated CpH because the cyclopentadienyl anion is abbreviated Cp−. This colorless liquid has a strong and unpleasant odor. At room temperature, ...

derivatives, Cp*H is not prone to dimerization.

Synthesis

Pentamethylcyclopentadiene is commercially available. It was first prepared from

tiglaldehyde ''trans''-2-Methyl-2-butenal is an organic compound with the formula CH3CH=C(CH3)CHO. This colorless liquid is a building block in organic synthesis. It is an α,β-unsaturated aldehyde related to the better-known crotonaldehyde. The European r ...

and 2-butenyllithium, via 2,3,4,5-tetramethylcyclopent-2-enone, with a

Nazarov cyclization reaction The Nazarov cyclization reaction (often referred to as simply the Nazarov cyclization) is a chemical reaction used in organic chemistry for the synthesis of cyclopentenones. The reaction is typically divided into ''classical'' and ''modern'' vari ...

as a key step. : Alternatively, 2-butenyllithium adds to

ethyl acetate Ethyl acetate commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This flammable, colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues, nail polish removers, ...

followed by acid-catalyzed dehydrocyclization: : :

Organometallic derivatives

Cp*H is a precursor to

organometallic Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

compounds containing the

, commonly called . Some representative reactions leading to such Cp*–metal complexes follow: Deprotonation with n-butyllithium: : Synthesis of (pentamethylcyclopentadienyl)titanium trichloride: : Synthesis of (pentamethylcyclopentadienyl)iron dicarbonyl dimer from

iron pentacarbonyl Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula . Under standard conditions Fe( CO)5 is a free-flowing, straw-colored liquid with a pungent odour. Older samples appear darker. This compound is a common precursor t ...

: : This method is analogous to the route to the related Cp complex, see

cyclopentadienyliron dicarbonyl dimer Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula ''η''5-C5H5)Fe(CO)2sub>2, often abbreviated to Cp2Fe2(CO)4, pFe(CO)2sub>2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crysta ...

. Some Cp* complexes are prepared using silyl transfer: : : A now-obsolete route to Cp* complexes involves the use of

hexamethyl Dewar benzene Hexamethyl Dewar benzene is a derivative of Dewar benzene with application in organometallic chemistry. It consists of the Dewar benzene core, with a methyl group substituent on each of its six carbon positions. Synthesis Hexamethyl Dewar benze ...

. This method was traditionally used for preparation of the chloro-bridged dimers and , but has been discontinued with the increased commercial availability of Cp*H. Such syntheses rely on a

hydrohalic acid In chemistry, hydrogen halides (hydrohalic acids when in the aqueous phase) are diatomic, inorganic compounds that function as Arrhenius acids. The formula is HX where X is one of the halogens: fluorine, chlorine, bromine, iodine, astatine, or ...

induced rearrangement of hexamethyl Dewar benzene to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either

iridium(III) chloride Iridium(III) chloride is the inorganic compound with the formula IrCl3. The anhydrous compound is relatively rare, but the related hydrate is much more commonly encountered. The anhydrous salt has two polymorphs, α and β, which are brown and r ...

rhodium(III) chloride Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring i ...

Comparison to other Cp ligands

Complexes of pentamethylcyclopentadienyl differ in several ways from the more common cyclopentadienyl (Cp) derivatives. Being more electron-rich, is a stronger donor and dissociation, like ring-slippage, is more difficult with Cp* than with Cp. The fluorinated ligand, (trifluoromethyl)tetramethylcyclopentadienyl (), combines the properties of Cp and Cp*: it possesses the steric bulk of Cp* but has electronic properties similar to Cp, the electron-donation from the methyl groups being "canceled out" by the electron-accepting nature of the trifluoromethyl substituent. 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 complex In organometallic chemistry, a tuck-in complex usually refers to derivatives of Cp* ligands wherein a methyl group is deprotonated and the resulting methylene attaches to the metal. The C5–CH2–M angle is acute. The term "tucked in" was coined ...

es".

Bulky cyclopentadienyl ligands In the area of organometallic chemistry, a bulky cyclopentadienyl ligand is jargon for a ligand of the type where R is a branched alkyl and ''n'' = 3 or 4. Representative examples are the tetraisopropyl derivative and the tris( ''tert''-bu ...

are known that are far more sterically encumbered than Cp*.

References

{{reflist Cyclopentadienes Ligands

Synthesis

Organometallic derivatives

Comparison to other Cp ligands

See also

References