1,2,3,4,5-Pentamethylcyclopentadiene
[CAS# 4045-44-7]

Identification

• Classification: Organic raw materials >> Hydrocarbon compounds and their derivatives >> Cyclic hydrocarbon
• Name: 1,2,3,4,5-Pentamethylcyclopentadiene
• Synonyms: 1,2,3,4,5-Pentamethyl-1,3-cyclopentadiene; Pentamethylcyclopentadiene
• Molecular Formula: C₁₀H₁₆
• Molecular Weight: 136.24
• CAS Registry Number: 4045-44-7
• EC Number: 223-743-4
• SMILES: CC1C(=C(C(=C1C)C)C)C

Properties

• Density: 0.8±0.1 g/cm³, Calc.^*, 0.87 g/mL (Expl.)
• Index of Refraction: 1.46, Calc.^*, 1.474 (Expl.)
• Boiling Point: 170.2 °C (760 mmHg), Calc.^*, 185.9 °C (Expl.)
• Flash Point: 44.4 °C, Calc.^*, 44 °C (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS02 Warning
• Risk Statements: H226
• Safety Statements: P210-P233-P240-P241-P242-P243-P280-P303+P361+P353-P370+P378-P403+P235-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Flammable liquids	Flam. Liq.	3	H226
Acute toxicity	Acute Tox.	4	H312
Skin irritation	Skin Irrit.	2	H315
Acute toxicity	Acute Tox.	4	H302
Specific target organ toxicity - single exposure	STOT SE	3	H335
Acute toxicity	Acute Tox.	4	H332
Eye irritation	Eye Irrit.	2	H319
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413

• Transport Information: UN 3295

Discovery and Applications

1,2,3,4,5-Pentamethylcyclopentadiene is an organometallic ligand precursor with the molecular formula C10H16. Structurally, it is a derivative of cyclopentadiene where each carbon of the five-membered ring is substituted with a methyl group, leading to a highly substituted and sterically hindered compound. The compound exists as a liquid under standard conditions and is known for its relatively high volatility and characteristic odor.

The compound is primarily used as a precursor to the pentamethylcyclopentadienyl ligand, abbreviated as Cp* (η⁵-C₅Me₅), which is widely employed in organometallic and coordination chemistry. This ligand exhibits strong electron-donating properties and provides significant steric bulk due to the five methyl groups, which influences both the stability and reactivity of the metal complexes it forms.

1,2,3,4,5-Pentamethylcyclopentadiene is typically synthesized through the alkylation of cyclopentadiene with methylating agents such as methyl iodide in the presence of a strong base, followed by thermal rearrangement and purification. The compound is prone to dimerization, like cyclopentadiene, and is often stored under inert atmosphere or freshly distilled prior to use.

Its most prominent application is in the preparation of metallocene and half-sandwich metal complexes, where the Cp* ligand coordinates to transition metals such as rhodium, iridium, titanium, or lanthanides. These complexes have found extensive use in homogeneous catalysis, including hydrogenation, hydroformylation, and C–H activation reactions. The Cp* ligand, derived from this compound, imparts thermal stability and controls the geometry and electronic environment around the metal center.

In synthetic chemistry, Cp*-based metal complexes derived from 1,2,3,4,5-pentamethylcyclopentadiene have been used in the development of catalytic systems for enantioselective transformations, transfer hydrogenation, and activation of inert bonds. Their stability and tunability have also made them subjects of academic and industrial research, particularly in the field of organometallic catalyst design.

The compound itself, while mainly serving as a ligand precursor, is also studied for its spectroscopic properties and reactivity in cycloaddition and electrophilic substitution reactions, due to the electron-rich nature of the methyl-substituted diene ring.

In conclusion, 1,2,3,4,5-pentamethylcyclopentadiene is a well-established and widely used compound in organometallic chemistry, primarily valued for its role in the formation of Cp*-metal complexes that are central to various catalytic processes.

References

1968. The syn-anti isomerism of adducts of 1,2,3,4,5-pentamethylcyclopentadiene with maleic anhydride. Bulletin of the Academy of Sciences of the USSR, Division of Chemical Science, 17(2), 285-287.
DOI: 10.1007/bf00908462

2023. Dominant changes in centre Fe atom of decamethyl-ferrocene from ferrocene in methylation. Theoretical Chemistry Accounts, 142(1), 13.
DOI: 10.1007/s00214-022-02949-8

2024. Enhancing blocking temperature using inverse hydrogen bonds for non-radical bridged dimeric Dy(III) single-molecule magnets. Science China Chemistry, Online First.
DOI: 10.1007/s11426-024-2157-7