Ferrocene
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Preferred IUPAC name
Ferrocene[1] | |||
Other names
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Identifiers | |||
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ChEBI | |||
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ECHA InfoCard
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100.002.764 | ||
PubChem CID
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UNII | |||
CompTox Dashboard (EPA)
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Properties | |||
C10H10Fe | |||
Molar mass | 186.04 g/mol | ||
Appearance | light orange powder | ||
Odor | camphor-like | ||
Density | 1.107 g/cm3 (0 °C) 1.490 g/cm3 (20 °C)[2] | ||
Melting point | 172.5 °C (342.5 °F; 445.6 K)[4] | ||
Boiling point | 249 °C (480 °F; 522 K) | ||
Insoluble in water, soluble in most organic solvents | |||
log P | 2.04050[3] | ||
Structure | |||
D5h (eclipsed) D5d (staggered) | |||
Sandwich structure with iron centre | |||
Hazards | |||
NFPA 704 (fire diamond) | |||
NIOSH (US health exposure limits): | |||
PEL (Permissible)
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TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp)[5] | ||
REL (Recommended)
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TWA 10 mg/m3 (total) TWA 5 mg/m3 (resp)[5] | ||
IDLH (Immediate danger) |
N.D.[5] | ||
Related compounds | |||
Related compounds
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Ferrocene is an
The first reported synthesis of ferrocene was in 1951. Its unusual stability puzzled chemists, and required the development of new theory to explain its formation and bonding. The discovery of ferrocene and its many analogues, known as metallocenes, sparked excitement and led to a rapid growth in the discipline of organometallic chemistry. Geoffrey Wilkinson and Ernst Otto Fischer, both of whom worked on elucidating the structure of ferrocene, later shared the 1973 Nobel Prize in Chemistry for their work on organometallic sandwich compounds. Ferrocene itself has no large-scale applications, but has found more niche uses in catalysis, as a fuel additive, and as a tool in undergraduate education.
History
Discovery
Ferrocene was discovered by accident twice. The first known synthesis may have been made in the late 1940s by unknown researchers at Union Carbide, who tried to pass hot cyclopentadiene vapor through an iron pipe. The vapor reacted with the pipe wall, creating a "yellow sludge" that clogged the pipe. Years later, a sample of the sludge that had been saved was obtained and analyzed by Eugene O. Brimm, shortly after reading Kealy and Pauson's article, and was found to consist of ferrocene.[7][8]
The second time was around 1950, when Samuel A. Miller, John A. Tebboth, and John F. Tremaine, researchers at British Oxygen, were attempting to synthesize amines from hydrocarbons and nitrogen in a modification of the Haber process. When they tried to react cyclopentadiene with nitrogen at 300 °C, at atmospheric pressure, they were disappointed to see the hydrocarbon react with some source of iron, yielding ferrocene. While they too observed its remarkable stability, they put the observation aside and did not publish it until after Pauson reported his findings.[7][9][10] Kealy and Pauson were later provided with a sample by Miller et al., who confirmed that the products were the same compound.[8]
In 1951,
Determining the structure
Pauson and Kealy conjectured that the compound had two cyclopentadienyl groups, each with a single covalent bond from the saturated carbon atom to the iron atom.[7] However, that structure was inconsistent with then-existing bonding models and did not explain the unexpected stability of the compound, and chemists struggled to find the correct structure.[10][12]
The structure was deduced and reported independently by three groups in 1952.[13] Robert Burns Woodward and Geoffrey Wilkinson deduced it by observing that ferrocene underwent reactions typical of aromatic compounds such as benzene.[14] Ernst Otto Fischer and Wolfgang Pfab noted that the compound was diamagnetic and centrosymmetric, also synthesizing nickelocene and cobaltocene and confirming they had the same structure.[15] Fischer described the strcuture as Doppelkegelstruktur ("double-cone structure"), though the term "sandwich" was preferred by British and American chemists.[16] Philip Frank Eiland and Raymond Pepinsky confirmed the structure through X-ray crystallography and later by NMR.[10][17][18][19]
The "sandwich" structure of ferrocene was shockingly novel, and required new theory to explain. Application of
Impact
Ferrocene was not the first organometallic compound to be discovered.
Structure and bonding
Each ring has six π-electrons, which makes them aromatic according to Hückel's rule. These π-electrons are then shared with the metal via covalent bonding. Since Fe2+ has six d-electrons, the complex attains an 18-electron configuration, which accounts for its stability. In modern notation, this sandwich structural model of the ferrocene molecule is denoted as Fe(η5-C5H5)2, where η denotes hapticity, the number of atoms through which each ring binds.
The carbon–carbon bond distances around each five-membered ring are all 1.40 Å, and all Fe–C bond distances are 2.04 Å. From room temperature down to 164 K, X-ray crystallography yields the monoclinic space group; the cyclopentadienide rings are a staggered conformation, resulting in a centrosymmetric molecule, with symmetry group D5d.[17] However, below 110 K, ferrocene crystallizes in an orthorhombic crystal lattice in which the Cp rings are ordered and eclipsed, so that the molecule has symmetry group D5h.[27] In the gas phase, electron diffraction[28] and computational studies[29] show that the Cp rings are eclipsed. While ferrocene has no permanent dipole moment at room temperature, between 172.8 and 163.5 K the molecule exhibits an "incommensurate modulation", breaking the D5 symmetry and acquiring an electric dipole.[30]
The Cp rings rotate with a low barrier about the Cp(centroid)–Fe–Cp(centroid) axis, as observed by measurements on substituted derivatives of ferrocene using 1H and 13C nuclear magnetic resonance spectroscopy. For example, methylferrocene (CH3C5H4FeC5H5) exhibits a singlet for the C5H5 ring.[31]
In solution, and at room temperature, eclipsed D5h ferrocene was determined to dominate over the staggered D5d conformer, as suggested by both Fourier-transform infrared spectroscopy and DFT calculations.[32]
Synthesis
Industrial synthesis
Industrially, ferrocene is synthesized by the reaction of
Via Grignard reagent
The first reported syntheses of ferrocene were nearly simultaneous. Pauson and Kealy synthesised ferrocene using iron(III) chloride and a Grignard reagent, cyclopentadienyl magnesium bromide. Iron(III) chloride is suspended in anhydrous diethyl ether and added to the Grignard reagent.[11] A redox reaction occurs, forming the cyclopentadienyl radical and iron(II) ions. Dihydrofulvalene is produced by radical-radical recombination while the iron(II) reacts with the Grignard reagent to form ferrocene. Oxidation of dihydrofulvalene to fulvalene with iron(III), the outcome sought by Kealy and Pauson, does not occur.[8]
Gas-metal reaction
The other early synthesis of ferrocene was by Miller et al.,[9] who reacted metallic iron directly with gas-phase cyclopentadiene at elevated temperature.[34] An approach using iron pentacarbonyl was also reported.[35]
- Fe(CO)5 + 2 C5H6(g) → Fe(C5H5)2 + 5 CO(g) + H2(g)
Via alkali cyclopentadienide
More efficient preparative methods are generally a modification of the original transmetalation sequence using either commercially available sodium cyclopentadienide[36] or freshly cracked cyclopentadiene deprotonated with potassium hydroxide[37] and reacted with anhydrous iron(II) chloride in ethereal solvents.
Modern modifications of Pauson and Kealy's original Grignard approach are known:
- Using sodium cyclopentadienide: 2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaCl
- Using freshly-cracked cyclopentadiene: FeCl2·4H2O + 2 C5H6 + 2 KOH → Fe(C5H5)2 + 2 KCl + 6 H2O
- Using an iron(II) salt with a Grignard reagent: 2 C5H5MgBr + FeCl2 → Fe(C5H5)2 + 2 MgBrCl
Even some amine bases (such as diethylamine) can be used for the deprotonation, though the reaction proceeds more slowly than when using stronger bases:[36]
- 2 C5H6 + 2 (CH3CH2)2NH + FeCl2 → Fe(C5H5)2 + 2 (CH3CH2)2NH2Cl
Direct transmetalation can also be used to prepare ferrocene from other metallocenes, such as manganocene:[38]
- FeCl2 + Mn(C5H5)2 → MnCl2 + Fe(C5H5)2
Properties
Ferrocene is an
Ferrocene readily
Reactions
With electrophiles
Ferrocene undergoes many reactions characteristic of aromatic compounds, enabling the preparation of substituted derivatives. A common undergraduate experiment is the Friedel–Crafts reaction of ferrocene with acetic anhydride (or acetyl chloride) in the presence of phosphoric acid as a catalyst. Under conditions for a Mannich reaction, ferrocene gives N,N-dimethylaminomethylferrocene.
Ferrocene itself can be used as the backbone of a ligand, e.g.
It is an
Protonation of ferrocene allows isolation of [Cp2FeH]PF6.[43]
In the presence of aluminium chloride, Me2NPCl2 and ferrocene react to give ferrocenyl dichlorophosphine,[44] whereas treatment with phenyldichlorophosphine under similar conditions forms P,P-diferrocenyl-P-phenyl phosphine.[45]
Ferrocene reacts with P4S10 forms a diferrocenyl-dithiadiphosphetane disulfide.[46]
Lithiation
Ferrocene reacts with butyllithium to give 1,1′-dilithioferrocene, which is a versatile nucleophile. In combination with butyllithiium, tert-butyllithium produces monolithioferrocene.[47]
Redox chemistry
Ferrocene undergoes a one-electron oxidation at around 0.4 V versus a saturated calomel electrode (SCE), becoming ferrocenium. This reversible oxidation has been used as standard in electrochemistry as Fc+/Fc = 0.64 V versus the standard hydrogen electrode,[48] however other values have been reported.[49] Ferrocenium tetrafluoroborate is a common reagent.[50] The remarkably reversible oxidation-reduction behaviour has been extensively used to control electron-transfer processes in electrochemical[51][52] and photochemical[53][54] systems.
Substituents on the cyclopentadienyl ligands alters the redox potential in the expected way: electron-withdrawing groups such as a
Stereochemistry of substituted ferrocenes
Disubstituted ferrocenes can exist as either 1,2-, 1,3- or 1,1′- isomers, none of which are interconvertible. Ferrocenes that are asymmetrically disubstituted on one ring are chiral – for example [CpFe(EtC5H3Me)]. This
Several approaches have been developed to asymmetrically 1,1′-functionalise the ferrocene.[57]Applications of ferrocene and its derivatives
Ferrocene and its numerous derivatives have no large-scale applications, but have many niche uses that exploit the unusual structure (
Ligand scaffolds
Chiral ferrocenyl
Fuel additives
Ferrocene and its derivatives are
Ferrocene has been found to be effective at reducing smoke and sulfur trioxide produced when burning coal. The addition by any practical means, impregnating the coal or adding ferrocene to the combustion chamber, can significantly reduce the amount of these undesirable byproducts, even with a small amount of the metal cyclopentadienyl compound.[64]
Pharmaceuticals
Ferrocene derivatives have been investigated as drugs,
The anticancer activity of ferrocene derivatives was first investigated in the late 1970s, when derivatives bearing amine or amide groups were tested against lymphocytic leukemia.[72] Some ferrocenium salts exhibit anticancer activity, but no compound has seen evaluation in the clinic.[73] Ferrocene derivatives have strong inhibitory activity against human lung cancer cell line A549, colorectal cancer cell line HCT116, and breast cancer cell line MCF-7.[74] An experimental drug was reported which is a ferrocenyl version of tamoxifen.[75] The idea is that the tamoxifen will bind to the estrogen binding sites, resulting in cytotoxicity.[75][76]
Ferrocifens are exploited for cancer applications by a French biotech, Feroscan, founded by Pr. Gerard Jaouen.
Solid rocket propellant
Ferrocene and related derivatives are used as powerful burn rate catalysts in ammonium perchlorate composite propellant.[77]
Derivatives and variations
Ferrocene analogues can be prepared with variants of cyclopentadienyl. For example, bisindenyliron and bisfluorenyliron.[60]
Carbon atoms can be replaced by heteroatoms as illustrated by Fe(η5-C5Me5)(η5-P5) and Fe(η5-C5H5)(η5-C4H4N) ("azaferrocene"). Azaferrocene arises from decarbonylation of Fe(η5-C5H5)(CO)2(η1-pyrrole) in cyclohexane.[78] This compound on boiling under reflux in benzene is converted to ferrocene.[79]
Because of the ease of substitution, many structurally unusual ferrocene derivatives have been prepared. For example, the penta(ferrocenyl)cyclopentadienyl ligand,[80] features a cyclopentadienyl anion derivatized with five ferrocene substituents.
In hexaferrocenylbenzene, C6[(η5-C5H4)Fe(η5-C5H5)]6, all six positions on a
The synthesis of hexaferrocenylbenzene has been reported using
The
Materials chemistry
Ferrocene, a precursor to iron nanoparticles, can be used as a catalyst for the production of carbon nanotubes.
See also
References
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External links
- Ferrocene at The Periodic Table of Videos(University of Nottingham)
- NIOSH Pocket Guide to Chemical Hazards (Centers for Disease Control and Prevention)