Ferrocene

Structural formula
Ecliptic conformation
General
Surname	Ferrocene
other names	Bis (η 5 -cyclopentadienyl) iron Bis (η 5 -cyclopentadienyl) iron (II) Di (cyclopentadienyl) iron [Fe (η 5 -C 5 H 5 ) 2 ] [Fe (Cp) 2 ]
Molecular formula	C 10 H 10 Fe
Brief description	orange needles with a camphor-like odor
External identifiers / databases
CAS number 102-54-5 EC number 203-039-3 ECHA InfoCard 100.002.764 PubChem 7611 ChemSpider 7329 Wikidata Q211972
properties
Molar mass	186.04 g mol −1
Physical state	firmly
density	1.49 g cm −3 (20 ° C)
Melting point	173-174 ° C
boiling point	249 ° C
solubility	almost insoluble in water soluble in many organic solvents
safety instructions
GHS labeling of hazardous substances danger H and P phrases H: 228-302-411 P: 210-260-273
Toxicological data	1320 mg kg −1 ( LD 50 ,  rat ,  oral )
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Ferrocene (named after IUPAC : bis (η ⁵ -cyclopentadienyl) iron ), with the semi- structural formula [Fe (Cp) ₂ ] or [Fe (C ₅ H ₅ ) ₂ ], is a metallocene , i.e. an organometallic compound with aromatic compounds Ring systems, in the center of which there is an iron atom (Latin: ferrum ). It was the first of these compounds to be called the sandwich compound because the two cyclopentadienyl rings were shown to be on opposite sides of the iron atom.

history

Ferrocene was discovered by chance in 1951 by Thomas J. Kealy and Peter L. Pauson at Duquesne University while trying to produce fulvalene by reacting cyclopentadienyl magnesium bromide with iron (III) chloride . In the course of the reaction they obtained, among other things, orange-colored crystals, which were surprisingly stable in air and could easily be sublimed.

${\displaystyle \mathrm {3\ C_{5}H_{5}MgBr+FeCl_{3}\longrightarrow [Fe(C_{5}H_{5})_{2}]+3\ MgBrCl+1/2\ H_{5}C_{5}-C_{5}H_{5}} }$

Independently of this, and also in 1951, a working group led by Samuel A. Miller (with John A. Tebboth and John F. Tremaine) at the British Oxgen Company had produced the same substance by reacting cyclopentadiene vapor with freshly reduced iron at 300 ° C described.

${\displaystyle \mathrm {2\ C_{5}H_{6}+Fe\longrightarrow [Fe(C_{5}H_{5})_{2}]+H_{2}} }$

The first on infrared spectroscopy based structure suggestions by Geoffrey Wilkinson and Robert B. Woodward , then both at Harvard University , was in 1952 by X-ray crystal structure analysis of Ernst Otto Fischer (Munich Technical University, both then) are confirmed and Wolfgang Pfab. Since Woodward postulated that the cyclopentadienyl rings in the Fe (C ₅ H ₅ ) ₂ an electrophilic substitution should be accessible led Whitning and Rosenblum the first Friedel-Crafts acylation on the cyclopentadienyl ring by means of the ferrocene. Because of this behavior, they suggested the name Ferrocene , with the ending -ene (analogous to benzene , English for benzene ), which underlines the aromaticity of the compound.

Ernst Otto Fischer and Geoffrey Wilkinson received the Nobel Prize in Chemistry in 1973 for their work on organometallic compounds , which also explained the bonding in ferrocene.

Extraction and presentation

In addition to the historical synthesis variants, ferrocene is easily accessible in the laboratory via the reaction of cyclopentadiene with iron (II) chloride in an inert solvent and an excess of potassium hydroxide , which serves both as a deprotonation reagent for the cyclopentadiene and as a dehydrating agent:

${\displaystyle {\ce {8 KOH + 2 C5H6 + FeCl2 * 4 H2O ->[N_2][] Fe(C5H5)2 + 2 KCl + 6 KOH * H2O}}}$

properties

Ferrocene (vacuum sublimated)

Physical Properties

Ferrocene forms orange-colored crystal needles with a melting point of 173 ° C and a boiling point of 249 ° C, but noticeable sublimation occurs from 100 ° C. Ferrocene is not soluble in water, but is good in non-polar solvents such as n- hexane or toluene . It has great thermal and chemical stability. Ferrocene can be cleaned well by vacuum sublimation because it has a relatively high vapor pressure for a solid .

Binding relationships

Ferrocene is formally composed of an iron (II) cation and two cyclopentadienyl anions (C ₅ H ₅ ^- ). Overall, there is an uncharged complex.

The bonding relationships can be explained in a simplified manner by the fact that the cyclopentadienyl anions, as aromatics, have a delocalized π-electron system. Each of the two ligands can provide the iron (II) cation with six π electrons. Since the iron (II) cation has six electrons and receives twelve electrons from the ligands, it has 18 electrons in the complex. In this way, following the 18-electron rule , it achieves the energetically favorable noble gas configuration of krypton .

The distance between the cyclopentadienyl rings is 332 pm, which corresponds to the van der Waals contact between two π systems, e.g. B. the distance between the layers in graphite of 330 pm or in bis (benzene) chromium with 322 pm.

The Fe-C distance is 204.5 ± 1 pm, the CC distance 140.3 ± 2 pm. It crystallizes at room temperature in the monoclinic crystal system in the space group P 2 ₁ / a (space group no. 14, position 3) with the lattice parameters a  = 1056.1  pm , b  = 759.7 pm, c  = 595.2 pm and β = 121.02 ° with two formula units per unit cell .

Ferrocene: staggered conformation (left) and ecliptic conformation (right)

The ecliptic conformation (“in line”) is preferred over the staggered conformation of the rings. Ferrocene crystallizes at room temperature in monoclinic, at T <164 K in triclinic and at T <110 K in orthorhombic modification. In the monoclinic form, a staggered conformation (D _5d ) of individual sandwich molecules is simulated by disorder . The triclinic shape deviates by 9 ° from the ecliptic arrangement (D ₅ ), the orthorhombic shape (D _5h ) is built exactly ecliptic.

Ferrocene is also ecliptic in the gas phase, but the rotation barrier is very small. Decamethylferrocene [Cp (CH ₃ ) ₅ ] ₂ Fe, which is methyl-substituted at all positions of the five-membered rings, realizes the staggered conformation in the crystal and in the gas phase. In cobaltocene Cp ₂ Co and nickelocene Cp ₂ Ni the Cp five-membered rings are on a gap (staggered).

use

Ferrocene can be added to heating oil (individual premium heating oil qualities) in order to achieve better oxygen binding and thus more effective and cleaner combustion. The combustion temperature of the oil is not increased by the additive. Substituted ferrocenes serve as monomers for the production of polyferrocenes .

In cyclic voltammetry , ferrocene is often used as a reference substance (E ⁰ = 0.400 V against a hydrogen electrode ) in non-aqueous solutions because of its reversible oxidation to the ferrocenium ion . This use goes back to a hypothesis made by Stehlow around 1960, which states that ferrocene is well suited for comparing redox potentials in different organic solvents because the electrostatic component of the solvation enthalpy of the ferricenium ion is very small and therefore the free enthalpy of solvation of the Ferrocene molecule is very similar to that of the ferrocenium ion. The same applies to the cobaltocene / cobaltocenium system, so that the redox potential of the cobaltocene in many different solvents has a value of −1.32 V vs. Fc assumes. For this reason, ferrocene, along with bis (benzene) chromium, was established in 1984 as the IUPAC potential standard for specifying potentials in organic solvents. However, the problem is the conversion of redox potentials, which were calibrated with ferrocene, to the SHE potential scale in the aqueous phase, since many very different values ​​exist in the literature for the redox potential (Fc vs. SHE). The reason for this strongly fluctuating data is the insolubility of ferrocene in the aqueous phase, the realization of the SHE in the organic phase and the intrinsic impossibility of the precise potentiostatic measurement of redox potentials between half-cells filled with different electrolyte systems, since the anionic and cationic distribution coefficients are not can be determined independently of one another and therefore the voltage drop that occurs at the interface is unknown.

More recent studies point to the advantageous properties of decamethylferrocene as a reference system in the organic phase, as this, like bisbiphenylchromium, has greater shielding against the solvent. The dependence of the redox potential on the solvent is to be assessed even less than with the ferrocene / ferricenium system, but since decamethylferrocene is just as insoluble in water as ferrocene, this approach does not provide a satisfactory solution to the problems when comparing redox potentials in the aqueous and organic phase.

Ferrocene derivatives have been studied as drugs since the 1990s, primarily as cytostatics and antimalarials.

In contrast to decamethylferrocene, ferrocene does not form a stable, isolatable dication.

Individual evidence

1. ↑ ^{a b c d e f g} Entry on ferrocene in the GESTIS substance database of the IFA , accessed on March 5, 2014(JavaScript required) .
2. ↑ ^{a b} entry on ferrocene. In: Römpp Online . Georg Thieme Verlag, accessed on April 15, 2014.
3. ↑ Entry on sandwich compounds . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.S05468 .
4. ↑ Thomas J. Kealy, Peter L. Pauson: A New Type of Organo-Iron Compound . In: Nature . tape 168 , no. 4285 , 1951, pp. 1039-1040 , doi : 10.1038 / 1681039b0 .
5. ^ Samuel A. Miller, John A. Tebboth, John F. Tremaine: 114. Dicyclopentadienylirone . In: Journal of the Chemical Society . 1952, p. 632-635 , doi : 10.1039 / JR9520000632 .
6. ^ ^{A b} Peter L. Pauson: Ferrocene — how it all began. In: J. Organomet. Chem. , 2001, 637-639. Pp. 3-6; (PDF, 103 kB) .
7. ↑ Pierre Laszlo , Roald Hoffmann : Ferrocene . Objective story or a rashomon tale? In: Angewandte Chemie . tape 112 , no. 1 , 2000, pp. 127-128 , doi : 10.1002 / (SICI) 1521-3757 (20000103) 112: 1 <127 :: AID-ANGE127> 3.0.CO; 2-2 .
8. ^ RB Woodward, M. Rosenblum, MC Whiting: A NEW AROMATIC SYSTEM . In: J. Am. Chem. Soc. tape 74 , no. 13 , 1952, pp. 3458-3459 , doi : 10.1021 / ja01133a543 .
9. ↑ ^{a b} William L. Jolly: Bis (cyclopentadienyl) iron (Ferrocene) . In: William L. Jolly (Ed.): Inorganic Syntheses . tape 11 . McGraw-Hill Book Company, Inc., 1968, p. 120-122 (English).
10. ^ Jack D. Dunitz , Leslie Orgel , Alexander Rich : The Crystal Structure of Ferrocene . In: Acta Crystallographica . tape 9 , no. 4 , 1956, pp. 373-375 , doi : 10.1107 / S0365110X56001091 .
11. ^ Norman Neill Greenwood , Alan Earnshaw : Chemistry of the elements . 1st edition. VCH Verlag , Weinheim 1988, ISBN 3-527-26169-9 , pp. 408-409 .
12. ^ Paul Seiler, Jack D. Dunitz: A new interpretation of the disordered crystal structure of ferrocene . In: Acta Crystallographica Section B . tape 35 , no. 5 , 1979, pp. 1068-1074 , doi : 10.1107 / S0567740879005598 .
13. ↑ Paul Seiler, Jack D. Dunitz: The structure of triclinic ferrocene at 101, 123 and 148 K . In: Acta Crystallographica Section B . tape 35 , no. 9 , 1979, pp. 2020–2032 , doi : 10.1107 / S0567740879008384 .
14. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 84th edition. (Internet version:), CRC Press / Taylor and Francis, Boca Raton, FL, Physical constants of organic compounds, pp. 3-282-3-283.
15. ↑ H.-M. Koepp, H. Wendt, H. Stkehlow: The comparison of the stress series in different solvents. II. In: Journal of Electrochemistry, Reports of the Bunsen Society for Physical Chemistry , Volume 64, No. 4, pp. 483-491, June 1960. doi : 10.1002 / bbpc.19600640406 (currently unavailable) .
16. ↑ Gerhard Gritzner , Jaroslav Kuta : Recommendations on reporting electrode potentials in Nonaqueous Solvents . Recommendations 1983. In: Pure and Applied Chemistry . tape 56 , no. 4 , 1984, pp. 461-466 ( PDF ). PDF ( Memento of the original from March 5, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
17. ↑ Indra Noviandri, Kylie N. Brown, Douglas S. Fleming, Peter T. Gulyas, Peter A. Lay, Anthony F. Masters, Leonidas Phillips: The Decamethylferrocenium / Decamethylferrocene Redox Couple: A Superior Redox Standard to the Ferrocenium / Ferrocene Redox Couple for Studying Solvent Effects on the Thermodynamics of Electron Transfer . In: The Journal of Physical Chemistry . tape 103 , no. 32 , 1999, p. 6713-6722 , doi : 10.1021 / jp991381 + .
18. ↑ Malay Patra, Gilles Gasser: The medicinal chemistry of ferrocene and its derivatives . In: Nature Reviews Chemistry . tape 1 , no. 9 , September 6, 2017, ISSN  2397-3358 , p. 1–12 , doi : 10.1038 / s41570-017-0066 ( nature.com [accessed June 12, 2020]).

Web links

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