Bis (benzene) chrome

Structural formula
General
Surname	Bis (benzene) chrome
other names	Dibenzene chrome Chromium dibenzene
Molecular formula	C 12 H 12 Cr
Brief description	brown to black, air-sensitive crystals
External identifiers / databases
CAS number 1271-54-1 EC number 215-042-7 ECHA InfoCard 100,013,675 PubChem 11984611 Wikidata Q421390
properties
Molar mass	208.22 g mol −1
Physical state	firmly
density	1.519 g cm −3
Melting point	284-285 ° C
Sublimation point	about 150 ° C (in a high vacuum )
solubility	soluble in benzene, poorly soluble in diethyl ether
safety instructions
GHS labeling of hazardous substances danger H and P phrases H: 228 P: 210
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Bis (benzene) chromium (precisely: bis (η ⁶ -benzene) chromium (0)) or dibenzene chromium is the organometallic compound with the empirical formula Cr (C ₆ H ₆ ) ₂ .

Bis (benzene) chromium is a brown to black, crystalline solid. It is sensitive to oxygen and must therefore be handled under protective gas.

The chromium is coordinated six times by two parallel molecules of benzene ( η ⁶ ). It is the prototype of a bis (arene) metal complex and played an important role in the development of sandwich complexes in organometallic chemistry . The structure is very similar to the sandwich structure of the metallocenes ( chromocene ).

history

Bis (benzene) chrome was first produced in 1955 by Ernst Otto Fischer and Walter Hafner . The method used is therefore also known as the "Fischer-Hafner method". EO Fischer received the Nobel Prize in Chemistry in 1973 with Geoffrey Wilkinson for his work on the chemistry of sandwich compounds (elucidation of the structure of ferrocene and the synthesis of bis (benzene) chromium). A sample of bis (benzene) chrome is part of the inventory of the Deutsches Museum Bonn .

Extraction and presentation

Sublimated crystals under protective gas

The synthesis of bis (benzene) chromium requires oxygen-free working methods (see Schlenk technology ). In the first synthesis, the reaction now known as the Fischer-Hafner method (also known as the reductive Friedel-Crafts method), chromium (III) chloride , aluminum and benzene react in the presence of aluminum chloride . The air-stable cation produced in this way is reduced in a further step with sodium dithionite to bis (benzene) chromium.

${\displaystyle {\ce {CrCl3 + {\frac {2}{3}}Al + AlCl3 + 2C6H6 -> [Cr(C6H6)2]AlCl4 + {\frac {2}{3}}AlCl3}}}$

${\displaystyle {\ce {[Cr(C6H6)2]AlCl4 + {\frac {1}{2}}Na2S2O4 -> [Cr(C6H6)2] + NaAlCl4 + SO2 ^}}}$

properties

Solid structure of bis (benzene) chromium from X-ray diffraction data

Physical Properties

Bis (benzene) chromium (18 valence electrons ) has no unpaired electrons and therefore no magnetic moment . The cation (17 valence electrons) has an unpaired electron and a magnetic moment of 1.77 Bohr magnetons .

As can be seen in the solid-state structure shown, bis (benzene) chromium is present in the crystal in an ecliptic conformation .

Chemical properties

Bis (benzene) chromium reacts with carboxylic acids to form chromium (II) carboxylates, such as B. Chromium (II) acetate with a Cr / Cr quadruple bond .

The oxidation of bis (benzene) chromium, e.g. B. by oxygen, leads to the green-colored cation [Cr (C ₆ H ₆ ) ₂ ] ⁺ . The half-sandwich complex η ⁶ - benzene (tricarbonyl) chromium is formed by carbonylation .

use

Bis (benzene) chromium is used in organic synthesis as a catalyst for dehydrogenation .

literature

• Historical appreciation: Wolfgang A. Herrmann: Dibenzechromium: Chemistry only for Chemists? , Z. Anorg. General Chem. , 2012, pp. 1245-1247; doi : 10.1002 / zaac.201210011 .

Individual evidence

1. ↑ ^{a b c} data sheet bis (benzene) chrome from Sigma-Aldrich , accessed on March 13, 2011 ( PDF ).
2. ↑ ^{a b c} Entry on organic chromium compounds. In: Römpp Online . Georg Thieme Verlag, accessed on April 4, 2014.
3. ↑ ^{a b} J. W. Herndon, Dibenzenechromium . In e-EROS Encyclopedia of Reagents for Organic Synthesis , 2001, Wiley, doi : 10.1002 / 047084289X.rd020 .
4. ↑ Guido Pampaloni: Aromatic hydrocarbons as ligands. Recent advances in the synthesis, the reactivity and the applications of bis (η ⁶ -arene) complexes . In: Coordination Chemistry Reviews . tape 254 , no. 5-6 , March 2010, pp. 402-419 , doi : 10.1016 / j.ccr.2009.05.014 .
5. ^ EO Fischer, W. Hafner: Di-benzol-chrom. About aromatic complexes of metals. In: Journal of Nature Research B . 10, 1955, pp. 665–668 ( PDF , free full text).
6. ↑ Dietmar Seyferth: Bis (benzene) chromium. 2. Its Discovery by EO Fischer and W. Hafner and Subsequent Work by the Research Groups of EO Fischer, HH Zeiss, F. Hein, C. Elschenbroich, and Others. In: Organometallics . tape 21 , no. 14 , 2002, p. 2800–2820 , doi : 10.1021 / om020362a (open access).
7. ^ Deutsches Museum Bonn: Dibenzolchrom und Ferrocene , accessed on April 16, 2018.
8. ↑ ^{a b} Christoph Elschenbroich: Organometallchemie . 6th edition. Vieweg + Teubner, Wiesbaden 2008, ISBN 978-3-8351-0167-8 .
9. ↑ Georg Brauer (ed.) U. a .: Handbook of Preparative Inorganic Chemistry. 3rd, revised edition. Volume III, Ferdinand Enke, Stuttgart 1981, ISBN 3-432-87823-0 , p. 1840.
10. ↑ Konstantin A. Lyssenko et al. : Estimation of the Barrier to Rotation of Benzene in the (η ⁶ -C ₆ H ₆ ) ₂ Cr Crystal via Topological Analysis of the Electron Density Distribution Function . In: J. Phys. Chem. A . tape 110 , no. 20 , 2006, pp. 6545-6551 , doi : 10.1021 / jp057516v .