Bis(benzene)chromium

Bis(benzene)chromium is the organometallic compound with the formula Cr(η6-C6H6)2. It is sometimes called dibenzenechromium. The compound played an important role in the development of sandwich compounds in organometallic chemistry and is the prototypical complex containing two arene ligands.

Bis(benzene)chromium
Bis(benzene)chromium
Bis(benzene)chromium
Names
IUPAC name
Bis(benzene) chromium
Other names
di(benzene) chromium
dibenzenechromium
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.013.675
EC Number
  • 215-042-7
RTECS number
  • GB5850000
Properties
C12H12Cr
Molar mass 208.22 g/mol
Appearance brown-black crystals
Melting point 284 to 285 °C (543 to 545 °F; 557 to 558 K)
Boiling point sublimes: 160 °C (320 °F; 433 K) in vacuo
insoluble
Solubility in other solvents slightly: benzene, THF
Structure
pseudooctahedral
0 D
Hazards
Main hazards flammable
GHS pictograms
GHS Signal word Warning
GHS hazard statements
 H228
P210, P240, P241, P280, P378
Flash point 82 °C; 180 °F; 355 K
Related compounds
Related compounds
 ferrocene
chromocene
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation

The substance is air sensitive and its synthesis requires air-free techniques. It was first prepared by Hafner and Fischer by the reaction of CrCl3, aluminium, and benzene, in the presence of AlCl3. This so-called reductive Friedel-Crafts method was pioneered by E.O. Fischer and his students.[1][2] The product of the reaction was yellow [Cr(C6H6)2]+, which was then reduced to the neutral complex. Idealized equations for the synthesis are:

CrCl3 + 2/3 Al + AlCl3 + 2 C6H6 → [Cr(C6H6)2]AlCl4 + 2/3 AlCl3
[Cr(C6H6)2]AlCl4 + 1/2 Na2S2O4 → [Cr(C6H6)2] + NaAlCl4 + SO2

Using the technique of metal vapor synthesis, bis(benzene)chromium and many analogous compounds can be prepared by co-condensation of Cr vapor and arene. In this way, the phosphabenzene complex [Cr(C5H5P)2] can be prepared.[3]

Structure elucidation

Compounds closely related to [Cr(C6H6)2]+ had been prepared many years before Fischer's work by Franz Hein by the reaction of phenylmagnesium bromide and CrCl3.[4] Hein's reaction affords cationic sandwich complexes containing bi- and terphenyl, which baffled chemists until the breakthrough by Fischer and Hafner.[5] (Indeed, although Harold Zeiss and Minoru Tsutsui of Yale University had earlier proposed the "sandwich" structure, they were unable to convince skeptical journal referees to publish their manuscript until Fischer and Hafner's results were made known.[6]) Fischer and Seus soon prepared Hein's [Cr(C6H5-C6H5)2]+ by an unambiguous route, thus confirming that Hein had unknowingly discovered sandwich complexes, a half-century ahead of the work on ferrocene.[7][8] Illustrating the rapid pace of this research, the same issue of Chem. Ber. also describes the Mo(0) complex.[9]

Reactions

The compound reacts with carboxylic acids to give chromium(II) carboxylates, such as chromium(II) acetate, which have interesting structures. Oxidation affords [Cr(C6H6)2]+. Carbonylation gives (benzene)chromium tricarbonyl.

The compound finds limited use in organic synthesis.[10]

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References
  1. King, R. B. Organometallic Syntheses. Volume 1 Transition-Metal Compounds; Academic Press: New York, 1965. ISBN 0-444-42607-8
  2. Elschenbroich, C.; Salzer, A. "Organometallics : A Concise Introduction" (2nd Ed) (1992) Wiley-VCH: Weinheim. ISBN 3-527-28165-7
  3. E. Schmidt, K. J. Klabunde, A. Ponce, A. Smetana, D. Heroux "Metal Vapor Synthesis of Transition Metal Compounds" Encyclopedia of Inorganic Chemistry 2006, John Wiley & Sons. doi:10.1002/0470862106.ia137
  4. Seyferth, D. (2002). "Bis(benzene)chromium. 1. Franz Hein at the University of Leipzig and Harold Zeiss and Minoru Tsutsui at Yale". Organometallics. 21 (8): 1520–1530. doi:10.1021/om0201056.
  5. Seyferth, D. (2002). "Bis(benzene)chromium. 2. Its Discovery by E. O. Fischer and W. Hafner and Subsequent Work by the Research Groups of E. O. Fischer, H. H. Zeiss, F. Hein, C. Elschenbroich, and Others". Organometallics. 21 (14): 2800–2820. doi:10.1021/om020362a.
  6. Werner, Helmut Landmarks in Organo-Transition Metal Chemistry: A Personal View; Springer Science & Business Media, 2008. ISBN 0-387-09848-8
  7. Fischer, E. O; Seus, D. (1956). "Zur Frage der Struktur der Chrom-phenyl-Verbindungen. Über Aromatenkomplexe von Metallen VI". Chemische Berichte. 89 (8): 1809–1815. doi:10.1002/cber.19560890803.
  8. Hein, F. (1956). "Zur Frage der Struktur der Chrom-phenyl-Verbindungen. Bemerkungen zur Abhandlung von E. O. Fischer und D. Seus". Chemische Berichte. 89 (8): 1816–1821. doi:10.1002/cber.19560890804.
  9. Fischer, E. O.; Stahl, H.-O. (1956). "Di-benzol-molybdän (O). Über Aromatenkomplexe von Metallen V". Chemische Berichte. 89 (8): 1805–1808. doi:10.1002/cber.19560890802.
  10. Herndon, J. W. "Dibenzenechromium" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.
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