Bis(benzene)chromium

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Bis(benzene)chromium
Bis(benzene)chromium Bis(benzene)chromium
Bis(benzene)chromium (sublimated, under nitrogen)
Identifiers
CAS number 1271-54-1
RTECS number GB5850000
Properties
Molecular formula C12H12Cr
Molar mass 208.22 g/mol
Appearance brown-black crystals
Melting point 284-285 °C
Boiling point sublimes:160 °C
in vacuo.
Solubility in water insoluble
Solubility in other solvents slightly: benzene, THF
Structure
Coordination
geometry
pseudooctahedral
Dipole moment 0 D
Hazards
R-phrases 11
S-phrases 16-33
Main hazards flammable
Flash point 82 °C; 180 °F; 355 K
Related compounds
Related compounds Ferrocene
chromocene
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

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.

Preparation[edit]

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

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.[3] Hein's reaction affords cationic sandwich complexes containing bi- and terphenyl, which baffled chemists until the breakthrough by Fischer and Hafner,[4] and thus Hein had unknowingly discovered sandwich complexes a half century ahead of the work on ferrocene. Fischer and Seus soon prepared Hein's [Cr(C6H5-C6H5)2]+.[5][6] Illustrating the rapid pace of this research, the same issue of Chem. Ber. also describes the Mo(0) complex.[7]

Reactions[edit]

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.[8]

References[edit]

  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. ^ 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. 
  4. ^ 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. 
  5. ^ 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. 
  6. ^ 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. 
  7. ^ 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. 
  8. ^ Herndon, J. W. "Dibenzenechromium" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.