Bis(benzene)chromium

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

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Preparation

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. Slightly idealized equations for the synthesis are:

CrCl₃ + 2/3 Al + 1/3 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

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]⁺.

The compound finds limited use in organic synthesis.^[8]

The substance is air sensitive and is handled using air-free techniques.

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. ^ Seyferth, D. (2002). "Bis(benzene)chromium. 1. Franz Hein at the University of Leipzig and Harold Zeiss and Minoru Tsutsui at Yale". Organometallics 21: 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: 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: 1809-1815.
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: 1816-1821.
7. ^ Fischer, E. O.; Stahl, H.-O. (1956,). "Di-benzol-molybdän (O). Über Aromatenkomplexe von Metallen V". Chemische Berichte 89: 1805-1808.
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.