Bis(benzene)chromium is the
organometallic compound with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...
Cr(
η6-C
6H
6)
2. It is sometimes called dibenzenechromium. The compound played an important role in the development of
sandwich compounds in
organometallic chemistry
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
and is the prototypical
complex containing two
arene ligands.
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. The product of the reaction was yellow
r(C6H6)2sup>+, which was then reduced to the neutral complex. Idealized equations for the synthesis are:
: CrCl
3 + 2/3Al + 1/3AlCl
3 + 2C
6H
6 →
r(C6H6)2lCl
4
:
r(C6H6)2lCl
4 + 1/2Na
2S
2O
4 →
r(C6H6)2 + NaAlCl
4 + SO
2
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
r(C5H5P)2can be prepared.
Structure elucidation
Compounds closely related to
r(C6H6)2sup>+ had been prepared many years before Fischer's work by
Franz Hein
Franz Hein (30 June 1892 – 26 February 1976) was a German scientist and artist.
History
Franz Hein was born in Grötzingen (Baden), Germany. His high school years were spent in Leipzig, as well as, his college years at the University of Leipzig. ...
by the reaction of
phenylmagnesium bromide
Phenylmagnesium bromide, with the simplified formula , is a magnesium-containing organometallic compound. It is commercially available as a solution in diethyl ether or tetrahydrofuran (THF). Phenylmagnesium bromide is a Grignard reagent. It is o ...
and CrCl
3. Hein's reaction affords cationic
sandwich complexes containing
bi- and
terphenyl, which baffled chemists until the breakthrough by Fischer and Hafner. (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.) Fischer and Seus soon prepared Hein's
r(C6H5-C6H5)2sup>+ by an unambiguous route, thus confirming that Hein had unknowingly discovered sandwich complexes, a half-century ahead of the work on
ferrocene
Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, a ...
. Illustrating the rapid pace of this research, the same issue of
''Chem. Ber.'' also describes the Mo(0) complex.
Reactions
The compound reacts with carboxylic acids to give chromium(II) carboxylates, such as
chromium(II) acetate, which have interesting structures. Oxidation affords
r(C6H6)2sup>+. Carbonylation gives
(benzene)chromium tricarbonyl
(Benzene)chromium tricarbonyl is an organometallic compound with the formula . This yellow crystalline solid compound is soluble in common nonpolar organic solvents. The molecule adopts a geometry known as “ piano stool” because of the plana ...
.
The compound finds limited use in
organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
.
[Herndon, J. W. "Dibenzenechromium" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. .]
References
{{DEFAULTSORT:Bis(Benzene)Chromium
Organochromium compounds
Sandwich compounds