Aromatic metal complexes

Aromatic metal complexes are organometallic compounds in which a metal atom is bound to an aromatic organic molecule. Well-known representatives of the aromatic metal complexes are the metallocene ferrocene , dibenzene chromium or uranocene .

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  Ferrocene: staggered conformation (left) and ecliptic conformation (right).

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  Structural formula of bis (benzene) chromium

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  Structure of uranocene

history

The first aromatic metal complex, ferrocene, was synthesized by chance independently by two different research teams in 1951. Tom J. Kealy and Peter L. Pauson obtained ferrocene in an attempt to prepare fulvalene by reacting ferric chloride with cyclopentadienyl magnesium bromide according to the following equation.

In the same year Samuel A. Miller, John A. Tebboth and John F. Tremaine had prepared and described the same substance by reacting cyclopentadiene vapor with freshly reduced iron at 300 ° C.

The first complex in which a neutral aromatic ligand is bound to a metal atom was dibenzene chromium, which was first produced in 1955 by Ernst Otto Fischer and Walter Hafner .

${\displaystyle \mathrm {CrCl_{3}+{\frac {2}{3}}\ Al+\ AlCl_{3}+2\ C_{6}H_{6}\longrightarrow [Cr(C_{6}H_{6})_{2}]AlCl_{4}+{\frac {2}{3}}\ AlCl_{3}} }$

${\displaystyle \mathrm {[Cr(C_{6}H_{6})_{2}]AlCl_{4}+{\frac {1}{2}}\ Na_{2}S_{2}O_{4}\longrightarrow [Cr(C_{6}H_{6})_{2}]+NaAlCl_{4}+SO_{2}} }$

Both ferrocene and dibenzene chromium belong to the so-called sandwich complexes , in which a metal atom, like in a sandwich , lies between two aromatic, cyclic molecules. Complexes in which the metal atom is only linked to one aromatic ligand are referred to by analogy as half-sandwich complexes . After the discovery of ferrocene, a number of other aromatic metal complexes were synthesized and described within a short period of time. The discovery of ferrocene is considered one of the milestones in organometallic chemistry and was the starting point for a rapid development of this new branch of chemistry. Over 80% of the organometallic complexes today are cyclopentadienyl complexes.

Ligands

Aromatic ligands in aromatic metal complexes are usually cyclic hydrocarbons, with the general formula C _x H _x (x ≥ 3) and 4n + 2 conjugated π-electrons (n ​​= 0, 1, 2). If one or more carbon atoms are substituted by another element (e.g. nitrogen, boron, oxygen), the homologous heterocycles are obtained.

aromatic ligands in aromatic complexes

Individual evidence

1. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 , pp. 1696-1715.
2. ^ TJ Kealy, PL Pauson: A New Type of Organo-Iron Compound . In: Nature . Vol. 168, No. 4285 , 1951, pp. 1039-1040 , doi : 10.1038 / 1681039b0 (English).
3. ↑ Samuel A. Miller, John A. Tebboth, John F. Tremaine: Dicyclopentadienyliron . In: J. Chem. Soc. 1952, p. 632-635 , doi : 10.1039 / JR9520000632 .
4. ^ EO Fischer, W. Hafner: Di-benzol-chrom. In: Journal of Nature Research B . 10, 1955, p. 665 ( PDF , free full text).
5. ↑ Anja Jaenschke: base adducts of Magnesocens . Hamburg 2006, urn : nbn: de: gbv: 18-30786 (207 pp., Dissertation).