Metal arene complexes are organometallic compounds of the formula xMLy. Common classes of are of the type ML3 and 2M. These compounds are reagents in inorganic and organic synthesis. The principles that describe arene complexes extend to related organic ligands such as many heterocycles and polycyclic aromatic compounds.
Synthesis
Fischer–Hafner synthesis
Also known as reductiveFriedel–Crafts reaction, the Fischer–Hafner synthesis entails treatment of metal chlorides with arenes in the presence of aluminium trichloride and aluminium metal. The method was demonstrated in the 1950s with the synthesis of bischromium by Walter Hafner and his advisor E. O. Fischer. The method has been extended to other metals, e.g. 2+. In this reaction, the AlCl3 serves to remove chloride from the metal precursor, and the Al metal functions as the reductant. The Fischer-Hafner synthesis is limited to arenes lacking sensitivefunctional groups.
Direct synthesis
By metal vapor synthesis, metal atoms co-condensed with arenes react to give complexes of the type M2. Cr2 can be produced by this method. Cr6 reacts directly with benzene and other arenes to give the piano stool complexes Cr3. The carbonyls of Mo and W behave comparably. The method works particularly well with electron-rich arenes. The reaction has been extended to the synthesis of +:
From hexadienes
Few Ru and Os complexes react directly with arenes. Instead, arene complexes of these metals are typically prepared by treatment of M precursors with cyclohexadienes. For example, heating alcohol solutions of 1,3- or 1,4-cyclohexadiene and ruthenium trichloride gives ruthenium dichloride dimer. The conversion entails dehydrogenation of an intermediate diene complex.
In most of its complexes, arenes bind in an η6 mode, with six nearly equidistant M-C bonds. The C-C-C angles are unperturbed vs the parent arene, but the C-C bonds are elongated by 0.2 Å. In some complexes, the arene binds through four carbons, in which case the arene is no longer planar. A well studied example is 0, formed by the reduction of 2+. In the fullerene complex Ru39, the fullerene binds to the triangular face of the cluster.
Reactivity
When bound in the η6 manner, arenes often function as unreactive spectator ligands, as illustrated by several homogeneous catalysts used for transfer hydrogenation, such as Ru. In cationic arene complexes or those supported by several CO ligands, the arene is susceptible to attack by nucleophiles to give cyclohexadienyl derivatives. Particularly from the perspective of organic synthesis, the decomplexation of arenes is of interest. Decomplexation can often be induced by treatment with excess of ligand.
