Science of Synthesis

Kündig, E. P.; Pache, S. H., Science of Synthesis, (2003) 2, 155.

The direct reaction of a metal with a ligand is conceptually the most straightforward synthetic route to homoleptic organometallic complexes containing the metal in the zero oxidation state. Strong metal—metal bonds in the bulk metal, the presence of oxide films on the metal, and unfavorable thermodynamics often render this route impossible. A solution to this problem is offered by metal-vapor synthesis whereby the metal vapor, consisting of atoms of transition metals, is generated and condensed with an excess of ligand before agglomeration to bulk metal can take place.[‌2‌] For arene sandwich complexes of group 6 metals this is the most versatile method, providing the sandwich complexes in yields up to 65% based on the amount of the metal evaporated.[‌3‌,‌4‌] Chromium can be sublimed at a useful rate for laboratory scale synthesis at a temperature of ca. 1200–1400°C. The most accessible procedure consists of the use of a rotary apparatus in which the condenser and rotary seal parts of a commercial rotavapor are replaced by two electrodes and an improved rotary seal.[‌5‌,‌6‌] Chromium is evaporated from a resistively heated, insulated alumina crucible in the center of a 2-L flask containing the arene in an inert solvent (often methylcyclohexane) and externally cooled so as to maintain a vacuum of preferably <1 × 10–4 Torr in the flask. The metal vapor condenses and reacts with the arene in the film of the mixture carried over by rotation of the flask. Alternative apparatus has been used whereby the metal vapor and the arene are co-condensed on a liquid nitrogen cooled wall of a reactor under vacuum. This method has also been applied for the synthesis of molybdenum and tungsten sandwich complexes. As the temperatures required for the evaporation of these metals are much higher (2500°C for Mo, 3300°C for W), the synthesis apparatus for the use of these metals is more sophisticated and focused electron beam technology is used for the metal evaporation.[‌7‌] Mixed sandwich compounds can also be obtained by the co-condensation method. A selection of complexes ‌1‌–‌4‌ prepared by metal-vapor synthesis is shown in ‌Scheme 1‌.

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References

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• 8.Meeeee-Meee, (8888) M 88-8, 888.