Sonolysis of transition metal complexes

The more extensive studies of reactions based on a sonolytic step were effected in transition metal chemistry, with theoretical and applied values as well. In his pioneering work, Suslick and his group studied essentially metal carbonyl complexes.Further developments in this domain were made in two directions, the complete decomposition of the complexes exploited for the production of amorphous metal powders (pp. 180 and 239), and the selective cleavage of ligands to generate reactive intermediates. 

An example of sonochemical switching was found with the sonolysis of iron pentacarbonyl (Fig. 23).Pyrolysis and photolysis form pyrophoric iron and diiron nonacarbonyl respectively, in contrast to sonolysis, which gives triiron dodecacarbonyl, a compound not easily accessible by other methods. A pyrolytic 

Using this model, the authors determined kinetically the temperature in the bubbles, 5200 K, and 1900 K in the limit layer. 3 the presence of ligands (phosphines, olefins), new complexes are formed. An olefin, e.g., 1-pentene, sonicated with catalytic amounts of iron pentacarbonyl is isomerized to a mixture of ( )- and (Z)-2-pentene. A similar reaction was observed with manganese, chromium, tungsten, and molybdenum complexes.  

In halocarbon solution, dinuclear complexes of manganese and rhenium are transformed to mixed halogenated complexes (Fig. 24). With respect to silent conditions, the rate enhancement is estimated at 10 . The vapor pressure of the starting material is very low as compared to that of the solvent, and its concentration in the bubble should be low. According to the authors, the metal-metal bond cleavage is unlikely despite its weakness, and the first reaction step is probably the solvent sonolysis. 

In support of this view, no halogenation reaction occurs with stoichiometric amounts of the halide in alkane solution, due to the trapping of the halogen atoms by the solvent. 

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Applications of the selective sonolysis of transition metal complexes are not numerous despite the obvious synthetic potential (Fig. 25). The palladium catalyzed addition-oxidation of halides to olefins (the Heck reaction) in its intramolecular version is improved by sonication. The sonochemical reaction can be conducted at lower temperatures than the silent one, and the usual deiodinated by-products are formed in smaller amounts (see p. 343). ... 

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