Sonochemistry and Sonocatalysis of Metal Carbonyls

Ultrasonic irradiation of liquids produces acoustic cavitation: the rapid formation, growth, and implosive collapse of vapor filled vacuoles. This generates short-lived “hot spots” with peak temperatures ~3000 K and nanosecond lifetimes. We have studied the effects of high intensity ultrasound on a variety of metal carbonyls and have observed the general phenomenon of sonochemical ligand dissociation, which often produces multiple CO substitution. Fe(CO)₅, for example, upon sonolysis, yields Fe₃(CO)₁₂ in the absence of additional ligands and Fe(CO)₃L₂ and Fe(CO)₄L (L = phosphine or phosphite) in their presence. Similar substitution patterns are observed for Fe₃(CO)₁₂, Mn₂(CO)₁₀, Cr(CO)₆, Mo(CO)₆, and W(CO)₆. In all cases examined the rates of sonochemical ligand substitution are first order in metal carbonyl concentration and independent of L concentration. In addition, In K_obsd correlates well with solvent system vapor pressure. These results are consistent with a dissociative mechanism in which coordinatively unsaturated species are produced by the cavitation process. Further use of these transient intermediates is made as alkene isomerization catalysts. Sonocatalysis by a wide range of metal carbonyls shows many similarities to photocatalysis, but different relative efficiencies and selectivities have also been observed.