The Sonochemistry of Some Metal Carbonyl Complexes (Iron, Chromium)
Ultrasonic irradiation of solutions produces accoustic cavitation; the formation, growth and violent collapse of gas bubbles in solution. During collapse of the bubbles, local temperatures of approximately 3300 K and pressures of approximately 300 atm may be produced. These high temperatures and pressures may give rise to high energy species in solution. Sonications were performed using a Heat-Systems Ultrasonics model W375 Sonicator cell disruptor with a sample cell equipped for cryogenic and inert atmosphere work. Sonication of the stable free radical diphenylpicryhydrazyl in a variety of solvents resulted in bleaching of its intense color, and demonstrated the ability of a wide range of organic solvents to support sonochemical reactions. Sonication of iron pentacarbonyl, neat or in hydrocarbon solvents, produces triiron dodecacarbonyl and finely divided iron powder. The ratio of these products may be varied by changing the vapor pressure of the solvent. The mechanism for formation of triiron dodecacarbonyl is believed to be initial formation of an iron tricarbonyl species which then reacts sequentially with two iron pentacarbonyls in solution to produce the triiron compound. Sonication of iron pentacarbonyl with phosphine and phosphite ligands yields a mixture of mono- and disubstituted products, which are formed in a constant ratio with time. Sonication of iron tetracarbonyl triphenylphosphine in excess ligand does not yield further substitution products. Sonication of chromium hexacarbonyl with phosphines gives similar results. Sonication of dimanganese decacarbonyl or dirhenium decacarbonyl in halogenated solvents gives the mononuclear metal pentacarbonyl halide complex. Only dimanganese decacarbonyl undergoes sonochemical substitution with phosphines. Sonication of one atmosphere of carbon monoxide in organic solvents leads to slow production of methane and methanol. The rates are not increased by addition of metal complexes or promoters. Sonication of ruthenium and osmium dodecacarbonyls under hydrogen and carbon monoxide produces ruthenium and osmium pentacarbonyls. Sonication of dicobalt octacarbonyl gives the triply bridged methlidyne tricobalt nonacarbonyl and tetracobalt dodecacarbonyl as soluble products.
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