Metal carbonyls photochemical reactions

With mononuclear transition metal carbonyls, photochemically induced substitutions of one or two carbonyl ligands by a diene can be expected to be the only reactions. Dinuclear transition metal carbonyls or carbonyl... 

In 1981, the first report on the sonochemistry of discrete organometallic complexes demonstrated the effect of ultrasound on iron carbonyls in alkane solutions (174). The transition metal carbonyls were chosen for these initial studies because their thermal and photochemical reactivities have been well characterized. The comparison among the thermal, photochemical, and sonochemical reactions of Fe(CO)5 provides an excellent example of the unique chemistry which homogeneous cavitation can... 

Over the last decade the spectroscopy, photochemistry and reactivity of metal carbonyls has been a subject of intense interest. As a result of this research it has been found that metal carbonyls undergo a wide range of facile photochemical reactions [1,2]. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

The dominant photochemical reaction of metal carbonyl compounds is loss of carbon monoxide, which is usually followed by substitution of another ligand to replace the expelled carbon monoxide. 

DR. ANTHONY POE (University of Toronto) As Dr. Geoffroy mentioned, there is a fair bit of work being done on dinuclear metal-metal bonded carbonyls but rather less on metal clusters [Geoffroy, G. L. Wrighton, M. S., "Organometallic Photochemistry," Academic Press New York, 1979]. We have been interested for some time in the thermal fragmentation of metal clusters, and have recently looked at some photochemical reactions as well. I would like to present some results here today which are very preliminary. 

A different application of eel is illustrated by the alternating current electrolysis of transition metal carbonyl complexes. Figure 6 shows the cycling of a complex A between oxidized and reduced forms. Use of alternating current electrolysis enables chemical use to be made of eel. For example, reaction (42) is known to be achievable by conventional photochemical means and can also be carried out by applying an alternating potential in an electrochemical cell.91... 

The range of alkenes that may be used as substrates in these reactions is vast Suitable catalysts may be chosen to permit use of ordinary alkenes, electron deficient alkenes such as a,(3-unsaturated carbonyl compounds, and very electron rich alkenes such as enol ethers. These reactions are generally stereospecific, and they often exhibit syn stereoselectivity, as was also mentioned for the photochemical reactions earlier. Several optically active catalysts and several types of chiral auxiliaries contained in either the al-kene substrates or the diazo compounds have been studied in asymmetric cyclopropanation reactions, but diazocarbonyl compounds, rather than simple diazoalkanes, have been used in most of these studies. When more than one possible site of cyclopropanation exists, reactions of less highly substituted alkenes are often seen, whereas the photochemical reactions often occur predominantly at more highly substituted double bonds. However, the regioselectivity of the metal-catalyzed reactions can be very dependent upon the particular catalyst chosen for the reaction. 

Photochemical Reactions of Metal Carbonyls with Dihydrogen... 

The second photochemical reaction which was studied was the reaction of CotCO NO with Lewis base ligands L (J 6 ). The observed solution phase photochemical reaction is carbonyl photosubstitution. This result initially did not appear to be related to the proposed excited state bending. Further reflection led to the idea that the bent molecule in the excited state is formally a 16 electron coordinatively unsaturated species which could readily undergo Lewis base ligand association. Thus, an associative mechanism would support the hypothesis. Detailed mechanistic studies were carried out. The quantum yield of the reaction is dependent on both the concentration of L and the type of L which was used, supporting an associative mechanism. Quantitative studies showed that plots of 1/ vs. 1/[L] Were linear supporting the mechanism where associative attack of L is followed by loss of either L or CO to produce the product. These studies support the hypothesis that the MNO bending causes a formal increase in the metal oxidation state. 

Photochemical activation of transition metal carbonyls has been used as a preparative tool for substitution of carbonyl ligands by donor molecules or unsaturated hydrocarbons for many years (7-6). The advantage of photochemical activation in comparison with thermal activation is the possibility of conducting reactions at fairly low temperatures. Hence even thermolabile products can be prepared and isolated by appropriate treatment of the reaction mixtures. However, due to the various activation modes of transition metal carbonyls by UV light, often more than one product is obtained, and chromatographic separation is necessary. Limitations are set primarily by the amount of substance which can be irradiated in solution at one time. 

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