Flash photolysis of metal carbonyls

It is not our intention in this section to provide a comprehensive review of flash photolysis of organometallic species rather, we summarize some key experiments which establish the timescales of different types of reactions. Understandably, much more work has been done on the flash photolysis of metal carbonyls in solution than in the gas phase, and so we begin with solution experiments. 

Flash photolysis of metal carbonyls in solution was pioneered by Kelly and Koerner von Gustorf 30). The most complete of these studies has been carried out on the photolysis of Cr(CO)6 30). The historical development of these experiments, which forms an intriguing story in its own right, has been recently retold (2). The salient features are as follows ... 

The use of supercritical fluids as reaction media for organometallic species is also investigated. Reactions include photochemical replacement of carbon monoxide with N2 and H2 in metal carbonyls, where the reaction medium is supercritical xenon. Also, photochemical activation of C-H bonds by organometallic complexes in supercritical carbon dioxide is investigated. More recent studies on photochemical reactions also include laser flash photolysis of metal carbonyls in supercritical carbon dioxide and ethane and laser flash photolysis of hydrogen abstraction reaction of triplet benzophenone in supercritical ethane and... 

Flash photolysis has now been applied to a wide range of metal carbonyl species in solution, including Mn2(CO)10 (37), [CpFe(CO)2]2 (38), and [CpMo(CO)3]2 (39). In almost every case, interesting data have emerged, but, as with Cr(CO)5, the structural information is usually minimal. Thus, the radical Mn(CO)5 has been generated in solution by flash photolysis (37), the rate constant for its bimolecular recombination has been measured, but the experiments did not show whether it had Z>3h or Qv symmetry. Some experiments have been unsuccessful. Although the fragment Fe(CO)4 is well known in matrices (15), it has never been... 

Relatively little work has been done on the flash photolysis of gas phase metal carbonyls, partly because of the low volatility of many of the compounds. Early work by Callear (41,42) provided some evidence for Ni(CO)3 generated from Ni(CO)4 in the gas phase (41) and Fe atoms produced from Fe(CO)5 (42). This latter process has even been used as the basis of an Fe atom laser (43). More recently Breckenridge and Sinai (44) studied the flash photolysis of Cr(CO)6. Their results, interpreted largely on the basis of data from matrix isolation experiments, were in broad agreement with Kelly and Bonneau s solution work (JJ). In particular, they found no evidence for loss of more than one CO group [Eqs. (4) and (5)]. 

Reviews have appeared of the photophysics of molybdenum complexes, primary and secondary processes in organometallic chemistry, flash photolysis of Pe(CO)5 and Cr(CO)g, dinuclear manganese carbonyl compounds, the photochemistry of metal complexes isolated in low temperature matrices, cluster complexes, diene complexes, photoproduction of coordinativeiy unsaturated species containing rhodium or iridium, and redox chemiluminescence of organometallic compounds.Synthetic and metal organic photochemistry in industry has also been reviewed. 

Time-resolved Infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR spectroscopy (ns), has been outstandingly successful in identifying reactive intermediates [5] and excited states [6] of metal carbonyl complexes in solution at room temperature. We have used infrared spectroscopy to probe the mechanism of photo-17] and electrochemical [8] catalytic reduction of COj. We have used TRIR to study organometallic reactions in supercritical fluids on a nanosecond time-scale [9-10]. 

Future progress may, however, be accelerated by recent and forthcoming developments. Recent flash photolysis studies of metal-carbonyls and metal-metal bonded systems should provide a foundation for more extensive development of this area. Applications of picosecond and nanosecond flash systems to a wide variety of problems is expected. Such techniques should be... 

There have also been a number of IR spectra of metal carbonyls using flash photolysis to investigate unsaturated intermediates. Three such papers are mentioned here. A paper by Church et al [20] looked at flash photolysis of Cr(CO)5 in a solution of cyclohexane, showing proof of a C v Cr(CO)5 photoproduct, another [21] looked at flash photolysis in cyclohexane solution saturated with H2, showing production of Cr(CO)5(H2), and a third looking at flash photolysis of Fe(CO)5 in a solution of benzene [22]. 

The formation of diphenylcarbene by laser flash photolysis of diphenyldiazomethane was studied in SC-CO2 and other supercritical media. [18] A laser-flash induced ring-closure reaction of a bipyridyl complex (Scheme 2) revealed solvation properties. [19] Laser-flash impact to metal carbonyl complexes activated hydrogen and simple alkanes like CH4, C2H4, C2H6, and further inorganic reactions in supercritical fluids have been reviewed. [20]... 

Intermediates (Alkane) M(CO)s are produced in predominant concentration in solution after flash photolysis of the metal carbonyls M(CO)6 (M = Cr,Mo, W). It has been proposed that the alkane (used as a solvent) is coordinated to the metal via a C-H Magostic interaction [21a,b] ... 

Photfrfysis of a-Diazo Carbonyl Compounds - Some recent advances in the matrix photochemistry of diazoketones, including some heterocyclic species, have been reviewed. Flash photolysis of 10-diazo-9(10//)-phenanthrenone (35) in aqueous solution led to the detection of two transient species on the pathway to the final product, fluorene-9-carboxylic acid. These were identified, from solvent isotope effects and the nature of the observed acid-base catalysis, as fluorenylideneketene (36, X = CO) and the enol of fluorene-9-carboxylic acid (36, X = C(0H)2), formed by hydration of the ketene. In related studies, fluorenylideneketene was found to react with amines to give ylides as intermediates on the route to the amide final products. The product distribution from the photochemical reactions of 2-diazo-3-oxo-5,10,15,20-tetraphenylchlorins with alcohols strongly depends on the central metal ion of the irradiated diazoketones. ... 

Q Ji, EM Eyring, R van Elchk, KP Johnston, SR Goates, ML Lee. Laser flash photolysis studies of metal carbonyls in supercritical CO2 and ethane. J Phys Chem 99 13461, 1995. 

Polynuclear Carbonyl Derivatives.-Variable temperature MAS n.m.r. methods have been used to probe the exchange processes occurring in solid homonuclear carbonyls containing 3 and 4 metal atoms.A quantitative investigation of the chemistry resulting from both continuous- and flash-photolysis of Ru3(C0)i2 has been carried out, and a new polymeric ruthenium carbonyl, Ru(C0)it n prepared from this precursor. The steric and electronic effects in associative substitution reactions of Iri (C0)i2 have also been examined. The combination of two OS3 units to yield an improved route to Ose raft-like clusters has been achieved, and a comprehensive analysis published on the different Mg cores of stacked Mg triangles in the [Mg C0)i8] "... 

Metal carbonyl derivatives with metal-oxygen bonds are not common. Flash photolysis of oxygen-iron pentacarbonyl mixtures produces a substance believed to be [Fc202(C0)] 226). Sulfur trioxide and iron pentacarbonyl afford an unisolable material thought to be the pyrophosphate Fe[0(S03)2](C0)3 141). However, the reaction between sulfur dioxide and enneacarbonyldiiron gives a substance [Fe2(C0)gS02], for which... 

Thus, overall, it is clear that flash photolysis with uv-visible detection is effective in establishing the broad outlines of the photochemistry of a particular metal carbonyl. Intermediates can be identified from their reaction kinetics, and sometimes, with the help of uv-vis data from matrix isolation experiments. Structural information from uv-vis flash photolysis is at best sketchy. Many questions remain unanswered. Time-resolved IR measurements can fill in some of these answers. 

Already a considerable number of transient organometallic species have been characterized by IR kinetic spectroscopy (see Table I). Like most other sporting techniques for structure determination, IR kinetic spectroscopy will not always provide a complete solution to every problem. What it can do is to provide more structural information, about metal carbonyl species at least, than conventional uv-visible flash photolysis. This structural information is obtained without loss of kinetic data, which can even be more precise than data from the corresponding uv-visible... 

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