Supercritical fluids organometallic reactions

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... 

Leitner W (1999) In Knochel P (ed) Reactions in Supercritical Carbon Dioxide (scC02) in Modern Solvent Systems. Top Curr Chem 206 107 Morita DK, David SK (1998) Chem Commun, p 1397 Wegner A, Leitner W (1999) Chem Commun, p 1583 Sellin M, Cole-Hamilton DJ (2000) J Chem Soc, Dalton Trans, p 1681 Solinas M, Pfaltz A, Leitner W (2004) J Am Chem Soc 126 16124 Leitner W, Scurto AM (1998) Imobilization of Organometallic Catalysts using Supercritical Fluids. In Cornils B, Herrmann WA (eds) Aqueous Organometallic Catalysis. WUey, Weinheim, p 664... 

Supercritical fluids have already provided routes to a considerable variety of new and unusual organometallic species. In each case, the reactions have been monitored and the products identified solely by non-destructive spectroscopic techniques. Although our reactions have only involved milligram quantities of organometallic species, supercritical fluids give us the freedom to manipulate such quantities with unusual delicacy and precision. 

In recent years there has been much interest in the use of supercritical fluids (SCFs) as replacements for conventional liquid solvents, particularly in separation science, but also as reaction media. In addition to their environmental benefits, SCFs have further advantages over conventional liquid solvents, and these are briefly outlined in Section 2. The remainder of the chapter describes the use of SCFs as a medium for NMR spectroscopic studies. First we look briefly at motives for such NMR studies and the techniques employed. We then examine in more detail chemical shifts and nuclear spin relaxation in SCFs. The lower relaxation rates associated with SCFs and consequent sharper lines obtained for quadrupolar nuclei make SCFs excellent solvents. Section 8 describes some NMR studies of organometallic reactions in SCFs. Here the miscibility of supercritical solvents with gaseous reagents proves to be a tremendously useful feature in, for example, homogeneous catalysis. Finally we comment on future possibilities for NMR studies in SCFs. 

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]. 

Reactions that otherwise would be carried out in more than one phase (heterogeneous reactions) can be transformed to homogeneous ones, with the aid of supercritical fluids, where interphase transport limitations are eliminated. This is realized due to enhanced solubilities of the supercritical fluids. Typical examples are reactions in water (supercritical water can solubilize organic compounds), homogeneous catalytic reactions, reactions of organometallic compounds. Homogenizing one compound more than the other may also affect relative rates in complex reactions and enhance the selectivity. 

Electrochemical methods have also been adopted for application of high pressure [41-43] (see Chapter 5). Correlations emerging from these investigations have valuable application in the interpretation of partial molar volume changes associated with electron transfer reactions (see Sect. 1.3.4). A potential future interest is in reactions carried out at elevated pressures in a suprercritical fluid medium in view of this a special optical cell has been developed for studying organometallic reactions initiated by flash photolysis in supercritical fluids [20] (see Chapters 12 to 14). 

As far as catalysis is concerned it is already benefiting from new developments such as supercritical fluids, ionic liquids, sol-gel technology, solid phase reactions, parallel and combinatorial chemistry and some of these can be coupled to microwave irradiation. Already a number of very efficient organometallic (mainly iridimn based) catalysts have been produced although we are still some way from the acid, base situation where tritia-tion (or detritiation) rates can be estimated from a knowledge of acid-base strengths. 

A third approach (Figure 6.14.9c) uses organometaDic catalysts in sohd modifications, for example, covalently bonded (see above) or immobilized in supported liquid form (see below). Here the reaction remains a sohd/supercritical fluid biphasic system during reaction and again the SCCO2 is used as the mobile phase to contact the reactant intimately with the immobilized organometallic complex. 

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