Supercritical metal complexes, solubility

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

Bhanage BM, Shirai M, Arai M et al (1999) Multiphase catalysis using water-soluble metal complexes in supercritical carbon dioxide. Chem Commun 14 1277-1278... 

The solubilities of S-diketonate complexes of Cu, Zn, Ni and Co in supercritical CO2 were determined. A linear correlation was found between the stability of the metal complex and extractant dissociation. This correlation helps finding extractants optimized for both extraction of metals and the recovery of valuble chemicals. Supercritical CO2 best extracted Cu S-diketonates and the free metal could be best recovered in nitric acid. The other metal complexes showed lower extraction levels in the decreasing order of Ni, Co and Zn. The metal removal efficiency from the stripping solution into CO2 varied between 5 and 90% and decreased in the order Cu, Co, Ni and Zn. The overall... 

The inventor proposes an alternate process in claim 1 in which the oxides are dissolved into an unspecified supercritical fluid at unspecified conditions but at 1 to 25 wt% below the solubility of the least soluble cranponent, rapidly expanding the fluid to precipitate a uniform and stoichiometrically accurate mixture, and thermally treating the powder. We wonder how this patent was allowed since the patent by Sievers and Hansen (U.S. 4,970,093) reviewed in this edition of the book, states explicitly the process and conditions neededlo dissolve metal complexes for superconductor fabrication. Sievers and Hansen are not quoted by the inventor nor is their quantitative data quoted. We also wonder if the inventor has ever tried to dissolve some of the materials that are listed in the claims section since the instant patent teaches to operate at 1 to 25 wt% less than the saturation amount of the least soluble component. Can we expect compounds such as yttrium to have solubilities at weight percent levels We do not believe so. 

Compressed liquid or supercritical carbon dioxide has been recognized as a useful alternative reaction medium for radical and ionic polymerization reactions (see Chapter 4.5). Many of the benefits associated with the use of SCCO2 in these processes apply equally well to polymerizations relying on a metal complex as the chain-carrying species. However, the solubility of the metal catalyst and hence the controlled initiation of chain growth add to the complexity of the systems under study. Furthermore, many of the environmental benefits would be diminished if subsequent conventional purification steps were needed to remove the metal from the polymer. Nevertheless, the interest in metal-catalyzed polymerizations is increasing, and some promising systems have been described. 

In an alternative approach, Morley et al. [50,51] describe a similar procedure, which is effective for the preparation of metal-SiOa nanocomposites based on the use of supercritical carbon dioxide for the impregnatimi of a preformed silica alcogel with a suitable metal complex and for the removal of the free ligand after the reduction step. Pd- and Ag-Si02 aerogels, which can be used as active catalysts, were effectively obtained by this procedure where supercritical carbon dioxide was used as a recyclable solvent, which does not alter the porous structure of the matrix. However, in addition to the many steps required, a major disadvantage of this procedure is that it requires solubility of metal complexes in sc-C02-... 

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