Metal complexation/supercritical

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. 

Jessop PG, Leitner W (1999) Metal-Complex Catalyzed Reactions. In Jessop PG, Leitner W (eds) Chemical Synthesis Using Supercritical Fluids. Wiley, Weinheim,... 

Langanke J, Leitner W (2008) Regulated Systems for Catalyst Immobilisation Based on Supercritical Carbon Dioxide. 23 91-108 Larock R (2005) Palladium-Catalyzed Annulation of Alkynes. 14 147-182 Larrow JF, Jacobsen EN (2004) Asymmetric Processes Catalyzed by Chiral (Salen)Metal Complexes 6 123-152... 

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... 

Krocher, O., Koppel, R.A., Froba, M. and Baiker, A. (1998) Silica hybrid gel catalysts containing group(VIII) transition metal complexes preparation, structural, and catalytic properties in the synthesis of N, N-dimethylformamide and methyl formate from supercritical carbon dioxide. Journal of Catalysis, 178, 284-298. 

Flow reactors offer considerable advantages over sealed autoclaves for supercritical reactions. Not only do flow-reactors require a much lower volume than a batch reactor for a given throughput of material (with obvious safety advantages) but also it is much easier to optimise reaction conditions in a flow reactor. We have already reported [4,5] the use of a miniature flow-reactor for the photochemical preparation of unstable metal complexes. We are now extending these techniques to the study of thermal and catalytic reactions. As an initial stage we... 

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... 

Although modifiers are added to supercritical fluids to increase their polarity, they can also impart decreased polarity, aromaticity, chirality and the ability to further complex organometallic compounds. Just as carbon dioxide is the most popular substance for use as a supercritical fluid, it is also that to which modifiers are most frequently added. This is so because modifiers are seen as the means for enabling the use of CO, in situations where it may not be the best solvent. For example, methanol is added to supercritical CO, to increase its polarity, aliphatic hydrocarbons to decrease it, toluene to impart aromaticity, [/ ]-2-butanol to add chirality and tributyl phosphate to enhance the solvation of metal complexes. The amount of modifier to be added depends on the properties of the extractant and those of the analyte and matrix usually, it ranges from a few... 

It is known that formic acid is synthesized from H2/CO2 as ester in alcohol solvent using metal complex catalysts such as HM(CO)5 (M W,Cr,Ru) in batch reactor system.[61] However, specific activity (TOF) of these system are relatively low. Recently, Noyori et al. found a significant increase of formic acid in a supercritical mixture of H2/CO2 with N(C2H5)3 using RuH2 P(CH3)3)4 complex at the condition of 20.5 Mpa, 5013 and H2/C02=l/1.4.[62] TOF increased one order of magnitude over that of conventional process because of high miscibdity of H2 with supercritical CO2. It is also noted that methyl formate produced from H2/CO2 is easdy converted to acetic acid by isomerization reaction. 

It is further characterized by rapid diffusion, low viscosity, and practically no surface tension. On the other hand, the supercritical phase still acts as a solvent for the solid metal complex, and both its heat capacity and its density can reach typical liquid-like values. 

Carbon dioxide, water, ethane, ethylene, propane, ammonia, xenon, nitrous oxide, and fluoroform have been considered useful solvents for SEE. Carbon dioxide has so far been the most widely used as a supercritical solvent because of its convenient critical temperature, 304°K, low cost, chemical stability, nonflammability, and nontoxicity. Its polar character as a solvent is intermediate between a truly nonpolar solvent such as hexane and a weakly polar solvent. Moreover, COj also has a large molecular quadrupole. Therefore, it has some limited affinity with polar solutes. To improve its affinity, additional species are often introduced into the solvent as modifiers. For instance, methanol increases C02 s polarity, aliphatic hydrocarbons decrease it, toluene imparts aromaticity, R-2-butanol adds chirality, and tributyl phosphate enhances the solvation of metal complexes. 

Canelas, D.A. Desimone, J.M. Polymerizations in liquid and supercritical carbon dioxide. Adv. Polym. Sci. 1997, 133 (Metal Complex Catalysts, Supercritical Fluid Polymerization, Supramolecu-lar Architecture), 103-140. 

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