Solubilized metal-ligand complexes

Fig. 9 Examples of C02-phUic ligands for solubilizing metal complexes and catalysts a refer to [81] b refer to Inorg Chem 38 5277 c refer to [83]...

In the aggregates, not all the extractant molecules are coordinated with the ion. According to coordination chemistry, this is equivalent to considering that some extractants are present as outer-sphere ligands in the complex. This can account for some discrepancies between the slope analysis and the stoichiometry of the complexes found by other methods. Extractants that are not directly bound to the metal can be considered to be involved in a process of solubilization of the complexes formed at the water/oil interface. 

This report gives results of uilibrium studies of the solubilization and metal-ion complexation behavior of three amphiphilic triamine ligands in micellar solutions. The separation selectivity has been evaluated using results of studies of solutions... 

The utility of micelles in spectroscopic measurements is derived from several possible effects. The well-documented effects of micellar systems on the acid-base chemistry of even slightly associated molecules can enhance (or degrade) the sensitivities of spectroscopic analysis [1,13-16]. In the field of metal ion complexation, much work has been carried out that suggests that the surfactant (within the micelle) takes part in the formation of a ternary complex with concomitant shifts in the wavelength of absorption [11,13,17]. Finally, as always, the ability of the micellar system to solubilize slightly insoluble or even very insoluble complexes and/or ligands has been used to enhance the sensitivities of spectroscopic analysis [12,13,18,19]. 

As mentioned in the previous section, there are good reasons to search for new reaction conditions for Heck and related reactions, which permit catalyst recovery, the use of less toxic solvents, and simpler product recovery. The use of liquid or supercritical (SC) CO2 addresses all of these issues [171]. Until recently, however, the use of supercritical COj had been limited to organometallic Pd complexes functionalized with perfluorinated ligands [172-174], due to the limited solubility of metal colloids in CO2, and often required the use of water as a co-solvent [175]. The work described here shows that dendrimers can be used to solubilize Pd nanoclusters in liquid and SC CO2. This new finding opens the door to the combined benefits of a catalyst that promotes Heck couplings, but without the need for toxic ligands or solvents. 

Rubredoxin is an electron-transfer protein with an Fe(IlI)/Fe(lI) redox couple at -0.31 V (SCE) in water (20). Our peptide model, [Fe( Cys-Pro-Leu-Cys-OMe)2] (Z = benzyloxycarbonyl) (21) exhibits its Fe(lll)/Fe(ll) redox couple at -0.50 V (SCE) in Mc2SO (9). This is similar to the value observed for the native protein when the difference of the solvent is taken into account. When the model complex is solubilized in water by formation of micelles with addition of the non-ionic detergent, Triton X-KX), we also observed a quasi-reversible redox couple at -0.37 V (SCE) (5). The Fe(lll) complexes of Cys-X-Y-Cys peptides also exhibit a characteristic MCD band at 350 nm due to ligand-to-metal charge transfer which has also been found in oxidized rubredoxin (4). 

The linking of a metal to an antibody could, in principle, be accomplished by forming the metal chelate either prior to or after attachment to protein. Success to date has been achieved only by formation of the protein-ligand conjugate before metal chelation. The complexation reaction has several general features. First, reactions between the metallic radionuclides and antibodies are almost always performed with sub-stoichiometric quantities of chelate and metal ion. It is therefore of the utmost importance that no carrier added metals obtained from commercial sources be exceedingly pure or else be purified prior to use. Reactions of "carrier added" metal solutions are not likely to be of use because of the ease with which available chelate sites become saturated. Because the formation of chelate complexes is usually a bimolecular reaction, the complexation will proceed optimally when more chelation sites are available. Similarly, the more isotope in solution, the faster the reaction. Employment of a carrier chelate to insure solubilization of the radiometal is of value to maximize available isotope and the acetate ion has proven useful. 

The general principle of two-phase catalysis in polar solvents, for example, in water, is shown in the simplified diagram of Fig. 1. The metal complex catalyst, which can be solubilized by hydrophilic ligands, converts the reactants A + B into the product C. The product is more soluble in the second than in the first phase and can be separated from the catalyst medium by simple phase separation. Excellent mixing and contacting of the two phases are necessary for efficient catalytic reaction, and thus the reactor is normally well stirred. 

Taken together, the results of these studies clearly demonstrate that efficient extraction of metal ions into SC-C02 (in particular, An and FP ions) need not require functionalization (in particular, fluorination) of the ligand employed. Rather, certain inexpensive, off-the-shelf extractants unexpectedly provide adequate metal complex solubilization and satisfactory extraction efficiency. This strongly suggests that large-scale SFC-based schemes for An and FP separations might indeed be practical. 

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