Trifluoroacetylacetone, chelates

The number of reported applications to analytical determinations at the trace level appear to be few, probably the best known being the determination of beryllium in various samples. The method generally involves the formation of the volatile beryllium trifluoroacetylacetonate chelate, its solvent extraction into benzene with subsequent separation and analysis by gas chromatography..61... 

Other fluorinated derivatives of acetylacetone are trifluoroacetylacetone (CF3COCH2COCH3) and hexafluoroacetylacetone (CF3COCH2COCF3), which form stable volatile chelates with aluminium, beryllium, chromium(III) and a number of other metal ions. These reagents have consequently been used for the solvent extraction of such metal ions, with subsequent separation and analysis by gas chromatography [see Section 9.2(2)]. 

Figure 14.7. Chemical structures of some chelators and enhancers used in time-resolved fluoroimmunoas-says 1, isothiocyanatophenyl-EDTA 2, Ari-p-(isothiocyanatobenzyl)-diethylenetriaminetetraaceticacid 3, diethylenetriaminepentaacetic anhydride (DTPA) 4, Tb3+-DTPA-/t-aminosalicylic acid S, Eu3+-pyridine-bisfmethylaminodiacetic acid) 6, benzoyltrifluoroacetone (BTA, a /i-diketone) 7, trifluoroacetylacetone (a/jt-diketone) 8,4,7-bis(chlorosulfonylphenyl)-l,10-phenanthroline-2,9-dicarboxylic acid (BCPDA).

Metal complexes with fluoro-/3-diketones have been comprehensively reviewed.1585 The introduction of electron-withdrawing groups in the chelate ring increases the Lewis acidity strength of the ML2 complexes, and consequently the bis adducts of the fluoro-/3-diketonato complexes are more stable than the corresponding complexes with /3-diketones. As an example of a nickel complex with 1,1,1-trifluoroacetylacetone which does not have a counterpart in the nickel acetylacetonate complexes we can mention the hexanuclear complex Ni6Lio(OH)2(H20)2.1586... 

Cobalt(III) trifluoroacetylacetonate, Co(tfac)3, is a six-coordinate, octahedral metal chelate in which three planar, bidentate tfac ligands are attached to a central Co atom ... 

The metal chelates are usually prepared from acetylacetone or trifluoroacetylacetone and are dissolved in a small quantity of stationary phase for the injection and separation. The UV absorbance of the derivatives is measured at 310 nm in a monitor equipped with a microflow cell. The separation of six metal acetylacetonates is shown in Fig.4.33 (column, SO cm X 2.7 mm I.D. particle diameter, 5-10 /am flow-rate, 1.8 mm/sec). 

Aluminium was analysed in a mixture with gallium and indium [618], An aqueous solution of these ions was buffered with acetate at pH 4—7 and extracted into benzene in the presence of trifluoroacetylacetone as a chelate-forming reagent for 4 h. The analysis was performed on 5% of DC-550 on silanized glass beads. At the milligram level of aluminium, the extraction yields obtained were 71—100%. From an aqueous medium of the acetate buffer, aluminium, copper and iron can be extracted at pH 4.5—5.5 with the aid of 1 M trifluoroacetylacetone in chloroform [619]. At the 0.5 mM level, a yield of 99.7% of aluminium was obtained. 

Beryllium was analysed by GC as a volatile chelate the most frequently of all elements. A rapid micro-analytical procedure for the determination of beryllium in biological fluids was developed and published by Black and Sievers [627], who devoted their attention to urine, blood, liver homogenates and plant extracts. The sample in ajsealed glass ampoule was treated directly with trifluoroacetylacetone, by which means the losses of the sample or possible contamination that occur in conventional ashing procedures were eliminated. 

Foreman et al. [631] compared the direct method of the chelate formation with the preliminary ashing method for the analysis of beryllium in rat urine. A detailed study showed that both of the methods are satisfactory, whereas testing of column material and packings showed the best results for a PTFE column packed with SE-52. Down to 1 ng/ml of the element could be detected in urine with the use of an ECD and EDTA as a masking reagent and a 0.05 M benzene solution of trifluoroacetylacetone. 

Chromium was measured in steels with high and low contents of carbon by Ross and Sievers [633]. A small amount of the sample (2 —4 mg) was allowed to react directly with trifluoroacetylacetone in the presence of nitric acid. If an undissolved residue occurred, it was dissolved in 70% nitric acid, evaporated to dryness and the procedure was repeated. The resulting red solution was extracted with benzene and excess of the chelating agent was removed by extraction with dilute sodium hydroxide solution. A PTFE column packed with 15% of SE-52 on Anakrom ABS and an ECD were used. The results showed a relative error of 1.4-1.7%. 

Savory et al. [635] determined chromium in serum. The sample was wet ashed by treatment with sulphuric, nitric and perchloric acids. Finally, the pH was adjusted to 6.0 with a buffer and chelation was effected with a dilute solution of trifluoroacetylacetone at 70°C. The analysis was carried out on 5% of QF-1 on Chromosorb W with the use of a 63Ni ECD. Booth and Darby [636] applied a very similar procedure to soft tissues and serum. They performed the chelation reaction also at 70°C for 1 h. Recoveries of chromium in serum and liver homogenates were 94 and 88—104%, respectively. Ross and Shafik... 

Beryllium chemistry includes its S-diketonate complexes formed from dimedone (9), acetylacetone and some other S-diketones such as a,a,a-trifluoroacetylacetone. However, unlike the monomeric chelate products from acetylacetone and its fluorinated derivative, the enolate species of dimedone (9) cannot form chelates and as the complex is polymeric, it cannot be distilled and is more labile to hydrolysis, as might be expected for an unstabilized alkoxide. However, dimedone has a gas phase deprotonation enthalpy of 1418 9 kJmoD while acetylacetone enol (the more stable tautomer) is somewhat less acidic with a deprotonation enthalpy of 1438 10 klmoD Accordingly, had beryllium acetylacetonate not been a chelate, this species would have been more, not less, susceptible to hydrolysis. There is a formal similarity (roughly 7r-isoelectronic structures) between cyclic S-diketonates and complexes of dimedone with benzene and poly acetylene (10). The difference between the enthalpies of formation of these hydrocarbons is ca... 

A procedure was developed for the determination of total and labile Cu and Fe in river surface water. It involved simultaneous solvent extraction of the metals as diethyldithio-carbamates (ddc) and tfac complexes. The complexes were extracted by isobutyl methyl ketone (ibmk) and the solution subjected to flame atomic absorption spectrometry. Variables such as pH, metal complex concentration, reaction time, ibmk volume and extraction time were optimized. Prior to the solvent extraction a microwave-assisted peroxydisulfate oxidation was used to break down the metallorganic matter complexes in the river surface waters . Trifluoroacetylacetone was used as a chelation agent for the extraction and quantitative determination of total Cr in sea water. The chelation reaction was conducted in a single aqueous phase medium. Both headspace and liquid phase extractions were studied and various detection techniques, such as capillary GC-ECD, EI-MS (electron-impact MS) and ICP-MS, were tested and compared. The LOD was 11-15 ngL Cr for all the systems examined. The method provided accurate results with EI-MS and ICP-MS, while significant bias was experienced with ECD °. ... 

The presence of water has been postulated to cause the thermal instability of many of the hydrates (3, 4, 20), and attempts to chromatograph neodymium (III) trifluoroacetylacetonate dihydrate have failed (23). There is some evidence that hydrolysis occurs at elevated temperatures (3, 4, 20). Furthermore, there are indications that certain complexes undergo hydrolysis when allowed to stand in vacuo, even at room temperature (20). For these reasons the chelates are difficult to dehydrate by conventional means. Many of the claims in the older literature that anhydrous tris chelates were obtained must be considered questionable because the assignments of composition were either arbitrary or were based on analytical methods relatively insensitive to the amount of water present. Some investigators, however, have reported reasonably well-characterized anhydrous tris complexes 4,11,15), but most of these are not sufficiently volatile and stable to be chromatographed. 

Extraction is used mainly for the preliminary separation of macro- and microquantities of metals which interfere in the determination of aluminium. After Fe(III), Ti, Zr, and Cu cupferronates have been extracted from dilute HCl into chloroform, aluminium cupferronate is extracted at pH 3.5 [5]. Some interfering metals are separated from Al by extraction with trifluoroacetylacetone in CHCI3 [6], and as chloride- or thiocyanate complexes in the presence of DAM [7]. Aluminium has been separated from various elements by extraction as chelates with BPHA [8], oxine [4,9], 8-hydroxyquinaldine [10], and acetylacetone [11]. 

Oxidative addition of elemental fluorine to appropriate 1,3-dicarbonyl compounds provides a convenient synthesis of perfluorinated 1,2-dioxolanes. In this way (20) may be formed from difluoromalonyl fluoride, F2C(COF)2 <92JST(274)163>, and (39) is similarly prepared from either hexafluoroacetylacetone or the copper(II) or nickel(II) chelate of trifluoroacetylacetone with concomitant replacement of all remaining hydrogen atoms by fluorine <65JOCI429>. 

The major breakthrough that transformed metal chelate GC into a useful analytical technique was the introduction of fluorinated beta-diketone ligands, which formed complexes of greater volatility and thermal stability. Trifluoroacetylacetone (l,l,l-trifluoro-2,4-pentanedione—HTFA) and hexafluoro-acetylacetone (l,l,l,5,5,5-hexafluoro-2,4,-pentanedione—HHFA) are the fluorinated ligands most frequently employed. HTFA extended the range of metals that may be gas chromatographed with little or no evidence of decomposition to include Ga3+, In3+, Sc3+, Rh3+ and V4+. An example of a recent application is the analysis for beryllium in ambient air particulates. After filter sampling and extraction/chelation, packed column GC with electron capture detection allowed ppm level beryllium quantitation in collected particulates which corresponded to levels of 2-20 x 10 5 pg/m3 in the sampled air. 

The /3-diketonates of di-, tri- and tetravalent metals have proved the most suitable and the ligands acetylacetone trifluoroacetylacetone and hexafluoro-acetone the most used. In general, the more fiuorinated the /3-diketone the more volatile the chelate also greater sensitivity can be achieved with the BCD. 

While its structure has not been determined, the first well-characterized bis(bridgc) complex was characterized by Ugo and co-workers, who noted that the reaction of [Rh2(02CMe)4(H20)2] with acac, trifluoroacetylacetone, or hfacac (Q) gives complexes of the form [Rh2(02CMe)2(Q)2] (H. 0)2. Adducts with py, 2-methylpyridine and 2-chloropyridine were characterized, and analysis of electronic and IR spectra, and the H and P NMR led Ugo to propose bridging acetates, with the acacs each chelated to a single metal. Mixed acetate/salicylate, acetate/phosphate and formate/ bipy complexes have also been reported, but the nature of the bridging has not been determined. 

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