Anhydrous diketonate complexes

Metal alkoxides constitute a useful class of starting materials for the synthesis of the metal / -diketonates. The ethoxides of Nbv, Tav and Uv react with diketones. Here, only partial substitution of the ethoxy groups occurs and materials of the type M(diketonate)3(OEt)2 are formed.194,195 Similar reactions with lanthanide alkoxides, however, provide pure, unsolvated lanthanide tris(diketonates). The virtue of such syntheses lies in their ability to yield anhydrous diketonate complexes. Removal of water from the hydrates without decomposition is sometimes difficult.196,197... 

To avoid the complication of hydrolysed product or partial decomposition during dehydration, anhydrous diketonate complexes can be prepared by the reaction of diketone with lanthanide 2-propanolate [47] in benzene, with a lanthanide hydride or with metallic europium [48]. 

Anhydrous / -diketonate complexes may be obtained by sublimation of the solid hydrated complexes [49]. Hydrated complexes may be dissolved in a solvent that forms an azeotrope with water and the solvent is cycled over molecular sieves to achieve complete dehydration [50]. 

Recent work by Cole-Hamilton and coworkers has demonstrated that stability and volatility are enhanced by the use of long-chain fluorinated p-diketonate ligands, that is those derived from Hdfhd (6) and Htdfhd (7). The hydrafed bis(/3-diketonate) complexes M(dfhd)2(H20) and M(tdftid)2(H20) were prepared for Cu, Ca, Sr, and Ba and were all found to sublime below 200 °C with weight loss ranging from 85 to 98% (see Table 15). In addition, the anhydrous compound Ba(tdfiid)2 was reported to melt at 196 °C and volatilize at 220 °C with essentially no decomposition (residual mass by TGA ca. 1%). 

The j8-diketones are excellent chelating ligands for the rare earth ions and typical examples of the types of complexes that are formed are presented in table 25.14. (The functioning of these complexes as lasers is discussed in ch. 35.) The complexes that might have been expected, R(jS-diketone)3, rarely form, except for scandium, because of the tendencies of these compounds to add one or more additional ligands, particularly water molecules. Attempts to dehydrate most of these hydrated species leads either to destruction of the complex or the formation of polymeric hydroxo species (Pope et al., 1961). If, however, the substituents on the j8-diketone are large, t-butyl groups as an example, then the anhydrous tris chelate can be prepared (eisentraut and Sievers, 1965). The anhydrous trisacetylacetonato complexes have been prepared, however, in the absence of any possible adduct formation by the reaction of acetylacetone directly with a rare earth hydride (Przystal et al., 1971). The anhydrous acetyl-... 

For both of these ligands the complexes formed when the acetates, chlorides, nitrates, and thiocyanates are used as the starting materials have a smaller number of organic ligands, coordination of some or all of the anions (the structure of the ten-coordinate complex [La(o-phen)2(N03)3] has been discussed earlier, section 3.9), and perhaps may be solvated as well (Forsberg, 1973). Most of the preparations can be carried out in aqueous or anhydrous ethanol. The existence of the lanthanide-nitrogen bond is clearly implied in these compounds and the infrared spectra (Sinha, 1964) and ultimately, complete structure determination, have verified that this interaction does take place. It should also be noted that both of these molecules form adducts with the tris-j8-diketone complexes and the structures of two of these with ortho-phenanthroline have been determined (Watson et al., 1972 Cunningham, 1973). 

More than 95% of the rare-earth y3-diketonate complexes described in the Uterature have been prepared by the metathesis reaction between the sodium or armnonium salt of a p-diketone and a rare-earth salt (chloride or nitrate) in water or ethanol as the solvent. In most cases, these methods work well, especially when the pH of the reaction mixture is controlled during the synthesis. Sometimes other synthetic routes have to be used, for instance when strictly anhydrous complexes are needed, or when complexes are wanted that are free of contaminating anions or cations. Complexes can be obtained by direct synthesis between a rate-earth metal and a j8-diketone in an 1 3 molar ratio in an inert solvent (for instance toluene). Hydrogen gas is evolved and a tris y3-diketonate complex is formed ... 

On treating diisobutene with acetic anhydride and anhydrous zinc chloride, A. C. Byrns and T. F. Doumani had isolated in 1943 a crystalline compound to which they had ascribed the structure of a zinc complex with a 1,3-diketone 40 the correct pyrylium chlorozincate structure was established by A. T. Balaban et al.41 in 1961, after extended investigation on the formation of pyrylium salts by alkene diacylation.42 This formation again had remained undetected for many decades during which alkenes had been acylated but only the water-insoluble monoacylation products had been investigated, whereas the water-soluble pyrylium salts went into the sink with the Lewis or Bronsted acid catalysts that had been used in the acylation. 

Mass spectral studies have been conducted on many of the yS-diketonate compounds discussed in this review. Mass spectra of the compounds in the series ML2 (M = Ca, Sr, and Ba L = tpm, ppm, and hpm) have shown that in the gas phase, from a source temperature of 200°C, many oligomeric species, such as [Ca4(tpm)7]+ and [Ca5(tpm)9]+, are present.151 The molecular ion peak corresponding to ML2 was reported to be very small or nonexistent for these compounds. The mass spectral studies of anhydrous Ba(thd)2 (presumably [Ba4(tmhd)g]) have shown that several oligomeric species also exist the ions [Ba4(thd)7]+, [Ba3(thd)5]+, [Ba2(thd)3]+, and [Ba(thd)]+ were detected in the mass spectrum.152 The complexes ML2 (M = Ca, Sr, and Ba L = hfac and tfac) have been... 

The fi-diketonates have been extensively studied, particularly since some of the fluorinated derivatives give complexes that are volatile and suitable for gas-chromatographic separation. The preparation of /3-diketonates by conventional methods invariably gives hydrated or solvated species such as [Ln(acac)3]-QHsOH SHzO. Anhydrous species obtained by vacuum dehydration appear to be polymeric, not octahedral. The neutral /3-diketonates can complex further to give anionic species such as the eight-coordinate thenoyltrifluoroacetate [Nd(TTA)4] The alkali metal salts of [Ln(/3-dike)4] are sometimes appreciably volatile and can be sublimed. 

Metal aUcoxides and alkyls are useful starting materials, particularly for the preparation of some unsolvated P-diketones that require anhydrous conditions (see Section 3.3). Addition of /3-diketonate ligands to metal alkoxides has also been used to produce heteroleptic metal complexes that are less reactive than the homoleptic metal alkoxides this simplifies the use of such complexes in chemical vapor deposition applications. ... 

Tris[as-(diacetyltetracarbonylmanganese)] aluminum is preparedreadily by treating acetylpentacarbonylmanganese with 1 molar-equivalent of methyllithium at 0° followed by the addition of t/a molar-equivalent of anhydrous aluminum chloride. This complex is isostructural with tris(2,4-pentanedionato)aluminum (where 2,4-pentanedione = acetylacetone) except that the methine group is replaced formally by a Mn(C0)4 group, which suggests that the title compound is one example of a metallo-j3-diketonate type complex. 

The / -diketonates resemble those of A1 rather than of the lanthanides thus the acetylacetonate is normally anhydrous and may be sublimed around 200°. The TTA complex can be extracted from aqueous solutions at pH 1.5-2 by benzene, while the 8-quinolinolate (cf. Al) can be quantitatively extracted by CHC13 Sc3 + can also be extracted from aqueous sulfate solutions by a quaternary ammonium salt.43... 

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