Acetylacetone, acid strength

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... 

To compare acid strength between acetone and acetylacetone, one considers the stabilities of the carbanions formed. The carbanion of acetone is similar to the carbanion formed by ionization of a hydrogen from carbon 1 or 5 of acetylacetone. Since the predominant carbanion for acetylacetone is formed by carbon 3 ionization, it is more stable than the acetone carbanion, therefore, more readily formed. This implies that acetylacetone is more acidic than acetone. 

Fig. 8.10. Plot of oscillator strength P of the 1 1 complexes of Ho3 with monobasic (+) and dibasic ( ) ligands vs. the pXa of the ligand acid (from ref. [741) (a) fluoride, (b) glycolate, (c) acetate, (d) propionate, (e) of-picolinate, (f) tropolonate, (g) kojate, (h) acetylacetonate, (I) IMAD, (2) dipicolinate, (3) methyl succinate, (4) malonate, (5) maleate, (6) fumarate, (7) sulfate.

Fig. 3. Rate coefficients (log, 0 kf) for proton transfer from acetylacetone (keto-form) to oxygen bases (B) and rate coefficients (log,0 kr) for proton transfer from oxygen acids (BH+) to the enolate ion to give acetylacetone. Reactions refer to ionic strength 0.1 M and 300.7 °K in aqueous solution. Redrawn with permission from M.-L. Ahrens et al., Bunsenges. Phys. Chem., 74 (1970) 380.

Since these reactions follow the BEP principle, substituents that stabilize carbanions also accelerate their formation by deprotonation of corresponding substituted methanes. Substituents of -h E type are therefore especially efficient (see Table 4.2), acetone (CH3COCH3) and nitromethane (CH3NO2) undergoing deprotonation quite rapidly with bases even of moderate strength. Reactions of this kind are, however, very much slower than corresponding ISOq reactions of acids of comparable strength, so acids such as acetylacetone do not obey the Br nsted relation. 

Post-synthesis alumination of siliceous forms of MCM-41 and MCM-48 can be readily achieved by reaction of parent materials at low temperatine in nonpolar solvents with various aluminum sources. Among the latter, trimethyl aluminum [113], aluminum trichloride [114], aluminum chlorohydrate [115] aluminum isopropoxide [116,117] and aluminum acetylacetonate [118,119] have been used After calcination, the resulting materials preserve the typical narrow pore distribution of the parent mesophase and Al NMR measurements reveal the presence of aluminum atoms in both tetrahedral and octahedral configurations generating Bronsted and Lewis acid sites with a strength comparable to that of mesostructures of similar composition prepared by direct synthesis. 

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