Acetylacetone is an organic compound with molecular formula C5H8O2. This diketone is formally named 2,4-pentanedione, although as discussed below, this name does not properly describe the predominant structure. It is a precursor to common bidentate ligand and a building block for the synthesis of heterocyclic compounds.
The keto and enol forms of acetylacetone coexist in solution; these forms are tautomers. The C2v symmetry for the enol form displayed on the right in scheme 1 has been verified by many methods including microwave spectroscopy. Hydrogen bonding in the enol reduces the steric repulsion between the cabonyl groups. In the gas phase K is 11.7. The equilibrium constant tends to be high in nonpolar solvents: cyclohexane is 42, toluene is 10, THF 7.2, dimethylsulfoxide (K=2), and water (K=0.23).
- (CH3CO)2O + CH3C(O)CH3 → CH3C(O)CH2C(O)CH3
The second synthesis involves the base-catalyzed condensation of acetone and ethyl acetate, followed by acidification
- NaOEt + EtO2CCH3 + CH3C(O)CH3 → NaCH3C(O)CHC(O)CH3 + 2 EtOH
- NaCH3C(O)CHC(O)CH3 + HCl → CH3C(O)CH2C(O)CH3 + NaCl
Because of the ease of these syntheses, many analogues of acetylacetonates are known. Some examples include C6H5C(O)CH2C(O)C6H5 (dbaH) and (CH3)3CC(O)CH2C(O)CC(CH3)3. Hexafluoroacetylacetonate is also widely used to generate volatile metal complexes.
C5H7O2−, is the conjugate base of 2,4-pentanedione. In reality, the free ion does not exist in solution, but is bound to the corresponding cation, such as Na+. In practice, the existence of the free anion, commonly abbreviated acac−, is a useful model.
The acetylacetonate anion forms complexes with many transition metal ions wherein both oxygen atoms bind to the metal to form a six-membered chelate ring. Some examples include: Mn(acac)3, VO(acac)2, Fe(acac)3, and Co(acac)3. Any complex of the form M(acac)3 is chiral (has a non-superimposable mirror image). Additionally, M(acac)3 complexes can be reduced electrochemically, with the reduction rate being dependent on the solvent and the metal center. Bis- and tris complexes of the type M(acac)2 and M(acac)3 are typically soluble in organic solvents, in contrast to the related metal halides. Because of this properties, these complexes are widely used as catalyst precursors and reagents. Important applications include their use as NMR "shift reagents" and as catalysts for organic synthesis, and precursors to industrial hydroformylation catalysts.
C5H7O2− in some cases also binds to metals through the central carbon atom; this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III).
Illustrative metal acetylacetonates
Chromium (III) acetylacetonate
Cu(acac)2, prepared by treating acetylacetone with aqueous Cu(NH3)42+ and is available commercially, catalyzes coupling and carbene transfer reactions.
Mn(acac)3, a one-electron oxidant, is used for coupling phenols. It is prepared by the direct reaction of acetylacetone and potassium permanganate. In terms of electronic structure, Mn(acac)3 is high spin. Its distorted octahedral structure reflects geometric distortions due to the Jahn-Teller effect. The two most common structures for this complex include one with tetrahedral elongation and one with tetragonal compression. For the elongation, two Mn-O bonds are 2.12 Å while the other four are 1.93 Å. For the compression, two Mn-O bonds are 1.95 and the other four are 2.00 Å. The effects of the tetrahedral elongation are noticeably more significant than the effects of the tetragonal compression.
"Nickel acac" is not Ni(acac)2 but the trimer [Ni(acac)2]3. This emerald green solid, which is benzene soluble, is widely employed in the preparation of Ni(O) complexes. Upon exposure to the atmosphere, [Ni(acac)2]3 converts to the chalky green monomeric hydrate.
C5H7O2− in some cases also binds to metals through the central carbon atom (C3); this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III). The complexes Ir(acac)3 and corresponding Lewis-base adducts Ir(acac)3L (L = an amine) contain one carbon-bonded acac ligand. The IR spectra of O-bonded acetylacetonates are characterized by relatively low-energy νCO bands of 1535 cm−1, whereas in carbon-bonded acetylacetonates, the carbonyl vibration occurs closer to the normal range for ketonic C=O, i.e. 1655 cm−1.
Other reactions of acetylacetone
- Deprotonations: Very strong bases will doubly deprotonate acetylacetone, starting at C3 but also at C1. The resulting species can then be alkylated at C-1.
- Precursor to heterocycles: Acetylacetone is a versatile precursor to heterocycles. Hydrazine reacts to produce pyrazoles. Urea gives pyrimidines.
- Precursor to related imino ligands: Acetylacetone condenses with amines to give, successively, the mono- and the di-diketimines wherein the O atoms in acetylacetone are replaced by NR (R = aryl, alkyl).
- Enzymatic breakdown: The enzyme acetylacetone dioxygenase cleave the carbon-carbon bond of acetyacetone, producing acetate and 2-oxopropanal. The enzyme is Fe(II)-dependent, but it has been proven to bind to zinc as well. Acetylacetone degradation has been characterized in the bacterium Acinetobacter johnsonii.
- C5H8O2 + O2 → C2H4O2 + C3H4O2
- Arylation: Acetylacetonate displaced halides from certain halo-substituted benzoic acid. This reaction is copper-catalyzed.
- 2-BrC6H4CO2H + NaC5H7O2 → 2-(CH3CO)2HC)-C6H4CO2H + NaBr
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