Acetylacetone

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, as it is a vinylogous carboxylic acid. It is a precursor to common bidentate ligand and a building block for the synthesis of heterocyclic compounds.

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Properties

The keto and enol forms of acetylacetone coexist in solution; these forms are tautomers. The C_2v symmetry for the enol form displayed on the right in scheme 1 has been verified by many methods including microwave spectroscopy.^[1] 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, dimethyl sulfoxide (K=2), and water (K=0.23).^[2]

Preparation

Two common procedures are used for synthesizing acetylacetone. Acetone and acetic anhydride react upon the addition of BF₃ catalyst.

(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.

Acetylacetonate "anion"

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.

Coordination chemistry

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,^[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.^[4] 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).

Metal acetylacetonates

Chromium(III) acetylacetonate

Cr(acac)₃ is used as a spin relaxation agent to improve the sensitivity in quantitative Carbon-13 NMR spectroscopy.^[5]

Copper(II) acetylacetonate

Cu(acac)₂, prepared by treating acetylacetone with aqueous Cu(NH₃)₄²⁺ and is available commercially, catalyzes coupling and carbene transfer reactions.

Copper(I) acetylacetonate

Unlike the copper(II) chelate, copper(I) acetylacetonate is an air sensitive oligomeric species. It is employed to catalyze Michael additions.^[6]

Manganese(III) acetylacetonate

  Mn(acac)₃, a one-electron oxidant, is used for coupling phenols.^[3] 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.^[7]

Nickel(II) acetylacetonate

"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.

Vanadyl acetylacetonate

  Vanadyl acetylacetonate is a blue complex with the formula V(O)(acac)₂. It is useful in epoxidation of allylic alcohols.

Zinc acetylacetonate

The monoaquo complex Zn(acac)₂H₂O (m.p. 138-140 °C) is pentacoordinate, adopting a square pyramidal structure.^[8] Dehydration of this species gives the hygroscopic anhydrous derivative (m.p. 127 °C). ^[9] This more volatile derivative has been used as a precursor to films of ZnO.

C-bonded acetylacetonates

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.^[10]

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

References

1. ^ W. Caminati, J.-U. Grabow (2006). "The C2v Structure of Enolic Acetylacetone". Journal of the American Chemical Society 128 (3): 854 - 857. doi:10.1021/ja055333g.
2. ^ Solvents and Solvent Effects in Organic Chemistry, Christian Reichardt Wiley-VCH; 3 edition 2003 ISBN 3-527-30618-8
3. ^ ^{a b} B. B. Snider, "Manganese(III) Acetylacetonate" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289
4. ^ W. Fawcett, M. Opallo (1992). "Kinetic parameters for heterogeneous electron transfer to tris(acetylacetonato)manganese(III) and tris(acetylacetonato)iron(III) in aproptic solvents". Journal of Electroanalytical Chemistry 331: 815-830. doi:10.1016/0022-0728(92)85008-Q.
5. ^ Caytan, Elsa & Remaud, Gerald S.; Tenailleau, Eve; Akoka, Serge (2007), " ", Talanta 71 (3): 1016-1021
6. ^ E. J. Parish, S. Li "Copper(I) Acetylacetonate" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289X.rc203
7. ^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.
8. ^ H. Montgomery and E. C. Lingafelter "The crystal structure of monoaquobisacetylacetonatozinc" Acta Crystallographica (1963), volume 16, pp. 748-752. doi:10.1107/S0365110X6300195X.
9. ^ G. Rudolph and M. C. Henry "Bis(2,4-Pentanedionato)zinc (Zinc Acetylacetonate)" Inorganic Syntheses, 1967, volume X, pp. 74-77.
10. ^ Straganz, G.D., Glieder, A., Brecker, L., Ribbons, D.W. and Steiner, W. "Acetylacetone-Cleaving Enzyme Dke1: A Novel C-C-Bond-Cleaving Enzyme." Biochem. J. 369 (2003) 573-581 doi:10.1042/BJ20021047

Further reading

• Bennett, M. A.; Heath, G. A.; Hockless, D. C. R.; Kovacik, I.; Willis, A. C. "Alkene Complexes of Divalent and Trivalent Ruthenium Stabilized by Chelation. Dependence of Coordinated Alkene Orientation on Metal Oxidation State" Journal of the American Chemical Society 1998: 120 (5) 932-941. doi:10.1021/ja973282k
• Albrecht, M. Schmid, S.; deGroot, M.; Weis, P.; Fröhlich, R. "Self-assembly of an Unpolar Enantiomerically Pure Helicate-type Metalla-cryptand" Chemical Communications 2003: 2526–2527. doi:10.1039/b309026d
• Charles, R. G., "Acetylacetonate manganese (III)" Inorganic Synthesis, 1963, 7, 183-184.
• Richert, S. A., Tsang, P. K. S., Sawyer, D. T., "Ligand-centered redox processes for manganese, iron and cobalt, MnL₃, FeL₃, and CoL₃, complexes (L = acetylacetonate, 8-quinolinate, picolinate, 2,2'-bipyridyl, 1,10-phenanthroline) and for their tetrakis(2,6-dichlorophenyl)porphinato complexes[M(Por)]"Inorganic Chemistry, 1989, 28, 2471-2475. doi:10.1021/ic00311a044
• Wong-Foy, A. G.; Bhalla, G.; Liu, X. Y.; Periana, R. A.. "Alkane C-H Activation and Catalysis by an O-Donor Ligated Iridium Complex." Journal of the American Chemical Society, 2003: 125 (47) 14292-14293. doi:10.1021/ja037849a
• Tenn, W. J., III; Young, K. J. H.; Bhalla, G.; Oxgaard. J.; Goddard, W. A., III; Periana, R. A. "CH Activation with an O-Donor Iridium-Methoxo Complex." Journal of the American Chemical Society, 2005: 127 (41) 14172-14173. doi:10.1021/ja051497l
• N. Barta, "Bis(acetylacetonato)zinc(II)" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289X.rb097