Among all iron catalysts employed in organic synthesis, tris(acetylacetonato)iron(III) or Fe(acac)3 is one the most popular. This is, in part, because it is air stable, easy to make, and soluble in organic solvents. It has found use in hydrogen atom transfer reactions, radical-based reactions, cross couplings, and CH functionalization reactions, among others.[1] The complex can be purchased at a modestly cheap price. However, it can also easily be synthesized on multigram scales. Could definitely be used as a morale-boosting synthesis for a rainy day. Your choice.
Procedure:[2] In an Erlenmeyer flask with magnetic stir bar was dissolved FeCl3-6H2O (3.30g, 12.0mmol) in 25mL of water. To this solution was subsequently added acetylacetone (3.80g, 38.0mmol) in 10mL of methanol dropwise, with vigorous stirring. Then a solution of sodium acetate (5.10g, deliquescent) in 15mL water was added dropwise, which caused near-immediate precipitation of a dark red solid. The reaction was pushed to completion by heating to 80degC for 15 minutes, and then allowed to cool back to room temperature. The product was isolated via vacuum filtration and washed with 100mL of water. After thoroughly drying the product (complete water removal can be checked by IR spectroscopy) it was isolated as a crystalline, dark red solid. Yield is near quantitative.
Notes:
- Fe(acac)3 is a dark-red, air-stable solid. It is very soluble in chlorinated solvents such as dichloromethane and chloroform, alcohols such as methanol and ethanol, and sparingly soluble in other polar organic solvents.
- The complex possesses an octahedral coordination environment, with each acetylacetonate ligand bound in kappa-2, chelating fashion, which gives rise to enantiomers. Furthermore, it is a paramagnetic, high-spin complex with five unpaired electrons (no Jahn-Teller distortion).
- Other bases can be used in the synthesis of the complex. They just need to be water soluble. Some procedures use sodium hydroxide, potassium hydroxide, or sodium acetate. Although not water-soluble, some procedures use FeO(OH) (iron(III) oxide-hydroxide) as the base.[3] There are several Youtube videos describing these procedures.
- The crystal structure of the complex has been subject to debate because of the compound’s chirality and the difficulties encountered in the compound’s optical resolution. It was first found to crystallize in the centrosymmetric, orthorhombic Pbca space group; later revised into the chiral, orthorhombic P212121 space group; and then back again to Pbca. Nevertheless, experimental and theoretical data seems to support Pbca.[4]
- Acetylacetone exhibits four proton resonances in the CDCl3 1H NMR, despite its high symmetry, because both enol and keto tautomers are present in solution in approximately (enol : keto) 6:1 ratio.[5] However, the equilibrium varies with solvent: Nonpolar solvents skew it towards the enol (roughly 40:1 in cyclohexane), whereas polar solvents tilt it towards the keto form (roughly 1:4 in water).[6] The enol form has the C3 proton at around 5.4ppm, while the keto form has C3 protons at 3.5ppm. It is one of these protons at the C3 position that gets deprotonated to form acetylacetonate, which is stabilized upon binding to iron. The pKa of acetylacetone is around 9 in water.
[1] a) Zettler, M. W.; Chen, Y.; Lee, C. Tris(acetylacetonato)iron(III). EROS, 2007. DOI: 10.1002/047084289X.rt396.pub2. b) Sherry, B. D.; Fürstner, A. The Promise and Challenge of Iron-Catalyzed Cross Coupling. Acc. Chem. Res. 2008, 41, 11, 1500-1511. c) Lübken, D.; Saxarra, M.; Kalesse, M. Tris(acetylacetonato) Iron(III): Recent Developments and Synthetic Applications. Synthesis 2019, 51, 161-177.
[2] Glidewell, C. Metal Acetylacetonate Complexes: Preparation and Characterization. In Inorganic Experiments, 3rd Edition. Wiley-VCH, 2010, 109-119.
[3] Chaudhuri, M. K.; Ghosh, S. K. Novel Synthesis of Tris(acetylacetonato)iron(III). J. Chem. Soc. Dalton Trans. 1983, 839-840.
[4] Ellis, T. K.; Kearley, G. J.; Piltz, R. O.; Jayasooriya, U. A.; Stride, J. A. Achirality in the low temperature structure and lattice modes of tris(acetylacetonate)iron(III). Dalton Trans. 2016, 45, 8278-8283.
[5] Seco, M. Acetylacetone: A versatile ligand. J. Chem. Educ. 1989, 66, 9, 779-780.
[6] Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3rd Edition. Wiley-VCH, 2003.