Metal acetylacetonates are
coordination complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es derived from the
acetylacetonate anion () and metal ions, usually
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s. The
bidentate ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
acetylacetonate is often abbreviated acac. Typically both oxygen atoms bind to the metal to form a six-membered chelate ring. The simplest complexes have the formula M(acac)
3 and M(acac)
2. Mixed-ligand complexes, e.g. VO(acac)
2, are also numerous. Variations of acetylacetonate have also been developed with myriad substituents in place of methyl (RCOCHCOR
−). Many such complexes are soluble in organic
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
s, in contrast to the related metal halides. Because of these properties, acac complexes are sometimes used as
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
precursors and reagents. Applications include their use as
NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
"shift reagents" and as catalysts for
organic synthesis, and precursors to industrial
hydroformylation
Hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes from alkenes. This chemical reaction entails the net addition of a formyl group (CHO) and a hydrogen atom to a carbon-carbon d ...
catalysts. 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).
Synthesis
The usual synthesis involves treatment of a metal salt with acetylacetone, acacH:
:M
''z''+ + ''z'' Hacac M(acac)
''z'' + ''z'' H
+
Addition of base assists the removal of a proton from acetylacetone and shifts the equilibrium in favour of the complex. Both oxygen centres bind to the metal to form a six-membered chelate ring. In some cases the
chelate effect
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
is so strong that no added base is needed to form the complex. Some complexes are prepared by metathesis using
Tl(acac).
Structure and bonding
In the majority of its complexes acac forms six-membered C
3O
2M chelate rings.
The M(acac) ring is planar with a symmetry plane bisecting the ring.
The acacM ring generally exhibits aromatic character, consistent with delocalized bonding in the monoanionic C
3O
2 portion. Consistent with this scenario, in some complexes, the acac ligand is susceptible to electrophilic substitution, akin to
electrophilic aromatic substitution
Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic n ...
(in this equation Me = CH
3):
:Co(O
2C
3Me
2H)
3 + 3NO
2+ → Co(O
2C
3Me
2NO
2)
3 + 3H
+
In terms of
electron count
Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and chemical bond, bonding. Many rules in chemistry rely on electron-counting:
*Octet rule is used with Lewis structures for mai ...
ing, neutral bidentate O,O-bonded acac ligand is an "L-X ligand", i.e. a combination of a Lewis base (L) and a pseudohalide (X).
An exception to the classical description presented above, the bis(pyridine) adduct of
chromium(II) acetylacetonate
Chromium(II) acetylacetonate is the coordination compound with the formula Cr(O2C5H7)2. It is the homoleptic acetylacetonate complex of chromium(II). It is an air-sensitive, paramagnetic yellow brown solid. According to X-ray crystallography, th ...
features
noninnocent acac
2- ligand.
Classification by triad
Titanium triad
Treatment of
TiCl4 with acetylacetone gives
TiCl2(acac)2, a red-coloured, octahedral complex with C
2 symmetry
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...
:
:TiCl
4 + 2 Hacac → TiCl
2(acac)
2 + 2 HCl
This reaction requires no base. The complex TiCl
2(acac)
2 is fluxional in solution, the NMR spectrum exhibiting a single methyl resonance at room temperature.
Unlike Ti(IV), both Zr(IV) and Hf(IV) bind ''four'' bidentate acetylacetonates, reflecting the larger radius of these metals.
Hafnium acetylacetonate and
zirconium acetylacetonate
Zirconium acetylacetonate is the coordination complex with the formula Zr(C5H7O2)4. It is a common acetylacetonate of zirconium. It is a white solid that exhibits high solubility in nonpolar organic solvents, but not simple hydrocarbons.
The compl ...
adopt square antiprismatic structures.
Regarding acetylacetonates of titanium(III),
Ti(acac)3 is well studied. This blue-colored compound forms from titanium trichloride and acetylacetone.
Vanadium triad
Vanadyl acetylacetonate
Vanadyl acetylacetonate is the chemical compound with the formula VO(acac)2, where acac– is the conjugate base of acetylacetone. It is a blue-green solid that dissolves in polar organic solvents. The coordination complex consists of the vanadyl ...
is a blue complex with the formula V(O)(acac)
2. This complex features the
vanadyl
The vanadyl or oxovanadium(IV) cation, VO2+, is a functional group that is common in the coordination chemistry of vanadium. Complexes containing this functional group are characteristically blue and paramagnetic. A triple bond is proposed to ex ...
(IV) group, and many related compounds are known. The molecule is square pyramidal, with idealized C
2v symmetry. The complex catalyzes epoxidation of allylic alcohols by peroxides.
Vanadium(III) acetylacetonate
Vanadium(III) acetylacetonate is the coordination compound with the formula V(C5H7O2)3, sometimes designated as V(acac)3. It is an orange-brown solid that is soluble in organic solvents.
Structure and synthesis
The complex has idealized D3 sy ...
is a dark-brown solid. Vanadium β-diketonate complexes are used as precatalysts in the commercial production of ethylene-propylene-diene elastomers (EPDM). They are often evaluated for other applications related to redox flow batteries, diabetes and enhancing the activity of insulin, and as precursors to inorganic materials by CVD.
Chromium triad
Chromium(III) acetylacetonate
Chromium(III) acetylacetonate is the coordination compound with the formula Cr(C5H7O2)3, sometimes designated as Cr(acac)3. This purplish coordination complex is used in NMR spectroscopy as a relaxation agent because of its solubility in nonpolar o ...
, Cr(acac)
3, is a typical octahedral complex containing three acac
− ligands. Like most such compounds, it is highly soluble in nonpolar organic solvents. This particular complex, which has a three unpaired electrons, is used as a spin relaxation agent to improve the sensitivity in quantitative
carbon-13 NMR spectroscopy.
Chromium(II) acetylacetonate
Chromium(II) acetylacetonate is the coordination compound with the formula Cr(O2C5H7)2. It is the homoleptic acetylacetonate complex of chromium(II). It is an air-sensitive, paramagnetic yellow brown solid. According to X-ray crystallography, th ...
is a highly oxygen-sensitive, light brown compound. The complex adopts a
square planar structure, weakly associated into stacks in the solid state. It is isomorphous with Pd(acac)
2 and Cu(acac)
2.
Manganese triad
![Lambda-tris(acetylacetonato)manganese(III)-3D-balls](https://upload.wikimedia.org/wikipedia/commons/1/1e/Lambda-tris%28acetylacetonato%29manganese%28III%29-3D-balls.png)
It has been prepared by the
comproportionation
Comproportionation or synproportionation is a chemical reaction where two reactants containing the same element but with different oxidation numbers, form a compound having an intermediate oxidation number. It is the opposite of disproportionation. ...
of the manganese(II) compound Mn(acac)
2 with
potassium permanganate
Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution.
Potassium permanganate is widely used in the c ...
in the presence of additional acetylacetone. Alternatively the direct reaction of acetylacetone with
potassium permanganate
Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution.
Potassium permanganate is widely used in the c ...
.
[Girolami, G.; Rauchfuss, T.; Angelici, R. ''Synthesis and Technique in Inorganic Chemistry'', 3rd ed.; University Science Books: Sausalito, CA, 1999; pp. 85-92. ] In terms of electronic structure, Mn(acac)
3 is
high spin Spin states when describing transition metal coordination complexes refers to the potential spin configurations of the central metal's d electrons. For several oxidation states, metals can adopt high-spin and low-spin configurations. The ambiguity o ...
. Its distorted octahedral structure reflects geometric distortions due to the
Jahn–Teller effect
The Jahn–Teller effect (JT effect or JTE) is an important mechanism of spontaneous symmetry breaking in molecular and solid-state systems which has far-reaching consequences in different fields, and is responsible for a variety of phenomena in sp ...
. The two most common structures for this complex include one with tetragonal 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 Å and the other four are 2.00 Å. The effects of the tetragonal elongation are noticeably more significant than the effects of the tetragonal compression.
:
![Mn(III) acac](https://upload.wikimedia.org/wikipedia/commons/f/f3/Mn%28III%29_acac.png)
In organic chemistry, Mn(acac)
3 has been used as a one-electron oxidant for coupling phenols.
Iron triad
Iron(III) acetylacetonate, Fe(acac)
3, is a red
high-spin complex that is highly soluble in organic solvents. It is a high-spin complex with five unpaired electrons. It has occasionally been investigated as a catalyst precursor. Fe(acac)
3 has been partially resolved into its
Δ and Λ isomers.
[ The ferrous complex Fe(acac)2 is oligomeric.
Like iron, Ru(III) forms a stable tris(acetylacetonate). Reduction of this Ru(III) derivative in the presence of other ligands affords mixed ligand complexes, e.g. Ru(acac)2(alkene)2.
]
Cobalt triad
Tris(acetylacetonato)cobalt(III)
Tris(acetylacetonato)cobalt(III) is the coordination complex with the formula Co(C5H7O2)3. Often abbreviated Co(acac)3, it is a green, diamagnetic solid that is soluble in organic solvents, but not in water. Owing to its solubility in organic solve ...
, Co(acac)3, is low-spin, diamagnetic complex. Like other compounds of the type M(acac)3, this complex is chiral
Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is distinguishable from ...
(has a non-superimposable mirror image).
:
The synthesis of Co(acac)3 involves the use of an oxidant since the cobalt precursors are divalent:
:2 CoCO3 + 6 Hacac + H2O2 → 2 Co(acac)3 + 4 H2O + 2 CO2
The complex "Co(acac)2", like the nickel complex with analogous stoichiometry, is typically isolated with two additional ligands, i.e. octahedral Co(acac)2L2. The anhydrous form exists as the tetramer 2">o(acac)2sub>4. Like the trimeric nickel complex, this tetramer shows ferromagnetic interactions at low temperatures.
Ir(acac)3 and Rh(acac)3 are known. A second linkage isomer of the iridium complex is known, ''trans''-Ir(acac)2(''C''H(COMe)2)(H2O). This ''C''-bonded derivative is a precursor to homogeneous catalysts for C–H activation and related chemistries.
Two well-studied acetylacetonates of rhodium(I) and iridium(I) are Rh(acac)(CO)2 and Ir(acac)(CO)2. These complexes are square-planar, with C2v symmetry.
Nickel triad
Nickel(II) bis(acetylacetonate) exists as the trimetallic complex 2">i(acac)2sub>3. Bulky beta-diketonates give red, monomeric, square-planar complexes. Nickel(II) bis(acetylacetonate) reacts with water to give the octahedral adduct
An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...
2(H2O)2">i(acac)2(H2O)2 a chalky green solid.
In contrast to the complicated magnetism and structures of Ni(acac)2, platinum(II) bis(acetylacetonate) and palladium(II) bis(acetylacetonate) are diamagnetic monometallic species.
Copper triad
Cu(acac)2 is prepared by treating acetylacetone with aqueous . It is available commercially, catalyzes coupling and carbene transfer reactions.
:
Unlike the copper(II) derivative, copper(I) acetylacetonate is an air-sensitive Air sensitivity is a term used, particularly in chemistry, to denote the reactivity of chemical compounds with some constituent of air. Most often, reactions occur with atmospheric oxygen (O2) or water vapor (H2O), although reactions with the other ...
oligomeric species. It is employed to catalyze Michael addition
In organic chemistry, the Michael reaction or Michael addition is a reaction between a Michael donor (an enolate or other nucleophile) and a Michael acceptor (usually an α,β-unsaturated carbonyl) to produce a Michael adduct by creating a carbon ...
s.
Zinc triad
The monoaquo complex Zn(acac)2H2O ( m.p. 138–140 °C) is pentacoordinate, adopting a square pyramidal structure. The complex is of some use in organic synthesis. Dehydration of this species gives the hygroscopic anhydrous derivative (m.p. 127 °C). This more volatile derivative has been used as a precursor to films of ZnO.
Acetylacetonates of the other elements
Colourless, diamagnetic Al(acac)3 is structurally similar to other tris complexes, e.g. 3">e(acac)3 The trisacetylacetonates of the lanthanide
The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yttr ...
s often adopt coordination numbers above 8.
Variants of acac
Many variants of acetylacetonates are well developed. Hexafluoroacetylacetonates and trifluoroacetylacetonates form complexes that are often structurally related to regular acetylacetonates, but are more Lewis acidic and more volatile. The complex Eufod, Eu(OCC(CH3)3CHCOC3F7)3, features an elaborate partially fluorinated ligand. This complex is a Lewis acid, forming adducts with a variety of hard
Hard may refer to:
* Hardness, resistance of physical materials to deformation or fracture
* Hard water, water with high mineral content
Arts and entertainment
* ''Hard'' (TV series), a French TV series
* Hard (band), a Hungarian hard rock supe ...
bases.
One or both oxygen centers in acetylacetonate can be replaced by RN groups, giving rise to Nacac and Nacnac ligands.
Image:Hfacenol.png, hexafluoroacetylacetone
Hexafluoroacetylacetone is the chemical compound with the nominal formula CF3C(O)CH2C(O)CF3 (often abbreviated as hfacH). This colourless liquid is a ligand precursor and a reagent used in MOCVD. The compound exists exclusively as the enol CF3C( ...
Image:Trifluoroacetylacetone.png, trifluoroacetylacetone
1,1,1-Trifluoroacetylacetone is the organofluorine compound with the formula CFC(O)CHC(O)CH. It is a colorless liquid. Like other 1,3-diketones, it is used as a precursor to heterocycles, e.g. pyrazoles, and metal acetylacetonates, metal chelates. ...
Image:Chem structure of Hnacnac.png, Tautomers and complexation of Nacnac
Image:Eufod.png, The "NMR shift reagent" Eufod
Image:Ac4C2H2.svg, Tetraacetylethane
Tetraacetylethane is the organic compound with the nominal formula H(C(O)CH3)2sub>2. It is a white solid that has attracted interest as a precursor to heterocycles and metal complexes. It is prepared by oxidation of sodium acetylacetonate:
:I2 + ...
, which forms bimetallic complexes
Image:Ac3CH.svg, Triacetylmethane
Triacetylmethane is the organic compound with the formula . It is a colorless liquid that is soluble in organic solvents and in alkaline water. It readily forms an enolate. The enolate forms a variety of metal complex
A coordination complex ...
Image:2,4,6-heptanetrione.svg, 2,4,6-Heptanetrione
2,4,6-Heptanetrione is the organic compound with the formula . It is a white or colorless solid. The molecule, which exists mainly in the enol form, undergoes condensation with 1,2-diketones. The compound contributes to the flavor of strawberri ...
, a binucleating ligand.
''C''-bonded acetylacetonates
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
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituen ...
) contain one carbon-bonded acac ligand. The IR spectra of ''O''-bonded acetylacetonates are characterized by relatively low-energy ''ν''CO 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.
References
{{Coordination complexes
Chelating agents
Ligands