Synthesis and NMR Characterization
of a Zinc Compound of Acetylacetone
Judith A. Walmsley and Frank Walmsley
Department of Chemistry, University of Texas at San Antonio
The compound acetylacetone (2,4-pentanedione) is an excellent ligand for the formation
of transition metal complexes. It is a bidentate ligand which chelates to the metal
through the two oxygen atoms, thus forming a six-membered ring, as shown in Fig. 1(a).
Acetylacetone (abbreviated acac) undergoes keto-enol isomerization, existing as a
mixture of the two tautomers at room temperature (Fig. 1(b)). The hydoxylic proton is
easily ionized and the negative charge in the resulting anion is delocalized over the
molecule (Fig 1(c)).
CH3
O
C
(a)
M
CH
O
C
CH3
(b)
H3C
O
O
C
C
C
H2
CH3
H3C
OH
O
C
C
keto
(c)
H3C
OH
O
C
C
C
H
C
H
CH3
enol
-H+
CH3
O-
O
C
C
O
O
_
H3C
C
H
CH3
H3C
C
C
C
H
Figure 1.
In homoleptic complexes, that is complexes in which all of the ligands are the same, the
metal typically bonds to 2 or 3 acac ions to form 4 or 6 coordinated complexes,
respectively. For example, a ball and stick model of Mn(acac)3 is shown in Fig. 2. The
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CH3
geometry about the Mn(III) is approximately octahedral with some Jahn-Teller distortion.
This complex and all homoleptic acac complexes that are octahedral are chiral.
Figure 2.
Mn(acac)3 octahedral complex.1
In this experiment you will synthesize and isolate a compound of Zn(II) and acac.
Although the procedure is designed to yield the monohydrate, Zn(acac)2∙H2O, the
anhydrous2 or dihydrate3 may be obtained. Using a variety of instrumental methods you
will analyze your product to determine which compound you have made.
The zinc acetylacetonate monohydrate is 5-coordinate (Fig. 3) and could have square
pyramidal or trigonal bipyramidal geometry. Xray analysis has shown the structure to be
square pyramidal with some distortion toward a trigonal bipyramidal structure. The
water molecule occupies the axial position.4 The anhydrous acetylacetone complex of
Zn(II) is a trimer containing both 5- and 6-coordinate zinc.2
Figure 3. Possible structures of zinc acetylacetone hydrates.
When H2O is present in a compound as a hydrate, or a material is just wet, it will appear
in the IR spectrum. If the compound contains water, a very broad band will appear in the
3200-3300 cm-1 region. This is the vibration that results from the stretching of the O-H
bond of water. The broadness and the position of the band are the result of hydrogen
bonding in the water molecule, either to other water molecules or to hydrogen bond
acceptor sites on another molecule. If the compound is anhydrous, this region of the
spectrum will be flat. The IR spectrum of Zn(acac)2∙H2O is shown in Fig. 4.
2
O-H stretch
3500
3000
2500
2000
1500
WAVENUMBERS/cm
1000
-1
Figure 4. IR spectrum of Zn(acac)2∙H2O
You will also obtain a 1H NMR spectrum of your compound and compare the spectrum
with that of the acetylacetone starting material. In the NMR spectrum of pure
acetylacetone, you will be able to observe the keto-enol equilibrium of this compound A
1H NMR spectrum of your sample will show how the spectrum of acetylacetone is
affected by the reaction with Zn(II) ion. A spectrum of acetylacetone is shown in Fig. 5
with assignments of the keto (k) and enol (e) forms.
CH3 (e)
DMSO
acetylacetone
CH3(k)
H2O
CH2 (k)
CH (e)
7
6
5
4
3
2
1
Chemical Shift/ppm
Figure 5. The 1H NMR spectrum of acetylacetone in dmso-d6. Obtained on a Varian
300 MHz NMR spectrometer.
Although it is possible to identify the presence of H2O in a sample by IR spectroscopy, it
is not possible to get a good quantitative measure. Another method, known as
Thermogravimetric Analysis (TGA) does give a quantitative measure of the amount of
H2O. In this method, a very small sample of material is placed in a platinum dish
suspended from a balance. The sample is placed in an oven and slowly heated, usually
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in an atmosphere of nitrogen. As the sample is heated it loses mass as it decomposes
and this mass loss is recorded at the temperature at which it occurs. A compound that
contains coordinated water will usually lose H2O around 100- 120 °C over a fairly small
temperature range, while a sample that is simply wet, will lose water gradually as soon
as heating is begun.
Figure 6. TGA spectrum of Zn(acac)2∙xH2O
In Fig. 6 the water of hydration is lost between 78-114 oC and further decomposition
(loss of acac) occurs until ~250 oC. A small amount of mass is additionally lost up to
~600 oC with metallic Zn as the ultimate product.
Experimental Procedure
CAUTION: Acetone is a highly flammable organic solvent and must not be used or kept
near an open flame. The 75% acetone solution that is used for recrystallization is also
flammable. You should avoid contact with the skin or inhalation of the vapors of
acetone and acetylacetone. It is advised that you wear gloves and work in a fume hood
or well ventilated area.
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Synthesis of Zn(acac)2•H2O. This portion of the experiment will require one period plus
a little time during a second period5. Weigh 0.80 g of zinc oxide and place it in a beaker
with 25 mL of distilled water. Heat the mixture to boiling and then remove it from the
heat. Add 3 mL of acetylacetone dropwise with continuous stirring to the ZnO mixture.
What changes did you observe? When the addition is complete, place the beaker in an
ice bath until its contents are thoroughly cooled. Suction filter the precipitate on a small
Buchner funnel with filter paper. Wash the precipitate with two or three portions of cold
water and suck out as much of the water as possible.
The solid that has been obtained is probably a mixture of Zn(C5H7O2)2.H2O,
Zn(C5H7O2)2.2H2O, and unreacted ZnO. It is necessary to recrystallize it in order to
obtain pure Zn(C5H7O2)2.H2O. To do this, prepare a solution of acetone and distilled
water that contains 35 mL of acetone plus 15 mL of water and add 1 mL of
acetylacetone to it. The acetylacetone serves to prevent decomposition of the product.
Finally, add the precipitate to the solution and stir thoroughly. What is the appearance
of the solution? Fit a long- or short-stem funnel with a piece of fine filter paper for a
gravity filtration and filter the mixture. The filtrate, which should be clear but not
necessarily colorless, is collected in a beaker. Cover the beaker with a watch glass or a
loosely fitting piece of aluminum foil and store it as directed by your instructor until the
next period. It is desirable that partial evaporation of the solvent occur during the
interim.
It is likely that crystals will have formed during the time in which the beaker of
solution sat between periods. The procedure for the next step depends on whether
there is still solvent in the beaker (Step (a)), or whether the solvent is all evaporated
(Step (b)).
Step (a). If there is still solvent present, cool the crystals and remaining solution by
placing the beaker in an ice bath and, concurrently, cool about 15-20 mL of acetone to
be used as a wash solvent. Suction filter the crystals and wash with two small portions
(5 and 10 mL) of cold acetone to remove impurities, mainly the acetylacetone that was
added during the recrystallization.
Step (b). If all of the solvent has evaporated, transfer the crystals to the filter funnel and
wash with three small portions of cold acetone.
In either case, the product should be air-dried. When it is thoroughly dry, place it in a
tared sample bottle and weigh it to obtain the mass of your product. Record this on the
report sheet. Label the bottle with your name and the formula of the product.
Melting Point. Determine the melting point and record it on the report sheet.
Infrared Spectrum. Obtain an infrared spectrum of your Zn complex as a solid.
TGA Analysis. In order to run a TGA analysis requires a considerable amount of time.
Therefore, it is likely that only one or two samples from the class can be run. If it is not
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possible to obtain the analysis of your sample, your instructor will give you a plot to
analyze or instruct you to use Fig. 5.
1H
NMR Spectra. Obtain two clean 5 mm NMR tubes from your instructor. Weigh
approximately 5 mg of your dry Zn(acac)2 product into a small vial or small test tube and
add 0.75 mL of DMSO-d6. Carefully swirl the liquid to dissolve your sample; it should
dissolve completely. Transfer the solution to one of the NMR tubes and stopper it. In
another NMR tube place 2 drops of pure acetylacetone and 0.75 mL of DMSO-d6,
stopper it and invert several times to mix the two liquids thoroughly.
See your instructor for directions on how to obtain a 1H NMR spectrum and do so for
each of your samples. Set the resonance of the residual DMSO protons at 2.50 ppm as
your internal reference. Integrate each line in each spectrum except the DMSO
resonance (a multiplet) at 2.50 ppm and a H2O resonance which you may observe at
about 3.3 ppm.
NMR Data Treatment. Assign each of the resonances in the spectra to a specific
proton in the compound. From the integrated intensities of the NMR lines in the
spectrum of pure acetylacetone determine the mole ratio of the two tautomers. Which
tautomer is more stable under the conditions at which you ran your spectrum? Examine
the spectrum of your product. The presence of a resonance for H2O may not
necessarily indicate that the compound is a hydrate because DMSO absorbs H 2O from
the air very easily or it could be that the sample was not completely dried of the
recrystallization solvent.
References
1. Wikimedia Commons: public domain.
2. Bennett, M. J.; Cotton, F. A.; Eiss, R. Acta Cryst. 1968, B24, 904-913.
3. Lippert, E. L.; Truter, M. R. J. Chem. Soc. 1960, 4996-5006.
4. Montgomery, H.; Lingafelter, E. C. Acta Cryst. 1963, 16, 748-752.
5. Adapted from Walmsley, J. A.; Walmsley, F. Chemical Principles, Properties, and
Reactions in the Laboratory, Addison-Wesley: Reading, MA, 1985, pp. 180-182.
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Report
1. Mass of Zn-acac compound obtained ________________________
2. Melting point of your product _______________________
3. Analysis of 1H NMR spectra.
A. Acetylacetone
(a) Keto form: Chemical shift of CH3 (in ppm) _____________
Integration of CH3 _______________________
Chemical shift of CH2 _____________________
(b) Enol form: Chemical shift of CH3______________________
Integration of CH3 ________________________
Chemical shift of CH ______________________
Chemical shift of OH ______________________
(c) Mole ratio of keto to enol form of acetylacetone in your sample. Show your
calculations.
B. Zn-acac Complex
(a)
Chemical shift of CH3 ______________________
Chemical shift of CH _______________________
Chemical shift of H2O ___________________
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(b) Suppose that the acac was bonded as a monodentate ligand to the Zn(II) ion
as shown here. How would the 1H nmr spectrum be different from your
spectrum?
4. Examine your IR spectrum.
(a) If present, what is the frequency of the O-H stretching vibration
in cm-1?__________________
(b) Does your product contain H2O?
5. (a) Calculate the percent mass loss corresponding to the loss of H2O (between ~
80-120 oC). See Fig. 5._________________
(b) Calculate the number of moles of H2O that this mass loss corresponds to.
____________
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6. Taking into account all of the data you have collected, what do you conclude is the
likely formula of your product?
7. Calculate the theoretical yield of your compound.
8. What is the percentage yield?
9. Attach your IR spectrum and your 1H NMR spectra to this report. Be sure to put your
name on each spectrum.
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