Results and Discussion
Synthesis of Cr(CH3COCHCOCH3)3 and Co(CH3COCHCOCH3)3
The product of the synthesis of Cr(CH3COCHCOCH3)3 and Co(CH3COCHCOCH3)3 was
crystals color purple and green respectively, with a yield of 98% for Cr(acac)3 and 6.66% for
Co(acac)3. Results of the Synthesis are showing in supplementary material.
Solubility Testing.
The solubility test was performed to have the most appropriate organic solvent to recrystallize the
compounds: Cr(CH3COCHCOCH3)3 and Co(CH3COCHCOCH3)3. It was necessary to choose two
organic solvents, one in which the compound was soluble and the other in which the compound
was insoluble, but these solvents should be miscible with each other. The results of the solubility
test are giving in supplementary material Table 1.
Recrystallization of Cr(CH3COCHCOCH3)3 and Co(CH3COCHCOCH3)3
The synthesis product was recrystallized, getting large crystals color dark purple for Cr(acac)3 and
dark green for Co(acac)3 as shown in Fig 2. The recrystallization of Cr(acac)3 was carried out with
the Layering method; the solvents used are: Toluene and Petroleum Ether in the second attend as
described in the Experimental Section. For with Co(acac)3 3 slow evaporation was perform using
methanol as the soluble solvent.
Characterization of paramagnetic complexes
Melting point
The melting point of a synthesis product can be used to assess its purity. A pure compound will
melt completely over a narrow temperature range. When impurities are present, the melting point
is lowered, and the range over which the compound melts is wider The melting point was
performed using the Melting Point Apparatus
Visible Spectroscopy
Another technique used to characterize the product obtained was UV-Vis spectroscopy, where
represents the electronic transitions that occur in the complex when it absorbs electromagnetic
radiation. In the spectrum, UV-Vis obtained experimentally at a wavelength 380nm a transition
d—d in Cr(acac)3 molecule are observed. The equation 𝜀 = A / c* L is used to get the molar
extension 𝜀, C is the concentration, which is 0.0096 M, and the path length is 1.00 cm.
IR
The IR spectra of Cr(acac)3 and Co(acac) 3 are shown in Fig. 5A/B. Based on the IR of 2,4Pentadione, we can observe the carbonyl group is located around 1600-1700 cm-1. In the infrared
spectrum obtained for Cr(acac)3, it is possible to observe a series of corresponding absorption bands
to the different vibrations of the functional groups of the synthesized molecule; most of those
bands are presented between 1600 cm−1 and 400 cm−1. Previous studies suggest 3 that the positions
of the bars in IR spectrum of bulk around 1276 cm−1 correspond to ν (C – C) + ν (C – O), 1515
cm−1 check to ν (C = C) and 1567 cm−1 correspond to ν (C = O) in Cr(acac) 3. In the infrared
spectrum obtained for Co(acac)3, most of those bands are presented between 1600 cm−1 and 400
cm−1, and the most peak representative is the band around 1567 cm-1 in Co(acac)3 was identified
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as a carbonyl stretch. More specifically, a study suggests 4 an assignment of the bands in the IR
spectra of Cr and Co tris(acetylacetonates)in detail:
According to a previous study 4 the usual vibrations in Cr and Co tris(acetylacetonates) are
basically similar, and the bans getting through the experiment are similar to the ones described in
this study.
H NMR
NMR spectra help to address the magnetic susceptibility of the metal complex Cr(acac)3. The goal
of calculations is to determine the number of unpaired d-electrons using the Evans Method. In this
experiment the paramagneic character of Cr(acac)3 was tested, therefore for Co(acac)3 we did not
report results for having diamagnetic properties.
Since it is the ratio of two magnetic fields, susceptibility is a dimensionless number. Diamagnetic
substances have negative susceptibilities (χ < 0); paramagnetic, superparamagnetic, and
ferromagnetic substances have positive susceptibilities (χ > 0)1 . In previous studies shown 5 in
octahedral complexes, like Cr(acac)3, the lobes in two of the five d orbitals, the dz2dz2 ,
and dx2−y2dx2−y2 orbitals, point toward the ligands.
However, the repulsions between the electrons in the eg orbitals and the ligands are greater than
those between the electrons in the t2g orbitals and the ligands. This is because the lobes of
the eg orbitals point directly at the ligands, whereas the lobes of the t2g orbitals point between them.
Thus, electrons in the eg orbitals of the metal ion in an octahedral complex have higher potential
energies than those of electrons in the t2g orbitals. A closer H NMR spectrum for Co(acac)3 ,peaks
that corresponds to acac complex in the region between the absorbance 0 and 3 ppm corresponding
to the methyl and hydrogen and on the left side of the spectrum around 7 ppm the Chloroform peak
is shown, demonstrating the diamagnetic character of Co(acac03
Experimental Section
Synthesis of Cr(CH3COCHCOCH3)3 or Cr(acac)3
For the synthesis, heat a water bath on a stirring hotplate and start by adding 1.30 g (0.0048 mol)
of CrCl3•6H2O into a 50-mL Erlenmeyer flask and 20.0 mL of distilled water and stir briefly
using a small magnetic stirring bar. When the chromium compound is formed, add 5.0 g of urea
and 4.0 mL(0.039 mol) of acetylacetone. Clamp the flask in place in the water bath, do not set the
flask directly on the bottom of the water bath, and heat the water to boiling. Heat the mixture,
uncovered and with vigorous stirring, for an hour. As the urea releases ammonia and the solution
becomes basic, deep maroon crystals will begin to form. After one hour, allow the reaction to cool
to room temperature and then place it in an ice bath to complete crystallization. Proceed to vacuum
filtration using the Büchner funnel to collect the crystals and wash out the flask and the crystals
on the filter with plenty of deionized water (about 3 x 10 mL). Allow the product to dry until the
next lab period and then weigh the dry final product to determine the yield of the dry product.2
Synthesis of Co(CH3COCHCOCH3)3 or Co(acac)3
2
To synthesize Co(acac)3, prepare a water bath on a stirring hotplate, add 0.38 g (0.0032 mol)
Co(II) carbonate into a 50-mL Erlenmeyer flask, and add 3 mL (0.03 mol) of acetylacetone, stir
and cover the solution with a watch glass. Place the Erlenmeyer flask into a water bath on a stirring
hot plate. Stir the reaction using the magnetic stirring bar and heat the reaction to about 80°C
keeping this temperature for 5 minutes. Slowly add 4.5 mL of 10% hydrogen peroxide (H2O2)
drop-wise at a rate of 1-2 mL/min using a disposable pipette while continuing to stir the reaction
mixture. Cover the flask with a watch glass between H2O2 additions. Cool the mixture to room
temperature and then place it in an ice bath for about 15 minutes or until crystallization appears to
be complete. Filter the product using a Büchner funnel, washing remnants out of the flask with
deionized water (2 x 3 mL). Allow the product to dry until the next lab period and then weigh the
dry final product to determine the yield of the dry product.2
Solubility Testing.
To facilitate the recrystallization is necessary to test the synthesis's products with different types
of organic solvents to find the best solvent to use in the recrystallization process. The first step is
to consult Bibliography or a Handbook to find the best solvents to use in the solubility test; pick
at least 4 or 5 solvents that the bibliography suggests are soluble in. For recrystallization, it is
necessary to choose two solvents, one in which the compound is soluble and the other in which it
is insoluble. Both solvents must be miscible to each other. For this experiment, in an assay tube
add, the completely dry Cr(acac) 3 in about a 1/2 mL of solvent to demonstrate its solubility in a
concentrated solution. To test solubility, use solvents: methanol, ethanol, acetone, toluene,
acetonitrile, water, pentene, and petroleum ether. The second compound was tested, Co(acac)3,
with Methanol, Acetone, Toluene, Petroleum ether, and water. The solubility test was done by
placing a small amount of Co(acac)3 in an assay tube and add about 1/2 mL of solvent. Classify
the solubility of the product insoluble, slightly soluble, and insoluble. Notice significant changes
that show that a solvent is ideal for the recrystallization process, such as color changes or the
turbidity of the solution.
Recrystallization
Layering: Consists of creating two layers of solvents, one denser than the other. The layers are
carefully made so that there is as little mechanical mixing as possible; they will mix, however,
over time due to diffusion. For the layering process is essential to find a solvent in which the
compound is soluble that is miscible with another solvent in which the compound is insoluble. For
the first compound Cr(acac)3, thanks to the solubility test and the density of the solvents, it was
decided to use methanol as a soluble solvent and pentane as an insoluble solvent, expecting that
methanol with a density of 0.7917 g / ml was the first layer that would be on the bottom. The
pentane with a density of 0.626 g / ml would be on top. Using a 50 ml Erlenmeyer flask with a
stopper, the Cr(acac)3 compound was completely dissolved in methanol; it is necessary to use a
minimal amount of solvent to be able to preserve a dark color of the solution, the Cr(acac)3 solution
was dark purple. Little by little, the pentane was added, trying to create two layers of solvent.
Although the solvents were miscible, there was no layer formation with these solvents due to
instabilities between both solvents; it was decided to evaporate the entire pentane on a hot water
bath; in this way, the Cr(acac)3 product was obtained again. A second recrystallization attempt
was made for Cr(acac)3 with toluene and petroleum ether. The densities of toluene and petroleum
ethers are 0.866 g / ml and 0.64 g / ml, respectively, expecting that the toluene by itself would be
denser in the upper layer and the petroleum ether in the lower layer. Both solvents were tested,
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turning out to be mixed. The solution was prepared with toluene, and little by little, this solution
of product underneath the insoluble solvent was added by taking the pipette directly to the bottom
of the Erlenmeyer flask and then dispensing the solution; this is the "bottom-up." Both layers
diffused together, and crystals were formed at the interface. To get the crystals, filtration with
Büchner funnel was used, washing remnants out of the flask with cold toluene (2 x 3 mL). For
Co(acac), three slow evaporation was performed. Using a 50 ml Erlenmeyer flask with a stopper,
the Co(acac)3 compound was completely dissolved in methanol, obtaining a concentrated solution
of Co(acac)3. The next step was heating the solution in a water bath to evaporate the methanol and
get more recrystallized product. Then the product was filtered with a Büchner funnel to remove
any undissolved particles and place them in a small vial. 1,2
Melting point
The characterization of the obtained compounds Cr (acac) 3 and Co (acac) 3 was tested by
determining the melting point of each of the compounds. Melting Point Apparatus - DigiMelt
MPA160 was used for this procedure. The melting point of our synthesized compounds was
previously investigated, and it was expected to obtain melting points close to that reported in the
literature, which are 210 ° Cr(acac)3 and 210-213 ° C for Co(acac)3. A small amount of the
compound was used, introduced into a capillary, and the capillary was placed in the designated
place on the measuring equipment. The melting point was taken twice; the first time it was brought
to the near melting temperature to observe if our compound had a melting point close to the one
reported in the literature, and the second test was carried out gradually to monitor the temperature
at the beginning and at the end of the melting point.2
Visible Spectroscopy
Shimadzu- 1800 UV-vis was used to take a visible spectrum. A compound sample needs to be
dissolved in a solvent of some sort. The absorbance of a compound is proportional to the amount
of that compound dissolved in the solution. It would help to aim for a maximum absorbance value
of about 1.0 (absorbance values do not have any units associated with them). The solvent used was
toluene. A small amount of each compound Cr(acac)3 and Co(acac)3 was dissolved in toluene:
10 mg of the complex in 5 mL of toluene, obtained a concentration of 0.00573 mol/L for Cr(acac)3
and 0.00561 mol/L for Co(acac)3 and Each solution was the filter through a disposable pipette
fitted with a Kimwipe plug to obtain a clear solution. To get a clear solution, an aliquot of the
solution was taken with 1 mL in 5 mL of toluene. The solution had a pale color but discernable
color. It obtained a spectrum for each compound from 390 nm to 900 nm using the Ocean Optics
Spectrometer.
Infrared Spectroscopy
Perkin Emer-Spectrum Two IR was used to determine the IR spectra of the metal complex. Before
using the measurement, it is necessary to clean the testing area (ATR crystal) and the tip of the
ATR clamp using IPA and Kimpwipes. Each compound: Cr(acac)3 and Co(acac)3 was placed
separately in the testing area (ATR crystal), and slowly the tip on the ATR clamp gently pressed
the sample until obtaining a graph for each of the compounds. The changes were observed due to
the deprotonation in IR spectra in the region between 1500-1700 cm-1, especially the carbonyl
band.
H NMR
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To perform a characterization of the compound, we used NMR if your complex is diamagnetic or
determine its magnetic moment by the Evans. We used an NMR spectrometer equipped with a
superconducting magnet, and CDCl3 will be used as the solvent in the measurements. Sealed
capillaries containing CDCl3 were already prepared. To prepare the sample, 3.5 mg of Cr(acac)3
and 5.1mg of Co(acac)3 were weighed and added to the NMR tube. Precisely 0.7 mL of CHCl3
was then added to the NMR tube; the tube was cap and gently shaken to dissolve the sample
completely. A sealed capillary that contains pure chloroform, CHCl3, was added to the
theoretically paramagnetic sample. The NMR spectrum of both solutions was recorded. In the
record, there should be two peaks near δ 7: one is due to CHCl3 in the capillary, and the other you
to CHCL3 that has been paramagnetically shifted by the dissolved Cr(acac)3. Determine the
chemical shifts of these two peaks, and measure their separation between them in hertz. This
experiment is characteristic for the paramagnetic complex: Cr(acac)3
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