Abstract:
Tris(2,4- pentanedionato) chromium(III) and Tris(2,4- pentanedionato) manganese(II) are metal complexes which were both synthesized in a similar way. The Chromium complex was synthesized with the use of chromium hexahydrate, urea, and acetylacetone. The resulting reaction gives dark red crystals. The manganese complex required different reagents. Manganese(II) chloride tetrahydrate is combined in conjunction with potassium permanganate and sodium acetatetrihydrate, producing a compound which is almost black in color. Magnetic susceptibility, IR spectra and Melting points were utilized in order to characterize these complexes.
Safety:
-Chromium (III) hexahydrate can be mildly toxic or irritating.
-Manganese (II) chloride tetrahydrate is harmful if swallowed, inhaled or absorbed through the skin. If it comes in contact with the skin or eyes flush with plenty of cold water.
-Potassium permanganate is a powerful oxidizing agent and should be handled with caution.
-Acetylacetonate should always be dispensed under a ventilation hood; fumes can be harmful .
Introduction:
An Organometallic compound is composed of a metal cation bound to organic anions or ligands. These metal complexes, also known as Coordination compounds, are formed via a simple acid base reaction; the acid in being the Metal and base being a strong base (2). The organic ligands attached can vary from something as simple as a methyl group or increasingly complex, such as an octadiene. Ligands that bond at only one atom are known as unidentate, one tooth bonded the metal (2). When there are two or more possible points of bonding the ligand will be known as bidentate or polydentate. When there are multiple points of bonding between the central metal and a single ligand the compound is identified as a chelate, or a claw complex.
Coordination complexes of Chromium and Manganese can be formed very simply, in relatively small quantities or in large, mass produced quantities. Acetylacetonate is formed when acetylacetone is in the presence of a strong base. The base is able to deprotenate acetylacetone terminally. The newly formed acetylacetonate ion is able to ionicly bond to the inorganic metal. The following figure demonstrates how the acetylacetone ion is formed.
Figure 1
Formation of acetylacetonate ion (7).
The product on the right is an ion stabilized by resonance. This ion is able to form an ionic bond to the metal and then subsequently deprotenated again on the other oxygen. The second oxygen then bonds at another site on the metal.
Figure 2
A general acetylacetonate complex (7).
The following reactions show how manganese(II) and chromium(III) form acetylacetonate complexes.
Tris(2,4pentanedionato)Chromium(III)
· + 3
Tris(2,4pentanedionato)Manganese(II):
· +
According to Sigma-Aldrich Co. Chromium acetylacetonate can be used in the modifications of polyurethanes. As stated in US patent 4571391, Chromium acetylacetonate can also be taken as a dietary supplement to lessen the effects of diabetes due to its hypoglycemic properties. Manganese (II) acetylacetonate can be used in the preparation of porous plasma films.
Procedure:
Chromium(III) acetylacetonate:
To begin synthesis of Tris(2,4-pentanedionato)Chromium(III), 2.0mL of water was combined with .128g of chromium (III) chloride hexahydrate. The mixture was heated to about 50 C and covered. Once all of the chromium was dissolved in the water, 500uL of urea was added and 400uL acetylacetone. Excess acetylacetone was be added to push the reaction to completion. Once all reagents were mixed in the reaction vessel was heated in a water bath and stirred for about an hour, under the hood. The reaction was allowed to proceed until dark red crystals completely formed in the solution. The crystals were separated by vacuum filtration and washed with three 200uL portions of cold water (6). The product was allowed to dry and then analyzed via IR and melting point.
Manganese(II)acetylacetonate:
Synthesizing the Tris(2,4-pentanedionato)Manganese(II) was synthesized by first adding 100mg of (.5mmol) Manganese(II) chloride tetrahydrate to 260mg of (1.9mmol) sodium acetate-trihydrate all in a 10mL flask fitted with a stir bar. 4mL of water was added to help the solution to mix easier. Once the all of the solute dissolved, 400uL of (3.84mmol) acetylacetone was stirred in. The reaction vessel was set aside and a potassium permanganate solution was prepared by mixing 20mg of (.127mmol) potassium permanganate in 1mL of water. The react in flask is placed under heat and the potassium solution was added drop wise. Once all of the potassium solution was added the flask is heated to almost boiling for 10 minutes, the mixture was allowed to cool to room temperature. The dark brown precipitate was collected via vacuum filtration(6). The product was left to dry and then characterized via IR.
Results and Discussion:
Once the products were finished drying the new metal complexes were ready to be analyzed.
The melting points were taken and masses were taken and are presented in table 1 on the next page.
Table 1
Physical qualities
Tris(2,4-pentanedionato)Chromium(III) / Tris(2,4-pentanedionato)Mn(II)Color / Dark red- Maroon / Dark Grey- Black
Mass / .34g / .33g
Melting point / 212.0 C / >250.0 C
Percent Yield / 52.0% / 52.9%
The mole to mole ratio was determined based on the equations previously mentioned.
The following equation was used to calculate percent yield:
· x 100%= Percent yield
The respective metal starting compounds were determined to be the limiting reactant for these syntheses. The percent yield was calculated to 3 significant figures.
· Tris(2,4pentanedionato) Chromium(III)- 52.0%
· Tris(2,4 pentanedionato) Manganese(II)- 52.9%
The products were characterized by melting point and both and Magnetic susceptibility measurements and IR spectra. Magnetic susceptibility tests of tris(2,4pentanedianato)Cr(III)acetylacetonate and tris(2,4pentanedionato)Mn(II) were conducted. Table 2 on the following page gives the results of the tests.
Table 2
Magnetic Measurements
Variable / Cr(III)acetylacetonate / Mn(II)acetylacetonateMW / 349.33 / 253.14
1084 / 1084
-025 / -032
R / 378 / 432
L / 1.2 / .4
Mass used / .023g / .02g
The figures from table 2 were used in the following calculations to determine magnetic susceptibility. Before the products could be analyzed the calibration constant was calculated. In order to calculate the Calibration constant, , a standard sample which is labeled with the value is used in order to set the balance. Using the values of table 2, the molar susceptibility and magnetic moment were calculated. Equation 2 was used to calculate which was then employed in later equations. Equations 3 and 4 were implemented to calculate mass susceptibility and magnetic moment.
The magnetic moments for Tris(2,4-pentanedionato)Chromium(III) and Tris(2,4-pentanedionato) Manganese(II) equal 4.19 and 2.83 respectively.
All of the mass susceptibility calculations were calculated according to equations 1 through 6 and table 3 gives the results of the magnetic calculations.The Mass susceptibility for the Chromium and Magnesium complexes equals 2.1 * and 9.2 * respectively. To calculate the molar susceptibility the mass susceptibility was simply multiplied by the molar mass. The calculated Molecular susceptibility was7.33 * for Chromium(III) acetylacetonate and 2.32 *for Manganese(II) acetylacetonate.
Eq. 1 Calculation of
Eq. 2- Calculation of Mass suceptibility :
Eq. 3: Molar Susceptibility
Eq. 4: Magnetic Moment calculation:
Note: there is a correction factor associated with which was obtained from the manual which was supplied with the Johnson and Mathey balance.
For first row transition metals the correction factor used was different for each complex.
For chromium the correction factor was 7.34 * and for Manganese 3.24 . The correction factor was added to , 13 * , which was also obtained from the manual which is supplied along with the balance. Using the magnetic moment from equation 4, the number of unpaired electrons was calculated for both complexes, according to equation 6.
Eq. 6: calculation of unpaired electrons:
Table 3 gives the values of , Mass susceptibility, magnetic moment and the number of unpaired electrons for both the chromium and magnesium complexes.
Table 3
Magnetic susceptibility Calculations
Variable / Tris(2,4pentanedianato)Cr(III) / Tris(2,4pentanedianato)Mn(II)correction / 7.353 * / 3.253 *
2.12 * / 9.22*
Molar susceptibility / 7.33 * / 2.33*
Magnetic moment / 4.19 / 2.83
Unpaired electrons / 3 / 2
Discussion:
The results analysis gives some positive feedback. The first measurements taken were strictly qualitative. According to Singh, the chromium complex should a dark red color and the manganese complex a dark brown to black color. The colors of both compounds were consistent with Singh’s observations( ).
According to U.S. patent 7282573 Chromium (III) acetylacetonate has a boiling point of 209 C- 213 C, and the measured melting point was 212 C which is consistent with the expected data(4) . According to American chemical elements web site the melting point was recorded at 214C(1). Manganese (II) - acetylacetonate has a melting point of but the measured melting point was above 250 C. on the American chemical elements website there is no recorded melting point for this compound(2). By this information I concluded that the products are in fact the desired products. For further support an IR spectrum was also utilized to examine the products.
The IR spectra for both compounds were also analyzed and the information was relatively consistent with the expected results. The observed stretching was very similar to the expected stretching of these compounds. Because both molecules had the same ligand their IR spectra look relatively similar. According to the IR table referenced the peaks between 1715cm- 1745cm demonstrate carbonyl stretching. Many of the peaks at2950cm and below show the various types of C-H stretching in the ligands. The stretching above 3800cm is indicative of a metal to oxygen bond.
According to calculations Tris(2,4 pentanedianato) Chromium(III) has 3 unpaired electrons and Tris(2,4pantanedianato) Manganese(II) has 2 unpaired electrons. This would be consistent with the expected oxidation numbers of each metal complex. By definition any metal atom with a permanent magnetic moment is paramagnetic. Because there are unpaired electrons on both complexes it can be deduced that both have a dipole moment and the both have paramagnetic qualities.
Erroneous results can arise from improper isolation of the products. Perhaps a recrystalization could give a more pure product; this would yield better data for the measurements of Mn(II) acetylacetonate. Because the products are so dark in color, there could be difficulty obtaining a good IR measurement, as it might be difficult for the waves to penetrate the sample. When taking magnetic susceptibility measurements, the machine should be well calibrated for the testing. If the balance is not calibrated correctly all of the data it produces will be erroneous and yield incorrect calculations for all equations. If any error occurred during the calculations of magnetic susceptibility they can be attributed to inexperience with that brand of balance.
Conclusion:
Although some of the results were inconclusive, I believe the synthesis was still successful. Many of the physical characteristics observed were concluded to be consistent with the expected results. Both of the compounds were in crystal form and the coloration matched the expected colors. The IR data following gives strong evidence for a successful synthesis. A good way to verify if a metal complex was formed would be to implement these products as catalysts in other reactions. If they perform in the same manner as the expected complexes then it would be fair to say that the products obtained are in fact the desired complexes.
These metal complexes have a freely mobile hydrogen which is the source of stored electrons. This very important electrochemical feature allows metal hydrides to be used as reagents and catalysts. Erroneous results could arise from improper synthesis of the product. Although the percent yield was only at 50% the structure of both complexes were determined and confirmed by the IR.
Bibliography
1.American Elements. “Chromium Acetylacetonate”. American Elements. 30 Oct. 2008 http://www.americanelements.com/cracac.html .
2. American Elements. “ Manganese Acetylacetonate”. American Elements. 30 Oct. 2008
http://www.americanelements.com/mnacac.html
3. Atkins, Peter; Overton Tina et al. Inorganic Chemistry fourth ed. W.H. Freeman and Company., New
York
4. Chaudhuri, Mihir K., Upasana Bora, and Sunjay K. Dehury. Process For making Acetylacetonates. Council Of scientific and Industrial Research, assignee. Patent 7282573. 2007.
5. Sigma- Aldrich. "245763 Manganese(II) Acetylacetonate." Sigma-Aldrich. 24 Sept. 2008 <http://www.sigmaaldrich.com/catalog/search/productdetail/aldrich/245763
6. Singh, M. Microscale Inorganic Chemistry. New York, 1991.
7. Answers.com. “Acetylacetone”. 25 Sept 2008 <content.answers.com/.../bb/150px-Cu(II)_acac.png> .
Thank you to Alex Smith and Stephen Benet for you assistance in the synthesis. And thank you to Christopher Neldon and Justin Huddleston for their reviews.
Mehul Patel
Synthesis of Tris(2,4pentandianato)Cr(III) and Tris(2,4pentandianato)Mn(II)
Under the Supervision of Dr. Julia Padden Metzker
Georgia College and State University
12/3/08