LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Coordination Complexes as Catalysts: The Oxidation of Anthracene by Hydrogen Peroxide in the Presence of VO(acac)2 Kimberly D. M. Charleton and Ernest M. Prokopchuk* Department of Chemistry, The University of Winnipeg, Winnipeg MB, Canada, R3B 2E9
bS Supporting Information ABSTRACT: A laboratory experiment aimed at students who are studying coordination chemistry of transition-metal complexes is described. A simple vanadyl acetylacetonate complex can be used as a catalyst in the hydrogen peroxide oxidation of anthracene to produce anthraquinone. The reaction can be performed under a variety of reaction conditions, ideally by different students in the same class, allowing for the accumulation of data that can be interpreted by students in their discussion of the reaction. Performed in the absence of the vanadium complex, the reaction does not produce any product. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Collaborative/Cooperative Learning, Hands-On Learning/Manipulatives, Aromatic Compounds, Catalysis, Coordination Compounds, IR Spectroscopy, Thin Layer Chromatography
U
ndergraduate chemistry students are familiar with catalyzed reactions, typically involving either an acid or base as a catalyst, from introductory organic chemistry courses. In inorganic chemistry courses, these students gain experience synthesizing various metal complexes, but often the product is the end point with little indication of the significance, if any, of the product. Many students are unaware that some coordination complexes can be used as catalysts in chemical reactions. This experiment was designed to demonstrate the use of a coordination complex as a catalyst and to introduce the students to assessing the performance of a catalyst, specifically relating reaction conditions to product yield, percent conversion, and turnover number (TON). We chose an oxidation reaction that could be done using standard glassware without needing any special apparatus to deal with air or moisture sensitivity. Furthermore, to enhance the impact of the experiment on the student, we chose an aromatic substrate, anthracene, for the oxidation (Scheme 1) because most students are familiar with the concept that aromatic rings are often resistant to many reactions. The catalyst in this oxidation reaction was VO(acac)2, which can either be purchased or synthesized easily.1 In the first 3-h lab period, all students prepared the catalyst (Scheme 1). After the synthesis of the catalyst was complete, a class discussion to determine the various reaction conditions for the oxidation of anthracene was conducted. In the second lab period, the students used the catalyst to oxidize anthracene. The oxidation procedure was based on work reported by Men’shikov and co-workers,2 but the reaction time was reduced so that the oxidation and separation could be completed in a 3-h period. To investigate the activity of VO(acac)2 as a catalyst for oxidation, students used different reaction conditions determined in the class discussion during the first week. The individual data generated from the catalysis reactions were shared by the entire Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Scheme 1. Catalytic Oxidation of Anthracene
Scheme 2. Preparation of the Catalyst
class. While the reaction was refluxing, the students examined the catalyst in different solvents. A portion of the third lab period was required for purifying the oxidation products and recording yields.3
’ EXPERIMENTAL DETAILS The preparation of VO(acac)2 was based on a known procedure1 and modified to use VOSO4 as the starting material (Scheme 2). The synthesis involved dissolving VOSO4 in water, adding 2,4-pentanedione, and then adding an appropriate base. We used saturated Na2CO3 solution and students continued to add this solution until there was no effervescence upon further addition. The solid product was filtered off and washed with water. The solid was allowed to dry on the filter before being Published: May 20, 2011 1155
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Journal of Chemical Education collected and stored in a sealed sample vial. The product was characterized by infrared (IR) spectroscopy. For the oxidation reactions, VO(acac)2 and anthracene were placed in a round-bottom flask and dissolved in ethyl acetate (Scheme 1). To this was added hydrogen peroxide and the reaction was allowed to proceed at the desired temperature for 2 h. When the reaction was done, water was added and the organic layer removed. The water layer was then extracted with chloroform or dichloromethane and the organic solution was combined with the ethyl acetate. The organic solvents were evaporated to isolate the solid product, which was then washed with toluene and allowed to dry. The product was collected, weighed, and then analyzed by thin-layer chromatography (TLC) using standard samples of anthracene and anthraquinone, running three spots side by side on the same plate. An ultraviolet (UV) lamp was used to visualize the spots on the TLC plate. If the product was not pure, further washings with toluene were performed until the TLC showed pure product. The capacity for the 5-coordinate structure to accommodate an additional ligand can be demonstrated by dissolving the compound in coordinating solvents such as pyridine or solutions containing aqueous ammonia or sodium carbonate. Coordination of a sixth ligand changes the color compared to solutions in noncoordinating solvents such as chloroform or dichloromethane. Given the workup that we use, the catalyst is not easily collected and recycled.
’ HAZARDS All chemicals used in this experiment should be handled with appropriate care. They are all considered to be irritants and contact with skin and eyes, and inhalation should be avoided. The organic compounds are flammable. In addition, chloroform and dichloromethane are considered possible carcinogens. Hydrogen peroxide is corrosive and may cause burns, as well as being a strong oxidizer, which may cause fire if brought into contact with flammable materials. Anthracene is a strong irritant and classified as an A1 carcinogen. ’ RESULTS AND DISCUSSION The synthesis of VO(acac)2 is straightforward. As the students add the carbonate solution, the evolution of CO2 is a useful indication that the deprotonation is occurring. When the addition of more carbonate solution does not produce any further effervescence, the deprotonation is complete. Other bases such as sodium acetate could have been used at this stage but carbonate was chosen to expose the students to a different method of determining when the appropriate amount of based had been added. There is potential for excess carbonate, or perhaps hydroxide formed due to the presence of carbonate, to coordinate to the vanadium and decrease the yield of product. During the addition of base, the product precipitates out of solution and in the end the mixture is a thick suspension that is filtered to isolate the product. After washing with water and allowing the product to dry, a blue-green solid is collected. Characterization by IR spectroscopy allows for the observation of the strong VdO peak at 995 cm 1. No special storage arrangements are necessary if the complex will be used within the span of a week. If it will be stored for several weeks or months, then the sample should be kept in a sealed container to avoid discoloration of the compound. No other precautions against air or moisture are required. In practice,
LABORATORY EXPERIMENT
Table 1. Typical Results from Instructor and Student Experiments for the Oxidation of Anthracene to Anthraquinone Average Yield Conditions Normala Half H2O2
Average TON
Anthraquinone (%)
12 ( 0.5 14 ( 2
40 ( 1 50 ( 1
Double catalyst
9(1
60 ( 2
55 °C
3.5 ( 1
12 ( 2
40 °C
1 ( 0.5
3 ( 0.5
1h
5 ( 0.5
17 ( 1
4h
22 ( 3
73 ( 5
No Catalyst
0
0
No H2O2 Double catalyst at 40 °C
0 0
0 0
Half everything
6(1
16 ( 1
7.5 mL H2O2, 300 mg
10 ( 0.5
53 ( 1
anthracene, 25 mg catalyst, and 10 mL ethyl acetate a
Normal conditions are 500 mg anthracene, 25 mg catalyst, 20 mL ethyl acetate, 15 mL hydrogen peroxide at reflux for 2 h.
this discoloration has not been observed in our teaching labs and its affect on the reaction has not been investigated. In addition to the oxidation reaction, it is possible to use VO(acac)2 for a series of qualitative tests to demonstrate the effect of coordination of other ligands at the vacant site in the square pyramid structure. This can also be used to illustrate the difference between coordinating and noncoordinating solvents. For example, in chloroform or dichloromethane, the complex formed a green solution, whereas in a coordinating solvent, such as pyridine, the complex forms a yellow-brown solution. Similarly, students can observe how coordination affects solubility because the complex is insoluble in water whereas it dissolves in aqueous ammonia. Using standard laboratory glassware, the oxidation reactions are performed under a variety of reaction conditions. Some possible variations include using different amounts of catalyst, anthracene, or peroxide, and using different reaction temperatures. After 2 h, the reaction was diluted with water and the organic layer removed. The aqueous layer was then extracted with dichloromethane. The organic solutions were combined and the solvent removed by rotovap. In the absence of a rotovap, alternative techniques for evaporating the solvent may be used. Purifying the product is easily achieved by washing with toluene, which dissolves anthracene but does not dissolve any significant quantity of anthraquinone. Our experience has been that there is always some anthracene present, so to save time and reduce handling losses, we have the students wash their product once with toluene before they perform the first TLC. Although we decided to use TLC on silica gel to detect when the product was pure anthraquinone, many instrumental techniques could be used to quantitatively determine the composition of the reaction product, which would allow for the percent conversion of anthracene to be determined. From the TLC, it was easy to determine if there was still anthracene present in the product, though the quantity is unknown. If anthracene was present, the product was washed again with toluene and the sample was again analyzed by TLC. This procedure was repeated until the product was pure anthraquinone, which typically took no more than two 1156
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Journal of Chemical Education
LABORATORY EXPERIMENT
washings. Because running the TLC consumes a small amount of product, it was important to weigh the washed compound before taking a sample for TLC. To compare the results of the different reaction conditions, turnover numbers (TON) were calculated to determine the number of moles of anthraquinone that were produced by each mole of catalyst. With this common benchmark, students could compare the effect of temperature and reactant ratios on the outcome of the reaction. Reactions performed without VO(acac)2 or without H2O2 did not produce any of the anthraquinone product whereas the other reactions produce varying amounts of anthraquinone. A TON around 14 with a yield of anthraquinone around 50% can be achieved with 25 mg of catalyst, 7.5 mL of 30% H2O2, and 500 mg of anthracene at reflux (77 °C). Yield and TON drop off dramatically if the reaction is performed at lower temperatures. Altering the amount of peroxide or catalyst relative to the anthracene alters the yield and TON. For example, doubling the amount of catalyst increases yield to 60% with a TON around 9. Typical student results are presented in Table 1. Extending the reaction time from 2 to 4 h increases the yield of anthraquinone. This suggests that the catalyst is capable of even higher TON and yields if given a longer reaction time.
’ SUMMARY Performing this experiment allowed students to use a relatively simple coordination complex to catalyze an oxidation reaction that does not occur in the absence of catalyst. Students enjoy making the complex and then using it to accomplish another chemical transformation rather than simply making a complex for its own sake. Students experience not just using a catalyst to accomplish a desired synthesis, but also study the catalyst itself and take part in determining the optimum reaction conditions, much as they would if they were beginning to study a newly discovered catalyst. Given the quantity of data generated across the entire class, students also have an opportunity to develop an in-depth discussion of the reaction. ’ ASSOCIATED CONTENT
bS
Supporting Information Student handout; instructor notes; detailed hazards. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) Morley, C. P. Inorganic Experiments, 2nd ed.; Wiley-VCH: Weinheim, 2003; pp 146 148. (2) Men’shikov, S. Y.; Vurasko, A. V.; Petrov, L. A.; Molochnikov, L. S.; Novoseiova, A. A.; Skryabina, Z. E.; Saloutin, V. I. Russ. Chem. Bull. 1992, 41, 619–622. (3) This experiment can be completed in one 6-h lab period if the time is well planned and the students are not waiting for chemicals or equipment, such as the rotovap. 1157
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