Undergraduate 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
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 beingcollected 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 chloroformor 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 cm1.
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,
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
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 m