Studying a Ligand Substitution Reaction with Variable Temperature 1H NMR
Spectroscopy
Adapted from:
Orvis, J.A.; Dimetry, B.; Winge, J.; Mullis, T.C. “Studying a Ligand Substitution Reaction with Variable Temperature
1
H NMR Spectroscopy: An Experiment for Undergraduate Inorganic Chemistry Students.” J. Chem. Educ. 2003, 8, 7, 803
Introduction:
Your goal in this lab is to synthesize and study the reactivity of the cobalt(III)
complex, trans-[Co(NH3)4Cl2]Cl:
+
NH3 Cl
NH3
Cl
Co
NH3
NH3
When this bright green complex is dissolved in water, it undergoes an aquation reaction,
where one chloride ligand is replaced by a water molecule, generating a product with the
formula [Co(NH3)4(H2O)Cl]Cl2. Ultimately, you will determine the mechanism of this
reaction.
Experimental Procedures:
Synthesis:
NOTE: YOU WILL BE USING CONCENTRATED AMMONIA AND 30%
HYDROGEN PEROXIDE IN THIS PROCEDURE. BOTH MATERIALS ARE
HAZARDOUS. WEAR GLOVES AND WASH ANY MATERIAL SPILLED ON
SKIN IMMEDIATELY. BOTH CAN CAUSE SEVERE BURNS ON THE SKIN.
ALL PROCEDURES MUST BE DONE IN THE FUME HOOD.
Step 1:
Dissolve 20.0 g of ammonium carbonate in 60.0 mL of water, and then add 60.0
mL of concentrated aqueous ammonia. In a second container, dissolve 15.0 g of cobalt
(II) nitrate hexahydrate in 30.0 mL of water. While stirring, pour these two solutions
together, and note any color changes that may occur. Keep the mixture stirring by using
a stir bar. SLOWLY, DROP BY DROP, add 8.0 mL of the 30% hydrogen peroxide
solution. If bubbling becomes too vigorous cease additions until the solution settles
down. Once the hydrogen peroxide has been added, evaporate the solution to a volume
of 90-100 mL. Turn up the heat on the stirring hot plate, but do not let the solution boil.
This will take some time, around an hour or so.
During the evaporation time, add about 5.0 g of ammonium carbonate to the
solution. Add it in small portions, say every ten minutes or so, making the 5.0 g last for
the duration of the evaporation step. When the solution volume is down to 90-100 mL,
suction filter the solution and cool the filtrate on ice. Red/purple crystals of product will
form. Collect these crystals by suction filtration, wash them with a few milliliters of icecold water and then ethanol, and allow them to dry (may leave overnight).
The product you should have synthesized is [Co(NH3)4CO3]NO3. You should
obtain a UV-Vis, IR, and 1H and 13C NMR spectra on your product. Perform the NMR
spectra in d6-DMSO for improved solubility. Calculate your yield and save the rest of
your product for the next synthesis step.
Step 2:
The next step is to produce the target complex from your newly synthesized
[Co(NH3)4CO3]NO3.
NOTE: CONCENTRATED ACID WILL BE USED IN THIS PORTION OF THE
PROCEDURE SO USE THE SAME SAFETY PRECAUTIONS AS IN STEP 1.
FOLLOW THE PROCEDURE CAREFULLY…TEMPERATURE AND TIMING
ARE IMPORTANT HERE.
Prepare a hot water bath, set to exactly 80C. Dissolve 1.0 g of
[Co(NH3)4CO3]NO3 in 5.0 mL of water in a 50.0 mL flask. Have a small stir bar and a
thermometer mounted in the flask as well. Heat the solution to between 50 and 60C
with stirring. Add 3.3 mL of concentrated HCl. Add the HCl very carefully, over a
period of 15 to 20 seconds. Do not let the solution “boil” over. Heat the solution to 80C
with vigorous stirring for five minutes. At this time, a dark green precipitate of transtetraamine(dichloro)cobalt(III) chloride is produced. After five minutes at 80C, cool the
solution to room temperature in an ice bath, and then suction filter the crude product.
Carefully rinse the green crystals with small portions of ice-cold water. Any purple solid
on the filter will likely be cis-tetraammine(dichloro)cobalt(III) chloride. Keep rinsing
until all of the purple solid has been removed, leaving only green crystals behind. Then,
wash the green crystals with 2.0 mL of ice-cold methanol and air-dry the product.
Obtain a percent yield for the product, and acquire a UV-Vis spectrum of it.
Remember, this material reacts immediately when dissolved in water. Have the UV-Vis
spectrometer ready to go, and make up your sample in the preparation area of the
instrument room, rather than in the inorganic lab.
Analysis:
NOTE- UNESSECARY DELAYS IN THIS PORTION OF THE LAB WILL
RESULT IN FAULTY DATA. YOU SHOULD BRING A STOPWATCH TO LAB
TO ENSURE THAT YOUR TIMING IS ACCURATE IN THE LAB.
The time between the preparation of the sample solution and the NMR analysis
should be minimized as much as possible in this section of the lab. You should begin the
analysis by preparing a sample containing 5.0 mg of your green crystalline product and
0.50 mL of D2O. You may use the sonicator to dissolve the sample. The solution should
be prepared in the organic lab so that the time between the mixing of the solution and the
acquisition of the data can be minimized. You should begin timing the reaction as soon
as the solvent comes into contact with the sample. Each group will be analyzing the
reaction at a different temperature and will have to modify the following procedures to
their specific temperature. The possible temperatures range from 10C to 35C.
First, the spectrometer should be tuned for 1H NMR. The sample you prepared
above should be gradient shimmed, and a normal 1H NMR should be collected at your
assigned temperature. You should record the autogain value.
After collecting the first 1H NMR spectra, you should set up a kinetics
experiment. Click on the Experiment button and look under the sub-heading, –More
1D Experiments, you should chose the experiment, Kinetics. Enter the autogain value
you recorded in the previous experiment as the value for Receiver Gain. DO NOT
CHOOSE THE AUTOGAIN OPTION; IT WILL NOT WORK WITH THIS
EXPERIMENT. Change the number of Scans to 8 regardless of the temperature at
which your experiment is collected. You should acquire 12 spectra over one hour. So if
your assigned temperature is room temperature then you should use allow a 300s period
between the collection of each spectrum. The period between collections can be adjusted
by entering a new value in the Period parameter space in the Experiment window. If
your assigned temperature is lower than room temperature, you may have to shorten the
period between collections in order to fit 12 spectra acquisitions into the reaction time.
At the bottom of the Experiment window, the parameter Num_time_points should look
like the following: y_acq{1[pnt]->12[pnt]: 1[pnt]}. After setting up the instrument, you
should begin preparing your sample. If your assigned temperature is substantially far
away from room temperature, then you should design a method for bringing your D2O
solvent to your temperature before making the solution. Be careful not to contaminate
your solvent with water. Use the temperature-hold icon to stabilize the instrument
temperature while you are changing samples. After inserting the new sample solution,
use the Z1/Z2 lock icon and begin your experiment as soon as the instrument has finished
shimming and locked on the sample. Remember to hit the GO button that appears after
you submit your experiment. The beginning of the first spectra acquisition should occur
when you click the GO button, so you should record this time. The average time
between the preparation of the sample solution and the first spectra acquisition should be
around three minutes. Click the View button in order to ensure that your spectra are
being collected. Each acquisition lasts only about 28 seconds so in order to see the first
one you must click the View button shortly after starting the experiment.
Looking at the NMR peaks of the final product, what can you say about the
structure of the of final cobalt complex? (include this in the discussion of your lab report)
After all of the spectra have been collected, you will have a set of time versus
concentration data, with the concentration expressed in terms of peak intensity. Since the
peaks associated with the formation of [Co(NH3)4(H2O)Cl]2+ make it difficult for
integrating the decrease in area of the [Co(NH3)4Cl2]+ peak, simply determine the change
in the peak maximum for the [Co(NH3)4Cl2]+ peak over time. Remember your first point
is not at zero time, but should include the time the time between when the D2O was first
added and when the first data point was collected. Analyze the data for zero, first, and
second order behavior. From the appropriate plot, extract the rate constant for this
reaction.
Use Microsoft Excel to produce an Eyring plot to determine the entropy and
enthalpy of activation for the aquation reaction. You will need the following equation:
ln(kobs/T) = (-H/ RT) + (S/ R) + ln(kb/h)
In the equation, H and S represent the enthalpy and entropy of activation, respectively,
kobs is the observed rate constant from the kinetic order plots, R is the gas constant, kb and
h are Boltzmann’s and Planck’s constants, respectively. Graph ln(kobs/T) vs T-1.