Experiment 30 Preparation and Study of a Cobalt(li) Oxygen Adduct ...

Szafran, Z.; Pike, R.; Singh, M., Microscale Inorganic Chemistry, John Wiley & Sons: New York, 1991, p. Experiment 257ff. 30 253 257
salenHz
The salen chelating ligand has two atoms (0, N) as the donor sites and belongs
to a special class of ligands known as Schiffs bases.
Co(salen) exists in two forms: One is the active species that absorbs oxygen,
and the other is the inactive variety that does not react with oxygen, both of
which are dimeric. A solution of Co(salen) in a donor solvent (e.g., DMSO and
DMF) readily absorbs oxygen, forming an adduct with a peroxo-bridged oxygen
ligand.
Peroxo J
bridge 1
In these diamagnetic compounds, the cobalt is present in three oxidation states.
In this experiment, the organic ligand salenHz is prepared, and subsequently
reacted with hydrated cobalt acetate to prepare Co(salen). The oxygen up-take
capacity of the compound is also investigated.
Prior Reading and Techniques
Section 5.C.2: Purging with an Inert Gas
Section 5.0.3: Isolation of Crystalline Products (Suction
FiltrationI

254 Szafran, Chapter Z.; Pike, 8 R.; I Chemistry Singh, M., Microscale of the Transition Inorganic Chemistry, John Wiley & Sons: New York, 1991, p. 257ff. 257
EXPERIMENTAL SECTION
Related Experiments
Cobalt Complexes: Experiments 7B, 17,26,27,35, and 47B
Part A: Preparation of N,N-Bls(sa"cylaldehyde Jethylenedlimlne, salenHz
CHEMICAL DATA Compound
Safety Recommendations
Salicylaldehyde (CAS No. 90-02-8): This compound is harmful if swallowed,
inhaled, or absorbed through the skin. ORL-RA T LD50: 520 mglkg. It
has been shown to have effects on fertility.
Ethylenediamine (CAS No. 107-15-3): This compound is harmful if swallowed,
inhaled, or absorbed through the skin. ORL-RA T LD50: 500 mg/kg. It
has an irritating ammonia odor (vapor pressure is 10 mm at 20 ec), so it should
only be used in the HOOD.
Salicylaldehyde 122.12 500 mg 4.09 1 197 1.146
Ethylenediamine 60.10 120 mg 2.00 8.5 118 0.899
Required Equipment
Magnetic stirring hot plate, sand bath, 1O-mL Erlenmeyer flask, magnetic stirring
bar, ice-water bath, Hirsch funnel, clay tile.
Time Required for Experiment: 0.5 h.
EXPERIMENTAL PROCEDURE Place 5.0 mL of 95% ethanol in a 1O-mL Erlenmeyer flask equipped with a
magnetic stirring bar. Put the flask in a sand bath, set atop a magnetic stirring
hot plate, and heat the ethanol to boiling. Immediately, with stirring, add
450 j.LL (4.09 mmol) of salicylaldehyde. To the boiling solution, add 140 J.LL
(2.0 mmol) of ethylenediamine.
Isolation of Product
Stir the solution for 3-4 minutes, and cool the solution in an ice-water bath.
Collect the yellow crystals by suction filtration using a Hirsch funnel. Wash the
product with 2 drops of ethanol and dry on a clay tile. Determine the melting
point and calculate a percentage yield.
Part B: Preparation of Co(salen)
Additional Safety Recommendations
Cobalt(lI) acetate tetrahydrate (CAS No. 6147-53-1): This compound
is harmful if inhaled or swallowed. ORL-RAT LD50: 708 mglkg.
CHEMICAL DATA Compound FW Amount _0101 mp (OC)
salenH2 268.32 230 mg 0.56
Cobalt(1I) acetate
tetrahydrate 249.09 200 mg 0.80 1. 705

256 Chapter 8 I Chemistry of the Transition
Side-arm
test tube
Szafran, Z.; Pike, R.; Singh, M., Microscale Inorganic Chemistry, John Wiley & Sons: New York, 1991, p. 257ff. 257
EXPERIMENTAL PROCEDURE
Measuring -----{:-:":-::-3
buret
Tygon tube
Movable
tube
Tygon tube
Figure 8.11. Movable tube apparatus.
Part C: Determination of Oxygen Absorption by Colsalenl
Additional Safety Recommendations
Dimethy. salfoxide (CAS No. 67-68-S): This compound is harmful if
swallowed, inhaled, or absorbed through the skin. ORL-RAT LDSO: 14,SOO mgl
kg.
Required Equipment
Graduated cylinder, 2­ x 18-cm side arm test tube with rubber stopper,
1­ x 7.S-cm test tube, forceps, two lO-mL burets, Tygon tubing.
Time Required for Experiment: 1.5 h.
Place SO-100 mg of Co(salen) (from Part 30.B) at the bottom of a
2­ x 18-cm side arm test tube. Using a graduated cylinder, transfer S mL of
DMSO to a 2S-mL beaker, and saturate the complexing solvent by bubbling
O2 gas through it. Fill the 1­ x 7.S-cm test tube with oxygen-saturated DMSO
to a level 2 cm from the rim. Using a pair of forceps, carefully lower the smaller
test tube into the side arm test tube. Make sure that no DMSO spills out from
the test tube. In case DMSO spills out, dry the side arm test tube, and try again.
Using Tygon tubing, connect the bottom of a 10-mL buret (henceforth called
a measuring buret) to the bottom of a second lO-mL buret (henceforth called
a movable tube), which will act as a water reservoir. Secure the arrangement
with damps to a strong ring stand. Fill the movable tube with water, and allow
water to drain so that the Tygon tubing is filled with water, and water begins
to enter the measuring buret. Attach the top of the measuring buret to the side
arm of the larger test tube. Adjust the height of the movable tube so that the
water level remains near the bottom of the measuring buret. This apparatus is
shown in Figure 8.11.
Rush the side arm test tube through its mouth for a few minutes with oxygen
gas. Tightly seal the mouth of the side arm test tube with a rubber stopper.
Adjust the height of the movable tube so that the water in the measuring buret
and in the water reservoir tube are at the same levels. Note the initial water
level from the measuring buret. Carefully invert the side arm tube in such a way
that DMSO pours out from the smaller test tube, allOwing it to react with the
Co(salen) at the bottom of the side arm test tube. Make certain that DMSO
does not enter into the Tygon tubingl Gently shake the side arm test
tube so as to dissolve the Co(salen) in the DMSO. As oxygen is absorbed by
this solution, the water level in the measuring buret will start rising. Continue
shaking the side arm tube until the water level no longer changes. The process
should not require more than 20 min. Readjust the height of the movable tube
so that the water levels in both the tubes are the same. Note the final water
level in the measuring buret.
Calculations
Calculate the decrease in the volume of water caused by the absorption of
oxygen gas by the Co(salen) in DMSO. This is equivalent to the volume of
oxygen uptake by Co(salen). Note the atmospheric pressure from a barometer.
Record the room temperature. Using the vapor pressure of water at that temperature,
calculate the number of moles of O2 gas absorbed using the ideal gas
law equation.
n = (P - f)VIRT
where f is the vapor pressure of water at room temperature.

Szafran, Z.; Pike, R.; Singh, M., Microscale Inorganic Chemistry, John Wiley & Sons: New York, 1991, experiment p. 257ff. 31 257 257
Part D: Reaction of the Oxygen Adduct with Chloroform
Time Required for Experiment: 0.25 h.
EXPERIMENTAL PROCEDURE Transfer the dark brown solid from Part 30.C into a centrifuge tube. Centrifuge
the mixture and remove the supernatant liquid. Layer the residue with a few
drops of chloroform and observe what happens. An evolution of oxygen gas
should be seen as the complex deoxygenates.
QUESTIONS 1. In terms of the chemistry of Fe(II) describe the triggering mechanism in
hemoglobin.
2. Why is CO so deadly to human respiration? (Hint: Consider the bonding
to the iron in hemoglobin.)
3. There is a reasonable correlation between the 0--0 bond length in metal
(dioxygen) complexes and the reversibility of the addition reaction. Discuss.
4. Cobalt is also found in other molecules of biological interest. Give an example
and discuss its biological properties.
5. Certain porphyrin complexes of cobalt(II) can be substituted into globin to
form cobalt analogs to hemoglobin. Discuss the similarities and differences
between these complexes. A useful initial literature reference is Hoffman, B.
M., Petering, D. H. Proc. Natl. Acad Sci. USA 1970.67,637.
REFERENCES 1. Appleton, T. G. J. Chem. Educ. 1977.54,443.
2. K1evan, J.; Peone, J.; Madan, S. K. J. Chem. Educ. 1973.50, 670.
3. Basolo, F.; Hoffman, B. M.; Ibers, J. A. Acct. Chem. Res. 1975.8,384.
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