Download Experiment 2

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SECTION 2:
Winter 2013
Preparation and Properties of Some "Classical"
Coordination Compounds
This part of the laboratory work deals with that part of transition metal chemistry called
"classical" coordination complexes. These complexes may be loosely defined as compounds
consisting of a central transition metal ion (Lewis acid) in a moderate oxidation state (viz. +2,
+3) bonded (coordinated) to a specific number (usually 4 or 6) of electron pair donor groups or
ligands (Lewis bases).
Experiment 2
Optical Isomers of [Co(en)3]3+
Introduction
The type of optical activity which you are probably most familiar with from organic
chemistry is that associated with a tetrahedrally coordinated carbon atom bearing four different
substituents. There are, however, many more geometries that exist that are not superimposable onto their mirror images. Transition metal complexes, which commonly show coordination numbers greater than 4, offer the possibility of preparing chiral complexes of a geometry
inaccessible in organic chemistry.
In this experiment, you will prepare one such complex, the cation [Co(en) 3]3+ (where en =
ethylenediamine, H2N-CH2CH2-NH2):
N
N
Co3+
N
N
N
N
N
N
N
D (+) [Co(en)3]3+
Co3+
N
N
N
L (-) [Co(en)3]3+
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Introduction (continued)
The resolving agent you will use is L (+) tartrate (i.e. (+) tart)
CO2
HCOH
HOCH
CO2
Two diastereomers, [(+) Co(en)3][(+) tart]Cl and [(-) Co(en)3][(+) tart]Cl, can be formed
when L (+) tartrate is added to the racemic solution of the cobalt complex; these have, of
course, different physical properties and in particular, different solubilities. With a proper
choice of conditions, it is possible to fractionally crystallize one diastereomer. In this case
[(+) Co(en)3][(+) tart]Cl, which happens to be the least soluble, is obtained from solution as the
pentahydrate:
[(+)Co(en)3]3+ (aq) + (+) tart (aq) + Cl- + 5 H2O → [(+) Co(en)3][(+) tart]Cl. 5 H2O (s)
[(-) Co(en)3]3+ (aq) + H2O
→
[(-) Co(en)3]3+ (aq)
The tartrate salt is then converted to the desired product, [(+) Co(en) 3]I3.H2O, by reaction
with I- (aq).
The other optical isomer, [(-) Co(en)3]I3.H2O, is obtained directly by adding I- to the
solution from which [(+) Co(en)3][(+) tart]Cl. 5 H2O was previously obtained. There is still some
(+) isomer of the cobalt complex remaining in solution at that point and it of course will also
precipitate when I- (aq) is added, contaminating the desired [(-) Co(en)3]I3.H2O product.
Fortunately the (+) enantiomer crystallizes in crystals of the racemate: crystals that contain both
the (+) and (-) enantiomers. The remaining [(-) Co(en)3]3+ then crystallizes in other crystals that
are optically pure. The crystals of the racemate and those of the optically pure complex can be
considered "diastereomeric" to each other and do show differing solubilities, which allow their
separation. The crystals of [(-) Co(en)3]I3.H2O are more soluble than those of the racemate in
warm water and purified enantiomer may therefore be obtained by recrystallizing it from warm
water.
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Introduction (continued)
To evaluate the optical purity of your products, you will measure the rotations of
solutions of known concentration and path length using a polarimeter.
Using this data, a
specific rotation, [α]D , can be calculated for an optically active material. [α]D , is given by
[α]D = α / lc
where
α
is the rotation angle of plane polarized light induced by the sample solution, as
measured using the polarimeter.
l
is the path length in dm
c
is the concentration in g of solute per mL of solution.
(ensure you record the length of the tube you use.)
(The D subscript indicates that the D emission line of sodium, λ = 589.3 nm, is being used.)
The optically pure enantiomers have [α]D s of + and - 89°.
You should also report the molecular rotation, [M]D = M x [α]D / 100, of your product
(where M is the molar mass). This unit is useful to allow comparison of the rotary power of
different compounds on a per mole basis.
A discussion of the rotation of plane polarized light by chiral compounds and of the
principles of a polarimeter are not given here, but are left for you to include as part of your
lab report.
The synthesis of [Co(en)3]3+ involves first the preparation of the corresponding Co 2+
complex, [Co(en)3]2+, from CoSO4.7H2O and ethylenediamine, followed by oxidation with Cl 2 (g)
in the presence of activated charcoal as a catalyst. The classic method for this experiment
involved the use of atmospheric oxygen to oxidize the cobalt, a procedure which lasted 4 hours
(2., 3.); however, it has been reported that this time can be reduced to a few minutes if chlorine
gas is used instead.
Procedure
Preparation of the Resolving Agent, Barium L(+)-Tartrate Monohydrate
Prepare solutions of BaCl2 and of L(+)-tartaric acid by dissolving 6.1 g of BaCl2. 2 H2O
and 3.7 g of L(+)-tartaric acid in minimum amounts of tap water. After heating these solutions to
90°C, mix them together and add the base ethylenediamine in a fume hood drop wise, using a
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Procedure (continued)
disposable pipette, until the resulting solution is neutral. Allow the solution to cool to room
temperature. Collect the precipitate using suction filtration and wash it with warm water. If you
dry this product, determine its yield.
Preparation and Resolution of [Co(en)3]3+
i)
[(+) Co(en)3][(+) tart]Cl. 5 H2O
Prepare a solution by first combining 6 mL of ethylenediamine and 12 mL of water and
then add 2.5 mL of concentrated HCl. (Boiling, splattering and splashing can occur in this last
step. Work in a fume hood and add the acid slowly, swirling as you do). Cool the resulting
solution in an ice bath.
In a 500 mL suction flask, dissolve 7.0 g of CoSO 4.7H2O in 12 mL of water. Add the
cooled ethylenediamine solution prepared above to this cobalt solution.
A thick yellowish
precipitate or chocolate-brown solution will form. Add 1 g of activated charcoal and bubble Cl 2
gas
(ask a laboratory instructor where the cylinder is located) through the mixture for ~5
minutes. Once the chlorine gas has been stirred into the mixture, stopper the flask with a large
rubber stopper and connect the side arm to the water aspirator in a fume hood. Slowly turn on
the water. The solution may froth as excess Cl 2 (g) is removed (be careful not to lose your
solution!). When frothing has stopped, stop the aspiration, remove the stopper, and heat the
mixture on a hot plate (do NOT boil!), with magnetic stirring, in a fume hood. Continue heating
until the last of the chlorine gas has been removed (test with moist starch-iodide paper).
Add ethylenediamine or dilute HCl to your solution, if necessary, to adjust the pH to 7.0–
7.5 (check with pH paper).
This addition may be performed at your bench.
Remove the
charcoal using suction filtration and wash the residue with 5 mL of water.
Transfer the filtrate to a beaker and to it add all of the barium L(+)-tartrate monohydrate
prepared previously. Heat the mixture, with stirring, to just below boiling for 30 minutes. Filter
off the precipitated BaSO4 using suction filtration and wash it with a small amount of hot water.
Reduce the volume of the filtrate to ~25 mL using a rotary evaporator. Allow the resulting
solution to cool to room temperature. Crystals of [(+) Co(en) 3][(+) tart]Cl.5H2O should form upon
upon cooling. If you do not have time to use the rotary evaporator or to allow the solution to air
cool to room temperature, cover the beaker with Parafilm to prevent further evaporation of the
solution. Store the beaker in your locker until the following lab period and then finish this portion
of the procedure.
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Procedure (continued)
i)
[(+) Co(en)3][(+) tart]Cl. 5 H2O
Once crystallization is complete, collect the crystals by suction filtration and save the
filtrate in another container for later isolation of the [(-) Co(en) 3]3+ enantiomer. (NOTE: Assume
a 50:50 mixture of [(+) Co(en)3]3+ and [(-) Co(en)3]3+ is produced). After removing the filtrate
from the suction flask and saving it for later use, wash the crystals with a 40% (by volume)
ethanol-water solution. Recrystallize the product by dissolving it in about 8 mL of hot water.
After performing a hot gravity filtration, cool the solution to room temperature and then chill it in
an ice bath. Collect the product by suction filtration and wash the collected crystals with a 40%
(by volume) ethanol-water solution and then with absolute ethanol. Air dry the crystals and
determine the yield of [(+) Co(en)3][(+) tart]Cl.5 H2O produced. Submit a sample of this product.
ii)
[(+) Co(en)3]I3.H2O
To prepare [(+) Co(en)3]I3.H2O, dissolve about 1 g of [(+) Co(en)3][(+) tart]Cl.5 H2O in 8 mL
of hot water and add 3 drops of concentrated ammonium hydroxide (14.8 M) solution. To this
solution add, with stirring, a solution of 8.5 g of NaI dissolved in 3.5 mL of hot water. After
cooling the resulting solution in an ice bath, suction filter and wash the crystals with an ice cold
solution of 3 g of NaI in 10 mL of water to remove any remaining L(+)-tartrate. After washing
with 95% ethanol and finally with acetone, allow the [(+) Co(en)3]I3.H2O to air dry and determine
its yield. Submit a sample of this compound.
iii)
[(-) Co(en)3]I3.H2O
To isolate [(-) Co(en)3]I3.H2O, add 3 drops of concentrated NH4OH solution to the filtrate
from which [(+) Co(en)3][(+) tart]Cl . 5H2O was precipitated [see part i) of this procedure]. Heat
the solution to 80°C and add, with stirring, 8.5 g of NaI. Upon cooling the mixture in an ice bath,
impure [(-) Co(en)3]I3.H2O precipitates. Collect this product by suction filtration and then wash
the product with a solution of 3 g of NaI dissolved in 10 mL of water. To purify this solid,
dissolve it with stirring in 17 mL of water at 50°C. Filter off the undissolved racemate by either
gravity or suction fitration and add 2.5 g of Na I with stirring to the filtrate. Crystallization of
[(-) Co(en)3]I3 .H2O occurs on cooling to room temperature. Use suction filtration to collect the
precipitate, wash with 95% ethanol and then with acetone, and finally air dry. Determine the
yield of product and submit a sample of it with your lab report.
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Procedure (continued)
Measurement of Specific Rotations
To determine the specific rotation for each compound, first prepare an aqueous solution
of each in separate 50.00 mL volumetric flasks using the approximate masses listed in the table
below. Determine each mass using an analytical balance.
Once the three solutions are prepared, obtain the instructions to use the polarimeter.
Determine a blank reading for the polarimeter. Then obtain measurements of the angle of rotation of plane polarized light for at least three aliquots of each solution. Calculate an average
angle of rotation for each solution and use the value to calculate [a]D for each solution.
Compound
Approximate Mass to be Used
(g)
[(+) Co(en)3][(+) tart]Cl. 5 H2O.
0.3
[(+) Co(en)3]I3.H2O
0.3
(-) Co(en)3]I3.H2O
0.25
Be sure to include the following in your report:
1.
balanced equations for each step in the preparations
2.
percentage yields of the isolated complexes
3.
[α]D and [M]D where appropriate
4.
a % optical purity value for [(+) Co(en)3]I3.H2O and [(-) Co(en)3]I3.H2O.
NOTE:
% Optical Purity =
{ ( [α]D (pure) + [α]D (observed) ) / 2 [α]D (pure) } x 100 %
Calculate the % optical purity of each enantiomer (assume in each case that the only
impurity is the other enantiomer).
Questions
1.
Why is a Co2+ complex the starting material for this synthesis of a Co3+ complex?
2.
Why is it not possible to resolve [Co(en)3]2+ (aq)?
3.
In the preparation of barium L(+)-tartrate, what was the purpose of adding
ethylenediamine?
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Questions
4.
In the purification of both (+) and (-) [Co(en)3]I3.H2O, the compounds were washed
with water containing NaI. What was the purpose of the NaI?
5.
Draw structures of all of the optical and geometric isomers of Co(gly) 3 where gly =
NH2–CH2–CO2- . Ensure ALL atoms and bonds are shown. Clearly indicate the
types of isomers that are present.
6.
The synthesis method you have used consisted of preparing a racemic mixture of
the two enantiomers, then resolving them later. Suggest a method of inducing
asymmetric synthesis: i.e. of selectively preparing an excess of one enantiomer
over the other. (Hint: see 2. and 3. below) Be sure to include the appropriate
reaction equations.
7.
The oxalate anion is also a bidentate ligand and complexes of the type [M(C2O4)3]n-,
where M is a metal, are chiral in the same way that [Co(en)3]3+ is. However, in this
example the complexes are anionic, so that tartrate is no longer useful as a
resolving agent. How could you resolve these anionic complexes?
(Hint: see 4.
below)
References
1.
Robert J. Angelici. Synthesis and Technique in Inorganic Chemistry. Philadelphia:
W.B. Saunders Company, 1969 and 1977. 1st ed., pp. 66–74;
2nd ed., pp. 71–76.
2.
J. A. Broomhead, F.P. Dwyer and J.W. Hogarth. Inorganic Syntheses VI, 186–188
(1960).
3.
C.F. Bell. Syntheses and Physical Studies of Inorganic Compounds. Oxford:
Pergamon Press, 1972, pp. 208–211.
4.
J.W. Vaughn, V.E. Magnuson and G.J. Seiler. Inorg. Chem., 8, 1201 (1969).