Experiment 10
Preparation of a Macrocyclic Ligand and its
Complexes
Macrocyclic Ligand Complexes - 1
Experiment 10: Preparation of a Macrocyclic Ligand and its
Complexes
Aims
to prepare a macrocyclic N4-donor ligand
to prepare a cobalt(III) macrocyclic complex
to compare the structures and spectroscopic properties of the above
Introduction.
Macrocyclic ligands are a very important group of compounds. They are found in nature
(particularly porphyrins) and enable metals to carry out a range of functions in the biological
environment. Synthetic macrocycles have been prepared by chemists, starting with Busch in
the early 1960’s and later by Pedersen (crown ethers) and others. Pedersen ultimately was
awarded the Nobel prize for his work in the area. Macrocycles are important because they
can confer unusual properties on metal centres. In particular they are able to stabilise
unusually high and low oxidation states and form complexes which are particularly inert to
substitution.
Macrocycle formation is not a favoured process and generally requires special conditions for
ring closure. However, in the example in this experiment no such conditions are necessary
and ring closure is the favoured process.
Experimental
a) Preparation of 1,2-diaminoethane dihydrobromide, H2N(CH2)2NH2.2HBr
- Dissolve 1,2-diaminoethane (3.75g, 0.0625 mol) in methanol (25 cm3).
- Dropwise, add 49% HBr (22.5 cm3) – be careful the reaction is vigorous – stirring all the
time.
- Collect the colourless salt by filtration on a glass sinter, wash with diethyl ether and air dry.
- Record the yield (g, %).
If you have insufficient product for the next step, reduce the volume of the filtrate and add
diethyl ether to obtain a further crop of the dihydrobromide.
b) Preparation of Hexamethyl-1,4,8,11-tetraazacyclotetradeca-7,14-diene dihydrobromide (L1.2HBr).
- To 1,2-diaminoethane dihydrobromide (5.6g, 0.025mol) add acetone (50 cm 3) and 1,2diaminoethane (1.5g, 0.025 mol).
- Gently reflux on a water bath for 30 mins.
- Cool the solution to room temperature.
- Collect the product by filtration, wash with ice-cold acetone followed by diethyl ether and air
dry.
- Record the yield (g, %) and the infra-red spectrum (nujol mull).
N
2 H2N(CH2)NH2.2HBr + 4
HN
O
.2HBr
NH
N
Macrocyclic Ligand Complexes - 2
c) Preparation of Na3[Co(CO3)3].3H2O
- Dissolve Co(NO3)2.6H2O (3.64g) in water (5 cm3).
- Slowly add 30% hydrogen peroxide (1.25 cm3).
- Prepare a slurry of sodium hydrogen carbonate, NaHCO 3, (5.25 g) in water (7 cm3) and cool
to 0ºC on an ice salt bath.
- Dropwise, add the cobalt solution to this slurry.
- Allow to stand at 0ºC for 1 hour.
- Filter off the olive-green solid, wash with cold water (3 x 2.5 cm 3), ethanol and diethyl ether.
- Dry in a vacuum desiccator.
- Record the yield (g, %).
d) Preparation of [Co(L1)Cl2]PF6.
- Heat a slurry of Na3[Co(CO3)3].3H2O (1.8g, 5 mmol) and L1.2HBr (3.0g, 6.25 mmol) in
methanol/water (1:1 v/v, 25cm 3) until effervescence ceases and a red solution results (ca 20
mins).
- Filter the resultant solution.
- Add conc. HCl (5 cm 3) to the filtrate and heat on a water bath until the volume has reduced
to ca 10cm3.
- Cool to room temperature.
- Add a saturated solution of NH4PF6 in H2O until no further precipitation occurs (ca 2-3 cm3).
- Collect the copious green precipitate by filtration, wash with water, ethanol and diethyl ether
and dry.
- Record the yield (g, %) and run an infra-red spectrum (nujol mull).
- Recrystallise a portion by dissolving in the minimum quantity of hot acetone (be patient),
adding a roughly equal volume of ethanol and leaving to stand overnight.
- Collect the deep green crystals by filtration, wash with ethanol and ether and dry.
Questions
1.
2.
3.
4.
5.
Briefly discuss the mechanism for formation of the ligand.
Assign the IR and NMR spectra of the ligand.
The ligand L1 is chiral. Draw the structure of the ligand and indicate the chiral centres.
Draw all the possible isomers of the ligand (Hint: concentrate on possible
combinations of conformations at the amine nitrogens).
Compare the IR and NMR spectra of the complex with those of the free ligand.
Explain any differences you see.
Macrocyclic Ligand Complexes - 3