Experiment 4
Preparation and Characterisation of a
Substituted Cyclotriphosphazene
Cyclotriphosphazene- 1
Experiment 4: Preparation and Characterisation of a
Substituted Cyclotriphosphazene
Aims
to functionalise hexachlorotriphosphazene
to characterise the product by spectroscopic means
to explore the bonding in inorganic ring systems
Introduction
Discovered as long ago as 1834, phosphazenes are formally unsaturated P(V) compounds
containing -P=N- units, which can self associate into rings and chains to give structures of
alternating phosphorus and nitrogen atoms. Of particular interest are the cyclic trimers, e.g.,
(NPCl2)3 because like for borazine (B3N3H6), it can be argued that these represent inorganic forms
of benzene (C6H6) since the six-membered (NP)3 rings are planar and the N-P bonds are all
equivalent in length, being shorter than single N-P bonds. However, this view is rather simplistic,
as unlike in benzene, the -bonding in cyclophosphazenes involves both p and d orbitals.
Experimental distinctions can also be seen, e.g., phosphazenes are much more difficult to reduce
than aromatic organic compounds.
One of the simplest methods of preparing a phosphazene is by the following reaction:
NH4Cl
+
PCl5

"1/n(NPCl2)n" + 4 HCl
However this procedure can give rise to a complex mixture of
oligomers with association states (n) ranging from 3 to 8. It is for
this reason that the cyclophosphazene skeletal ring in this
experiment is pre-formed, and so the synthetic task here involves
the functionalisation of the P ring atoms. This is achieved by
treating hexachloro-cyclotriphosphazene (right) with potassium 4nitrophenoxide in a 1:6 stoichiometric ratio to give the desired
product, hexakis(4-nitrophenoxy)cyclotriphosphazene.
Cl
Cl
N
Cl
P
P
N
Cl
N
P
Cl
Cl
Experimental
a) Preparation of Potassium 4-Nitrophenoxide
- Place 4-nitrophenol (6 g) in a 100 ml conical flask equipped with a stirring bar.
- Add ethanol (20 cm3) by pipette and stir to dissolve the solid.
- Prepare an ethanolic solution of KOH (3 g in 25 cm3). Once the KOH is completely dissolved (if
the KOH doesn’t dissolve completely, warm the mixture on a hot plate or steam bath or add a little
more ethanol).
- Add this solution in a dropwise manner over a period of about 15 minutes to the 4-nitrophenol
solution prepared initially. A yellow micro-crystalline solid should separate out during the course of
this addition.
- When the addition is complete, cool the flask, with stirring, in an ice-water bath for 10 minutes.
- Collect the yellow crystalline product by filtration through a sintered glass crucible and wash
with two 5 ml aliquots of chilled ethanol and leave the crystals to dry on a clean watchglass.
Cyclotriphosphazene- 2
b) Preparation of the New Cyclotriphosphazene
- Place hexachlorotriphosphazene (1.16 g) and potassium 4-nitrophenoxide (3.85 g; prepared in
part a) in a 100 cm3 round bottom flask equipped with a stirring bar.
- Add a catalytic amount of tetra-n-butylammonium bromide (about 0.2 g) and THF (40 cm 3).
- Attach a water condenser, fitted with a CaCl2 drying tube, to the flask and mount the flask in a
water bath on a stirrer/hotplate.
- Reflux the mixture for one hour during which time a fine precipitate should separate.
- Allow the mixture to cool to room temperature.
- Pour the contents of the flask into a beaker containing 150 cm 3 of ice cold water to precipitate
the desired product.
- Filter the precipitate through a sintered glass crucible (this process may be slow!), then wash the
solid with three 10 cm3 aliquots of water, followed by three 10 cm 3 aliquots of ethanol.
- Dry the solid in an oven at 100C for about 30 minutes. This is the crude product.
- Record the yield (g, %) and its melting point.
- Save a small portion of the crude product.
- To purify the remaining crude product, recrystallise by dissolving in the minimum amount of hot
DMF to which is then added a small volume of 1-butanol (about one-tenth of the amount of DMF
used). Polymorphic crystals of the purified product should precipitate out as the DMF solution
cools.
- Record the yield (g, %).
c) Characterisation of the New Cyclotriphosphazene
- Record the IR spectra of the starting hexachlorophosphazene and of the products obtained in
Parts A and B.
- Obtain the melting point of both the crude and recrystallised product.
Questions
1.
Write balanced equations for the reactions occurring in Parts (a) and (b).
2.
Compare the IR spectra of hexachlorophosphazene, potassium 4-nitro-phenoxide and
your substituted phosphazene.
3.
Discuss the nature of the bonding found in phosphazenes. Reading material on
compounds of this type can be found in the references cited below.
References
1. Greenwood and Earnshaw, "The Chemistry of the Elements", p. 622.
2. F.A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry", Fifth Edition, p. 406.
3. H.R. Allcock, Chem. Rev., 1972, 72, 315.
4. Bronstein et al., Inorg. Chem., 1985, 24, 625.
Cyclotriphosphazene- 3