Conclusions

Three ligands have been synthesised and characterised as models for the polymeric analogues, and a fourth compound synthesised as a crystallisation model. Each ligand comprises a cyclotriphosphazene ring with one spiro-2,2'-dioxybiphenyl moiety blocking one ring-phosphorus site. The remaining two phosphorus atoms are fully substituted with a 2-pyridyloxy derivative. The choice of stereochemistry completes the available scenarios for cyclotriphosphazene substitution with pyridine moieties for the Aincough/Brodie group.30, 31, 33, 34

Reaction times for synthesis of the ligands appear to be determined by steric factors dependent on the presence and position of the pyridyl methyl group, which had been introduced to assess solubility factors that might assist in ensuring good solubility in the polymeric analogues. Reaction time for the chloro analogue is comparatively fast, which may be due to exocyclic electronic effects although the factors affecting the rate have not been investigated further.

The P···N interaction, whereby one or more pyridyloxy nitrogen atoms appear to interact with the phosphazene ring nitrogen atom to which they are attached via their respective hinge oxygen atoms, is a common feature in these ligands. The conclusion by Jung et al.,11,

12

that this interaction was due to steric constraint of the pyridyl nitrogen atoms by chloro groups in the 1,1-bis(pyridyl-2-thio)-3,3,5,5-tetrachlorocyclotriphosphazene system may be specific to that system only. That a similar P···N interaction is observed in the new ligands, where the 2-oxypyridine moieties do not sterically constrain the nitrogen atoms of each other may point to an electronic, rather than steric, mechanism for this behaviour as proposed by Davidson.41 As yet, no evidence can be found in the literature for a

cyclotriphosphazene containing a combination of thio- and 2-oxypyridine moieties that might further elucidate matters.

Given the high concentration of aryloxy moieties, the degree of stacking within the cyclotriphosphazene models is unexpectedly low and it is difficult to predict the effect of stacking in the polyphosphazene analogues, given the low bond torsion of the

polyphosphazene skeleton. This serves to emphasise, or question, the validity of arguments regarding the use of the trimeric ring structure as an appropriate model for the polymer architecture, rather than the tetrameric or higher cyclic structures.

It is noteworthy that the use of 2-pyridyloxy moieties has resulted in the successful syntheses of stable ligands, with no apparent degradation. The ligand design provides a potentially nitrogen rich environment for coordination to transition metals, having four nitrogen donors from the pyridyloxy moieties and three available nitrogen donors from the cyclotriphosphazene ring. Flexibility of the ligands is evident from the conformations adopted in the X-ray crystal structures and rotation of the 2-pyridyloxy rings about the hinge oxygen atoms may afford greater coordination ability.

A number of questions arise from the ligand synthesis:

(i) Will the methyl groups added to afford greater solubility in the polymer actually hinder coordination by their steric bulk?

(ii) While it is obvious that the methyl groups affect the packing forces in the solid state, will coordination to metals result in a higher degree of stacking?

(iii) Each ligand displays some degree of interaction between the pyridyloxy nitrogen atoms and the phosphorus to which they are attached via the hinge oxygen atom. Will this interaction compete for control of the metal coordination site?

(iv) Given that the ligands hexakis(2OPy)cyclotriphosphazene and hexakis(2O-4- Py)cyclotriphosphazene demonstrate a range of coordination geometries to transition metals, how will the coordination modes be affected by the presence of the 2,2'- dioxybiphenyl moiety?

(v) To what extent are the ligand results applicable to the polyphosphazene analogues?

2.5 References

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González, P. A., Macromolecules 1996, 29(16), 5320.

29. Ainscough, E. W.; Brodie, A. M.; Derwahl, A., Polyhedron 2003, 22, 189.

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Chapter 3: Reactions of Phosphazene Ligands with Selected