DFT calculations on the interaction of phosphazenes with transition metals : a thesis presented in partial fulfillment of the requirements for the degree of Master of Science in Chemistry at Massey University, Palmerston North
The electronic structure of substituted cyclic phosphazenes has been investigated using
Density Functional Theory (DFT) and Natural Bond Order (NBO) analysis. NBO
analysis shows covalent, ionic and negative hyper-conjugation interactions all
contribute to the electronic structure of cyclic phosphazenes.
The geometric and electronic structural changes that occur when transition metals are
coordinated to the nitrogen atom of the phosphazene ring have been analyzed using the
NBO model. The bonding of transition metal ions with the ring nitrogen on the
phosphazene was investigated by modeling hexakis(2-pyridyloxy)cyclotriphosphazene,
hexakis(4-methyl-2-pyridyloxy)cyclotriphosphazene and octakis(2-
pyridyloxy)cyclotetraphosphazene with different metal ions (Co(II), Ni(II), Cu(II),
Zn(II)) in their assorted configurations with DFT as implemented in the Gaussian03
First-row transition metals bind to the phosphazene ring with simple s donor behaviour
via the ring nitrogen. The lengthening of the PN bonds adjacent to the coordinated
metal centre is a result of electron density being removed from the PN bonding orbitals
and going into the 4s orbital of the metal ion.
Investigating the pyridine substituents on the phosphazene ring showed that these can
affect the PN bonds in a similar fashion, although weaker, to the transition metals. This
effect is the result of the pyridine nitrogen lone pair affecting the negative hyperconjugation
component of the PN bond.
Coupling between two metal atoms coordinated to the phosphazene ring was
investigated by DFT calculations, which showed molecular orbitals in both the tricyclic
and tetracyclic phosphazene capable of providing an ‘electron density bridge’ between
the metal centres. These results are in accord with ESR and magnetic susceptibility
results, which can be explained in terms of weak antiferromagnetic coupling between
The cyclic phosphazenes are model compounds for polyphosphazenes and the results
obtained from this work will provide insight into the electronic properties of this
important class of inorganic polymers.