Synthetic spectroscopic and structural investigations on cyclic and acyclic phosphazanes and their transition metal complexes

Abstract

The electronic origin of the structural differences between the two types of the bicyclic P$_4$N$_5$ systems represented by 1 and X presumably lies in the maximization of negative hyperconjugative interactions between the nitrogen lone pairs and the P–X $\sigma^*$ (X = N or O) orbitals (see Sec. 2.2.5). These negative hyperconjugative interactions will be maximum in 1 only if the nitrogen lone pairs are parallel to the P–O bonds (or P=O bonds). However, such a relationship is not possible in any of the usually observed ring conformations of the six-membered rings. Hence, the molecule twists around one of the ring P–N bonds to maximize the parallel relationship, and the result is the observed ring conformation wherein the nitrogen lone pairs have an orthogonal relationship with the P=O bonds and a parallel relationship with the P–O bonds.

It appears from the crystal structure of 1 and the three crystal structures determined for A$_5$-cyclotriphosphazanes that the nitrogen lone pair prefers a parallel orientation with the P–X bonds rather than the P=X bond. This would in turn mean that negative hyperconjugative interactions are more effective with P–X $\sigma^$ orbitals than P=X $\sigma^$ orbitals. This preference is in fact reflected in the pyramidal geometry of the bridging nitrogen atom of the bicyclic phosphazenes, X. If this nitrogen atom in X were to have a trigonal-planar geometry, then the lone pair would be oriented parallel to the phosphazene formal P=N double bond, unlike in the case of 1, where the bridging nitrogen atom lone pair is oriented parallel to the formal P–N single bond.

Quantitative theoretical calculations on the electronic structures of this type of molecules would shed further light on the nature of the P–N multiple bonding.