Unexpected formation of ansa isomers enabled by phosphazene ring flexibility in the reactions of cyclotetraphosphazene with 1,2-ethanediol

Abstract

In this work, the nucleophilic substitution reactions of the cyclotetraphosphazene, tetramer, (1) (N₄P₄Cl₈) with the disodium salt of 1,2-ethanediol in THF solution at different mole ratios were demonstrated. Surprisingly, one double bridged (2) and three ansa derivatives [bis (3), tris (4) and tetrakis (5)] were formed demonstrating two competing pathways in these reactions. The new type cyclotetraphosphazene compounds (2–5) consisting of multiple rings were characterized by elemental analysis, mass spectrometry, ¹H and ³¹P NMR spectroscopy and X-Ray crystallography. Although 1,2-ethanediol, a short chain diol, can only produce spiro type products with cyclotriphosphazene, (N₃P₃Cl₆), it was able to give ansa type products with cyclotetraphosphazene which suggests the role of the ring flexibility of 1. Crystallographic evidence shows the distortion of the cyclotetraphosphazene ring.