Synthesis, spectral characterisation and single-crystal structure of a bicyclic tetraphosphapentazane tetraoxide, N5P4ET5O4(OC6H3Me2-2,6)2

Reaction of the bicyclic phosphazane N5P4Et5Cl2 with 2,6-dimethylphenol and subsequent oxidation of the product by aqueous hydrogen peroxide yields N5P4Et5O4(OC6H3Me2-2,6)2 in 85% yield. Its structure has been established by NMR spectroscopy and single-crystal X-ray diffraction. The compound crystallises in the monoclinic space group C2/c with a= 21.245(5), b= 10.879(2), c= 16.450(6)Å, ?= 123.94(2)°, Z= 4, R= 0.066. The structural features are compared with those of bicyclic ?5-phosphazenes of type N5P4R3(NR1R2)5(NHR3)(R1,R3= Me or Et, R2= H or Me). The observed conformation of the N3P3 rings in the present compound is mainly dictated by the maximisation of the stabilising influence of �negative hyperconjugative interactions� between the nitrogen lone pairs and the adjacent P�X ?* orbitals.