Structure and Conformation of the Medium-Sized Chlorophosphazene Rings

Medium-sized cyclic oligomeric phosphazenes [PCl₂N]_<i>m</i> (where <i>m</i> = 5–9) that were prepared from the reaction of PCl₅ and NH₄Cl in refluxing chlorobenzene have been isolated by a combination of sublimation/extraction and column chromatography from the predominant products [PCl₂N]₃ and [PCl₂N]₄. The medium-sized rings [PCl₂N]_<i>m</i> have been characterized by electrospray ionization–mass spectroscopy (ESI-MS), their ³¹P chemical shifts have been reassigned, and their T₁ relaxation times have been obtained. Crystallographic data has been recollected for [PCl₂N]₅, and the crystal structures of [PCl₂N]₆, and [PCl₂N]₈ are reported. Halogen-bonding interactions were observed in all the crystal structures of cyclic [PCl₂N]_<i>m</i> (<i>m</i> = 3–5, 6, 8). The crystal structures of [P­(OPh)₂N]₇ and [P­(OPh)₂N]₈, which are derivatives of the respective [PCl₂N]_<i>m</i>, are also reported. Comparisons of the intermolecular forces and torsion angles of [PCl₂N]₈ and [P­(OPh)₂N]₈ with those of three other octameric rings are described. The comparisons show that chlorophosphazenes should not be considered prototypical, in terms of solid-state structure, because of the strong influence of halogen bonding