Irrespective of the order of the addition of reagents, the reactions of [PCl2N]3 with MX3 (MX3 = AlCl3, AlBr3, GaCl3) in the presence of water or gaseous HX give the air- and light-sensitive superacid adducts [PCl2N]3·HMX4. The reactions are quantitative when HX is used. These reactions illustrate a Lewis acid/Brønsted acid dichotomy in which Lewis acid chemistry can become Brønsted acid chemistry in the presence of adventitious water or HX. The crystal structures of all three [PCl2N]3·HMX4 adducts show that protonation weakens the two P–N bonds that flank the protonated nitrogen atom. Variable-temperature NMR studies indicate that exchange in solution occurs in [PCl2N]3·HMX4, even at lower temperatures than those for [PCl2N]3·MX3. The fragility of [PCl2N]3·HMX4 at or near room temperature and in the presence of light suggests that such adducts are not involved directly as intermediates in the high-temperature ring-opening polymerization (ROP) of [PCl2N]3 to give [PCl2N]n. Attempts to catalyze or initiate the ROP of [PCl2N]3 with the addition of [PCl2N]3·HMX4 at room temperature or at 70 °C were not successful.

Medium-sized cyclic oligomeric phosphazenes [PCl2N]m (where m = 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]m 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]m (m = 3–5, 6, 8). The crystal structures of [P(OPh)2N]7 and [P(OPh)2N]8, which are derivatives of the respective [PCl2N]m, 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.

Phosphazene polymers are classically synthesized by the high-temperature, ring-opening polymerization (ROP) of [PCl2N]3 to give [PCl2N]n, followed by functionalization of [PCl2N]n with different side groups. We investigated the interactions of [PCl2N]3 with Lewis acids because Lewis acids have been used to induce the high-temperature ROP of [PCl2N]3. The reactions of [PCl2N]3 with MX3 (M = group 13, X = halides), under strict anaerobic conditions gave adducts [PCl2N]3·MX3. Adducts were characterized by X-ray crystallography and multinuclear and variable-temperature NMR studies, and mechanistic understanding of their fluxional behavior in solution was achieved. The properties of the [PCl2N]3·MX3 adducts at or near room temperature strongly suggests that such adducts are not involved directly as intermediates in the high-temperature ROP of [PCl2N]3.

The reactions of HCl, PCl5, and a crown ether (12-crown-4 or 18-crown-6) in CHCl3 under anaerobic conditions give complexes of the superacid HPCl6: [H(12-crown-4)][PCl6 ] and [H(18-crown-6)2][PCl6]. The crystal structures indicate that the proton lies roughly in the center of the 12-crown-4 molecule in [H(12-crown-4)][PCl6 ] whereas it lies between two oxygen atoms of two different 18-crown-6 molecules in [H(18-crown-6)2][PCl6].

The by-product chlorophosphazenes obtained from the reaction of PCl5 and NH4Cl at ∼ 140°, have been characterized by 2D 31P–15N HMQC and DOSY NMR spectroscopy. These NMR spectra show that the byproducts are a very complex mixture.