Group 13 Superacid Adducts of [PCl₂N]₃

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.