Bis(triphenylphosphine)iminium chloride

PPNCl is prepared in two steps from triphenylphosphine:[1]

Ph₃P + Cl₂ → Ph₃PCl₂

This triphenylphosphine dichloride is related to phosphorus pentachloride. Treatment of this species with hydroxylamine in the presence of Ph₃P results in replacement of the P-Cl bonds by P=N bonds:

2 Ph₃PCl₂ + NH₂OH·HCl + Ph₃P → {[Ph₃P]₂N}Cl + 4HCl + Ph₃PO

The PNP angle in the cation is flexible, ranging from ~130 to 180° depending on the salt. Bent and linear forms have been observed in the same unit cell.[2] The same shallow potential well for bending is observed in the isoelectronic species bis(triphenylphosphoranylidene)methane, (Ph₃P)₂C, as well as the more distantly related molecule carbon suboxide. For the solvent-free chloride salt, the PNP bond angle was determined to be 133°.[3] The P-N bond lengths are equivalent at 1.597(2) Å.

In the laboratory, PPN chloride is the main precursor to PPN⁺ salts. Using salt metathesis reactions, nitrite, azide, and other small inorganic anions can be obtained with PPN⁺ cations. The resulting salts PPNNO₂, PPNN₃ etc. are soluble in polar organic solvents.

PPN⁺ forms crystalline salts with a range of anions that are otherwise difficult to crystallize. Its effectiveness is partially attributable to its rigidity, reflecting the presence of six phenyl rings. Often PPN⁺ forms salts that are more air-stable than salts with smaller cations such as those containing quaternary ammonium or alkali metal cations. This effect is attributed to the steric shielding provided by this voluminous cation. Illustrative PPN⁺ salts of reactive anions include PPN[HFe(CO)₄], PPN[Co(CO)₄], and PPN[Fe(CO)₃NO]. The role of ion pairing in chemical reactions is often clarified by examination of the related salt derived from PPN⁺.

A phosphazenium cation related to PPN⁺ is {[(CH₃)₂N)₃P]₂N}⁺.[4]