# Bis(triphenylphosphine)iminium chloride

Bis(triphenylphosphine)iminium chloride is the chemical compound with the formula [((C$_6$H$_5$)$_3$P)$_2$N]Cl, often abbreviated [(Ph$_3$P)$_2$N]Cl, where Ph is phenyl C$_6$H$_5$, or even abbreviated [PPN]Cl or [PNP]Cl or PPNCl or PNPCl, where PPN or PNP stands for (Ph$_3$P)$_2$N. This colorless salt is a source of the [(Ph$_3$P)$_2$N]$^+$ cation (abbreviated PPN$^+$ or PNP$^+$), which is used as an unreactive and weakly coordinating cation to isolate reactive anions. [(Ph$_3$P)$_2$N]$^+$ is a phosphazene.

## Synthesis and structure

[(Ph$_3$P)$_2$N]Cl is prepared in two steps from triphenylphosphine Ph$_3$P:
    Ph$_3$P + Cl$_2$ → Ph$_3$PCl$_2$
This triphenylphosphine dichloride Ph$_3$PCl$_2$ is related to phosphorus pentachloride PCl$_5$. Treatment of this species with hydroxylamine in the presence of Ph$_3$P results in replacement of the two single P–Cl bonds in Ph$_3$PCl$_2$ by one double P=N bond:
    2 Ph$_3$PCl$_2$ + NH$_2$OH·HCl + Ph$_3$P → [(Ph$_3$P)$_2$N]Cl + 4HCl + Ph$_3$PO
Triphenylphosphine oxide Ph$_3$PO is a by-product.
Bis(triphenylphosphine)iminium chloride is described as [(Ph$_3$P)$_2$N]$^+$Cl$^−$. The structure of the bis(triphenylphosphine)iminium chloride [(Ph$_3$P)$_2$N]$^+$Cl$^−$ is [Ph$_3$P=N=PPh$_3$]$^+$Cl$^−$. The P=N=P angle in the cation is flexible, ranging from ~130 to 180° depending on the salt. Bent and linear forms of the P=N=P connections have been observed in the same unit cell. The same shallow potential well for bending is observed in the isoelectronic species bis(triphenylphosphoranylidene)methane, Ph$_3$P=C=PPH$_3$, as well as the more distantly related molecule carbon suboxide, O=C=C=C=O. For the solvent-free chloride salt [(Ph$_3$P)$_2$N]Cl, the P=N=P bond angle was determined to be 133°. The two P=N bonds are equivalent, and their length is 1.597(2) Å.

## Use as reagent

In the laboratory, [(Ph$_3$P)$_2$N]Cl is the main precursor to [(Ph$_3$P)$_2$N]$^+$ salts. Using salt metathesis reactions, nitrite, azide, and other small inorganic anions can be obtained with [(Ph$_3$P)$_2$N]$^+$ cations. The resulting salts [(Ph$_3$P)$_2$N]$^+$NO−2, [(Ph$_3$P)$_2$N]$^+$N−3, etc. are soluble in polar organic solvents.
[(Ph$_3$P)$_2$N]$^+$ 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 [(Ph$_3$P)$_2$N]$^+$ forms salts that are more air-stable than salts with smaller cations such as those containing quaternary ammonium cation [NR$_4$]$^+$, or alkali metal cations. This effect is attributed to the steric shielding provided by this voluminous cation. Illustrative [(Ph$_3$P)$_2$N]$^+$ salts of reactive anions include [(Ph$_3$P)$_2$N]$^+$[HFe(CO)$_4$]$^−$, [(Ph$_3$P)$_2$N]$^+$[Co(CO)$_4$]$^−$, ([(Ph$_3$P)$_2$N]$^+$)$_2$[M$_2$(CO)$_{10}$]$^{2+}$ (M = Cr, Mo, W), and [(Ph$_3$P)$_2$N]$^+$[Fe(CO)$_3$(NO)]$^−$. The role of ion pairing in chemical reactions is often clarified by examination of the related salt derived from [(Ph$_3$P)$_2$N]$^+$.

## Related cations

A phosphazenium cation related to [(Ph$_3$P)$_2$N]$^+$ is [(((CH$_3$)$_2$N)$_3$P)$_2$N]$^+$.