Bis(triphenylphosphine)iminium chloride

Synthesis and structure

PPNCl is prepared in two steps from triphenylphosphine:

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:

In PPN⁺ salts, P-N bond lengths are equivalent, 1.58 Å. The cation is bent at the central nitrogen, and has no inversion centre. Its connectivity is indicated by Ph₃P⁺–N^––P⁺Ph₃, which explains non-linear arrangement of P-N-P moiety owing to sp³-hybridization of the nitrogen center. Alternative depiction Ph₃P=N=PPh₃⁺ would imply sp-hybridization at N-atom and thus linear structure which is not the case.

Applications

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⁺.