Bis(triphenylphosphine)iminium chloride

Bis(triphenylphosphine)iminium chloride is the chemical compound with the formula [(C₆H₅)₃P)₂N]Cl, often written [(Ph₃P)₂N]Cl and abbreviated PPNCl. This colorless salt is a useful source of the PPN⁺ cation, which is used to isolate reactive anions. PPN⁺ is a phosphazene.

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Synthesis and structure

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

Ph₃P + Cl₂ → Ph₃PCl₂

This phosphorus(V) chloride 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

In PPN⁺ salts, P-N bond lengths are equivalent, 1.58 Å. The cation is centrosymmetric; its connectivity is indicated by Ph₃P=N=PPh₃⁺.

Applications

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. These salts 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 attributed 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⁺.

References

1. ^ Ruff, J. K.; Schlientz, W. J. “μ-nitrido-Bis(triphenylphosphorus)(1+ (“PPN”) Salts with Metal Carbonyl Anions” Inorganic Syntheses, 1974, volume XV, pages 84-90.