Bis(triphenylphosphine)iminium chloride is the chemical compound with the formula [((C6H5)3P)2N]Cl, often abbreviated [(Ph3P)2N]Cl, where Ph is phenylC6H5, or even abbreviated [PPN]Cl or [PNP]Cl or PPNCl or PNPCl, where PPN or PNP stands for (Ph3P)2N. This colorless salt is a source of the [(Ph3P)2N]+ cation (abbreviated PPN+ or PNP+), which is used as an unreactive and weakly coordinating cation to isolate reactive anions. [(Ph3P)2N]+ is a phosphazene.
Bis(triphenylphosphine)iminium chloride is described as [(Ph3P)2N]+Cl−. The structure of the bis(triphenylphosphine)iminium cation [(Ph3P)2N]+ is [Ph3P=N=PPh3]+. 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.[2] The same shallow potential well for bending is observed in the isoelectronic species bis(triphenylphosphoranylidene)methane, Ph3P=C=PPH3, as well as the more distantly related molecule carbon suboxide, O=C=C=C=O. For the solvent-free chloride salt [(Ph3P)2N]Cl, the P=N=P bond angle was determined to be 133°.[3] The two P=N bonds are equivalent, and their length is 1.597(2) Å.
thermal ellipsoid model the bis(triphenylphosphine)iminium cation
Use as reagent
In the laboratory, [(Ph3P)2N]Cl is the main precursor to [(Ph3P)2N]+ salts. Using salt metathesis reactions, nitrite, azide, and other small inorganic anions can be obtained with [(Ph3P)2N]+ cations. The resulting salts [(Ph3P)2N]+NO−2, [(Ph3P)2N]+N−3, etc. are soluble in polar organic solvents.
[(Ph3P)2N]+ 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 [(Ph3P)2N]+ forms salts that are more air-stable than salts with smaller cations such as those containing quaternary ammonium cation [NR4]+, or alkali metal cations. This effect is attributed to the steric shielding provided by this voluminous cation. Illustrative [(Ph3P)2N]+ salts of reactive anions include [(Ph3P)2N]+[HFe(CO)4]−, [(Ph3P)2N]+[Co(CO)4]−, ([(Ph3P)2N]+)2[M2(CO)10]2+ (M = Cr, Mo, W), and [(Ph3P)2N]+[Fe(CO)3(NO)]−.[1] The role of ion pairing in chemical reactions is often clarified by examination of the related salt derived from [(Ph3P)2N]+.
Related cations
A phosphazenium cation related to [(Ph3P)2N]+ is [(((CH3)2N)3P)2N]+.[4]
References
^ abRuff, J.K.; Schlientz, W.J. (1974). "μ‐Nitridobis(triphenylphosphorus)(l+) ("PPN") Salts with Metal Carbonyl Anions". Inorganic Syntheses. Vol. 15. pp. 84–90. doi:10.1002/9780470132463.ch19. ISBN9780470132463. ((cite book)): |journal= ignored (help)
^Hardy GE, Zink JI, Kaska WC, Baldwin JC (December 1978). "Structure and triboluminescence of polymorphs of hexaphenylcarbodiphosphorane". Journal of the American Chemical Society. 100 (25): 8001–8002. doi:10.1021/ja00493a035.
