Bis(triphenylphosphine)iminium chloride
Encyclopedia
Bisiminium chloride is the chemical compound
with the formula [(C6H5)3P)2N]Cl, often written [(Ph3P)2N]Cl and abbreviated PPNCl. This color
less salt is a source of the PPN+ cation, which is used to isolate reactive anions. PPN+ is a phosphazene
.
:
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=PPh3+.
, 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+.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with the formula [(C6H5)3P)2N]Cl, often written [(Ph3P)2N]Cl and abbreviated PPNCl. This color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
less salt is a source of the PPN+ cation, which is used to isolate reactive anions. PPN+ is a phosphazene
Phosphazene
Phosphazenes are a class of chemical compounds in which a phosphorus atom is covalently linked to a nitrogen atom by a double bond and to three other atoms or radicals by single bonds. While other substitutions produce relatively persistent compounds, in organic synthesis the term largely refers to...
.
Synthesis and structure
PPNCl is prepared in two steps from triphenylphosphineTriphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...
:
- Ph3P + Cl2 → Ph3PCl2
This triphenylphosphine dichloride
Triphenylphosphine dichloride
Triphenylphosphine dichloride, Ph3PCl2, is a chlorinating agent widely used in organic chemistry. Applications include the conversion of alcohols and ethers to alkyl chlorides, the cleavage of epoxides to vicinal dichlorides and the chlorination of carboxylic acids to acyl chlorides.- Structure :In...
is related to phosphorus pentachloride. Treatment of this species with hydroxylamine
Hydroxylamine
Hydroxylamine is an inorganic compound with the formula NH2OH. The pure material is a white, unstable crystalline, hygroscopic compound. However, hydroxylamine is almost always provided and used as an aqueous solution. It is used to prepare oximes, an important functional group. It is also an...
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 + Ph3POTriphenylphosphine oxideTriphenylphosphine oxide is the chemical compound with the formula OP3. Often chemists abbreviate the formula by writing Ph3PO or PPh3O . This white crystalline compound is a common side product in reactions involving triphenylphosphine...
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=PPh3+.
Applications
In the laboratory, PPN chloride is the main precursor to PPN+ salts. Using salt metathesis reactions, nitriteNitrite
The nitrite ion has the chemical formula NO2−. The anion is symmetric with equal N-O bond lengths and a O-N-O bond angle of ca. 120°. On protonation the unstable weak acid nitrous acid is produced. Nitrite can be oxidised or reduced, with product somewhat dependent on the oxidizing/reducing agent...
, azide
Azide
Azide is the anion with the formula N3−. It is the conjugate base of hydrazoic acid. N3− is a linear anion that is isoelectronic with CO2 and N2O. Per valence bond theory, azide can be described by several resonance structures, an important one being N−=N+=N−...
, 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+.