Phosphazenes refer to classes of

organophosphorus compound Organophosphorus compounds are organic compounds containing phosphorus. They are used primarily in pest control as an alternative to chlorinated hydrocarbons that persist in the environment. Some organophosphorus compounds are highly effective in ...

s featuring phosphorus(V) with a double bond between P and N. One class of phosphazenes have the formula . These phosphazenes are also known as iminophosphoranes and phosphine imides. They are

superbase A superbase is a compound that has a particularly high affinity for protons. Superbases are of theoretical interest and potentially valuable in organic synthesis. Superbases have been described and used since the 1850s.''Superbases for Organic ...

s. Another class of compounds called phosphazenes are represented with the formula , where X = halogen, alkoxy group,

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...

and other

organyl group In organic and organometallic chemistry, an organyl group is an organic substituent with one (sometimes more) free valence(-s) at a carbon atom.. The term is often used in chemical patent literature to protect claims over a broad scope. Exampl ...

s. One example is hexachlorocyclotriphosphazene .

Bis(triphenylphosphine)iminium chloride Bis(triphenylphosphine)iminium chloride is the chemical compound with the formula , often abbreviated , where Ph is phenyl , or even abbreviated PNl or NPl or PPNCl or PNPCl, where PPN or PNP stands for . This colorless salt is a source of the c ...

is also referred to as a phosphazene, where Ph =

phenyl group In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydroge ...

. This article focuses on those phosphazenes with the formula .

Phosphazene bases

Phosphazene bases are strong non-metallic non-ionic and low-nucleophilic bases. They are stronger bases than regular

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent su ...

amidine Amidines are organic compounds with the functional group RC(NR)NR2, where the R groups can be the same or different. They are the imine derivatives of amides (RC(O)NR2). The simplest amidine is formamidine, HC(=NH)NH2. Examples of amidines inclu ...

bases. Protonation takes place at a doubly bonded nitrogen atom. Related to phosphazene bases are the Verkade bases, which feature P(III) with three amido substituents and a transannular amine. The p''K''_a's of , where R = Me and pyrrolidinyl, are 42.7 and 44, respectively. These are the highest p''K''_a measured for the

conjugate acid A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...

of charge-neutral molecular base. P4-t-Bu_Strukturformel.svg, ''t''-Bu-P₄ BEMP Phosphazene.svg, BEMP Phosphazene bases are established reagents in organic synthesis. Perhaps the best known phosphazene bases are BEMP with an acetonitrile p''K''_a of the

of 27.6 and the phosphorimidic triamide ''t''-Bu-P₄ (p''K''_BH⁺ = 42.7) also known as Schwesinger base after one of its inventors. In one application ''t''-Bu-P₄ is employed in a

nucleophilic addition In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, such that the double or triple bond is broken. Nucleophilic additions ...

converting the pivaldehyde to the

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

: The active

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they a ...

is believed to be a highly reactive phosphazenium species with full negative charge on the

arene Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past grouping ...

sp² carbon. Besides organic synthesis, phosphazene bases are used as basic titrants in non-aqueous

acid–base titration An acid–base titration is a method of quantitative analysis for determining the concentration of an acid or base by exactly neutralizing it with a standard solution of base or acid having known concentration. A pH indicator is used to monito ...

. Their advantages for this are: they are very strong bases in many

solvents A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

and their conjugate acids are inert and non-HBD cations.

References

{{reflist Nitrogen compounds Phosphorus compounds Non-nucleophilic bases Superbases

Phosphazene bases

See also

References