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 imide In chemistry a phosphine imide (sometimes abbreviated to phosphinimide) also known as a iminophosphorane is a functional group with the formula R3P=NR. While structurally related to phosphine oxide its chemistry has more in common with phosphonium ...

s. 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 The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...

alkoxy group In chemistry, the alkoxy group is an alkyl group which is singularly bonded to oxygen; thus . The range of alkoxy groups is vast, the simplest being methoxy (). An ethoxy group () is found in the organic compound ethyl phenyl ether (, also k ...

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. Example ...

s. One example is

hexachlorocyclotriphosphazene Hexachlorophosphazene is an inorganic compound with the formula . The molecule has a cyclic, unsaturated backbone consisting of alternating phosphorus and nitrogen centers, and can be viewed as a trimer of the hypothetical compound . Its classif ...

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 hydrogen ...

. 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 substituen ...

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 includ ...

bases. Protonation takes place at a doubly bonded nitrogen atom. Related to phosphazene bases are the

Verkade base In chemistry, a Verkade base (or Verkade superbase) is a superbase with the formula P(MeNCH2CH2)3N. A colorless oil, it is an aminophosphine although its inventor John Verkade called it proazaphosphatrane. The trimethyl derivative or 2,5,8,9-tet ...

s, which feature P(III) with three amido substituents and a

transannular In organic chemistry annulation (from the Latin ''anellus'' for "little ring"; occasionally annelation) is a chemical reaction in which a new ring is constructed on a molecule. : Examples are the Robinson annulation, Danheiser annulation and cert ...

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 Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not clas ...

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 di ...

converting the

pivaldehyde Pivaldehyde is an organic compound, more specifically an aldehyde. Shown in the image is a line-angle representation of this organic aldehyde, whose systematic name, 2,2-dimethylpropanal, is based on the longest carbon chain (three carbon atoms), ...

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 are ...

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 monitor ...

. 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 p ...

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