Phosphazene
Organophosphorus compound with pentavalent phosphorus having P=N bonds
Phosphazenes refer to various classes of organophosphorus compounds featuring phosphorus(V) with a double bond between P and N. One class of phosphazenes have the formula R−N=P(−NR2)3. These phosphazenes are also known as iminophosphoranes and phosphine imides. They are superbases.
01BEMP and t-Bu-P4
Well known phosphazene bases are BEMP (2-tert-Butylimino-2-diEthylamino-1,3-diMethylperhydro-1,3,2-diazaPhosphorine) with an acetonitrile pKa of the conjugate acid of 27.6 and the phosphorimidic triamide t-Bu-P4 (pKBH+ = 42.7) also known as Schwesinger base. BEMP and P4-t-Bu|t-Bu-P4 have attracted attention because they are low-nucleophilic, which precludes their participating in competing reactions. Being non-ionic ("charge-neutral"), they are soluble in nonpolar solvents. Protonation takes place at a doubly bonded nitrogen atom. The pKa's of tert-Bu−(H)N=P(−N=P(−NR2)3)3]+, where R = Me and pyrrolidinyl, are 42.7 and 44, respectively. These are the highest pKa recorded for the conjugate acid of charge-neutral molecular base.
In one implementation, t-Bu-P4 catalyzes the conversion of pivaldehyde to the alcohol: Phosphazene bases have been used as basic titrants in non-aqueous acid-base titrations.
02Other classes of phosphazenes
Also called phosphazenes are represented with the formula (−N=P(−X)2−)n, where X = halogen, alkoxy group, amide and other organyl groups. One example is hexachlorocyclotriphosphazene (−N=P(−Cl)2−)3. Bis(triphenylphosphine)iminium chloride [Ph3P=N=PPh3]+Cl−is also referred to as a phosphazene, where Ph = phenyl group. The present article focuses on those phosphazenes with the formula R−N=P(−NR2)3.
Sources and credits
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