Alkylation and acylation of cyclotriphosphazenes

Phosphazenes (RNH)6P3N3 (R = n-propyl, isobutyl, isopropyl, cyclohexyl, tert-butyl, benzyl) are readily alkylated at ring N sites by alkyl halides forming N-alkyl phosphazenium cations. Alkylation of two ring N sites occurred after prolonged heating in the presence of methyl iodide or immediately at room temperature with methyl triflate yielding N,N'-dimethyl phosphazenium dications. Geminal dichloro derivatives Cl2(RNH)4P3N3 are methylated by methyl iodide at the ring N site adjacent to both P centers carrying four RNH groups. X-ray crystal structures showed that the alkylation of ring N sites leads to substantial elongation of the associated P-N bonds. Both N-alkyl and N,N'-dialkyl phosphazenium salts form complex supramolecular networks in the solid state via NH...X interactions. Systems carrying less-bulky RNH groups show additional NH...N bonds between N-alkyl phosphazenium ions. N-Alkyl phosphazenium halides form complexes with silver ions upon treatment with silver nitrate. Depending on the steric demand of RNH substituents, either one or both of the vacant ring N sites engage in coordination to silver ions. Treatment of (RNH)6P3N3 (R = isopropyl) with acetyl chloride and benzoyl chloride, respectively, yielded N-acyl phosphazenium ions. X-ray crystal structures revealed that elongation of P-N bonds adjacent to the acylated ring N site is more pronounced than it is in the case of N-alkylated species. Salts containing N-alkyl phosphazenium ions are stable toward water and other mild nucleophiles, while N,N'-dialkyl and N-acyl phosphazenium salts are readily hydrolyzed. The reaction of (RNH)6P3N3 with bromoacetic acid led to N-alkylation at one ring N site in addition to formation of an amide via condensation of an adjacent RNH substituent with the carboxylic acid group. The resulting bromide salt contains mono cations of composition (RNH)5P3N3CH2CONR in which a CH2-C(O) unit is embedded between a ring N and an exocyclic N site of the phosphazene.