The Mechanism of the Reaction of Alkai Metal Phenoxides with Hexahalocyclotriphosphazenes

• Published in: Phosphorus, sulfur, and silicon and the related elements Vol. 183; no. 2-3; pp. 479 - 482
• Main Authors: Allen, Christopher W., Calichman, Michael, Freund, Amy S.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis Group 01.02.2008
• Subjects: Cyclophosphazenes; kinetics; phenoxyphosphazenes; phosphazene reaction mechanisms
• ISSN: 1042-6507; 1563-5325
• DOI: 10.1080/10426500701761425

In both specific reaction rate studies and in intermolecular competition reactions, sodium phenoxide preferentially reacts with hexafluorocyclotriphosphazene (1) over hexachlorocyclotriphosphazene (2). In an intramolecular competition experiment using difluorotetrachlorocyclotriphosphazene (3), sodium phenoxide reacts exclusively at the fluorinated phosphorus site. These results are consistent with the NBO charges at the phosphorus centers. DFT calculations also indicate that in the case of pentacoordinate intermediate formation, an associative mechanism is kinetically favored. The variations of the rate of reaction of 2 with temperature, solvent and alkali metal cation have been determined. The activation parameters for the chlorophosphazene system show positive enthalpies and entropies of activation. The reaction rates of 2 increase with solvent basicity. The rates of reaction for the heavier alkali metal cations are greater than those for the lighter cations. The addition of a crown ether results in a significant rate increase. Consideration of all of these data suggest a mechanism where the rate has a major contribution from a preequilibrium wherein the metal phenoxide cluster dissociates into kinetically active speicies which adds, in the rate determining step, to the phosphorus (V) center.