Ansa isomer selectivity in the reactions of cyclotetraphosphazene with octafluorohexane-1,6-diol
The reaction of octachlorocyclotetraphosphazene, N4P4Cl8 (1), in three stoichiometries (1:1, 1:2 and 1:3) with the sodium derivative of the fluorinated diol, 2,2,3,3,4,4,5,5-octafluorohexane-1,6-diol (2), in THF solution at room temperature produced five products: the open chain compound N4P4Cl7[OCH2(CF2)4CH2OH] (3), the spiro compound N4P4Cl6[OCH2(CF2)4CH2O] (4b), the 1,5-ansa compound N4P4Cl6[OCH2(CF2)4CH2O] (4c), the cis-1,3;5,7-bis-ansa derivative N4P4Cl4[OCH2(CF2)4CH2O]2 (5a) and the 1,3;1,3;5,7;5.7-tetrakis-ansa compound, N4P4(OCH2(CF2)4CH2O)4 (6a). The structures were determined by elemental analysis, mass spectrometry, 1H, 19F and 31P NMR spectroscopies and for compounds 4c and 5a, where suitable single crystals were obtained, the structures were characterised by X-ray crystallography. It was found that the (PN)4 ring in compound 4c is in a slightly-twisted tub conformation but in compound 5a the first symmetrical crown conformation of the (PN)4 ring is observed. The results of all the reactions were compared with previous work on the reaction of hexachlorocyclotriphosphazene, N3P3Cl6, with the sodium derivative of the diol (2), in a 1:1.2 mole ratio in the same solvent, where intermolecular bridging reactions were preferred. The regio-selective formation of mainly ansa derivatives of the fluorohexanediol with cyclotetraphosphazene was rationalised in terms of non-bonded 1,3 and 1,5 P…P distances of the tetramer ring.