Basicity of some P1 phosphazenes in water and in aqueous surfactant solution

Abstract

The pK_a values in water and in dilute surfactant solution for 15 ring-substituted phenyl P₁ pyrrolidino phosphazenes PhNP(NC₄H₈)₃ and the phenyl P₁ dimethylamino phosphazene PhNP(NMe₂)₃ previously studied in acetonitrile (AN) and tetrahydrofuran (THF) are reported. The nonionic surfactant Tween 20 was used for the basicity measurements of some compounds to overcome the solubility problems. Measurements with a control group of phosphazenes in both media were used to validate the use of the obtained pK_a values as estimates of aqueous values. The pK_a values of the studied phosphazenes in aqueous medium vary from 6.82 (2,6-dinitro-) to 12.00 (4-dimethylamino-). The basicity span is 5.18 pK_a units. The aqueous pK_a values of the P₁ phosphazenes were correlated with the respective basicity data in AN and THF and from these correlations the pK_a values in water for the parent compounds HNP(NC₄H₈)₃ and HNP(NMe₂)₃ were estimated as 13.9 and 13.3. Also a comparison of the basicity of phosphazenes and some guanidines, amines and pyridines was made. In water the parent phosphazenes and guanidines are the strongest of all the groups of bases studied. In AN and THF the parent phosphazenes are clearly the strongest bases followed by guanidines, amines and pyridines which are bracketed between the basicities of phenyl phosphazenes. In the gas phase the phosphazenes for which data are available are clearly more basic than the other compounds referred to here. Comparison of the basicity data of P₁ phosphazenes and some guanidines confirms earlier conclusions about the partly ylidic character of the NP double bond.