Intrinsic 31P NMR Chemical Shifts and the Basicities of Phosphate Groups in a Short-Chain Imino Polyphosphate

Abstract

The stepwise protonation constants of two linear triphosphate ligand anions, triphosphate, {text{P}}_{3} {text{O}}_{10} ^{5 - } and di-imidotriphosphate, {text{P}}_{3} {text{O}}_{8} left( {text{NH}} right)_{2}^{5 - } , were investigated by potentiometric titration, and the intrinsic chemical shifts of the stepwise protonated species of these anions were determined from the pH-dependence of the 31P NMR chemical shifts. All stepwise protonation constants of {text{P}}_{3} {text{O}}_{8} left( {text{NH}} right)_{2}^{5 - } were found to be larger than those of {text{P}}_{3} {text{O}}_{10}^{5 - } , and the 31P NMR signals due to P3O8(NH)2 always appeared at a lower magnetic field compared to the signals due to P3O10. These results indicate higher basicity of the P3O8(NH)2 ligand, because it contains two imino groups in the ligand molecule. The 31P NMR signals for the end phosphate groups appear at a lower magnetic field than those for the middle phosphate groups, indicating that the basicity of the end phosphate group is higher than that of the middle phosphate group. It can be expected that the high basicity of the P3O8(NH)2 ligand brings about the formation of high stability complexes with various metal ions. Furthermore, the only 31P NMR signal due to the middle phosphate group of P3O8(NH)2 ligand molecule clearly showed a low-field shift in the range of pH < 2.5. The reason for this peculiar low-field shift should be the change of the localization of imino protons around the nitrogen atom in P3O8(NH)2.Graphical