Metal ion-binding properties of the diphosphate ester analogue, methylphosphonylphosphate, in aqueous solution

Abstract

The stability constants of the 1:1 complexes formed between methylphosphonylphosphate (MePP3-), CH3P(O)2--O-PO32-, and Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Zn2+, or Cd2+ (M2+) were determined by potentiometric pH titration in aqueous solution (25°C; l = 0.1 M, NaNO3). Monoprotonated M(H;MePP) complexes play only a minor role. Based on previously established correlations for M2+-diphosphate monoester complex-stabilities and diphosphate monoester β-group basicities, it is shown that the M(MePP)- complexes for Mg2+ and the ions of the second half of the 3d series, including ZN2+ and Cd2+, are on average by about 0.15 log unit more stable than is expected based on the basicity of the terminal phosphate group in MePP3-. In contrast, Ba(MePP)- and Sr(MePP)- are slightly less stable, whereas the stability for Ca(MePP)- is as expected, based on the mentioned correlation. The indicated increased stabilities are explained by an increased basicity of the phosphonyl group compared to that of a phosphoryl one. For the complexes of the alkaline earth ions, especially for Ba2+, it is suggested that outersphere complexation occurs to some extent. However, overall the M(MePP)- complexes behave rather as expected for a diphosphate monoester ligand.