Table of Contents
Metal-Based Drugs
Volume 6, Issue 6, Pages 321-328

Metal Ion-Binding Properties of the Diphosphate Ester Analogue, Methylphosphonylphosphate, in Aqueous Solution

1Institute of Inorganic Chemistry, University of Basel, Spitalstrasse 51, Basel CH-4056, Switzerland
2Faculty of Pharmacy, Comenius University, Kalinčiaková 8, Bratislava 83232, Slovakia
3Faculty of Chemistry, Slovak University of Technology, Radlinskeho 9, Bratislava 81237, Slovakia

Received 17 June 1999; Accepted 6 July 1999

Copyright © 1999 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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+,Cu2+,Zn2+,orCd2+(M2+) were determined by potentiometric pH titration in aqueous solution (25C° ; 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.