METAL-ION-COORDINATING PROPERTIES OF A VIRAL INHIBITOR, A PYROPHOSPHATE ANALOG, AND A HERBICIDE METABOLITE, A GLYCINATE ANALOG - THE SOLUTION PROPERTIES OF THE POTENTIALLY 5-MEMBERED CHELATES DERIVED FROM PHOSPHONOFORMIC ACID AND (AMINOMETHYL)PHOSPHONIC ACID

The pyrophosphoric-acid-analogue phosphonoformic acid (pfa) and the amino-acid-analogue (aminomethyl)phosphonic acid (ampa) both form, in the deprotonated state, i.e., as (OOC)-O---PO32- and H2N-CH2-PO32-, respectively, five-membered chelate rings with metal ions. pfa inhibits both phosphate transport and virus replication, while ampa is a metabolic product of the common herbicide glyphosate (= N-(phosphonomethyl)glycine). The acidity constants of H(2)pfa(-) and H(2)ampai(+/-) as well as the stability constants of the [M(Hpfa)], [M(pfa)]-, [M(Hampa)](+), and [M(ampa)] complexes, where M(2+) = Mg2+, Ca2(+), Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Cu(2,2'-bipyridyl)(2+), Cu(1,10-phenanthroline)2+, Zn2+ or Cd2+, have been determined by potentiometric pH titrations in aqueous solution at 25 degrees and I=0.1M (NaNO,). The structures of isomeric complexes and the connected intramolecular equilibria are deduced and evaluated based on the equilibrium constants measured and those calculated via the pK(a), values of the above mentioned ligands and previously established log K vs. pK(a) straight-line plots (H. Sigel et nl., Helv. Chim. Acta 1992, 75, 2634) for a simple phosphonate-M(2+) coordination. pfa forms stronger complexes than ampa with ah the above mentioned metal ions, with the single exception of [Cu(ampa)] which is slightly more stable than [Cu(pfa)](-). In neutral solutions, more precisely at pH of ca. 6, pfa complexes of alkaline-earth-metal ions retain one phosphonate-bound proton, [M(Hpfa)], while those of the transition-metal ions chelate with the trianionic ligand, pfa(3-). In accord with increasing ligand-basicity, the stability-constant order for all metal-ion complexes is oxalate <pfa <pyrophosphate but, owing to proton competition in pyrophosphate, in neutral solutions metal-ion complexation of pfa(3-) competes with P2O74-. With ampa alkaline-earth-metal ions interact only with the phosphonate group of even the dianionic ligand (though Mg2+ appears to form a low fraction of a [Mg(ampa)] chelate) while transition-metal ions form chelates which are comparable in stability to those of glycinate.