Metal ion-binding properties of (1H-benzimidazol-2-yl-methyl)phosphonate (Bimp2-) in aqueous solution. Isomeric equilibria, extent of chelation, and a new quantification method for the chelate effect.

The acidity constants of the 2-fold protonated (1H-benzimidazol-2-yl-methyl)phosphonate, H2(Bimp)(+/-), are given, and the stability constants of the M(H;Bimp)+ and M(Bimp) complexes with the metal ions M2+ = Mg2+, Ca2+, Ba2+, Mn2+, Co2+, Cu2+, Zn2+, or Cd2+ have been determined by potentiometric pH titrations in aqueous solution at I = 0.1 M (NaNO3) and 25 degrees C. Application of previously determined straight-line plots of log KM(M(Bi-R)) versus pKH(H(Bi-R)) for benzimidazole-type ligands, Bi-R, where R represents a residue which does not affect metal ion binding, proves that the primary binding site in the M(H;Bimp)+ complexes is (mostly) N3 and that the proton is located at the phosphonate group; outersphere interactions seem to be important, and the degree of chelate formation is above 60% for all metal ion complexes studied, except for Zn(H;Bimp)+.

Synthesis and acid-base properties of (1H-benzimidazol-2-yl-methyl)phosphonate (Bimp2-). Evidence for intramolecular hydrogen-bond formation in aqueous solution between (N-1)H and the phosphonate group.

The synthesis of (1H-benzimidazol-2-yl-methyl)phosphonic acid, H2(Bimp)+/-, is described: 2-chloromethylbenzimidazole was reacted with ethylchloroformate to give 1-carboethoxy-2-chloromethylbenzimidazole which was treated with trimethyl phosphite and after hydrolysis with aqueous HBr H2(Bimp)+/- was obtained. In H2(Bimp)+/- one proton is at the N-3 site and the other at the phosphonate group; both acidity constants were determined in aqueous solution by potentiometric pH titrations (25 degrees C; I = 0.1 M, NaNO3) and this furnished the pKa values of 5.