On the Metal-Ion-Coordinating Properties of the Benzimidazolate Residue in Aqueous Solution-Extent of Acidification of Benzimidazole-(N3) H Sites by (N1)-Coordinated Divalent Metal Ions

The stability constants of the 1:1 complexes formed between Mg2+, Mn2+, Co2+, Ni2+ or Cd2+ (= M2+) and the anionic 5(6)-nitrobenzimidazolate [= (NBI-H)(-)] or 5,6-dinitrobenzimidazolate [= (DNBI-H)(-)] were determined by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.5 M, NaNO3). The acidity constants for the deprotonation of the (N1)H sites in neutral NBI and DNBI (= L) were measured by the same method. The comparison of the stability constants determined for the M(L-H)(+) complexes with those calculated from log K-ML(M) versus pK(HL)(H) straight-line plots, which were established recently for neutral benzimidazole-type ligands, reveals that the stabilities of the M(L-H)(+) complexes are significantly enhanced, as one might have expected due to the negative charge present on the Ligands. Of course, the electron-withdrawing properties of (N3)-bound metal ions facilitate the release of the proton from the (N1)H site in the M(NBI)(2+) and M(DNBI)(2+) complexes, if compared to the situation in the free ligands. The effect of a metal ion bound to a benzimidazolate or imidazolate residue on the coordination tendency toward a further metal ion (giving rise to an imidazolate bridge) was estimated.