Metal ion-coordinating properties of imidazole and derivatives in aqueous solution, interrelation between complex stability and ligand basicity

The stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+ (M2+) and the simple, sterically unhindered imidazole-type ligands, imidazole, 1-methylimidazole, 5-chloro-1-methylimidazole, N-(2,3,5,6-tetra-fluorophenyl)imidazole or 4'-(imidazol-1-yl)acetophenone (L), were determined by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.5 M, NaNO3). The construction of log K-ML(M) versus pK(HL)(H) plots results in straight lines; the equations for the least-squares lines are calculated and listed. These data allow calculation of the expected stability constant for a complex of any imidazole-type ligand, provided its pK(HL)(H) value (in the pK(a) range 4-8) is known. For the stabilities of Fe2+ complexes with imidazole-type ligands an estimation procedure is provided. It is shown further that the complex formation between 1-methylbenzimidazole (MBI) and Mn2+, Ni2+, Cu-2 divided by or Zn2+ is sterically hindered, i.e. the data points for these M(MBI)(2+) complexes do not fall on the straight lines defined by the imidazole-type ligands. (C) 1998 Elsevier Science S.A. All rights reserved.