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

Abstract

The stability constants of the 1:1 complexes formed between Mg 2+, Ca 2+, Sr 2+, Ba 2+, Mn 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+ or Cd 2+ (M 2+) and the simple, sterically unhindered imidazole-type ligands, imidazole, 1-methylimidazole, 5-chloro-1-methylimidazole, N-(2,3,5,6-tetrafluorophenyl)imidazole or 4′-(imidazol-1-yl)acetophenone (L), were determined by potentiometric pH titrations in aqueous solution (25°C; I = 0.5 M, NaNO 3). The construction of log K ML M versus p K 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 p K HL H value (in the p K a range 4–8) is known. For the stabilities of Fe 2+ complexes with imidazole-type ligands an estimation procedure is provided. It is shown further that the complex formation between 1-methylbenzimidazole (MBI) and Mn 2+, Ni 2+, Cu 2+ or Zn 2+ is s 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.