Acid-Base and Metal-Ion-Coordinating Properties of Benzimidazole and Derivatives (= 1, 3-Dideazapurines) in Aqueous Solution: Interrelation between Complex Stability and Ligand Basicity

摘要

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 benzimidazole-type ligands sterically unhindered at the N3 position (= L), namely, benzimidazole, 1-methylbenzimidazole, 5 (6)-chlorobenzimidazole, 6-chloro-5-fluorobenzimidazole, 5 (6)-nitrobenzimidazole, 5, 6-dichloro-1-(#beta#-D-ribofuranosyl) benzimidazole, and 5, 6-dinitrobenzimidazole (DNBI). were determined by UV/Vis spectrophotometry for DNBI and for all the other ligands by potentiometric aH titration in aqueous solutions (25 deg C, I = 0.5M, NaNO_3). The acidity constants for the monoprotonated ligands HL~+ were also measured. For the HL~+ species which are symmetric with respect to the H(N1) and H(N3) sites, the corresponding micro acidity constants are also given. Plots of log K_(ML)~M versus pK_(HL)~H (taking into account the micro acidity constants where appropriate) give straight lines. The equations for these least-squares lines allow calculation of the expected stability constant for a complex of any benzimidazole-type ligand, provided its pK_(HL)~H value (in the pK_a range 2-6) is known. For the stabilities of Fe~(2+) complexes with benzimidazole-type ligands an estimation procedure is described. The effect of steric inhibition resulting from annelation (the fusion of a benzene ring to C4 and C5 of imidazole), is quantified and compared to that of a methyl group in the ortho position to the N atom binding the metal ion. The effect of annelation is considerable for the complexes of the divalent 3d metal ions (-0.3 to -0.7 log units) but practically nonexistent for those of the alkaline earth ions, which indicates outer-sphere complex formation for the latter. This interpretation agrees with the observation that the stability of the ML~(2+) complexes of Ca~(2+), Sr~(2+), and Ba~(2+) is practically independent of the basicity of the benzimidazole derivative. The regression lines obtained for the complexes of the benzimidazoletype ligands now permit the determination of the extent of the steric inhibition of the (C6) NH_2 group on metal-ion binding at N7 of the adenine residue.