Acidity of some 2-substituted-1H-benzimidazoles and their affinity towards Cu(II) ion

Abstract

Twenty eight 2-substituted-1H-benzimidazole derivatives have been synthesized, of which nine of them are novel. The benzimidazoles, which are potential biologically active chemicals, consist of 2-alkyl-1H-benzimidazole, 2-aryl-1H-benzimidazole and bis-(2-benzimidazolyl)- derivatives. The protonation constants and analytical stabilities of Cu(II) complexes of the benzimidazole derivatives were determined quantitatively by means of the known potentiometric method. It was found that: the protonation constant value, log beta, of the imidazole ring lies between 3.17 and 6.37. The wide interval of the acidity on the imidazole ring arises from the unique characteristic of substituents in the benzimidazole species' Because of that, the delta effects of chloro-, nitro- and methyl- substituents on acidity of the berizimidazole ring were investigated, and it was also determined that the substituents having delta- effects lead to lower log beta values. The stability of Cu(II)- complexes of the monodentate benzimidazoles can be described with log K values between 3 and 4. For bidentate benzimidazoles, which have a secondary donor substituent, log K values greater than 4.7 were obtained. 2-(ortho-hydroxyphenyl) substituted IH-benzimidazoles form quite stable copper complexes due to the chelate effect. The stability constants (log K) of such chelates increase to 9.1- 10.2.