Metal complexes of some peptide derivatives. Part-XV. Mixed ligand complex formation of copper(II) with some N-benzenesulfonyldipeptides as primary ligands and acetate, 2-methylbenzimidazole, glycinate and bipyridine as secondary ligands

Potentiometric investigation on the complex formation equilibria of Cu I I with N-benzenesulfonyl derivatives of some dipeptides, viz. glycylglycine, glycyl-β-alanine and β-alanylglycine, hereafter, abbreviated as NBSGG, NBSGB and NBSBG (AH 2 ) respectively, in the presence of typical secondary ligands, viz. acetate (ac-), 2-methylbenzimidazone (bz), glycinate (gly-) and bipyridine (bipy), hereafter, B, provides evidence of formation of varieties of binary and ternary complexes of the types: Cu(AH), Cu(H - 1 AH), Cu(H - 1 A), Cu(H - 1 A)(OH), Cu(B), Cu(B)(AH), Cu(B)(H - 1 AH) and Cu(B)(H - 1 A). Stability constants of the complexes at 25 ′ 1°, in aqueous solution at a constant ionic strength, I = 0.1 M (NaNO 3 ) have been determined and the complex formation equilibria elucidated on the basis of speciation curves. The sulfonamidodipeptide ligands (AH 2 ) provide three coordinating groups: -COOII, -CONH-, and -SO 2 NH-, and show ambidentism in forming the binary and the ternary complexes depending upon the denticity, σ-basicity and π-acidity of the B-ligands. The ternary complexes are found to be stable in the intermediate pH range (5-8). Above pll 8, the mixed ligand complexes derived from all the AH 2 and B-ligands decompose to produce either the fully deprotonated binary complexes, Cu(H - 1 A) - or the ternary hydroxo complexes Cu(H - 1 A)(OH) - 2 depending upon the nature of the B-ligands.