Stability of Binary and Ternary Copper (II) Complexes of 2((4-methyl-5-nitro-6-(pyrrolidine-1-yl)pyrimidine-2-yl)amino) propionic acid, in Aqueous Solution

The acidity and stability constants of M (MNPPAP)[1] M: Cu2+, Cu (Bpy[2])2+, or Cu(Phen[3])2+ complexes, were determined by potentiometric pH titration. It is shown that the stability of the binary Cu (MNPPAP) complex is determined by the basicity of the carboxylate group on one side and amine group on the other side. It is demonstrated that the equilibrium, Cu(Har[4])2+ + Cu(MNPPAP) Cu(Har)(MNPPAP) + Cu2+, is displacement due to the well known experience that mixed ligand complexes formed by a divalent 3d ion, a heteroaromatic N base and an O donor ligand possess increased stability. The other part of this displacement, which amount on average to no increased stability of the mixed ligand Cu (Bpy)(MNPPAP) and Cu(Phen)(MNPPAP) complexes. The stability constants were determined by potentiometric pH titration in aqueous solution. The order of the stability constants was reported. A comparative investigation between ternary complexes of MNPPAP, Trp[5] and Gly[6] is made. The comparison of stability constants of these ternary complexes show that Cu (Har) (MNPPAP) and Cu (Har) (Gly) exist in open form but Cu (Har)(Trp) is found near 100% in closed form. The differences between the above mentioned stability constants based on stacked form of Cu (Har) (Trp). The last provides for increased stability compared with Cu (Har)(MNPPAP).