AMINOHYDROXAMIC ACIDS AS METAL SEQUESTERING AGENTS: COPPER(II) AND NICKEL(II) COMPLEXES OF 2,6-DIAMINO-N-HYDROXYHEXANAMIDE, 2-AMINO-5-[(AMINOIMINOMETHYL)AMINO]-N-HYDROXYPENTANAMIDE, 2-AMINO-4-METHYLTHIO-N-HYDROXYBUTANAMIDE IN AQUEOUS SOLUTION

Abstract

Abstract Aqueous solution equilibria of the systems H+, copper(II) and nickel(II) with L-lysinehydroxamic acid (2,6-diamino-N-hydroxyhexanamide) (dahhe), L-argininehydroxamic acid (2-amino-5-[(aminoiminomethyl)amino]-N-hydroxypentanamide) (aimahp), and DL-methioninehydroxamic acid (2-amino-4-methylthio-N-hydroxybutanamide)(amthb) have been investigated in the metal-ligand ratio range 1:1–1:4 using potentiometric and spectrophotometric methods. The protonation and metal-complex formation constants for each system were calculated using the SUPERQUAD program and are reported. Complexation with copper(II) begins at pH ca. 3.0 for each system with simultaneous formation of [CuA]+ and [Cu2A2H−1]+. Only copper-dahhe forms complexes with the terminal amino group in the protonated form. Between pH 5.5–9.5 the bis complex is dominant. Above pH 8 deprotonated complexes are detected. In the different bis complexes ((CuA2H2]2+, [CuA2H]+, [CuA2] and [CuA2H−1]− in the case of dahhe, [CuA2] and [CuA2H−1]− with aimahp and amthb) only the nitrogens are coordinated but both the oxygen and the nitrogen donor atoms of the ligand are involved in the coordination in complexes [CuAH]2+ and [Cu2A2H]3+ formed with dahhe and in [CuA]+ and [Cu2A2H−1]+ formed with the other two ligands. Spectroscopic evidence shows no significant coordination through the terminal group of the ligand for any of the systems investigated. Complexation with nickel(II) starts at pH ca. 5.0 with simultaneous formation of 1:1 and 1:2 species. The [NiA]+ ([NiAH]2+ in the case of dahhe) is a very minor species. The [NiA2] ([NiA2H2]2+ with dahhe) complex is the dominant species with a planar geometry. Deprotonation of one of the coordinated hydroxamates occurs above pH 8 and overlaps with deprotonation of the terminal NH3 + groups of the dahhe ligand.