Complex Formation Between Zinc(II) and Alkyl-N-iminodiacetic Acids in Aqueous Solution and Solid State

Removal of metal compounds from wastewater using processes where metals can be removed and valuable chemicals recycled is of significant industrial importance. Chelating surfactants are an interesting group of chemicals to be used in such applications. Carboxylated polyamines are a promising group to be used in such processes. To apply carboxylated polyamines as chelating surfactants, detailed knowledge of the solution chemistry, including complex formation, kinetics and structures of pure fundamental systems, is required. In this study zinc(II) alkyl-N-iminodiacetate systems with varying length of the alkyl chain have been studied. Acidic and stability constants have been studied by potentiometry, and the structures of both solids and aqueous solutions have been determined by EXAFS. Zinc(II) forms two strong complexes with alkyl-N-iminodiacetates in aqueous solution. In an attempt to determine the acidic constants of these complexes, the deprotonation of the nitrogen atom in the complex bound ligands, it was observed that this reaction is very slow and no accurate values could be obtained. The bis(alkyl-N-iminodiacetato)zincate(II) complexes take, however, up two protons in the pH region 3–7, which means that this complex is approximately singly protonated in the pH region 3–7 and doubly protonated at pH < 3. The bis(n-hexyl-N-iminodiacetato)zincate(II) complex at pH = 13 has a distorted octahedral configuration with four short strong Zn–O bonds at 2.08(1) Å, while the Zn–N bonds are weaker at much longer distance, 2.28(2) Å. Similar configurations are also found in most reported structures of zinc(II) complexes with carboxylated amines/polyamines. The singly protonated complex seems to be five-coordinate, with four Zn–O bond distances at ca. 2.03 Å, and a single Zn–N bond distance in the range 2.15–2.25 Å. The relationship between the structure of the protonated bis(n-hexyl-N-iminodiacetato)zincate(II) complex and the slow kinetics in the region pH = 3–7 are discussed