Ketopiperazinediacetate zinc and copper complexes in the degradation of ethylenediaminetriaceta

Polymeric ketopiperazinediacetate complexes [M(kpda)(H2O)2]n·nH2O (M = Cu, 1; Zn, 2. H2kpda = ketopiperazinediacetic acid, C8H12N2O5) were obtained from the reactions of copper chloride and zinc nitrate with ethylenediaminetriacete at pH 5.0 (H3ed3a = ethylenediaminetriacetic acid), where ed3a is in equilibrium with amidation product of kpda. 1 or 2 consists of a monomeric neutral unit [M(kpda)(H2O)2]n, where ketopiperazinediacetate anion serves as bridged ligand through the amine nitrogen and the acetate oxygen forming a glycinate metallocycle, and the other acetate coordinates bidentately with the neighboring metal atom. The former could be converted to [CuII(ox)(phen)(H2O)]·H2O (3) after the addition of phenanthroline (H2ox = oxalic acid, phen = phenanthroline), which shows degradation of ethylenediaminetriaceta, where [CuI(phen)2Cl]·6H2O (4) and [CuII(phen)2(H2O)](NO3)2 (5) were isolated in the reaction sequence.