Chelating dihydroxamic acids: study of metal speciation and co-ordination compounds with Ni2+ and Cu2+

Abstract

Solution (potentiometric, absorption, EPR) as well as solid state studies (X-ray, EPR, magnetic susceptibility) have shown that the binding ability of oxalodihydroxamic acid (H2oxha) with two hydroxamic groups next to each other differs distinctly from that observed for aliphatic dihydroxamic acids, HONHOC(CH2)nCONHOH (n = 3–8). H2oxha was found to be a very powerful ligand for NiII and CuII due to an N,N′-chelating binding mode. The availability of the mixed nitrogen–oxygen donor systems results in formation of polymeric complexes with CuII. The crystal structure of K2[Ni(oxha)2]·2H2O has shown two very characteristic features of the dihydroxamic ligand: (i) two short intramolecular hydrogen bonds stabilising very effectively the complex formed and (ii) very specific binding modes of K+ ions, which play a bridging role between [Ni(oxha)2]2− ions. The latter finding demonstrates a capacity of H2oxha-based anionic complex species to form polynuclear complexes with different bridging modes.