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 Ni^II and Cu^II due to an N,N′-chelating binding mode. The availability of the mixed nitrogen–oxygen donor systems results in formation of polymeric complexes with Cu^II. 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.