Interaction of desferrioxamine B (DFB) model dihydroxamic acids with some essential and toxic metal(ii) ions: effects of the structure and length of connecting chains on the metal ion selectivity

Abstract

Complexation of desferrioxamine B (DFB) model dihydroxamic acids (HO(CH₃)NCO(CH₂)_xCONH(CH₂)_yCON(CH₃)OH where x = 2, 3, y = 5, 4, 3, 2, and the compounds are abbreviated as 2,5-DIHA, 2,4-DIHA, 2,3-DIHA, 2,2-DIHA, 3,4-DIHA and 3,3-DIHA, respectively) with Cu(II), Ni(II), Zn(II), Pb(II) and Cd(II) was studied by pH-potentiometric and spectroscopic (UV-VIS, NMR and ESI-MS) techniques. The effects of the position of the peptide group, the chain length and the geometry on the stability and stoichiometry of the complexes formed were evaluated. It was concluded that metal ions preferring regular octahedral geometry in their complexes form the most stable bis-chelated mononuclear complexes, [ML], with 2,5-DIHA having the same connecting chain structure and length as those of DFB. This benefit of 2,5-DIHA, however, almost disappears in the case of Cu(II). With this metal, which prefers the equatorial coordination of two hydroxamates, the parallel formation of both [CuL] and [Cu₂L₂] was found. ESI-MS results indicate that the latter complex is exclusively formed with 2,2-DIHA involving the shortest linker. All these dihydroxamic acids are excellent chelating agents for Pb(II). The special geometry determined by the lone pair electrons should be responsible for the somewhat unique preference order of the ligands towards the Pb(II) ion, for the favoured formation of the monomeric bis-chelated complexes and also for the unexpectedly high stability of the species [Pb(2,2-DIHA)].