Solution species of Fe(iii), Ga(iii), In(iii) or Ln(iii) and suberodihydroxamic acid from electrospray ionization mass spectrometry

Abstract

Positive ion electrospray ionization mass spectrometry (ESI-MS) was used to characterize complexes formed in aqueous solution between Fe(III), Ga(III), In(III), Ce(III), Eu(III) or Yb(III), and the linear dihydroxamic acid suberodihydroxamic acid (HONHC(O)(CH₂)_nC(O)NHOH), n = 6; L⁶H₄) at pH 2.5, and at metal (M) to ligand (L) ratios of 1 : 1 or 1 : 2. In the Ga(III)–L⁶H₄ system, complexes with five different types of M : L stoichiometries were identified, with the major signals assigned to [Ga(L⁶H₂)·CH₃OH]⁺ (M : L, 1 : 1), [Ga(L⁶H₂)(L⁶H₃)] adducts (H⁺, Na⁺, K⁺) (M : L, 1 : 2), [Ga₂(L⁶H₂)₂(μ-OCH₃)]⁺ (M : L, 2 : 2), [Ga₂(L⁶H₂)₃] adducts (H⁺, Na⁺, K⁺) (M : L, 2 : 3), or [Ga₃(L⁶H₂)₄]⁺ (M : L, 3 : 4). Where M = Ce(III), Eu(III) or Yb(III), signals for [M(L⁶H₂)(L⁶H₃)] (H⁺ adduct) were dominant, signals for [M₃(L⁶H₂)₄]⁺ were weak, and signals for [M₂(L⁶H₂)₂(μ-OCH₃)]⁺ were either weak (Ce(III)) or absent (Eu(III), Yb(III)), which reflected that the higher coordination number demands of lanthanide ions were not satisfied by the latter two stoichiometry types. An improved understanding of the solution speciation between different types of metal ions and hydroxamic acids is important in the context of the roles played by these ligands in bioremediation/biogeochemistry and biomedicine/imaging.