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.