Stoichiometric diversity of Ni(II) metallacrowns with β-alaninehydroxamic acid in aqueous solution

The complex-formation equilibria of the system Ni(II)/β-alaninehydroxamic acid (β-AlaHA, LH) have been studied in solution by using potentiometry, calorimetry, ESI mass spectrometry and UV-Visible spectrophotometry. A series of mononuclear and polynuclear species have been identified at different pH and for different Ni(II)/ligand ratios. Among the polymetallic species, we have identified the 12-metallacrown-4 (12-MC-4) species [Ni5(LH(-1))4](2+), which is analogous to the previously characterized [Cu5(LH(-1))4](2+) 12-MC-4 complex, and which corresponds to the metallacrown complex predicted by the Metallacrown Structural Paradigm. Besides this species, we have very unexpectedly identified in solution three other polynuclear species, corresponding to the stoichiometries [Ni4(LH(-1))4], [Ni5(LH(-1))5] and [Ni6(LH(-1))6]. A metallacrown with the same stoichiometry as [Ni5(LH(-1))5] containing α-AlaHA as the ligand has been previously reported, while the other two species are new for the speciation systems of Cu(II) or Ni(II) aminohydroxamic acids. Intriguingly, all the three latter stoichiometries correspond to those of vacant 12-MC-4, 15-MC-5 or 18-MC-6 species. The thermodynamic parameters related to their formation in solution are discussed, together with the speciation of the mononuclear complexes, on the basis of the structural and thermodynamic information available on metallacrowns.