The peculiar behavior of Picha in the formation of metallacrown complexes with Cu(II), Ni(II) and Zn(II) in aqueous solution

The thermodynamic stability of the metallacrown complexes formed by picolinehydroxamic acid (Picha) with Cu(II), Ni(II) and Zn(II) in aqueous solution has been determined by potentiometry, and the speciation models were validated by ESI-MS and UV-visible spectrophotometry. Cu(II) and Zn(II) form 12-MC-4 species as the unique metallacrowns present in the solution. While for Cu(II) the 12-MC-4 is slightly less stable than that obtained with alaninehydroxamic acid (Alaha), the opposite was found for Zn(II). Moreover, with Cu(II) unprecedented 15-MC-5 and 18-MC-6 species were identified under ESI-MS conditions. Picha with Ni(II) forms, in contrast, a 15-MC-5 complex as a unique metallacrown species. Structural studies of the framework of the 12-MC-4 complexes by ab initio methods were also carried out. The results of our investigations allowed us to rationalize not only the different behaviour of Picha in the formation of metallacrowns with the three metal ions, but also the reasons which underpin the strategies for stabilization of these species reported in the literature using ancillary ligands such as pyridine.