Metallacrowns of Ni(II) with α-Aminohydroxamic Acids in Aqueous Solution: beyond a 12-MC-4, an Unexpected (Vacant?) 15-MC-5

Growing attention has been devoted in the recent years to a class of metallamacrocycles known as metallacrowns (MCs). They are structural analogues of crown ethers where the methylene bridges have been substituted by coordinative bonds formed by a transition metal ion (“ring” metal) and a nitrogen atom. The cavity of the metallacrown can accommodate an additional metal ion (“core” metal) either identical or different from the ring metal, thus forming a homo- or hetero-metallic MC. The most studied ring metal ion is certainly Cu2+ and among the most suitable ligands to form MCs have proved to be aminohydroxamic acids. The behavioural analogies between Cu2+ and Ni2+ in forming complexes, along with recent literature data in the solid state, prompted us to investigate the possible MC formation between Ni2+ and both (S)-α-alaninehydroxamic acid and (S)-valinehydroxamic acid, in aqueous solution. Two 12-MC-4 and 15-MC-5 complexes have been found to form, confirmed by ESI-MS results. Their structures are discussed on the basis of potentiometric, calorimetric, spectroscopic data and DFT calculations. The existence of a vacant 15-MC-5 species in solution might be put forward for the first time, making the present metal/ligand systems very interesting from the point of view of their potential applications in cation recognition and separation. Finally, the crystal structure of the binary complex K[NiL2H-1] • 5/3 H2O of (S)-α-alaninehydroxamic acid (LH) is also reported.