title: Metallacrowns of Ni(II) with α-aminohydroxamic acids in aqueous solution: beyond a 12-MC-4, an unexpected (vacant?) 15-MC-5.
abstract: 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 Cu(2+) and the aminohydroxamic acids have proved to be very suitable ligands to form MCs. The behavioural analogies between Cu(2+) and Ni(2+) in forming complexes, along with recent literature data in the solid state, prompted us to investigate the possible MC formation between Ni(2+) and both (S)-α-alaninehydroxamic acid and (S)-valinehydroxamic acid, in aqueous solution. Two metallacrowns, a 12-MC-4 and an unexpected 15-MC-5 have been detected by potentiometry and 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 can be put forward for the first time, making the present metal/ligand systems very interesting for their potential applications in cation recognition and separation. Finally, the crystal structure of the binary complex K[NiL(2)H(-1)]·5/3 H(2)O of (S)-α-alaninehydroxamic acid (LH) is also reported.
authors.affiliation: Dipartimento di Chimica, Università di Ferrara, Via L. Borsari 46, 44121, Ferrara, Italy.
authors.forename: Dimitri
authors.initials: D
authors.lastname: Bacco
authors.forename: Valerio
authors.initials: V
authors.lastname: Bertolasi
authors.forename: Francesco
authors.initials: F
authors.lastname: Dallavalle
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Galliera
authors.forename: Nicola
authors.initials: N
authors.lastname: Marchetti
authors.forename: Luciano
authors.initials: L
authors.lastname: Marchiò
authors.forename: Maurizio
authors.initials: M
authors.lastname: Remelli
authors.forename: Matteo
authors.initials: M
authors.lastname: Tegoni
mesh_terms: D002151:Calorimetry; D003461:Crystallography; D006877:Hydroxamic Acids / Q000737:chemistry; D047028:Macrocyclic Compounds / Q000737:chemistry*; D013058:Mass Spectrometry / Q000379:methods; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D011199:Potentiometry; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D006877:Hydroxamic Acids; D047028:Macrocyclic Compounds; D014867:Water; D009532:Nickel