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.

Clarifying the Mechanism of Cation Exchange in Ca(II)[15-MCCu(II)Ligand-5] Complexes

“…A dissociative pathway would in fact go through the formation of an intermediate vacant 15-MC-5 containing five negative oxygen atoms in the cavity that are not bound to a core metal. This species, although found for Ni II , 50 has never been observed with Cu II and it is likely a high-energy species for this metal. 3,15 The dissociative mechanism results, therefore, in a high activation energy, which makes the associative (or interchange associative) pathway preferred over a dissociative substitution.…”

Supramolecular Maleate Adducts of Copper(II) 12‐Metallacrown‐4: Magnetism, EPR, and Alcohol Sorption Properties

“…The commonest structural types of hydroxamate metallacrowns are 9‐MC‐3, 12‐MC‐4, and 15‐MC‐5. The planar copper(II) and nickel(II) 12‐MC‐4 and 15‐MC‐5 complexes demonstrate stability in solutions under a range of conditions,, while the availability of vacancies in the coordination spheres of their metal ions (or potentially vacant positions occupied by labile solvent molecules) makes them promising candidates for use as building blocks for coordination polymers and supramolecular assemblies. Additional reasons for using this class of coordination compounds as building blocks are their interesting properties, such as ability for selective recognition of cations, anions, and molecules,, catalytic activity, non‐trivial magnetism,, , luminescence, and so on.…”

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

“…A recent investigation concerning a study on the Ni(II)/ α-alaninehydroxamic acid (AlaHA) system, 36 very unexpectedly revealed that a vacant 15-MC-5 is formed under appropriate conditions and in the absence of a suitable core metal. A "vacant" 12-MC-4 complex is also formed, although it is not markedly stable.…”