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

Abstract

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.

Explaining How α-Hydroxamate Ligands Control the Formation of Cu(II)-, Ni(II)-, and Zn(II)-Containing Metallacrowns

“…This behavior is in agreement with the results reported previously for α-AlaHA, ValHA, β-AlaHA. 34,35 Here again, the presence of 15-MC-5 and NiL 2 species is in agreement with the structures of the crystallized solid compounds (vide supra).…”

“…The complexation starts with the mononuclear [NiL] + , followed by the formation of the polynuclear [Ni 5 (LH −1 ) 4 ] 2− (12-MC-4) complex present in a rather narrow pH-range (5− 6.5), the behavior typical for Ni(II) 12-MC-4 complexes. 34,35 The formation of 12-MC-4 is almost concomitant with another S9). UV−vis characteristics of the system are given in Figure S11.…”

“…33 Formation of both 12-MC-4 and 15-MC-5 complexes of Ni(II) ions with αand β-aminohydroxamic acids, specifically α-alaninehydroxamic acid (α-AlaHA), β-alaninehydroxamic acid (β-AlaHA), and valinehydroxamic acid (ValHA), has also been observed. 34,35 One of the MC-forming ligands most extensively described in the literature, 22,27,36−44 both in solution and in the solid state, is o-picolinehydroxamic acid (PicHA). Inspired by the results obtained for PicHA, we decided to extend the studies to other aromatic hydroxamate derivatives that have recently been used to prepare extremely bright, near-IR-emissive MCs.…”

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.…”

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

“…Our research group presented in the past few years a study on the thermodynamics of formation of both 12-MC-4 and 15-MC-5 complexes of copper(II) and nickel(II) with hydroxamic derivatives of α-, βand γ-derivatives of amino acids. [28][29][30][31][32][33][34][35] Perhaps the most important observation was that in the absence of cations such as calcium(II) or lanthanide(III) ions the α-aminohydroxamates form 12-MC-4 complexes with copper(II), but not 15-MC-5 complexes, which represents an exception to the metallacrown structural paradigm. 12,34 By addition of larger Ln(III) or UO 2 (VI) ions (M′ in Scheme 1) the 12-MC-4 species of α-derivatives rearrange into 15-MC-5 complexes.…”