Thermodynamics of Self-Assembly of Copper(II) 15-Metallacrown-5 of Eu(III) or Gd(III) with (S)-α-Alaninehydroxamic Acid in Aqueous Solution

Abstract

The equilibria of self-assembly of 15-metallacrown-5 (15-MC-5) complexes of Cu(2+) and (S)-alpha-alaninehydroxamic acid (alpha-Alaha, HL) with the lanthanide (Ln) ions Eu(3+) or Gd(3+) in aqueous solution are described. The binary Ln(3+)/alpha-Alaha systems were first studied by potentiometric and calorimetric in-cell titrations; the latter technique allowed us to define the most suitable speciation model. On the contrary, because the kinetics of formation of the Ln(3+) 15-MC-5 complexes is slow, their stability constants were determined by out-of-cell (batch) potentiometric titrations. Two 15-MC-5 complexes are formed with both Eu(3+) and Gd(3+), namely, {Ln[Cu(5)L(5)H(-5)]}(3+) and {Ln[Cu(5)L(5)H(-5)](OH)}(2+), with the latter being the hydroxo species of the former. The acidity of the former to give the hydroxo species is remarkably high (log K = 4.40-4.69). Moreover, our potentiometric and spectrophotometric investigations clearly indicate that the hydroxide ion is coordinated to the central Ln ion, as was reported for several 15-MC-5 in the solid state. The formation of {Ln[Cu(5)L(5)H(-5)]}(3+) starts at ca. pH 3.5, which converts at ca. pH 4.5 into the {Ln[Cu(5)L(5)H(-5)](OH)}(2+) species, which predominates up to pH 7, where a purple precipitate occurs. The coexistence of both 15-MC-5 species and the copper(II) 12-MC-4 species of alpha-Alaha ([Cu(5)L(4)H(-4)](2+)) was observed under appropriate experimental conditions (pH and ligand and metal concentrations). A complete ESI-MS investigation of the Ln(3+)/Cu(2+)/alpha-Alaha system at different pH's confirmed the formation of the two 15-MC-5 species. The 15-MC-5 stability constants were employed to quantitatively evaluate the solution behavior of Ln(III) MCs regarding their integrity, ligand substitution, and transmetalation processes. In particular, EDTA or DOTA, added in equimolar amounts, should not appreciably interfere with the MC integrity, as found in previous experimental investigations, although it is expected that at higher amounts of EDTA, the MC should be disrupted. Our results also demonstrate that an excess of alpha-aminohydroxamate does not interfere with the integrity of the MC, and the disappearance of the CD spectra upon addition of the R enantiomer to 15-MC-5 containing the S enantiomer is due to a very rapid ligand exchange with formation of all possible isomers with no selectivity. The stability of the 15-MC-5 complexes in the presence of transferrin, serum albumin, or an excess of Zn(2+) is also discussed. With regards to the latter metal ion, we found that the MCs are stable toward Gd(3+)/Zn(2+) transmetalation. although the presence of a phosphate buffer promotes the disruption of the MC scaffold by formation of stable Gd(3+)/phosphate species.

pH‐Responsive Switching Properties of a Water‐Soluble Metallamacrocyclic Phenylalaninehydroximate La(III)–Cu(II) Complex: Insight into Tuning Protonation Ligand States

“…At the same time, unlike the Ca(II)–Cu(II) and Sr(II)–Cu(II) [15‐MC‐5] complexes the absorption band at about 640 nm typical of Cu[12‐MC‐4] disassembly species, does not appear. Such a behavior agrees with the earlier potentiometric and calorimetric studies of various Ln(III)–Cu(II) [15‐MC‐5] systems in acidic solutions evidenced only containing the Ln ions solution species . The discoloration can be assigned probably to protonation of the aminohydroximate ligands.…”

New insights into water‐soluble and water‐coordinated copper 15‐metallacrown‐5 gadolinium complexes designed for high‐field magnetic resonance imaging applications

“…There is essentially no dependence of proton chemical shifts on concentration. The simplicity of the spectra is indicative of the fivefold pseudo‐symmetry of these molecules as was observed with other 15‐metallacrown‐5 complexes . Note that the NMR line widths of the 1 (Gd)– 4 (Gd) complex signals (1600–2000 Hz) are 5–8 times larger than those of the corresponding La complexes ( ca 200–400 Hz).…”

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.…”