Thermodynamics of Core Metal Replacement and Self-Assembly of Ca²⁺ 15-Metallacrown-5

Abstract

The equilibria for core Ca2+ replacement by Ln3+ in copper(II) 15-MC-5 complexes have been investigated using a series of visible spectrophotometric titrations of calcium(II) metallacrowns ({CaII[15-MCCuII(N)(L)-5]}2+) with Ln3+ ions (H2L = pheha, (S)-α-phenylalaninehydroxamic acid, or trpha, (S)-α-tryptophanhydroxamic acid). These studies allowed the determination of the equilibrium constants for the reaction {CaII[15-MCCuII(N)(L)-5]}2+ + Ln3+ → {LnIII[15-MCCuII(N)(L)-5]}3+ + Ca2+ in methanol/water 9:1 (Ln3+ = La3+, Gd3+, Dy3+, Er3+) or 99:1 (Ln3+ = La3+, Nd3+, Gd3+, Dy3+, Er3+, Yb3+), respectively. The log K for these reactions decreases with increasing atomic number of the lanthanide(III), ranging from 6.1 to 3.91 in methanol/water 9:1. The same behavior is observed in methanol/water 99:1, although the constants are uniformly lower (log K = 4.09−2.52). A significant thermodynamic selectivity was observed for the later lanthanides (Gd3+−Yb3+) while a smaller selectivity is present throughout the beginning of the series (La3+−Gd3+). This observation has been interpreted on the basis of the size correspondence between the metal ions and the metallacrown cavity. The overall stability of the {CaII[15-MCCuII(N)(L)-5]}2+ in methanol/water 9:1 has been determined by pH-spectrophotometric titrations with HCl. The resulting log K values are 63.46(12) and 65.05(13) for pheha and trpha, respectively (Ca2+ + 5Cu2+ + 5HL− = {CaII[15-MCCuII(N)(L)-5]}2+ +5H+). The stability of both the La3+ and Ca2+ 15-metallacrown-5 complexes in the presence of high Na+ concentrations has also been demonstrated by spectophotometric studies. Based upon these observations, the preference of the 15-MC-5 for Ca2+ complexation compared to crown ethers has been quantitatively demonstrated for the first time.

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

“…L-Tryptophanhydroxamic acid and L-phenylalaninehydroxamic acid were synthesized as previously reported in the literature. 24 The purity of the ligands was checked by NMR and elemental analysis. 1 (1).…”

“…Metallacrowns generally have been shown to sequester ions including Li I , 6 Na I , 6−9 K I , 6,8 Mn II , 10 Cu II , 10−17 and Ni II , 18,19 as 12-MC-4 structures. Although 15-MC-5 complexes with salicylhydroxamic acid could be prepared solely as mixed valence manganese complexes, 20 the copper 15-metallacrowns-5 complexes of α-aminohydroxamic acids (Scheme 1) were shown to be able to encapsulate a number of metal ions such as Y III , 21−23 Na I , 23 Ag I , 23 Pb II , 23 Hg II , 23 Ca II , 24,25 Ln III , 1−3,26−29 and UO 2 VI . 30−32 For the last three core cations it was established that the order of stability of the corresponding 15-MC-5 is Ca II < Ln III < UO 2 VI .…”

“…24 The thermodynamic selectivity in the formation of these complexes by substitution of the core Ca II ion was explained by taking into account the different dimension of the Ln III ions, their capability to interact with the MC cavity and their desolvation energies. 24 Recently, the effect of the dimension of the central lanthanide ion in Ln(III)[15-MC Cu(II)N(Pheha) -5] 3+ complexes in the solid state has been examined, 29 leading to the conclusion that the fine structural features (planarity, cavity dimension etc.) and likely the complexation capabilities of the 15-MC-5 scaffold arise from a delicate balance of several factors, among which the nature and dimension of the central lanthanide are of paramount importance.…”

Synthesis, Structural, and Magnetic Characterization of a Mixed 3d/4f 12-Metallacrown-4 Family of Complexes

“…The formed repeating unit of [Mn III -N-O], along with the triply deprotonated salicylhydroximate, the central Mn II ion and the two acetate bridges were responsible for the aforementioned configuration. Usually, in the central MC cavity sits a transition metal ion, even though there are also reports where alkali and alkaline earth metals occupy the cavity [15,16]. Lately, the focus has been turned into the incorporation of lanthanide ions in the center, since these compounds have been proposed as excellent candidates for molecular recognition [17,18], molecular magnetism [19] and luminescent [1,[19][20][21][22] technologies.…”

Heterometallic Complexes Containing Lanthanides

“…Their structures are entirely predictable based on the coordination angle of the aminohydroxamate ligands and the transition metal ions [49]. They have been shown to bind carboxylate anions through coordination to the lanthanide and they lend themselves to further functionalization through chemical modification of peripheral ligands [50,51]. This property could be exploited in the formation of supramolecular architectures through interactions with other kinetically stable building blocks to form heteromultimetallic arrays.…”