Selective Interaction of Lower Rim Calix[4]arene Derivatives and Bivalent Cations in Solution. Crystallographic Evidence of the Versatile Behavior of Acetonitrile in Lead(II) and Cadmium(II) Complexes

Abstract

The interaction of lower rim calix(4)arene derivatives containing ester (1) and ketone (2) functional groups and bivalent (alkaline-earth, transition- and heavy-metal) cations has been investigated in various solvents (methanol, N,N-dimethylformamide, acetonitrile, and benzonitrile). Thus, 1H NMR studies in CD3OD, C3D7NO, and CD3CN show that the interaction of these ligands with bivalent cations (Mg2+, Ca2+, Sr2+, Ba2+, Hg2+, Pb2+, Cd2+) is only observed in CD3CN. These findings are corroborated by conductance measurements in these solvents including benzonitrile, where changes upon the addition of the appropriate ligand (1 or 2) to the metal-ion salt only occur in acetonitrile. Thus, in this solvent, plots of molar conductance against the ligand/metal cation ratio reveal the formation of 1:1 complexes between these ligands and bivalent cations. Four metal-ion complex salts resulting from the interaction of 1 and 2 with cadmium and lead, respectively, were isolated and characterized by X-ray crystallography. All four structures show an acetonitrile molecule sitting in the hydrophobic cavity of the ligand. The mode of interaction of the neutral guest in the cadmium(II) complexes differs from each other and from that found in the lead(II) complexes and provides evidence of the versatile behavior of acetonitrile in binding processes involving calix(4)arene derivatives. The thermodynamics of complexation of these ligands and bivalent cations in acetonitrile is reported. Thus, the selective behavior of 1 and 2 for bivalent cations is for the first time demonstrated. The role of acetonitrile in the complexation process in solution is discussed on the basis of 1H NMR and X-ray crystallographic studies. It is suggested that the complexation of 1 and 2 with bivalent cations is likely to involve the ligand-solvent adducts rather than the free ligand. Plots of complexation Gibbs energies against the corresponding data for cation hydration show a selectivity peak which is explained in terms of the predominant role played by cation desolvation and ligand binding energy in complex formation involving metal cations and macrocycles in solution. A similar peak is found in terms of enthalpy suggesting that for most cations (except Mg2+) the selectivity is enthalpically controlled. The ligand effect on the complexation process is quantitatively assessed. Final conclusions are given highlighting the role of the solvent in complexation processes involving calix(4)arene derivatives and metal cations.

An Enchiridion of Supramolecular Thermodynamics: Calix[N]arene (N=4,5,6) Tertiary Amide Derivatives and their Ionic Recognition

“…Given that this cavity is larger for the cyclic pentamer than for the tetramer, this interaction is expected to be greater for the latter relative to the former. As previously shown the presence of acetonitrile in the hydrophobic cavity of the ligand 22 has implications on the cation complexing ability of these macrocycles in this solvent relative to others. (Table 5) i) Chemical shift changes (Δδ, ppm) of L1 with monovalent metal cations observed in CD 3 CN relative to the free ligand show significant changes in the aromatic (H-2), the axial and the equatorial (H-3 and H-4), the methylene bridge (H-5) and the terminal protons (H-7).…”

“…A similar behaviour has been previously found for other calix [4]arene derivatives. 2,14,15,22 This has been attributed to an interaction of the macrocycle through its hydrophobic cavity with the solvent. Given that this cavity is larger for the cyclic pentamer than for the tetramer, this interaction is expected to be greater for the latter relative to the former.…”

“…These studies show that the arrangement of minimum energy for this system is that in which one Na (I) is held by 6 coordination sites provided by the oxygen donor atoms and the solvent molecule (MeCN), while the second Na (I) is coordinated to 5 oxygen donor atoms in the complex, as shown in Figure 8. A detailed investigation by X-ray diffraction studies on heavy metal cations (Cd (II) and Pb (II)) 22 complexes with a calix [4]arene ester showed that the solvent is hosted in the hydrophobic cavity, thus, the solvent can provide a coordinating site for the cation through its nitrogen donor atom. Such is the case for the cadmium complex and this may be the case here.…”

Formation of a Unique 1:2 Calcium-Calixquinone Complex in Aqueous Media

“…5 Danil de Namor et al investigated complexation process between divalent cations and calix [4]arene derivatives with ester and ketone functional groups in various solvents. 6 They found that the medium was a very important factor that affects complex stability and selectivity. [7][8][9] The calix [4]arene derivatives bound with Ca 2+ ion in acetonitrile while they formed no complex in methanol and N,N-dimethylformamide.…”

"Estudo do processo de complexação de calixarenos com íons metálicos e espécies neutras por simulações de dinâmica molecular"

“…Uma cavidade hidrofílica pode ser criada ligando-se resíduos apropriados ao oxigênio fenólico. A cavidade hidrofóbica apresenta propriedades seletivas sobre moléculas de solvente dependendo da interação de íons metálicos dentro da cavidade hidrofílica [12]. Este tipo de reação acoplada, na qual a ligação de um íon metálico com um sítio hidrofílico na supramolécula produz mudanças conformacionais que alteram as propriedades seletivas da cavidade hidrofóbica com moléculas orgânicas é conhecida como efeito alostérico.…”