Lanthanide cation binding to a phosphoryl-calix[4]arene: the importance of solvent and counterions investigated by molecular dynamics and quantum mechanical simulations

Abstract

Molecular dynamics simulations on the 1 : 1 M3`lanthanide (La3`, Eu3`and Yb3`) "" inclusion ÏÏ complex of a t-butyl-calix [4]arene L substituted at the narrow rim by four arms demonstrate the role of CH 2 ÈP(O)Ph 2 hydration and counterions on the cation binding mode and shielding. In dry chloroform and in the absence of counterions, the cation is "" endo ÏÏ, fully encapsulated within the pseudo-cavity delineated by the four phosphoryl arms and the four phenolic oxygens. This "" endo ÏÏ bidentate binding mode is supported by full ab initio quantum mechanical optimization of the calixarene M3`complexes. In biphasic solution, the complexes are shown to be surface active and to adsorb at an "" oil ÏÏ/water interface with the cationic site pointing towards water and the hydrophobic t-butyl groups in "" oil ÏÏ. The cation is not encapsulated, but adopts an "" exo ÏÏ position, coordinated to the four P2O oxygens of L, to water molecules, and to counterions. This complex is too hydrophilic to be extracted from the interface to an organic phase. The unexpected binding mode has important implications concerning the mechanism of liquidÈliquid ion extraction and the microscopic state of the extracted complex in the organic phase.

Binding Mechanisms in Supramolecular Complexes

“…[112] Consistent with such findings, equivalent or often even better results are obtained in chromatographic separations using open-chain cyclodextrin analogues rather than with cyclodextrin itself. [117] The mobility of a guest molecule within a cavity can be so limited that in the presence of two binding centers in the guest, two orientations will arise as a result of different interactions with the two complementary centers in the host cavity, a phenomenon that can be observed by NMR spectroscopy (Section 10). Thus, upon dissolution in water, the Eu 3+ ion from a crystalline [222]cryptand complex wanders out of the cavity into the solution.…”

Bindungsmechanismen in supramolekularen Komplexen

“…[114] NMR-Untersuchungen [115] und Moleküldynamiksimulationen [116] von ähnlichen Komplexen indizieren ebenfalls Bindungen in Outer-Sphere-Komplexen. [117] Innerhalb eines Hohlraums kann die Beweglichkeit von Gastmolekülen so eingeschränkt sein, dass bei Vorhandensein zweier Bindungszentren im Gast zwei NMR-spektroskopisch sichtbare Orientierungen durch unterschiedliche Wechselwirkungen mit komplementären Zentren des Wirthohlraums auftreten (Abschnitt 10).…”

Complexation, luminescence and energy transfer of Ln(III) ions with phenylphosphonic acid

“…Molecular lanthanide coordination complexes have attracted attention due to their potential as phosphors in thin film electroluminescent (EL) displays [16]. Organophosphorus ligands are of great importance in the field of liquid-liquid extraction of lanthanide and actinide ions from aqueous solutions [17,18]. Properties of polyaza rings with coordinating arms have successfully been explored in designing the magnetic resonance imaging (MRI) contrast agents [19,20] based on Gd 3+ and diagnostic/therapeutic radiopharmaceutics utilising metal radionuclides [21,22].…”