Hydroxyquinoline-Calix[4]arene-Conjugates as Ligands for Polynuclear Lanthanide Complexes: Preparation, Characterization, and Properties of a Dinuclear Eu(III) complex

The synthesis of a calixarene-based N₄O₄ donor ligand H₆L, in which two 8-hydroxyquinoline-2-carbaldehyde units are appended via hydrazone-carbonylmethoxy linkages in a 1,3-arrangement to the lower rim of tert-butyl-calix[4]arene, and its coordination properties towards Eu(III) are described. The reaction of H₆L with Eu(NO₃)₃.6H₂O produces a dinuclear complex [Eu₂(H₃L)₂] (4). The Eu(III) ions in 4 are well-separated from each other (d > 8 Å) each being coordinated by two tetradentate hydroxyquinoline-2-carbaldehyde-hydrazone substituents from two triply deprotonated molecules (H₃L)^3- in a dodecahedral geometry. As a consequence there are no exchange interactions between the Eu(III) ions, and the magnetic properties are solely associated with thermal population of excited (⁷F₁-⁷F₆) terms. The spin-orbit coupling constant lambda is 325.2(4) cm^-1. Moreover, the ligand H₆L was found to extract Eu(III) and Tb(III) at pH 7-8 from aqueous solution, as established by radiotracing using the short-lived radionuclides ¹⁶⁰Tb and ¹⁵²Eu.