Synthesis, Characterization, and Molecular Structure of Some Uranyl Complexes Supported by Hybrid...

Three new hybrid bis(salicylaldiminato)/calix[4]arene ligands H₄L1-H₄L3 have been synthesized and investigated with regard to their coordination behavior toward the UO₂²⁺ cation. The ligands H₄L1 and H₄L2, derived from bis-1,3-amino-ethoxy-functionalized calix[4]arenes and 3-methoxy-2-hydroxy-salicylaldehydes, react readily with uranyl nitrate in the presence of NEt₃ to support mononuclear neutral complexes with a 1:1 metal:ligand stoichiometry, namely [UO₂(H₂L1)] (6) and [UO₂(H₂L2)] (7). Ligand H₄L3, with an additional alanyl linker connecting the bis(2-amino-ethoxy)-calix[4]arene backbone and the 3-methoxy-2-hydroxy-salicylaldehyde arms, supports a neutral, mixed-ligand dinuclear uranyl complex [(UO₂)₂(MeO)₂(H₂L3)] (8). The ligands H₄L1 and H₄L2 act as pentadentate O₄N ligands for the UO₂²⁺ ion to produce a distorted pentagonal bipyramidal coordination environment (O₆N donor set). The ligand H₄L3 supports a binuclear [UO₂(μ-OMe)₂UO₂]²⁺ core unit, whose terminal coordination sites are occupied by the donors of the two pendant arms of H₄L3. The spectroscopic properties (NMR, IR, UV-Vis, ESI-MS) suggest that the complexes 6 and 7 retain their integrity in solution state. The structures are further stabilized by intramolecular hydrogen bonding interactions, as implied by computational analyses (NCI plots). These findings provide valuable insight into the influence of spatial flexibility and donor arrangement on the uranyl coordination chemistry of calix[4]arene-based ligand systems.