Abstract

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.

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