4-(Ammoniomethyl)benzoate, a protic zwitterionic bifunctional linker in uranyl ion carboxylate complexes

Abstract

4-(Aminomethyl)benzoic acid (Hamb) has been found to complex the uranyl ion in its neutral, zwitterionic form in a series of mixed-ligand complexes synthesized under solvo-hydrothermal conditions with several dicarboxylate coligands. [UO₂(tdc)(Hamb)]·5H₂O (1), [UO₂(pda)(Hamb)]·CH₃CN (2), and [UO₂(cam)(Hamb)]·CH₃CN (3), where tdc²⁻, pda²⁻ and cam²⁻ are 2,5-thiophenedicarboxylate, 1,2-phenylenediacetate and (1R,3S)-(+)-camphorate, respectively, crystallize as simple chains in which the dicarboxylate ligand is bridging and Hamb is terminal, all carboxylates being κ²O,O′-chelating. [UO₂(pht)(Hamb)]·0.5H₂O (4), where pht²⁻ is phthalate, is a ribbonlike chain in which both ligands are bridging, while the presence of the coordinated solvent N,N-dimethylacetamide (dma) prevents polymerization in [(UO₂)₄(O)₂(pht)₂(Hamb)₂(dma)₂]·2H₂O (5), a bis(μ₃-oxo)-bridged tetranuclear assembly. 1,2-Phenylenedioxydiacetate (pdda²⁻) gives [UO₂(pdda)(Hamb)]·0.5CH₃CN (6), another ribbonlike chain in which only pdda²⁻ is bridging. Finally, [UO₂(pim)(Hamb)] (7), involving the pimelate ligand (pim²⁻) is a double-stranded, ribbonlike chain in which two UO₂(pim) linear polymers are bridged by the Hamb ligands. In all these complexes, the ammonium group of Hamb is involved in extended hydrogen bonding giving rise to weakly bonded assemblies of higher periodicity. All complexes except 5 and 6 are emissive, with photoluminescence quantum yields between 2 and 11%, and with the exception of a broad signal associated with 1, all display the usual vibronic fine structure, with peak positions clearly related to the uranium coordination numbers.