Bifunctional ligands in uranyl chemistry: Metalloligands and zwitterions

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

“…In contrast to previous work involving aprotic dizwitterionic dicarboxylates of large size, 3–11 which are conducive to the formation of mixed-ligand complexes with periodicities varying from 0 to 3 displaying entanglement in several cases, the present work with the Hamb monozwitterionic ligand has only yielded monoperiodic coordination polymers and one tetranuclear discrete species. The reduced metal ion-bridging ability of Hamb is of course a drawback here, but, due to the presence of the primary ammonium group, its bifunctionality is more obvious than with pyridinium-based zwitterionic carboxylates.…”

“…55–59,61 The fact that complex 1 is formed preferentially to a honeycomb network involving only tdc 2− ligands may be seen as further indication of the particular ease with which true mixed-ligand species can be obtained through mixing anionic and zwitterionic carboxylates. 3 Chains with double bridges between uranyl centres are found in complexes 4 and 6 , with either both ligands or only the dicarboxylate, respectively, being bridging, and complex 5 illustrates the effect of a coordinating solvent (deleterious in regard to polymer formation in the present case, but not necessarily so in general 82 ). Complex 7 is a double-stranded chain in which Hamb links the two linear uranyl pimelate polymers.…”

“…All the seven complexes presently obtained are true mixed-ligand species, with both anionic dicarboxylate and Hamb (always in its zwitterionic form) bound to uranium, which confirms the ease with which true heteroleptic complexes can be generated from mixtures of anionic and zwitterionic carboxylates. 3 Six complexes, [UO 2 (tdc)(Hamb)]·5H 2 O ( 1 ), [UO 2 (pda)(Hamb)]·CH 3 CN ( 2 ), [UO 2 (cam)(Hamb)]·CH 3 CN ( 3 ), [UO 2 (pht)(Hamb)]·0.5H 2 O ( 4 ), [UO 2 (pdda)(Hamb)]·0.5CH 3 CN ( 6 ) and [UO 2 (pim)(Hamb)] ( 7 ), crystallize as monoperiodic coordination polymers, while [(UO 2 ) 4 (O) 2 (pht) 2 (Hamb) 2 (dma) 2 ]·2H 2 O ( 5 ), involving the same carboxylate ligands as 4 , contains an additional coordinated solvent (dma) and is a discrete, tetranuclear species. Prior to the examination of the coordination differences and supramolecular arrangement in these compounds, an interesting point to address is that of the relative strength of the anionic dicarboxylate and zwitterionic carboxylate ligands.…”

“…Extended hydrogen bonding however associates the chains into di- or triperiodic assemblies, making Hamb a true bifunctional ligand. A possible development of this work would be to use partial or pseudo-zwitterionic ligands 3 possessing two or more carboxylates and one primary ammonium group, which would however not be neutral but anionic.…”

“…The fact that the presence of a positively charged centre adjacent to a carboxylate-donor group, as in the well-known ligand “betaine” (trimethylammonioacetate), appears to have negligible influence on M–O bond lengths in comparison to those in anionic carboxylate donors 1,2 has been the basis of our efforts to characterize mixed zwitterionic/anionic ligand uranyl ion complexes. 3,4 The intention there was to determine how the structure of uranyl ion coordination polymers based on polycarboxylate ligands might be modified by the co-coordination of neutral oligozwitterionic carboxylates. To avoid possible complications in synthesis due to additional acid–base equilibria and coordination modes, the oligozwitterions employed in this work 5–11 were those with quaternary nitrogen centres, thus lacking dissociable protons.…”