Bifunctional ligands in uranyl chemistry: Metalloligands and zwitterions

Structural studies of uranyl ion complexes of metalloligands and zwitterions, ligand classes with some degree of overlap, constitute a major contribution to more recent developments in uranium chemistry. To some extent, they have been focussed on basic product characterization but more generally on gaining an understanding of how particular features of ligand structure may influence the complete crystal structure and thus the potential for any applications. Metalloligands provide an obvious means of generating heterometallic complexes but since it is not always necessary to preform a metalloligand prior to its interaction with uranyl ion, they can be identified by somewhat arbitrary excision of appropriate components from a very large number of complete crystal structures, so that only selected examples are considered here. Zwitterion (and oligozwitterion) complex structures are treated more comprehensively, two subclasses here being those where the positive site is aprotic and those where it is protic, differing in the weak interactions involved. Association of zwitterionic ligands with classical, anionic polycarboxylates and with cucurbiturils are recent developments covered extensively.