**3. Conclusions**

While the lack of complete luminescence and porosity measurements for known capsular oligomers of uranyl carboxylates creates some uncertainty as to their potential value as photo-oxidation catalysts, there seems little likelihood that any would be selective due to their capacity to encapsulate substrates of moderate molecular size. Practical aspects of application, such as stability under reaction conditions are also completely unexplored. Geometrical analysis analogous to that applied to other metallacapsule design [108,109] could of course be used along with the "extended ligand" approach [110] to prepare ligands suited to the formation of larger cavities, although this involves the danger of generating interpenetration in the structures, already seen in numerous uranyl coordination polymers [111]. Another potential drawback with most known capsules is that they are anionic and thus favour interaction with cations, so that one objective of continuing efforts of synthesis would be to couple a neutral bridging ligand, such as a bis(naphthyridine), with two carboxylate units on every uranium. The focus of this brief review has been on solid materials containing capsular species, in part because known examples are all solids of low solubility in any solvent and in part because product separation is more straightforward with heterogeneous catalysis but soluble capsular species would also be of interest. The attraction of a capsular species as a reaction vessel is that any selectivity depends on the structure of the capsule itself and not upon the environment in which it is found and capsular species which align in crystals so as to define channels, as found in various instances described herein, could offer heterogeneous catalysts of this type. Here, tubular complexes as found with selenates [71,72] and phosphonates [52–55] as well as with polycarboxylates such as tricarballylate [112], iminodiacetate [69] and phenylenediacetates [68], would also be of interest, especially if a better understanding of metal ion quenching of uranyl ion luminescence in solids could be attained, since many tubular systems are

those involving heterometallic species [47]. As solvothermal synthesis [113,114] is widely applied for the isolation of crystalline uranyl ion complexes, another need is for more data concerning kinetics and equilibria of complex formation under conditions of high temperature and pressure, particularly in mixed solvents. Finally, it is essential to note that the crystal structures of known uranyl ion complexes are frequently seen [115–118] to be sensitive to a wide range of weak interactions, so that the supramolecular behaviour of a bound ligand is a crucial aspect of its design but one ye<sup>t</sup> to be mastered.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2624-8549/2/1/7/s1, Figure S1: Perspective view of one helical tube within the crystal of [UO2(dipic)(OH2); Figure S2: Views, down a, of the triperiodic structure of [H2NMe2]2[(UO2)2(adc)3]·1.5H2O; Figure S3: Views of one of the diperiodic sheets found in the crystal of [H2NMe2]2[(UO2)2(ada)3]·1.5H2O; Figure S4: Views of the diperiodic anionic polymer sheets (counter cations not shown) in the crystal of [H2NMe2][PPh3Me][(UO2)2(ada)3]·H2O; Figure S5: Views of the trough-like anionic monoperiodic polymers and their closest cations found in the crystals of (a) [PPh4]2[(UO2)2(adc)3]·2H2O and (b) [PPh4]2[(UO2)2(ada)3].

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
