**4. Concluding Remarks**

Complementary interactions provide a foundation for generating a wide range of discrete supramolecular assemblies or extended structures within crystals. In this work, we discovered an unexpected example of extreme complementarity in which six components are brought together in order to produce an aesthetically pleasing, highly symmetrical network structure. Through recognition of the nature of the complementary interactions that underpin this system, it was found that the same structural type could be replicated with different alkali metal ions and anions. It is clear that the reported system is amenable to at least some degree of structural tinkering. The insights provided

by this work offer opportunities to generate further members of this family which may include the incorporation of a wider range of tetrahedral anions and the employment of other metal ions.

Whilst the relatively simple cations, anions and solvent molecules from which the crystals have been formed have all participated in the generation of the four supramolecular crystalline materials described above, the cyclotricatechylene has played the key structural role in this system. In its fully protonated neutral form, it has been able to coordinate to a trio of K<sup>+</sup> or Rb+ ions through hydroxyl oxygen atoms to form the cage. In addition, each H6ctc also serves as a hydrogen bond donor and forms six identical hydrogen bonds to three different anions by employing the same hydroxyl groups. Furthermore, its ability to interact with large alkali metal cations i.e., Rb+ and Cs+ through π-interactions allows the cage to accommodate the requisite number of alkali cations needed for charge balance. It is interesting to note that both cations and anions prefer to form bonds (hydrogen bonds or coordinate bonds) with the neutral H6ctc molecule rather each other. Clearly, the symmetry of cyclotricatechylene (*C*3v in its most symmetrical conformation) and versatility in being able to participate in a variety of different interactions makes it a particularly useful molecular building block for supramolecular assemblies and there would appear to be grea<sup>t</sup> scope for expanding the type of assemblies that may be formed in its combination with cations, anion and neutral species.

**Author Contributions:** Conceptualization, J.L.H., S.M.R., B.F.A.; methodology, J.L.H., S.M.R., B.F.A.; investigation, J.L.H., S.M.R., B.F.A., T.A.H., K.F.W.; writing—original draft preparation, J.L.H., B.F.A.; writing—review and editing, J.L.H., S.M.R., B.F.A., T.A.H., K.F.W.; funding acquisition, B.F.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors gratefully acknowledge the funding by the Australian Research Council (DP180101414).

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