**4. Discussion**

It is well known that copper forms a paddlewheel motif when combined with carboxylates, as reported for a large number of cage complexes [32–42]. For this reason, it has been the mainstay of our e fforts to form lantern-type cages. Unfortunately, we were only able to isolate an identifiable product using **H2L1** in combination with copper, with the reactions using **H2L2** and copper giving solutions that could not be properly characterized. NMR, in particular, proves challenging, due to the paramagnetism of the copper(II) ion. Perhaps it can be reasoned that a copper (II) analogue of compound **2** would not form, as the coordination environment is not ideally suited to the coordination preferences of the ion (presumably necessitating the N-donor DABCO ligand in a Jahn–Teller distorted axial position). Similarly, a cobalt (II) analogue of compound **1** could not be prepared, again with characterization issues regarding paramagnetic nuclei. Cobalt–carboxylate paddlewheels are somewhat rarer in the literature than their copper counterparts, and perhaps the DABCO ligand assists in stabilizing the paddlewheel that we do observe. Whilst it is unfortunate that direct comparisons between complexes with identical metal ions can be made, there does appear to be some rationale for why this may be the case.

The cage-type complexes obtained using **H2L1** and **H2L2** demonstrate both the versatility of the synthetic approach and its limitations. It is clear that there is a drive towards the formation of lantern-type structures, a well-known M4L4 motif for achiral cages (in which rigid ligands connect the paddlewheels in a mesocate-type arrangemen<sup>t</sup> with the ligands running straight between the paddlewheels), but one which is less explored for helical complexes [35–42]. The [Cu4(**L1**)4(solvent)4] cage forms as anticipated. Whilst the conformation of the cage is distorted away from an idealized C4 symmetric species in the solid-state structure, it is likely that there is some structural relaxation in solution (one must always be careful not to extrapolate solution behaviour from a static crystallographic model).

It seems that the anticipated M4L4-type motif in compound **2** is being disrupted due to the use of a shorter and more rigid ligand than **H2L1** and those used in previous studies. Although the change is most pronounced at the terminal coordination sites, there are other features in the complex, namely, the extreme bowing of the ligands and subtle changes to Co-O bond lengths, which indicate that the system is under some strain. Clearly the rigidity of the ligand, combined with size, has a substantial impact. The earlier report by Chen et al. demonstrated that short diimides can be used to form cages, in this case using a bicyclooctene core [36]. Given the similarity in length between bicyclooctene and phenyl cores, it is therefore evident that the rigidity of the pyromelliticdiimide is the governing factor in limiting the formation of a complete lantern-type cage. Perhaps this is also borne out by our published use of larger, NDI-based ligands that formed octahedral cages, as the rigidity presumably disfavoured the formation of M4L4 complexes [43,44]. The internal bridging DABCO may help to stabilize the cages in [{Co4(**L2**)4(DABCO)(H2O)4}2(DABCO)]. Whilst it is far from definitive, we were unable to obtain any crystalline material using **H2L2** in the absence of DABCO, other than analogues of previously reported coordination polymers, thus suggesting that the role of DABCO is essential [79].
