**3. Results**

### *3.1. Design and Synthesis of Ligands*

Two dicarboxylate ligands, containing amino acid residues, were synthesized as analogues of the sulfone–diphthalimide species that we previously used to form helical [Cu4(LeuBPSD)4] cages [35]. The two core groups used represent a longer, bent ligand using a 9, 10-dimethylethanoanthracene derivative (**H2L1**) and a shorter, more rigid ligand using a pyromellitic acid building block (**H2L2**). Pyromelliticdiimides are well known in the literature. For example, isophthalate derivatives have been used in MOFs with high sorption capacities [60], dipyridyl derivatives have been used in a number of coordination polymers [61–64], as have thioether derivatives [65], and the diimide core has also been incorporated into a number of macrocycles and receptors [66–71]. Pyromelliticdiimides based on amino acid derivatives have been reported in hydrogen-bonding systems and coordination polymers [72–79]. The applicability of the dihydroanthracene core group is hinted at in the literature with metallocages reported containing shorter ligands using bicyclooctene as a core group [36]; the ethanodihydroanthracene moiety can be viewed as an analogous, extended core group with the same geometry. This core group, as a bis-catechol, has been used in the synthesis of a uranyl lantern-type cage, discrete silane assemblies, and a Mn12 metallacycle [80–82].

9,10-Dimethyl-9,10-ethanoanthracene-2,3,6,7-tetracarboxylic acid was synthesised in three steps by a literature procedure [51]. A low yielding cyclisation step proved rather prohibitive for the large scale synthesis of this material. The tetra-acid was reacted with leucine in acetic acid under reflux for four nights, resulting in **H2L1** being isolated as a pale, yellow solid, as shown in Scheme 1. **H2L2** was synthesized by the slight amendment of a known method [52].

### *3.2. Synthesis and Structure of Helicate-Type Complexes*

The reaction of **H2L1** with copper acetate at an elevated temperature in DMF yielded a blue/green crystalline material suitable for analysis by single crystal X-ray diffraction (compound **1**). The structure, solved and refined in the orthorhombic setting *C*2221, reveals the anticipated lantern-type helical complex, [Cu4(**L1**)4(solvent)4]. The asymmetric unit contains one half of a cage (comprising one copper paddlewheel and one and two half ligands). The overall cage has the two halves related by a two-fold proper rotation that bisects the mid points of two ethanoanthracene core groups. The external facing sites of the two paddlewheels are occupied by aqua ligands. The internal sites are occupied by one DMF and one aqua ligand, which are disordered within the crystal structure. The cage has an internal Cu···Cu distance of 9.225(3) Å, substantially longer than the prior sulfone-based analogue (ca. 7.2 Å). Taking the internal cavity, minus coordinated solvent, as an approximate prolate spheroid, the internal volume is ca. 380 Å3. The persistence of the cage in solution was confirmed by mass spectrometry as a H2O/MeCN solvated complex. The cage does not possess a C4 axis along the Cu2···Cu2 vector, with the paddlewheels themselves not aligned, although a pseudo four-fold rotation can be seen in Figure 2 on the left. The complex has a helical pitch that results in the ligands coordinating at sites 90◦ apart between the two paddlewheels, analogous to the structures observed in studies of the smaller sulfone-based ligand. It seems that the use of a longer ligand results in a complex that is somewhat less regular in shape, presumably due to the enhanced flexibility of the core group, ye<sup>t</sup> the overall structural design is retained. The arrangemen<sup>t</sup> around the paddlewheels is not distorted, with imide N···N distances between adjacent ligands in the range 6.29–6.39 Å.

**Figure 2.** Two views of the cage complex [Cu4(**L1**)4] showing only the coordinating oxygen atoms in the solvent positions. Hydrogen atoms are omitted for clarity.

The reaction of **H2L2** with cobalt nitrate at an elevated temperature in DMF yielded a few royal blue crystals (compound **2**) that were suitable to obtain moderate resolution single crystal X-ray diffraction data using synchrotron radiation. Unfortunately, the yields were so low as to prohibit bulk analysis other than by mass spectrometry of the reaction solution, as described below. The crystal structure was solved and refined in the orthorhombic space group *P*21212 and contains halves of two

crystallographically unique complexes in the asymmetric unit. The nature of the complex is somewhat complicated to determine unambiguously due to crystallographic disorder, with several possible sites for (de)protonation. However, the gross formulation is clear as a "dumbbell-like" complex consisting of two pseudo-[Co(**L2**)4(DABCO)] units, bridged by a central DABCO ligand with an overall formulation of [{Co4(**L2**)4(DABCO)(H2O)x}2(DABCO)], as shown in Figure 3. The two crystallographically unique dumbbell complexes both contain an axis of rotation passing through the central DABCO ligand, meaning that the two ends are symmetry equivalent. The two complexes appear to be compositionally identical, although the crystallographic disorder is different and, for brevity, only the complex with the more minor disorder is discussed below—see Appendix A for further crystallographic discussion.

**Figure 3.** The "dumbbell-type" complex [{Co4(**L2**)4(DABCO)(H2O)4}2(DABCO)]. Hydrogen atoms are omitted for clarity.

In both complexes, there is one major component of the disorder. The ends of this dumbbell complex are a variation of the M4L4 cage motif, with the cage closed by the expected paddlewheel motif at one end (that closest to the centre of the complex) ye<sup>t</sup> capped by an unusual coordination environment at the other end (the far ends of the complex, Figure 4). The two ends are bridged by a DABCO ligand within the cavity of the cage. The Co2 unit at the end of the complex consists of two octahedral cobalt ions. The one nearest the centre of the cavity is coordinated by four carboxylate oxygen atoms and the DABCO ligand, as expected, but also by an aqua ligand. This aqua ligand forms a bridge to the second Co ion, which is coordinated by only two of the carboxylate groups and has three aqua ligands to complete its coordination sphere. The remaining two carboxylates of this M4L4 unit are non-coordinating. Ambiguity arises from the nature of the bridging ligand (H2O, OH<sup>−</sup>, or <sup>O</sup>2−) and the protonation state of the carboxylate groups (i.e., carboxylate vs. carboxylic acid); bond lengths and crystal colour confirm the oxidation state of the metal ions as cobalt (II). The distances between the bridging oxygen atom and non-coordinated oxygen atoms of the carboxylate groups are ca. 2.7 Å and the angle subtended at the bridging oxygen atom is 104◦. These measurements strongly sugges<sup>t</sup> that this is a μ2 bridging H2O, acting as a hydrogen bond donor to the two carboxylates, and therefore has the formula [{Co4(**L2**)4(DABCO)(H2O)4}2(DABCO)]. There are minor components in both instances for which solvation within the coordination sphere cannot be fully resolved due to their small occupancy and proximity to heavier elements. What can be found suggests tetrahedral or non-standard coordination geometries for these positions, supported by the observation of crystal colour (blue), which suggests some amount of tetrahedral CoII is present.

The two crystallographically unique complexes are different in terms of the distortion of the cage, presumably a consequence of accommodating the disorder around the terminal Co2 units. One useful way of measuring this distortion is by the distances between the imide nitrogen atoms of the adjacent **L2** ligands. The complex containing two disordered Co positions adopts a "pinched" conformation, with three N···N distances of 6.2 Å and one of 7.2 Å (measured at both ends of the cage). The measurements are the same at both ends of the cage (i.e., there is no enhanced distortion of the N···N distance at the non-paddlewheel node). The result is that there is a larger "window" on one face of the cage. The other complex, with three disordered cobalt positions, contains N···N distances in the range 6.19–6.54 Å, again with no significant di fference between the distances at the two ends of the cage. There is some evidence of strain in the cages by examination of the Co-O bond lengths and the appreciable bowing of the **L2** ligands. The average of the Co-O bond lengths in the two paddlewheels is 2.03 Å, whereas those in the pseudo-paddlewheels (measured only at the bottoms of these motifs, which are analogous to a paddlewheel) average 2.06 Å. Whilst only a slight di fference, this does sugges<sup>t</sup> some strain at the extremities of the complexes. The **L2** ligands are significantly bowed away from planarity, which is normally observed for pyromelliticdiimides [60–79]. Calculating a mean plane through the C and N atoms of the pyromelliticdiimide core of **L2** highlights that there is significant deviation away from planarity, with the nitrogen atoms ca. 0.2 Å removed from the plane. The carbon atoms of the alanine groups (formerly the α-carbon of the amino acid) are even further removed from the plane by 0.55–0.90 Å, highlighting the convex nature of the ligands in this complex and the apparent strain.

ESI mass spectrometry of the reaction solution shows both singly- and doubly-charged signals corresponding to one end of the dumbbell complex, [Co4(**L2**)4(DABCO)] with some degree of solvation, indicating that the cage is stable in solution, although not the dumbbell complex (not unexpected, given the rather curious nature of the complex). A signal for a 1- ion matches for [Co4(**L2**)4(DABCO)]NO3, suggesting the that the complex can exist in a non-solvated form, presumably with the implication that the paddlewheels are fully closed. The 2- ion has a *m*/*z* value that corresponds to [Co4(**L2**)4(DABCO)(H2O)3(OH)]NO3, which matches with the formulation seen in the crystal structure (with the deprotonation of one water). These results sugges<sup>t</sup> that the cage complex is retained in the bulk, but not the dumbbell complex.

**Figure 4.** One end of the "dumbbell-type" complex shown in Figure 3, in which the coordination environment of the terminal end (top of picture) and the appreciable bowing of the **L2** ligands can be seen (hydrogen atoms are omitted for clarity) alongside a schematic of the unusual terminal coordination environment.
