A crystal of 65Cd was subjected to single crystal X-ray diffraction, with the data solved and the structural model refined in the monoclinic space group P21/c. The asymmetric unit of 65Cd,
shown in Figure 3.10, contains three unique cadmium environments and two unique ligands within the asymmetric unit, and confirms that the in situ hydrolysis of the ester groups took place. The six independent carboxylate groups all undergo coordination to the cadmium ions, with the cadmium coordination spheres completed by one coordinating amide oxygen atom (O16), three unique aqua ligands and one DMF ligand. Each cadmium ion assumes a unique coordination number and geometry, with Cd1, Cd2 and Cd3 adopting 6-, 7- and 8-coordination geometries respectively. A trinuclear cluster mode best describes the metal coordination environment, where the metal centres are linked by four bridging carboxylate groups, as seen in Figure 3.10. Three of the bridging carboxylates adopt a μ2-κO,O′:κO binding mode, with the
remaining bridging carboxylate engaged in a triply bridging μ3-κO:κO:κO′ coordination mode.
The non-bridging carboxylates adopt purely chelating coordination modes. Around the periphery of the trinuclear cluster, numerous O- H···O hydrogen bonds (from the aqua ligands) and N-H···O hydrogen bonds from adjacent amide groups serve as further supports for the cluster geometry.
The extended structure of 65Cd is a densely connected three-dimensional coordination polymer. The connectivity of this polymer is quite complex and cannot be easily related to a Figure 3.10 (a) the coordination sphere of the trinuclear cadmium node, with atom labelling scheme; (b) connectivity of the two unique 65 groups. Hydrogen atoms and atom labels are omitted for clarity.
Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides
known net; as seen in Figure 3.11. The complexity of this extended polymer is largely as a result of the coordination from the amide oxygen of one of the two unique ligand species, giving different connectivity to the two ligand nodes. Disregarding this interaction does not restore symmetry between the two nodes and fails to simplify the network description.
The structure of 65Cd contains no amide-amide hydrogen bonding, unlike 64Cd and other BTA-containing structures. In place of this, five N-H hydrogen bond donors are occupied in hydrogen bonds to carboxylate oxygen atoms, and one instance of an N-H···O hydrogen bond to a lattice water molecule is present. This is comparable behaviour to that encountered previously in the study of short-chain BTA carboxylate species,148 discussed in Chapter 2, where competitive hydrogen bonding functionality is present. The BTA core groups, however, are still aligned in stacks in the structure of 65Cd despite the absence of amide-amide interactions. The columns are aligned parallel to the a axis and proceed in an AABBAA fashion with respect to the two unique ligand molecules and these are supported by N-H···O (carboxylate) hydrogen bonds at their periphery. The aromatic cores are 3.66, 3.77 and 3.59 Å apart for the two symmetric and one unsymmetric pairings respectively, with the ring centroids slipped by 0.4-0.6 Å for the symmetric pairings and 1.6 Å for the unsymmetric pairing. By considering these columns as structural features, the extended network can be described as an approximately hexagonal rod packing motif, where each column is connected to one another through six sets of radially distributed cadmium cluster nodes, as depicted in Figure 3.11. Figure 3.11 Extended structure of 65Cd; (a) the linear stacks of 65 groups encircled by trinuclear cadmium clusters; (b) topological representation of the network viewed parallel to the stacks; yellow represents 65 nodes and pink represents cadmium clusters.
Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides
Besides the DMF and aqua ligands within the asymmetric unit, the structure of 65Cd contains five lattice water molecules; minor crystallographic disorder required modelling these over a total of seven positions. These aqua ligands mainly take part in hydrogen bonding interactions with one another and the aqua ligands and carboxylate groups within the structure. These groups arrange to form narrow one-dimensional channels that are parallel to the crystallographic c axis and perpendicular to the propagation of the BTA stacks. These unbound water molecules account for an approximate solvent-accessible volume of 9%, increasing to 21% if the metal-bound solvent molecules are considered. Further investigations into the presence of solvent in these channels will be described in the next section. Phase purity was confirmed by x-ray powder diffraction and is shown in Figure 3.19 in 3.6.
As mentioned previously, in a study by Yan and co-workers,145 they reported they reported the structures of 65 with zinc, manganese, nickel and cobalt. In each of these structures, each metal ion coordinates to three ligands and each ligand bridges three metal ions, as well as resulting in an infinite 2D anionic layer with honeycomb-like cavities. Two adjacent layers are staggered in such a way that non-interpenetrating double-layer structures are formed. These double layers are separated by a layer of hexaqua-metal cations through hydrogen bonds. Here it is reported that 65Cd forms a 3D polymer and does not contain the anionic layer reported by Yan, thus highlighting the effects of different metals on the structures formed.