Structural study of [Cd 3 (64) 2 (DMF) 2 ]·5H 2 O·2DMF

Single crystals of 64Cd were grown in DMF/H2O (1:1) using the solvothermal method

described in 3.2.1 and 3.2.2. A single crystal of 64Cd was analysed by single crystal X-ray diffraction at 100 K, with the data solved and the structural model refined in the triclinic space group P-1. The asymmetric unit contains three unique cadmium environments and two complete 64 species, with each of the carboxylates deprotonated. There was a small proportion of disorder present in the vicinity of cadmium sites Cd2 and Cd3, (further details in appendix). The coordination spheres of each of the three unique cadmium ions were filled by oxygen donors, either from 64 carboxylates or two unique DMF ligands, as depicted in Figure 3.6. The cadmium ion Cd1 adopts a 7-coordinate geometry, while Cd2 and Cd3 possess distorted octahedral 6-coordinate geometries. As is often the case for Cd-carboxylate systems, bridging coordination modes are widely adopted here; Cd1-Cd2 and Cd2-Cd3 are each bridged by three unique carboxylates, while Cd3-Cd1 are doubly bridged. A one-dimensional cadmium- carboxylate chain that extends parallel to the a axis results from this bridging.

The ligand species in the structure of 64Cd adopts a splayed T-shaped conformation and the two crystallographically unique moieties alternate in a stack aligned parallel to the a

axis and parallel to the cadmium-carboxylate chains. The two species exhibit similar conformations with the slight differences mainly manifested in variations in the orientation of the acetate fragments. The adjacent molecules align into slipped stacks, however, the traditional BTA hydrogen bonding mode is not observed between directly adjacent molecules; only three Figure 3.6 Connectivity and chemical environment of the three nodes in 64; (a) Cadmium coordination geometry with heteroatom labelling scheme, where O26 corresponds to a coordinating DMF molecule; (b and c) Connectivity of the two crystallographically independent 64molecules within the structure. Hydrogen atoms, framework disorder, lattice solvent molecules and selected atom labels are omitted for clarity.

Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides

of a possible six unique amide-amide hydrogen bonds are observed in each stack. The adjacent BTA ring planes are separated by ca. 3.2 Å in the direction of the plane normal vector, with the rings horizontally slipped by ca. 3.7 Å, thus differentiating this packing mode from the typical BTA helix, Figure 3.7. The remaining amide N-H groups that are not involved in amide-amide hydrogen bonding fulfil their bonding requirements by bonding to two lattice DMF molecules and a lattice water molecule.

The T-shaped conformation of the 64 molecules linking the one-dimensional cadmium carboxylate chains give rise to a corrugated double layer two-dimensional framework that is oriented parallel to the ac plane and shown in Figure 3.8. Two crystallographically independent, rectangular intra-layer solvent channels are present within each sheet, with maximum interatomic dimensions ca. 12.5 × 8 Å (omitting the solvent molecules). In total, 5 DMF (2 coordinating and 3 non-coordinating) sites were located in the crystallographic model, with a total occupancy of 4.2 DMF molecules per 3 cadmium ions, and three water sites with a total occupancy of 2.5 molecules per 3 cadmium ions. Further DMF sites are presumed to be present within the structure, associated with the 15 % disorder contribution of the two cadmium sites, however, this could not be sensibly modelled. Adjacent layers associate in the b direction through various π-π and C-H interactions, along with a defined hydrogen bonding interaction through lattice water molecules between cadmium-carboxylate chains of adjacent sheets. While all solvent molecules were included in the final refinement, removal of the lattice solvent from the model gave an estimate of 25 % solvent accessible volume. This solvent was mostly Figure 3.7 Hydrogen bonding interactions between alternating adjacent core units of 64Cd molecules in the structure of complex 64Cd, showing the slipped-stack packing arrangement.

Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides

localised in the intralayer channels with a small contribution from the narrow channels between the adjacent layers. Elemental analysis of a sample of 64Cd revealed the presence of both H2O

and DMF molecules. The sample used for elemental analysis was freshly prepared and was not dried in vacuo before analysis. This was done in order to probe the presence of solvent molecules in the channels. Further details of the elemental analysis can be found in the experimental section. The presence of these solvent accessible channels probed further studies which will be discussed in 3.6.

Figure 3.8 (a) structure of an individual network of 64Cd showing the 1-dimensional intralayer solvent channels (non-coordinating solvent molecules omitted); (b) the 3-dimensional structure of 64Cd showing the intralayer void volume and smaller interlayer pores.

Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides

The phase purity of a bulk sample of 64Cd was confirmed using X-ray powder diffraction as shown in Figure 3.9, where the peaks of the measured pattern coincide with those of the simulated pattern derived from the single crystal data.

In a report by Zhao et al., the coordination polymers formed upon the reaction of the analogue of 64, 41, with a variety of metals was discussed.102 The authors reported that the zinc polymer had a layer type structure, while the coordination polymer generated from the reaction of the ligand with cobalt chloride displayed a doubly interpenetrated three-dimensional network. The reaction of the ligand with cadmium nitrate and cobalt nitrate resulted in isomorphous structures where a grid layered architecture in which the secondary building unit was a Cd3 cluster was observed. This study demonstrates the influence of the various metals on the structure of the resulting coordination polymers and also highlights the structural effects of incorporating an extra CH2 group into 41. As mentioned previously, one of the aims of this

chapter was to investigate the effect of bridging distances and the presence of aromatic groups on two BTA derivatives with similar flexibility, thus the structure of coordination compound formed from the derivative without aromatic side arms will be discussed in the next section.

Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides