Structural studies of 63

63 is the shortest alkyl chain carboxylic acid derivative in this family of compounds and single crystals were prepared from a 2:1 H2O/THF mixture. The data obtained from X-ray diffraction

were refined and solved in the triclinic space group P1̅, with the asymmetric unit shown in Figure 2.17. The entire molecule is present in the asymmetric unit, with each of the three unique arms exhibiting a slightly different conformation. The arm containing the amide group C7, O1 and N1 displays a slightly twisted configuration with respect to the central phenyl ring (amide- ring interplanar angle 26.9°), compared to the other two unique amides which are effectively coplanar with the central ring. In addition, the arm originating from C17, O3 and N3 has fully staggered dihedral angles which results in an overall linear arrangement, while the remaining two arms have a combination of staggered and gauche conformations resulting in non- equivalent twisted arrangements.

Hydrogen bonding interactions are the dominant mode of intermolecular interactions within this structure with there being six non-equivalent hydrogen bonding donors and six possible acceptors. The hydrogen-bonded network is quite symmetrical in nature, with each of the three unique arms interacting with its symmetry equivalent to generate four hydrogen bonds per pairing, as shown in Figure 2.18. Arising from the hydrogen bonding interactions, each molecule of 63 interacts with three others to form an extended 2-dimensional hydrogen bonded network. Adjacent layers of the network interact through π-π type interactions, with an overlap of π systems occurring between the phenyl ring and the amide groups from adjacent layers at minimum (interatomic) distance of 3.352(4) Å for C1-C12. One-dimensional hexagonal channels parallel to the crystallographic a axis, of approximately 10 Å (interatomic) diameter, Figure 2.17 The structure of 63 with heteroatom labelling scheme. Selected hydrogen atoms are omitted for clarity.

Chapter 2. Benzene-1,3-5-tricarboxamide n-alkyl ester and carboxylic derivatives

are the by-product of this packing arrangement. Unlike for 62, it was not possible to sensibly model the contents of these channels from the crystallographic data obtained due to poor ordering of the guest molecules, presumably owing to the lack of strongly interacting groups (hydrogen bond donors or otherwise) directed into the channels. Due to this disorder, the contribution from the disordered guest molecules to the measured structure factors was accounted for using the SQUEEZE routine in PLATON,129 and elemental analysis was used to determine the solvent content in the channels. For an air-dried sample, elemental analysis revealed the presence of 1.2 H2O molecules per molecule of 63. A freshly isolated, crystalline

sample of 63 displayed a mass loss of approximately 39% below 100 °C by TGA, with the onset of decomposition occurring at 250 °C, as seen below in Figure 2.19. These results would suggest greater occupancy of the solvent channels immediately after removal from solution, Figure 2.18 (a) Hydrogen bonding interactions between adjacent molecules in the structure of 63. Hydrogen atoms not involved in hydrogen bonding interactions are omitted for clarity. (b) Extended structure of 63 viewed parallel to the solvent channels

Figure 2.19 TGA of 63 showing a weight loss of 39 wt% before 100 °C, with the onset of decomposition at 250

Chapter 2. Benzene-1,3-5-tricarboxamide n-alkyl ester and carboxylic derivatives

with additional guest molecules gradually lost or exchanged for atmospheric water. X-ray powder diffraction analysis of 63 revealed that the phase identified from the single crystal analysis was the predominant in the bulk sample and was unchanged on standing in air, as seen in Figure 2.20. In powder diffraction, a random orientation is usually assumed, however, in some cases such as with plates or needles it can be difficult for the samples to adopt random orientations. The preferred orientations of these samples can cause systematic errors in the peak intensities of the powder patterns, thus, this can be accounted for by applying the March-Dollase parameter for preferred orientations, as was the case for 63 due to its needle-like nature.130

To further probe the stability of these pores, some gas adsorption studies were undertaken. A sample of 63 was subjected to exhaustive evacuation by heating at 100 °C under dynamic vacuum overnight, followed by testing for uptake of N2 at 77 K and adsorption of CO2

at 273 K. Similarly, to 62, negligible uptake was recorded in both cases, suggesting that the framework collapsed upon complete desolvation. The crystal structure of the ester analogue of 63 was reported in 2015 by Haldar et al,77in which 60 was reported to exhibit a supramolecular columnar structure that is stabilized by threefold intermolecular H-bonding interactions in the solid state. The individual discotic molecules stack one on top of another and are connected by intermolecular H-bonding interactions between the neighbouring amides to form a helical Figure 2.20 X-ray powder diffraction of 63 measured at room temperature compared to the simulated pattern from the single-crystal data collected at 100 K. To account for the preferred orientation caused by the needle-like crystalline morphology, the data were simulated with preferred orientation (011) with a March-Dollase parameter of 2.

Chapter 2. Benzene-1,3-5-tricarboxamide n-alkyl ester and carboxylic derivatives

columnar assembly along the crystallographic b axis. There was also some π-π interactions

between adjacent aromatic rings, with the centre-centre distance between the rings being 3.486 Å. A similar type structure was also observed for 59.

To conclude this section, single crystal X-ray structures were obtained of the two ester derivatives and two of the carboxylic acid derivatives, with the end group functionality and length of alkyl chain having an impact on the solid-state structure. The carboxylic acid derivatives, 62 and 63, formed crystalline materials that contain well-resolved solvent channels, which were destroyed upon drying under dynamic vacuum. The carboxylic acid derivatives synthesised in this chapter did not form the classical columnar structures associated with BTA structures. This is due to the competitive hydrogen bonding interactions that are possible due to the presence of the carboxylic acids and prevent the three-fold amide-amide interactions necessary for the columnar structures. On the other hand, the ester derivatives, 58 and 59, were found to have weakly associated columnar packing behaviour in the crystalline state. The shorter chain derivative, 59, forms the classical helical structure common for BTA compounds, while the longer chain was found to be polymorphous, with one X-ray crystal structure obtained and found to have be a densely packed network and possess an unusual loop interaction. This loop interaction occurred due to the length of the alkyl chain enabling the chain to engage in two hydrogen bonding interactions with the same adjacent molecule, spanning the width of the central phenyl ring. Thus, the length of the side chains of the BTAs has a great impact on structure, with a difference of just one carbon leading to great structural changes. As expected, the shorter chains, both ester and carboxylic acid, have a greater tendency towards crystallinity, with this decreasing as the alkyl chain length increases, with it not being possible to obtain single crystals of suitable quality for diffraction of 61. Many attempts were made to obtain metal complexes of the carboxylic acid derivatives, with the intention of these being more stable to evacuation and possessing useful adsorption properties. Unfortunately, however, these attempts were fruitless, only resulting in amorphous materials.