Towards the synthesis of hydrogen bonded frameworks

Abstract

Hydrogen-Organic frameworks (HOFs) are supramolecular materials constructed by linking organic molecules through intermolecular hydrogen bonds. HOFs are a lightweight alternative to MOFs and have been shown to be effective in a wide range of applications including gas uptake. HOFs are commonly formed from rigid organic building units as this can promote strongly directional hydrogen bonding and framework stability, creating permanently porous structures. However, the field of flexible HOFs has yet to be thoroughly explored. This thesis will present work towards the synthesis of flexible hydrogen bonded materials.

The origin of HOF materials and an examination of hydrogen bonding interactions is discussed in Chapter 1.

Chapter 2 describes the design, synthesis and characterisation of the flexible organic compound 2,2',5,5'-tetrakis(2''-carboxyethyl)butadiene sulfone (H4L1) and the attempted synthesis of 2,2',5,5'-tetrakis(4-carboxybenzyl)butadiene sulfone (H4L2). H4L1 was synthesised in two steps and was characterised using NMR and IR spectroscopy and microanalysis. H4L1 contained a butadiene sulfone core which undergoes a retro-Diels Alder reaction to produce the corresponding diene. This was supported by thermogravimetric analysis (TGA). The precursor 2,2',5,5'-tetrakis(4-iodobenzyl)butadiene sulfone (7) was synthesised in three steps and a variety of reactions were attempted to add a carboxyl functionality, however, only trace amounts of the desired compound were able to be isolated.

Chapter 3 presents the X-ray crystal structures of ten polymorphic and pseudopolymorphic structures (H4L1 to H4L10) of the compound H4L1. These structures were found to contain different H4L1:solvent ratios and hydrogen bonding motifs which resulted in vastly different assemblies. The structures were further analysed using powder X-ray diffraction and DFT calculations. Exposure experiments to air, vacuum, temperature and solvents were also carried out in attempt to transition from one structure to another. While solvent was unable to be removed from a structure, solvent exchange of MeCN to acetone and acetone to H2O was achieved.

Chapter 4 further explores hydrogen bonding interactions by using charge-assisted hydrogen bonding in an attempt to extend the structures of H4L1. The H4L1 compound was deprotonated and then reacted with amidinium hydrochloride salts to produce the structures L1-1-1, L1-1-2, HL1-1-1 and L1-2-1. These structures were analysed using single crystal and powder X-ray diffraction.