Synthesis, characterisation and postsynthetic modification of zirconium and hafnium metal-organic frameworks

This thesis focuses on the self-assembly and functionalisation of Zr and Hf MOFs demonstrating unique structural, physical and mechanical properties, while also improving fundamental aspects such as their syntheses. Following an introduction to prototypical MOF structures, their applications, coordination modulation and postsynthetic modification (specifically of Zr MOFs) in Chapter 1, in Chapter 2 amino acids are investigated as synthetic modulators for the syntheses of a range Zr and Hf MOFs with different pore dimensions and functionality. Surprisingly it is found that upon addition of small amounts of L-proline to synthetic mixtures, the crystallinity of the products is dramatically improved and fine-tuning reaction conditions results in single crystals, a task that is routinely difficult for Zr and Hf MOFs. A range of experimental techniques are used to demonstrate that the amino acid modulated MOFs are phase pure, porous and thermally stable. Accessing single crystals of Zr MOFs, either with rigid (linear) or flexible (kinked) ligands, allows specialised techniques such as high-pressure single crystal X-ray diffraction and nanoindentation to be used to show that ligand flexibility imparts the MOFs with greater overall mechanical stability. To further investigate the effect of flexibility within Zr and Hf MOFs, extended carboxylate based ligands were designed and synthesised by Sonogashira cross couplings. The length of the bridging organic ligands was varied, with alkene, alkyne and butadiyne derivatives obtained. Incorporation of the series of ligands into Zr and Hf MOFs results in highly porous frameworks, and when the longest ligand is used 2-fold interpenetrated MOFs are obtained. When contained within Zr and Hf MOFs, the integral unsaturated sites of the extended ligands are potential sites for chemical transformation. In Chapter 3, it is shown that the integral unsaturated sites can be halogenated, resulting in a mechanical contraction as the hybridisation of the integral carbon atoms is altered during the transformation, ultimately affecting ligand length and geometry. Using the Zr-alkyne MOF (Zr-L9) as a typical example, then addition of chloroform solutions containing Br2 to the MOF transforms the alkyne units to dibromoalkene units, with the overall ligand geometry changing from linear to kinked. The transformation proceeds stereoselectively, resulting exclusively in the trans-dibromoalkene, while the solution-phase reaction of the free ligand results in a mixture of both the cis and trans isomers. Enhanced stereoselectivity is obtained as a result of mixed-phase reactivity, with the topological constraints of the MOF reducing the conformational freedom of the ligands. The transformation is possible in a rare single-crystal to single-crystal manner, with the cubic unit cell edge a contracting by ~0.38 Å (3.7% reduction in volume), proving the high mechanical stability of the MOFs. The facile addition of bromine across the integral unsaturated sites suggested the potential of the MOFs for vapour phase iodine capture. An interpenetrated Zr MOF demonstrates an I2 storage capacity of 279% w/w though a combination of both physisorption (compact organisation of I2 within the pores) and chemisorption (reactivity of the integral unsaturated sites). The very high iodine uptake suggests that the MOFs could be used for the sequestration of radioactive iodine released from nuclear accidents. In Chapter 4, a series of interpenetrated Zr and Hf MOFs containing functionalised ligands, with pendant methyl, fluorine, naphthalene or benzothiadiazole moieties are reported. Twelve MOFs are synthesised, eleven of them as single crystals and it is found that they demonstrate interesting structural behaviour under different external stimuli. High pressure crystallography reveals that in the unfunctionalised derivative (Hf-L11), the phenylene core of the ligands twists by ~90° upon application of pressure, enabling bifurcated hydrogen bonds to form between the two interpenetrated nets, resulting in an overall stabilisation of the material. When pendant methyl groups are incorporated onto the ligands, a structural perturbation is observed where the ligands twist out of the plane of conjugation to minimise steric interactions in the ambient structure. Variable temperature powder X-ray diffraction and solid state fluorescence emission spectroscopy reveal that the ligands are able to rotate to adopt a linear geometry, resulting in a structural change and an associated change of symmetry. Fluorescence emission spectroscopy is used to probe host-guest interactions, investigating the effect of a range of analytes on the MOFs solid-state fluorescence, with sensitive emission behaviour upon wetting suggesting their possible use as water vapour sensors