Crystal engineering of mixed-ligand metal-organic frameworks

• Main Author: Tahier, Tayyibah
• Other Authors: Oliver, Clive L
• Format: Dissertation
• Language: English
• Published: University of Cape Town 2017
• Subjects: Chemistry
• Online Access: http://hdl.handle.net/11427/22913

Research of solid state complexes has grown and developed exponentially over the past few years in terms of supramolecular chemistry and crystal engineering. The synthesis and characterisation of metal organic frameworks (MOFs) have attracted widespread attention owing to their potential in various applications. This includes gas sorption, which could aid in alleviating serious environmental issues such as global warming by sequestrating greenhouse gases. Advances in the design of these materials using the mixed ligand approach add to variation in structures and thus provide a further means of tailoring of properties. A novel two dimensional, mixed ligand MOF has been synthesised based on 1,3,5 benzenetricarboxylic acid, 4,4' bipyridine N,N' dioxide and zinc sulfate with the formula [Zn3(BTC)(4,4' bpdo)(OH)(SO4)(H2O)3]n·n(H2O)2.33 (1). The 2D layers of 1 arrange in a polar fashion with adjacent layers forming isolated cavities. Variable temperature powder X ray diffraction (VT PXRD) analysis showed that the crystallinity of the compound was retained and the crystalline phase remained unchanged as the temperature was increased. Variable temperature single crystal X ray diffraction (VT SCXRD) analysis of 1 revealed that the dehydration and rehydration processes occur via single crystal to single crystal transformations. Water vapour sorption experiments showed a type I isotherm, typical of microporous materials. A two dimensional, interpenetrated, mixed ligand MOF has been synthesised based on 5 nitro 1,3 benzenedicarboxylic acid, 1,2 bis(4 pyridyl)ethane and cadmium nitrate with the formula [Cd(bpe)1.5nbdc]·DMF (2). VT PXRD analysis shows subtle differences in the compound as the temperature is increased. VT SCXRD experiments show that the most notable change in the structure occurs at 373 K. These changes include the removal of the guest molecule and a change in the crystal system, along with changes in the orientation of the pyridyl ring of the organic ligand. Carbon dioxide sorption experiments at 195 K showed a type IV isotherm, which is usually associated with mesoporous materials. Both 1 and 2 were synthesised using the solvothermal method and fully characterised using X ray diffraction studies (SCXRD, PXRD, VT SCXRD and VT PXRD), thermal analysis (thermogravimetric, differential scanning calorimetry, hot stage microscopy), elemental analysis and FT IR spectroscopy. The porosity of the compounds was tested using carbon dioxide (273 K and 193 K), nitrogen, water vapour and liquid sorption experiments.