The past decade has witnessed an explosion of interest in metal-organic framework (MOF) materials, primarily as an important class of porous zeolite analogues. The inherent advantages of MOFs over traditional inorganic zeolites are numerous and include greater compositional and structural diversity and easier tailoring of properties by modification of the organic ligands. Chemical functionality is also easier to introduce to the frameworks themselves, as is the ability to introduce chiral centers to allow preparation of chiral porous solids. In this thesis, the initial motivation was the preparation of chiral MOFs using one of the simplest chiral organic ligand, L-tartaric acid.
Chapter One surveys the background to MOFs successful design and preparation. In general, chiral organic compounds contain carboxylate and hydroxyl O-donor and even pyridyl N-donor ligands can be utilized in synthesize chiral MOFs. An introduction to photochromic compounds as well as viologens in particular, is discussed.
Chapter Two explores the MOFs formed from metal dications and various tartrates under both ambient and a variety of hydrothermal conditions. A total of 9 phase types were formed by systematic variation of reaction conditions, temperature and solvent. Ambient conditions led to lower dimension polymers whilst hydro/solvothermal synthesis afforded higher framework dimensionality. By increasing the reaction temperature or using organic solvent, ancillary aquation was reduced and thus polymer dimensionality typically increased. Interestingly, two examples of solid-state phase transitions between phase types via dehydration were discovered. One of these from [Mg(L-TAR)(H 2O)]1.5H2O to [Mg(L-TAR)] can occur by a single crystal to single crystal phase transformation due to the structural similarity of two phases. In general the 3D MOF phases formed had limited channel size, however framework stabilities in excess of 200°C were found from variable temperature powder X-ray diffraction evidence.
Chapter Three explores one approach to channel engineering of such chiral MOFs based on addition of 4, 4’-bipyridine and related neutral N-donor ligands as a spacers to be incorporated into the transition metal tartrate framework. In order for this approach to be successful the metal should prefer a mixed N/O coordination sphere. Mixed 4,4-bipyridine-tartrate polymers for several first row transition metals such as Mn, Fe, Co, Ni, Cu as well as Zn were therefore investigated. The syntheses and crystal structures of 17 new phases are reported. The approach has been successful in allowing the dual incorporation of the chiral and spacer components into the MOF. The chiral frameworks with enlarged porosity were successfully synthesized with typical framework stabilities of 150°C.
Two promising open framework MOF families have been found for octahedral metals and square pyramidal copper. In the cases of copper-tartrate-4,4’-bipy systems, channels with guest molecule selectivity were found. These MOF polymers were demonstrated to be robust to the loss of channel guest molecules and channel wall engineering, though variation of bipyridine spacer and solid solution of tartrate, malate or succinate appears possible. These materials are thus excellent candidates for selective, or even enantio-selective sorption. It was noticed that one chiral MOF material [Zn(L-Mal)(4,4’-bipy)] was strongly photo-chromic and further studies were made to investigate the nature and mechanism of its photo-chromism. Chapter Four scrutinizes the mechanism through the synthesis and characterization of 18 related organic solids compositionally related to the photo-chromic MOF. The solids were characterized both before and after photo-irradiation by EPR, XPS, UV-vis and RR spectroscopies. Photo-chromic phases incorporating [4,4’-bipy] moieties show a variety of forms, including neutral, mono and diprotonated states. However the colored species appear to have a common [4,4-bipyH2]•+ radical cation based on Resonance Raman spectral data. Mechanisms proposed for some of the solids are thus more complex than simple electron transfer as might be found in methyl-viologen analogues. They apparently also involve concomitant hydrogen atom and proton transfer. These pathways may be mediated by strong N-H-O hydrogen bonded interactions, which are commonly found in these organic co-crystals. The photo-chromism is of “type T” in most cases is indefinitely stable at room temperature, but thermally reversible. The purple color bleaches out upon heat treatment or infra-red irradiation of the solids.