Metal-organic frameworks: functional materials by form and design

Abstract

A range of metal-organic framework (MOF) materials, consisting of extended structures of coordination polymers incorporating paramagnetic metal ions (MnII,III, FeII, CoII,III, NiII and CuII) ,have been synthesized using bridging ligands such as carboxylate-decorated and/or nitrogen containing aromatics. The in situ. ligand transformation of 1,2,3-benzene tricarboxylic acid into isophthalate to form four isomorphous coordination polymers was found to be highly dependent upon the reaction temperature, only occuring at higher temperatures such as 220°C. The structural and magnetic properties of all of the compounds have been studied in detail, with many showing bulk magnetic ordering and magnetic phase transitions. In [Co(bIM)(acetate)]n, benzimidazole complexation led to antiferromagnetic superexchange interactions between the cobalt(II) ions, however at very low temperatures the magnetic behaviour changed from antiferromagnetic to weak ferromagnetic ordering as a result of spin canting. Similarly, another 3D MOF [NiII 2(pda)(OH)2(H2O)]n based on linear 1D NiII chains connected via Ni4O4- cubanes, has been synthesized, offering a rare example of a magnetic material in which the magnetic phase transitions (MPTs) are potentially coupled to slow relaxation phenomena. This particular material showed an antiferromagnetic ordering at 21 K, with a magnetic phase transition from AF to CAF at 2.8 K. We were also able to design a topology in which the bulk magnetic ordering to higher temperatures were found to be shifted to considerably higher temperatures. For example, two 3D isostructural MOFs based on μ3-O centered metal clusters {[MIII 3(pydc)3(μ3-O)]n.Cl, M = Mn, Co}, have been synthesized, displaying bulk magnetic ordering at around 45 K; whilst only the cobalt based MOF displayed any spin canting behavior, and then only below 10 K. Other aspects of this thesis involved the introduction of flexible ligand systems such as 1,4-phenylene diacetic acid (pda) and di-(4-pyridyl)-ethane (dpe) in the synthesis of MOFs. Four novel isostructural MOFs {[M(pda)(dpe)]n, where M = MnII, CoII, NiII and CuII} were synthesised using pda and dpe, and were found to show bulk magnetic ordering. We also have reported a microporous framework, [Cu3(ipO)2(pyz)2]n, which exhibits a remarkable preferential adsorption for H2 over N2, having a ratio of H2 selectivity in excess of 5 at sub-ambient pressures.