Use of chelating ligands in the synthesis of paramagnetic frameworks
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Chapter 1 of this thesis contains an extensive literature review of magneto-structural correlations (MSCs) of a range of transition metal dimers. Different structural parameters are considered and a critical assessment is given of the principal factors influencing the exchange interactions in each case. Following this, two different synthetic strategies for the synthesis of polymetallic systems are compared and contrasted. Their role in the formation of paramagnetic frameworks is also considered. Chapter 2 describes the synthesis of a family of MnIII dimers and highlights the importance of chelating ligands in their formation and magnetic properties. A family of MnIII dimers of general formula of [MnIII2(R-sao)2(dpa)2](ClO4)2 (1-5) has been synthesised using derivatised phenolic oximes (R-saoH2, where R=H, Me, Et, Ph) in combination with di-(2-picolyl)-amine (dpa). The structures reveal a double-oxime bridged [MnIII(NO)]2 magnetic core in which the Jahn-Teller axes lie perpendicular to the bridging plane, in contrast to two previously reported family members (6 and 7). The switch in the orientation of the Jahn-Teller axes is enforced through the use of the chelating ligand which is present in 1-5 and absent in 6-7. Dc magnetic susceptibility measurements reveal that the exchange interactions between the MnIII metal centres in 1-5 are antiferromagnetic in contrast to that observed for 6 and 7 which are ferromagnetic. DFT calculations have also been undertaken in order to confirm these findings. Chapter 3 describes the synthesis of a family of novel cages from the metalloligand [FeIIIL3], where HL = 1-(4-pyridyl)butane-1,3-dione and metal salts CuBr2, Cu(NO3)2, NiCl2, CoCl2 Co(SCN)2, ZnBr2 and Pd(OTf)2 (where OTf = triflate anion). The flexibility in the design of these structures offers huge potential to tune their physical properties as the constituent parts of the cage can be altered without any change to the structure. Due to the large size of the internal cavity, it is possible that the cage could play host to different species, such as magnetic guests, which would then exert control over the exchange interaction between the metal ions in the host framework and between the host and guest(s). Chapter 4 presents the synthesis and crystallographic studies of two novel metal organic framework (MOF) [CuIIL2]n and HL = 1-(4-pyridyl)butane-1,3-dione. An in-depth high-pressure crystallographic study of structure shows that the framework undergoes a single-crystal phase transition as well as a switching of the orientation of the Jahn-Teller axis, which is dependent on the hydrostatic media and also the presence of negative linear compressibility (NLC) behaviour.