Synthesis and properties of model systems for magnetic resonance studies and new substituted terephthalic acid MOF linkers

Abstract

The interest in complex biomolecules containing more than two spin labels has increased in the recent years. Therefore, also the measurements as well the analysis of their distances became more relevant. In this work, organic model systems with more than two paramagnetic centres were designed to validate criteria for extracting complex distance distributions by using pulse dipolar electron paramagnetic resonance (EPR). One organic model system contains one nitroxide spin label and three terpyridines, which can coordinate paramagnetic copper(II) ion to investigate the occurring multi spin effect in pulse electron-electron double resonance and relaxation induced dipolar modulation enhancement (RIDME) experiments. The synthesis of the model systems was divided into four precursor molecules which can be connected to each other by Sonogashira cross coupling reactions and esterification. It was possible to isolate all four precursors but the attachment of those was more challenging than expected and the desired organic model system could not be obtained. Further investigations of the cross coupling conditions need to be done to overcome the problem of orthogonal cross coupling reactions. Nevertheless, with those precursor molecules it was possible to synthesise terpyridine ligands of different length with one nitroxide spin label attached, for dimerisation studies of the ligand binding to metal(II) ions, which can be investigated by pulse dipolar EPR spectroscopy. Also, a paramagnetic holmium complex, which can accelerate relaxation times of nitroxide spin labels could be synthesised and a novel crystal structure of the paramagnetic complex could be obtained. Preliminary EPR measurements were carried out, which show the accelerations of the nitroxide relaxation time by adding the holmium complex. Further studies are necessary to understand the process of the relaxation time acceleration by the holmium complex. Finally, the synthesis and purification of several oxime derivatives and their corresponding bis(oximato)copper(II) complexes for continuous wave (EPR) were successfully established, in order to extract EPR parameters as a complementary methods to solid state nuclear magnetic resonance (ssNMR) and density functional theory (DFT) to predict the assignment of the spectra from those paramagnetic complexes. Also, a bis(oximato)nickel(II) crystal doped with 1% paramagnetic bis(oximato)copper(II) complex could be isolated for further pulse dipolar EPR studies.

The organic linker 2,5-dihydroxyterephtalic acid (DHTP) in combination with bivalent metal ions, like Zn²⁺, Mg²⁺, Ni²⁺, Co²⁺ etc. is used for the formation of CPO-27 metal organic framework (MOF) structures. One of the most interesting MOF applications is the delivery of nitric oxide for biomedical applications. Therefore, there is a demand for regulating the release and binding of the medical gas on the porous material, by varying the pore size and binding affinity. Therefore, several novel derivatives of the DHTP linker were successfully synthesised in an attempt to alter the accessibility of coordinated unsaturated metal sites on the framework and to alter the hydrophilicity of the material. For the substitution on the remaining two benzene ring positions, halogen, alkyl chains and alcohol groups could be introduced in the DHTP motive with several different organic named reactions. The corresponding single crystal structures of these novel compounds were determined. Different MOF synthesis approaches with the novel linker, i.e. water based, hydrothermal/solvothermal, post synthetic modification (PSM) and mixed linker synthesis were subsequently investigated using the chlorinated and alkylated DHTP analogues. The obtained materials were analysed by powder X-ray diffraction (PXRD), infrared spectroscopy (IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and solid state nuclear magnetic resonance (ssNMR). It was possible to reproduce CPO-27-Zn/Ni material with the water based and solvothermal synthesis approaches but transferring those conditions on the novel DHTP linkers did not lead to the desired CPO-27 structure and an unknown material whose structure could not be identified was obtained. The material obtained from PSM did not show any changes in the PXRD pattern compared to the original CPO-27-Zn structure. An exception was the chloro derivative, whose PXRD pattern is significantly different with features of the original CPO-27-Zn structure. Further investigation by SEM revealed that the hexagonal morphology of CPO-27-Zn was degraded instead of incorporation of the chlorine derivative by this approach. For the CPO-27-Ni the PSM leads to no structural changes observed by IR and PXRD, indicating that the PSM approach was not successful to incorporate the novel linker into the CPO-27 structure. The material obtained from the approach, using standard DHTP as an initiator in combination with the novel DHTP analogues, leads to material with CPO-27 structure and additional reflections in the PXRD. By varying the ratio between standard DHTP and the novel linker it was observed that the additional reflection became more distinct and the reflections of CPO-27 less pronounced, indicating that a secondary phase was rather formed than the incorporation of the novel linker into the CPO-27 structure, and this could be confirmed by SEM measurements. Further analysis should be done to reveal the structure of the unknown second phase. Therefore, it was not successful to functionalise CPO-27 with those synthetic approaches.

Date of Award	14 Jun 2023
Original language	English
Awarding Institution	University of St Andrews
Supervisor	R Alan Aitken (Supervisor)