LANTHANIDE SINGLE-MOLECULE MAGNETS CONTAINING NITROGEN-BASED RADICAL AND BULKY AMIDE LIGANDS
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| Title: | LANTHANIDE SINGLE-MOLECULE MAGNETS CONTAINING NITROGEN-BASED RADICAL AND BULKY AMIDE LIGANDS |
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| Authors: | Benner, Florian |
| Committee Members: | Demir, Selvan; McCusker, James K.; McCracken, John L.; Blanchard, Gary J. |
| Summary: | Lanthanide single-molecule magnets (SMMs) are a fascinating class of functional molecules that have received considerable interest from the scientific community over the course of the past two decades while substantial progress has been made within the realm of mononuclear SMMs, successful design strategies towards well-performing multinuclear SMMs are still in development. Multinuclear SMMs require strong magnetic coupling between lanthanide ions which is difficult to achieve due to the contracted nature of the 4f ions. An effective method of generating strong coupling is through the implementation of open shell ligands (e.g. organic radicals) between lanthanide ions, where the diffuse spin orbitals of the radical allow strong magnetic interactions. However, so far, the number of radical bridged SMMs is small necessitating the systematic exploration new redox-active bridging ligands and their utility in multinuclear SMM design. The best-performing mononuclear SMMs are composed of cyclopentadienyl derivatives, which prompts the question whether other sterically demanding ligands are suitable to generate axial coordination spheres to lanthanide ions. Chapter 1 contains an overview of electronic structure, general reactivity, and SMM properties of the lanthanides. Common design criteria for mono- and multinuclear SMMs are introduced, and relaxation mechanisms are described. Chapter 2 describes the introduction of 2,2ʹ-bisimidazole (bim) into rare earth chemistry, the synthesis and characterization of bim-bridged dinuclear complexes via various methods such as density functional theory (DFT) and SQUID magnetometry. The Dy congener is a SMM, however the magnetic moments are essentially uncoupled due to the diamagnetic nature of the bim ligand. Chapter 3 and 4 introduce 2,2ʹ-bisbenzimidazole (Bbim) into rare earth chemistry, and the first structurally authenticated bbim trianionic radical of any type stabilized as dinuclear rare earth metallocene complexes. The radical character was confirmed via electron paramagnetic resonance (EPR) spectroscopy, DFT and ab initio calculations. The Gd, Tb, and Dy congeners exhibited strong magnetic coupling stemming from the interaction between the Ln and radical magnetic moments. The observation of open hysteresis loops below 5.5 K proved real magnetic memory effect in the Dy complex. Chapters 5 and 6 present a series of dinuclear complexes bearing the deprotonated fluoflavine (H2flv), ligand as the bridge in various oxidation states. This bridge represents an extremely rare case of multi redox reactivity, giving rise to the isolation of two distinct new radical oxidation states. The radical character was confirmed via EPR spectroscopy and DFT calculations. With the two radical oxidation states, SMM behavior was observed for the Dy congener, where the flv3−-bridged complex showed open magnetic hysteresis loops at higher temperatures than the variant bearing the monoanionic flv ligand. Chapter 7 explores the magnetic properties of dinuclear Dy complexes containing smaller tetraazaacene radical ligands. Chapter 8 describes the potential of Bi-containing ligands to promote magnetic exchange coupling between Er ions. Here, the diffuse Bi valence orbitals were proven to promote significantly improved coupling between two Er ions through a diamagnetic Bi66− bridge. Chapters 9 and 10 introduce sterically demanding triarylamide ligands in mononuclear SMM design. A cationic bis(amide) Dy complex yielded the first bis(amide) SMM with open magnetic hysteresis. The triarylamide ligands are also able to stabilize a formally divalent Tb complex that exhibited slow magnetic relaxation under applied magnetic fields. Chapter 11 summarizes all aforementioned chapters. Chapter 12 provides an outlook into future research directions based on the presented research. |
| URL: | https://doi.org/doi:10.25335/vc7k-j951 |
| Database: | OpenDissertations |
| Abstract: | Lanthanide single-molecule magnets (SMMs) are a fascinating class of functional molecules that have received considerable interest from the scientific community over the course of the past two decades while substantial progress has been made within the realm of mononuclear SMMs, successful design strategies towards well-performing multinuclear SMMs are still in development. Multinuclear SMMs require strong magnetic coupling between lanthanide ions which is difficult to achieve due to the contracted nature of the 4f ions. An effective method of generating strong coupling is through the implementation of open shell ligands (e.g. organic radicals) between lanthanide ions, where the diffuse spin orbitals of the radical allow strong magnetic interactions. However, so far, the number of radical bridged SMMs is small necessitating the systematic exploration new redox-active bridging ligands and their utility in multinuclear SMM design. The best-performing mononuclear SMMs are composed of cyclopentadienyl derivatives, which prompts the question whether other sterically demanding ligands are suitable to generate axial coordination spheres to lanthanide ions. Chapter 1 contains an overview of electronic structure, general reactivity, and SMM properties of the lanthanides. Common design criteria for mono- and multinuclear SMMs are introduced, and relaxation mechanisms are described. Chapter 2 describes the introduction of 2,2ʹ-bisimidazole (bim) into rare earth chemistry, the synthesis and characterization of bim-bridged dinuclear complexes via various methods such as density functional theory (DFT) and SQUID magnetometry. The Dy congener is a SMM, however the magnetic moments are essentially uncoupled due to the diamagnetic nature of the bim ligand. Chapter 3 and 4 introduce 2,2ʹ-bisbenzimidazole (Bbim) into rare earth chemistry, and the first structurally authenticated bbim trianionic radical of any type stabilized as dinuclear rare earth metallocene complexes. The radical character was confirmed via electron paramagnetic resonance (EPR) spectroscopy, DFT and ab initio calculations. The Gd, Tb, and Dy congeners exhibited strong magnetic coupling stemming from the interaction between the Ln and radical magnetic moments. The observation of open hysteresis loops below 5.5 K proved real magnetic memory effect in the Dy complex. Chapters 5 and 6 present a series of dinuclear complexes bearing the deprotonated fluoflavine (H2flv), ligand as the bridge in various oxidation states. This bridge represents an extremely rare case of multi redox reactivity, giving rise to the isolation of two distinct new radical oxidation states. The radical character was confirmed via EPR spectroscopy and DFT calculations. With the two radical oxidation states, SMM behavior was observed for the Dy congener, where the flv3−-bridged complex showed open magnetic hysteresis loops at higher temperatures than the variant bearing the monoanionic flv ligand. Chapter 7 explores the magnetic properties of dinuclear Dy complexes containing smaller tetraazaacene radical ligands. Chapter 8 describes the potential of Bi-containing ligands to promote magnetic exchange coupling between Er ions. Here, the diffuse Bi valence orbitals were proven to promote significantly improved coupling between two Er ions through a diamagnetic Bi66− bridge. Chapters 9 and 10 introduce sterically demanding triarylamide ligands in mononuclear SMM design. A cationic bis(amide) Dy complex yielded the first bis(amide) SMM with open magnetic hysteresis. The triarylamide ligands are also able to stabilize a formally divalent Tb complex that exhibited slow magnetic relaxation under applied magnetic fields. Chapter 11 summarizes all aforementioned chapters. Chapter 12 provides an outlook into future research directions based on the presented research. |
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