Metal Semiquinoid Magnets: From Molecules to Materials

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In this dissertation, efforts are detailed to utilize semiquinoid bridging ligands to impart strong magnetic coupling between metal centers. Chapter 1 introduces the synthetic challenge of realizing molecule-based magnets with high operating temperatures due to weak magnetic coupling between spin centers through large, diamagnetic ligands. An alternative strategy is described where the use of redox-active bridging ligands to generate stabilized organic results in direct exchange magnetic coupling between the ligand radical and metal center.', '\tChapter 2 focuses on the use of chloranilate-radical ligands to synthesize a two-dimensional, porous Fe-based metal-organic framework in two redox states. Heterogenous reduction of this material reveals a magnetic ordering temperature of Tc = 105 K, the first metal-organic magnetic to order above 100 K that is not based on organonitrile ligands. The framework in both the pristine and reduced states is also shown to be electrically conductive. Chapter 3 explores a one-dimensional material composed of FeII centers bridged by diamagnetic, dianionic ligands. However, below 220 K the chain undergoes a spontaneous and reversible electron transfer to form FeIII centers bridged by the trianionic, radical ligand form. The ensuing strong metal-radical magnetic coupling leads to long-range correlation along the chain and the first example of a semiquinoid-bridged single-chain magnet. Finally, Chapter 4 details work to utilize a bridging ligand with exclusively nitrogen donor atoms. A series of dinuclear molecules are synthesized with CrIII, MnII, FeII, and CoII and the radical bridged species generated through in situ or postsynthetic reduction. Comparison of magnetic data across the series reveals very strong metal-radical magnetic coupling in all molecules but also large variation in coupling strength depending on metal identity. These results are rationalized using ligand field and effective nuclear charge arguments. Together, these results demonstrate the versatility of semiquinoid ligands in promoting strong magnetic coupling in two-, one-, and zero-dimensional systems.

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  • 10/14/2019
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