Manipulating the Electronic Spin State of Transition Metal Complexes in Pursuit of Advanced Molecular Materials

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The electronic spin state (S) of metal ions is fundamental to the performance of magnets, protein cofactors, and magnetic resonance imaging (MRI) contrast agents. The ability to manipulate the spin state of transition metals allows for the development of advanced materials with emergent properties. This following chapters will introduce two different methods of tuning S, targeting metal-organic magnetic materials and responsive magnetic resonance probes. One way to tune the spin state of transition metal ions is through the introduction of magnetic coupling. Strong coupling between metal centers is particularly important for the design of magnetic materials, including metal-organic magnets. Metal organic magnets offer several advantages over inorganic magnets, but strong magnetic coupling between metal centers is difficult to achieve across large organic linkers. To address this issue, we targeted a strong magnetic coupling mechanism called double-exchange which stems from fast electron transfer between metal ions in different oxidation states. While double-exchange coupling is well documented in inorganic materials like perovskites, it is exceedingly rare in organic-bridged molecules and materials. As a proof of concept, we synthesized a mixed-valence dinuclear Fe2+Fe3+ complex with a diiminobenzoquinone-based bridging ligand to allow electron transfer and magnetic coupling between metal centers. Mössbauer spectroscopy, single crystal X-Ray crystallography, and SQUID magnetometry confirmed the presence of double-exchange coupling and a well-isolated S = 9/2 spin ground state. This work represents the largest metal-metal separation observed for double-exchange coupling. The spin state of transition metal ions can also be manipulated through the modification of its ligand environment. With the appropriate ligand strength and geometry, some iron(II) complexes can undergo a thermally-induced spin transition, a process called spin-crossover. These spin-crossover compounds occupy a diamagnetic S = 0 ground state at low temperature, but are thermally excited to the S = 2 state at high temperatures. Considering the resonance frequencies of NMR-active nuclei are highly dependent on S, we have developed a 19F magnetic resonance pH sensor in which the value of S depends on its protonation state. This was accomplished by introducing mildly acidic hydroxylpyridine groups to our previously developed spin-crossover 19F MR thermometer. Deprotonation of the hydroxylpyridine ligands increases S and causes a dramatic 35.25 ppm shift in the 19F resonance frequency at 25 °C as the pH is increased from 4.74 to 7.82. A plot of resonance frequency vs pH can then be used as a standard curve for the direct measurement of pH in samples or tissues.

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  • 01/11/2019
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