Metal-Radical Magnetic Exchange: From Molecules to Metal–Organic Frameworks
This dissertation describes several investigations into the use of paramagnetic ligands to drive new electronic behaviors in molecules and materials. In the field of single-molecule magnetism, radical bridging ligands are primarily of interest as a mode of generating strongly exchange-coupled magnetic units for use in high-density data storage. Incorporation of radical ligands into metal–organic frameworks may provoke a more diverse set of behaviors, including electronic conductivity, redox activity, and bulk magnetic ordering. Chapter 1 provides a perspective on the field of metal-radical coordination chemistry, highlighting some of the most notable metal-radical materials in the literature.
Chapter 2 details the study of a three-dimensional metal-radical solid composed of FeIII centers and paramagnetic semiquinoid linkers, (NBu4)2FeIII2(dhbq)3 (dhbq2–/3– = 2,5-dioxidobenzoquinone/1,2-dioxido-4,5-semiquinone). UV-Vis-NIR diffuse reflectance measurements reveal that this framework exhibits Robin-Day Class II/III ligand mixed valence, one of the first such observations in a metal–organic framework. The mixed-valence ligand manifold is shown to facilitate high electronic conductivity in addition to strong metal-ligand magnetic exchange. Slow-scan cyclic voltammetry is used to probe the redox activity of the framework, leading to synthesis of the reduced framework material Na0.9(NBu4)1.8FeIII2(dhbq)3 via a post-synthetic chemical reduction reaction. Differences in electronic conductivity and magnetic ordering temperature between the two compounds are correlated to the relative ratio of the two different ligand redox states. Overall, the transition metal-semiquinoid system is established as a particularly promising scaffold for achieving tunable long-range electronic communication in metal–organic frameworks.
In Chapter 3, a series of two-dimensional lanthanide-quinoid metal–organic frameworks of the formula Ln2(dhbq)3(DMF)x･yDMF (Ln = Y, Sm–Yb, DMF = N,N-dimethylformamide) is synthesized and post-synthetically reduced to produce the series of lanthanide-semiquinoid frameworks NaxLn2(dhbq)3(DMF)y(THF)z (THF = tetrahydrofuran). This set of lanthanide-radical frameworks is investigated using IR and UV-Vis-NIR spectroscopies, which confirm the presence of radical ligands. Magnetic susceptibility measurements indicate that lanthanide-radical magnetic exchange is relatively weak and localized. The systematic analysis of magnetic behaviors of lanthanide-radical coordination solids incorporating a series of lanthanide ions allows for new discussion regarding enhancement of lanthanide-radical magnetic exchange in extended solids.
Chapter 4 reports the synthesis and characterization of the trinuclear 4d-4f complexes [(C5Me5)2Ln(μ-S)2Mo(μ-S)2Ln(C5Me5)2][Co(C5Me5)2] (Ln = Y, Gd, Tb, Dy), containing the highly polarizable and paramagnetic MoS43− bridging unit. UV-Vis-NIR diffuse reflectance and electron paramagnetic resonance spectroscopies reveal substantial charge transfer between MoV and LnIII centers. This metal-to-metal charge transfer enables strong ferromagnetic Ln–Mo exchange, giving rise to one of the largest GdIII magnetic exchange constants, JGd–Mo, observed to date, +16.1(2) cm–1. Both the TbIII and DyIII complexes are shown to exhibit slow magnetic relaxation via ac magnetic susceptibility measurements, with the DyIII congener exhibiting the largest thermal relaxation barrier yet reported for a complex containing a 4d metal center, 68 cm–1. These results demonstrate a generalizable route to enhanced nd-4f magnetic exchange, revealing opportunities for the design of new nd-4f single-molecule magnets.