Chemical, Electrochemical and Physical Properties of Metallosupramolecular Architectures with Neutral and Radical Bridging Ligands and the Influence of Non-Covalent Interactions
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From common refrigerator magnets to tiny magnetic particles on which data are recorded in hard drives, magnets are pervasive in everyday life. The reduction in size of magnetic particles over the past several decades has led to significant increase in the storage capacity but the limit is being reached. Professor Dunbar''s group at Texas A&M University Main Campus explores new types of chemical structures that display magnetic behavior at the nanoscale level. Through systematic investigation of the structure-property relationships of these magnetic compounds, the team aims to gain a better fundamental understanding of the factors that contribute to superior magnetic properties and provide insight to inform design of the next generation magnets for computing or electronic storage devices. The versatility of skills required to work on this project is an excellent opportunity for students, including members of underrepresented groups, to learn chemical synthesis as well as numerous methods of characterization.The Macromolecular, Supramolecular and Nanochemistry Program of the NSF Chemistry Division supports Prof. Dunbar''s group to synthesize and study the properties of multinuclear transition and lanthanide metal supramolecular architectures with neutral and radical bridging ligands. Whereas more commonly used closed-shell (diamagnetic) bridging ligands engage in superexchange (indirect) interactions between the metal spins, radical bridging ligands facilitate direct exchange interactions which results in much stronger magnetic coupling. For transition metal complexes, this situation results in modest coupling and in lanthanide complexes these interactions are very weak. Magnetic coupling in both transition metal complexes and lanthanide rare earth complexes is greatly improved by using a radical bridging ligand that can engage in direct exchange with the metal ions. In order to further improve upon these systems, the Dunbar group seeks to generate new classes of radical-bridged magnetic complexes using supramolecular strategies. This project focuses on using N-heterocyclic neutral ligands that feature interactions between anions and electron deficient aromatic rings, namely the anion-pi interaction. The neutral ligand can undergo a reversible one-electron reduction at accessible potentials to form a stable radical. Computatonal studies are performed to identify the most synthetically feasible targets and guide the direction of research as it progresses. Spontaneous self-assembly of paramagnetic metal ions with the ligands gives multinuclear supramolecular complexes in high yields due to anion and solvent templates. A plethora of techniques (including single-crystal X-ray crystallography, SQUID magnetometry, electrochemistry, and electron paramagnetic resonance) is used to study the rich electronic, redox, and magnetic properties of these supramolecular architectures. By comparing the properties of the complexes with a variety of ligands with different electron donating ability and coordinating geometry, this research aims to unveil the structural and electronic factors that contribute to superior magnetic coupling.This award reflects NSF''s statutory mission and has been deemed worthy of support through evaluation using the Foundation''s intellectual merit and broader impacts review criteria.