Chemical, Electrochemical and Physical Properties of Metallosupramolecular Architectures with Neutral and Radical Bridging Ligands and the Influence of Non-Covalent Interactions

从普通的冰箱磁铁到硬盘中记录数据的微小磁性颗粒，磁铁在日常生活中无处不在。在过去的几十年里，磁性颗粒尺寸的减小导致了存储容量的显着增加，但正在达到极限。邓巴教授在德克萨斯农工大学主校区的小组探索了新型的化学结构，这些化学结构在纳米水平上显示出磁性行为。通过对这些磁性化合物的结构-性质关系的系统研究，该团队旨在更好地了解产生优异磁性的因素，并为下一代计算或电子存储设备磁铁的设计提供见解。这个项目所需要的技能的多样性对学生来说是一个很好的机会，包括代表性不足的群体的成员，学习化学合成以及许多表征方法。美国国家科学基金会化学部的大分子、超分子和纳米化学项目支持邓巴教授的团队合成和研究具有中性和自由基桥接配体的多核跃迁和镧系金属超分子结构的性质。而更常用的闭壳（抗磁性）桥接配体参与金属自旋之间的超交换（间接）相互作用，自由基桥接配体促进直接交换相互作用，导致更强的磁耦合。对于过渡金属配合物，这种情况导致了适度的耦合，而在镧系配合物中，这些相互作用非常弱。在过渡金属配合物和镧系稀土配合物中，通过使用自由基桥接配体与金属离子直接交换，磁性偶联得到了极大的改善。为了进一步改进这些系统，邓巴小组试图利用超分子策略产生新的基团桥接磁性复合物。本项目重点利用阴离子与缺电子芳环相互作用的n-杂环中性配体，即阴离子- π相互作用。中性配体可以在可接近的电位下进行可逆的单电子还原，形成稳定的自由基。进行计算研究以确定最综合可行的目标，并指导研究的方向。顺磁性金属离子与配体的自发自组装在阴离子和溶剂模板下得到了高产量的多核超分子配合物。大量的技术（包括单晶x射线晶体学、SQUID磁强计、电化学和电子顺磁共振）被用于研究这些超分子结构的丰富电子、氧化还原和磁性。通过比较具有不同给电子能力和配位几何的配体的配合物的性质，本研究旨在揭示导致优越磁耦合的结构和电子因素。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.