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将数据记录在硬盘驱动器上的微小磁性颗粒，磁铁在日常生活中无处不在。在过去的几十年里，磁性颗粒的尺寸减小导致了存储容量的显着增加，但正在达到极限。德州农工大学主校区的邓巴教授的团队探索了在纳米尺度上显示磁性行为的新型化学结构。通过系统地研究这些磁性化合物的结构-性能关系，该团队旨在更好地从根本上了解导致优异磁性的因素，并为设计用于计算或电子存储设备的下一代磁体提供洞察力。从事这一项目所需的多才多艺的技能是学生，包括代表人数不足的群体的成员，学习化学合成以及各种表征方法的绝佳机会。NSF化学部的大分子、超分子和纳米化学计划支持Dunbar教授的团队合成和研究具有中性和自由基桥联配体的多核过渡金属和稀土金属超分子结构的性质。虽然更常用的闭壳(抗磁性)桥联配体参与金属自旋之间的超交换(间接)相互作用，但自由基桥联配体促进直接交换相互作用，从而导致更强的磁耦合。对于过渡金属络合物，这种情况导致适度的耦合，而在稀土络合物中，这些相互作用非常弱。通过使用能与金属离子直接交换的自由基桥联配体，大大改善了过渡金属络合物和稀土稀土络合物中的磁性耦合。为了进一步改进这些体系，Dunbar小组寻求使用超分子策略来生成新的自由基桥接磁性络合物。本项目的重点是使用N-杂环中性配体，其特征是阴离子与缺电子芳香环之间的相互作用，即阴离子-pi相互作用。中性配体可以在接近的电位下进行可逆单电子还原，形成稳定的自由基。进行计算机研究是为了确定最综合可行的目标，并在其进展过程中指导研究方向。由于阴离子和溶剂模板的存在，顺磁性金属离子与配体的自发自组装形成了高产率的多核超分子络合物。大量的技术(包括单晶X射线结晶学、SQUID磁学、电化学和电子顺磁共振)被用来研究这些超分子结构的丰富的电子、氧化还原和磁性。通过比较具有不同电子给体能力和配位几何构型的各种配体的络合物的性质，这项研究旨在揭示促成优异磁耦合的结构和电子因素。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.