Regulating Lanthanide Single-Molecule Magnets with Coordination Geometry and Organometallic Chemistry

ConspectusSingle-molecule magnets (SMMs), particularly those based on lanthanide ions, have emerged as a revolutionary class of molecular nanomaterials with potential applications in quantum computing, high-density information storage, and spintronic devices. The key to unlocking their full potential lies in the precise engineering of ligand fields to control the magnetic anisotropy and slow magnetic relaxation dynamics. This Account presents our group's systematic investigations into advanced coordination geometry regulation strategies and organometallic ligand design for optimizing the performance of lanthanide SMMs, with particular focus on establishing clear magneto-structural correlations and developing innovative coordination approaches.Central to our design philosophy is the fundamental understanding that for Dy(III) and Tb(III) ions with oblate electron density strong axial ligand fields coupled with minimized equatorial interactions are crucial for achieving maximum magnetic axiality. Our research has developed two synergistic strategies to realize this ideal coordination environment: (1) pseudo-two-coordinate model with symmetry control and (2) conjugated chelating organometallic ligand engineering. In the first approach, we have constructed a series of Werner-type complexes with well-defined local symmetries (D_4h, D_5h, D_6h, etc.), creating model systems that feature weak equatorial crystal fields while maintaining strong axial ones. These carefully designed architectures have yielded exceptionally large energy barriers for magnetization reversal with some complexes approaching those of state-of-the-art SMMs. Beyond symmetry considerations, we have demonstrated how subtle modifications of the geometry can fine-tune crystal field parameters, while the introduction of rigid axial ligands effectively suppresses quantum tunneling of magnetization and Raman relaxation processes. This dual control strategy has led to significant improvements in magnetic blocking temperatures of the pseudo-two-coordinate system. Our second strategy involves the development of novel π-delocalized organometallic ligands, including carboranyl, amidinate, and guanidinate systems. These ligands offer advantages comparable to those of cyclopentadienyl derivatives. For instance, carboranyl anions provide very strong ligand fields due to their unique electronic structures, while amidinate ligands exhibit a labile chelating capability to stabilize Dy(II) and Tb(II) ions, opening new frontiers in nontraditional low-valent lanthanide chemistry as well as magnetochemistry. These works highlight the importance of coordination geometry and the ligand field in engineering high-performance SMMs and provide insights into the magneto-structural correlations. While challenges remain in truly understanding the relaxation mechanism and further improving blocking temperatures, these strategies offer clear pathways for advancing lanthanide-based SMMs.