From geometric to charge-distribution symmetry: deeper insights into lifting the performance of dysprosium single-ion magnets

Crystal-field symmetry of lanthanide ions plays a critical role in suppressing quantum tunneling of magnetization (QTM) in single-molecule magnets (SMMs), but high-performance SMM design and modulation remain challenging only in view of the geometric symmetry of the first coordination sphere. Herein, two bis(semicarbazone)/bis(thiosemicarbazone)dysprosium single-ion magnets with pentagonal bipyramid geometry were reported, and bis(thiosemicarbazone)lanthanide complexes have never been reported to the best of our knowledge. They served as good archetypes to study the magneto-structural relationships based on the charge distribution The complex with more ideal geometric symmetry displays fast zero-field QTM with negligible "effective barrier", owing to the defective charge distribution. By modulating the transverse crystal field via the replacement of the O sites with the less charged and larger radius S atoms, it results in lower geometric but higher charge-distribution symmetry, giving rise to the significant suppression of QTM and the enhancement of reversal barrier up to ca. 1,000 K. These results demonstrate that the charge-distribution symmetry can be chemically tailored by modification of the crystal field, which is essential for designing high-performance SMMs.