Quenching the Quantum Tunneling of Magnetization in Heterometallic Octanuclear {TM^III₄Dy^III₄} (TM=Co and Cr) Single-Molecule Magnets by Modification of the Bridging Ligands and Enhancing the Magnetic Exchange Coupling

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• 作者：Kuduva R. Vignesh ; Dr. Stuart K. Langley ; Prof. Keith S. Murray and Prof. Gopalan Rajaraman
• 刊名：Chemistry - A European Journal
• 出版年：2017
• 出版时间：January 31, 2017
• 年：2017
• 卷：23
• 期：7
• 页码：1654-1666
• 全文大小：5310K
• ISSN：1521-3765

文摘

We report the synthesis, structural characterisation, magnetic properties and provide an ab initio analysis of the magnetic behaviour of two new heterometallic octanuclear coordination complexes containing Co^III and Dy^III ions. Single-crystal X-ray diffraction studies revealed molecular formulae of [Co^III₄Dy^III₄(μ-OH)₄(μ₃-OMe)₄{O₂CC(CH₃)₃}₄(tea)₄ (H₂O)₄]⋅4 H₂O (1) and [Co^III₄Dy^III₄(μ-F)₄(μ₃-OH)₄(o-tol)₈(mdea)₄]⋅ 3 H₂O⋅EtOH⋅MeOH (2; tea³⁻=triply deprotonated triethanolamine; mdea²⁻=doubly deprotonated N-methyldiethanolamine; o-tol=o-toluate), and both complexes display an identical metallic core topology. Furthermore, the theoretical, magnetic and SMM properties of the isostructural complex, [Cr^III₄Dy^III₄(μ-F₄)(μ₃-OMe)_1.25(μ₃-OH)_2.75(O₂CPh)₈(mdea)₄] (3), are discussed and compared with a structurally similar complex, [Cr^III₄Dy^III₄(μ₃-OH)₄(μ-N₃)₄(mdea)₄(O₂CC(CH₃)₃)₄] (4). DC and AC magnetic susceptibility data revealed single-molecule magnet (SMM) behaviour for 1–4. Each complex displays dynamic behaviour, highlighting the effect of ligand and transition metal ion replacement on SMM properties. Complexes 2, 3 and 4 exhibited slow magnetic relaxation with barrier heights (U_eff) of 39.0, 55.0 and 10.4 cm⁻¹ respectively. Complex 1, conversely, did not exhibit slow relaxation of magnetisation above 2 K. To probe the variance in the observed U_eff values, calculations by using CASSCF, RASSI-SO and POLY_ANISO routine were performed on these complexes to estimate the nature of the magnetic coupling and elucidate the mechanism of magnetic relaxation. Calculations gave values of J_Dy–Dy as −1.6, 1.6 and 2.8 cm⁻¹ for complexes 1, 2 and 3, respectively, whereas the J_Dy–Cr interaction was estimated to be −1.8 cm⁻¹ for complex 3. The developed mechanism for magnetic relaxation revealed that replacement of the hydroxide ion by fluoride quenched the quantum tunnelling of magnetisation (QTM) significantly, and led to improved SMM properties for complex 2 compared with 1. However, the tunnelling of magnetisation at low-lying excited states was still operational for 2, which led to low-temperature QTM relaxation. Replacement of the diamagnetic Co^III ions with paramagnetic Cr^III led to Cr^III⋅⋅⋅Dy^III coupling, which resulted in quenching of QTM at low temperatures for complexes 3 and 4. The best example was found if both Cr^III and fluoride were present, as seen for complex 3, for which both factors additively quenched QTM and led to the observation of highly coercive magnetic hysteresis loops above 2 K. Herein, we propose a synthetic strategy to quench the QTM effects in lanthanide-based SMMs. Our strategy differs from existing methods, in which parameters such as magnetic coupling are difficult to control, and it is likely to have implications beyond the Dy^III SMMs studied herein.