Quenching the Quantum Tunneling of Magnetization in Heterometallic Octanuclear {TM^III<inf>4</inf>Dy^III<inf>4</inf>} (TM=Co and Cr) Single-Molecule Magnets by Modification of the Bridging Ligands and Enhancing the Magnetic Exchange Coupling

Vignesh, KR, Langley, SK, Murray, KS and Rajaraman, G (2017) Quenching the Quantum Tunneling of Magnetization in Heterometallic Octanuclear {TM^III<inf>4</inf>Dy^III<inf>4</inf>} (TM=Co and Cr) Single-Molecule Magnets by Modification of the Bridging Ligands and Enhancing the Magnetic Exchange Coupling. Chemistry - A European Journal, 23 (7). pp. 1654-1666. ISSN 0947-6539

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Abstract

© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 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 CoIIIand DyIIIions. Single-crystal X-ray diffraction studies revealed molecular formulae of [CoIII4DyIII4(μ-OH)4(μ3-OMe)4{O2CC(CH3)3}4(tea)4(H2O)4]⋅4 H2O (1) and [CoIII4DyIII4(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, [CrIII4DyIII4(μ-F4)(μ3-OMe)1.25(μ3-OH)2.75(O2CPh)8(mdea)4] (3), are discussed and compared with a structurally similar complex, [CrIII4DyIII4(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 (Ueff) of 39.0, 55.0 and 10.4 cm−1respectively. Complex 1, conversely, did not exhibit slow relaxation of magnetisation above 2 K. To probe the variance in the observed Ueffvalues, 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 JDy–Dyas −1.6, 1.6 and 2.8 cm−1for complexes 1, 2 and 3, respectively, whereas the JDy–Crinteraction was estimated to be −1.8 cm−1for 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 CoIIIions with paramagnetic CrIIIled to CrIII⋅⋅⋅DyIIIcoupling, which resulted in quenching of QTM at low temperatures for complexes 3 and 4. The best example was found if both CrIIIand 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 DyIIISMMs studied herein.