Asymmetric Binuclear Dysprosium Single-Molecule Magnets\nSynthesized from a Peripheral-Protected Nonequivalent Chelation Strategy

As\na kind of special binuclear dysprosium single-molecule magnet\n({Dy<sub>2</sub>}-SMM), the asymmetric {Dy<sub>2</sub>}-SMMs feature\ntwo Dy centers that locate in distinct ligand fields. Such an asymmetric\nstructure allows to locally modulate a special part of the magnet\nwhile maintaining original features of other parts, thus favoring\nmore delicate and targeted tuning of the single-ion anisotropy of\nthe Dy center and magnetic interactions between Dy centers. Yet, they\nare still rare due to the difficulty in synthesis. Herein, we report\nthree asymmetric {Dy<sub>2</sub>}-SMMs, namely, [Dy<sub>2</sub>(tmsa)(MeOsa)(H<sub>2</sub>O)<sub>2</sub>] (NO<sub>3</sub>)<sub>2</sub>·CH<sub>3</sub>OH (<b>1</b>), [Dy<sub>2</sub>(tmsa)(sa)<sub>2</sub>(H<sub>2</sub>O)]NO<sub>3</sub> (<b>2</b>), and [Dy<sub>2</sub>(tmsa)(Fsa)(H<sub>2</sub>O)<sub>2</sub>] (NO<sub>3</sub>)<sub>2</sub>·5H<sub>2</sub>O·CH<sub>3</sub>OH (<b>3</b>), where H<sub>3</sub>tmsa,\nHMeOsa, Hsa, and HFsa represent tris[(3-methoxysalicylidene)aminoethyl]amine,\n3-methoxysalicylaldehyde, salicylaldehyde, and 3-fluorinesalicylaldehyde,\nrespectively. They were prepared by a peripheral-protected nonequivalent\nchelating strategy, wherein a single tmsa<sup>3–</sup> ligand\nuses two distinct chelating pockets to encapsulate two Dy ions while\nproviding peripheral protection. Significantly, salicylaldehyde-based\nauxiliary ligands bearing different substituents, including HMeOsa,\nHsa, and HFsa, have fine-tuned the local ligand fields of the Dy ions\nsuccessfully. <b>1</b>–<b>3</b> thus show different\nslow magnetic relaxation behaviors, with the highest efficient energy\nbarriers (<i>U</i><sub>eff</sub>) of 79, 154, and 110 K,\nrespectively. This work provides an efficient strategy for the achievement\nof new asymmetric {Dy<sub>2</sub>}-SMMs and the fine-tuning of the\nrelaxation behavior of these special SMMs.