Significantly Enhancing the Single-Molecule-Magnet Performance of a Dinuclear Dy(III) Complex by Utilizing an Asymmetric Auxiliary Organic Ligand

In this work, we employed an asymmetric auxiliary organic ligand (1,1,1-trifluoroacetylacetone, Htfac) to further regulate the magnetic relaxation behavior of series of Dy-2 single-molecule magnets (SMMs) with a N1,N3-bis(3-methoxysalicylidene)diethylenetriamine (H2L) ligand. Fortunately, an air-stable Dy-2 complex, [Dy-2(L)(2)(tfac)(2)] (1; Htfac = 1,1,1-trifluoroacetylacetone) was obtained at room temperature. A structural analysis indicated that some Dy-O or Dy-N bond lengths for 1 are not in the range of those for the complexes [D-2(III)(L)(2)(acac)(2)]center dot 2CH(2)Cl(2) (Dy-2-acac; Hacac = acetylacetone) and [Dy-2(III)(L)(2)(hfac)(2)] (Dy-2-hfac; Hhfac = hexafluoroacetylacetone), although the electron-withdrawing ability of tfac(-) is stronger than that of acac(-) but weaker than that of hfac(-). Additionally, the Dy-O3/O3a (the two O atoms bridged to Dy-III ions) bond lengths are also affected by the asymmetrical Htfc ligand. This indicated that the charge distribution of the coordination atoms around D-III has been modified in 1, which leads to the fine-tuning of the magnetic relaxation behavior of 1. Magnetic studies indicated that the values of effective energy barrier (U-eff) for 1 and its diluted sample (2) are 234.8(3) and 188.0(6) K, respectively, which are both higher than the reported value of 110 K for the complex Dy-2-hfac. More interestingly, 1 exhibits a magnetic hysteresis opening when T < 2.5 K at zero field, while the hysteresis loops of 2 are closed at a zero dc field. This discrepancy is due to the weak intramolecular exchange coupling in 2, which cannot overcome the QTM of the single Dy-III ion. Ab initio calculations for 1 revealed that the charge distributions of the coordination atoms around Dy-III ions were regulated and the intramolecular exchange coupling was indeed improved when the asymmetrical Htfc was employed as a ligand for the synthesis of this kind of Dy-2 SMM.

Automated summary

In this study, an asymmetric organic ligand was employed to regulate the magnetic relaxation behavior of Dy-2 single-molecule magnets. A structurally unique Dy-2 complex was successfully synthesized, exhibiting different bond lengths compared to other similar complexes. This led to the fine-tuning of the magnetic relaxation behavior and improved intramolecular exchange coupling.