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 Dy2 single-molecule magnets (SMMs) with a N1,N3-bis(3-methoxysalicylidene)diethylenetriamine (H2L) ligand. Fortunately, an air-stable Dy2 complex, [Dy2(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 [DyIII2(L)2(acac)2]·2CH2Cl2 (Dy2-acac; Hacac = acetylacetone) and [DyIII2(L)2(hfac)2] (Dy2-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 DyIII ions) bond lengths are also affected by the asymmetrical Htfc ligand. This indicated that the charge distribution of the coordination atoms around DyIII 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 (Ueff) 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 Dy2-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 DyIII ion. Ab initio calculations for 1 revealed that the charge distributions of the coordination atoms around DyIII 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 Dy2 SMM.