Double and triple pyridine-N-oxide bridged dinuclear Dysprosium(III) dimers and single-molecule magnetic properties

Abstract

A series of dinuclear lanthanide complexes bridged by three pyridine-N-oxide (PyNO) ligands with formula [Dy2(hfac)6(4-cpyNO)3]·4H2O (Dy1), [Gd2(hfac)6(4-cpyNO)3]·4H2O (Gd1), [Dy2(hfac)6(4-mepyNO)3] (Dy2), [Gd2(hfac)6(4-mepyNO)3] (Gd2) (hfac− = hexafluoroacetylacetonate, 4-cpyNO = 4-cyanopyridine N-oxide, 4-mepyNO = 4-Methylpyridine N-oxide), and series of dinuclear lanthanide complexes bridged by two PyNO ligands with formula [Dy2(hfac)6(4-cpyNO)2] (Dy3), [Y2(hfac)6(4-cpyNO)2] (Y1), [Gd2(hfac)6(4-cpyNO)2] (Gd3), [Dy2(hfac)6(4-mepyNO)2] (Dy4), [Y2(hfac)6(4-mepyNO)2] (Y2), [Gd2(hfac)6(4-mepyNO)2] (Gd4), were structurally and magnetically characterized. The crystal structures revealed that pyridine-N-oxide serves as an effective bridge to link two Ln(III) centers, and periphery β-diketonate ligands complete the coordination sphere. The number of PyNO bridge can be rationally controlled by the stoichiometric ratio of reagents, which leads to different local coordination models and intramolecular Ln-Ln distances. Furthermore, elaborate modulation on the strength of magnetic coupling transmitted by PyNO bridge was successfully carried out by introducing electron-donating or -withdrawing substituents on the bridge ligand backbone. Magnetic measurements revealed that complex Dy3 displayed significant double zero-field slow magnetic relaxation process, while Dy1 and Dy2 showed field-induced SMM behavior. Significantly enhanced SMMs performance was observed in Dy4 with a hysteresis temperature of 2.5 K. The distinct slow magnetic relaxation behaviors were strongly related to their different individual Dy(III) ion magnetic anisotropy and Dy(III)-Dy(III) coupling, which were further confirmed by ab initio calculation.