Modulating the Magnetic Dynamics of Five-Coordinate Dy^III Single-Molecule Magnets by the Substitution Effect on the Axial Ligand

Two five-coordinate mononuclear Dy-III single-molecule magnets [(Trapen(TMS))Dy(L)] (Trapen=tris(2-aminobenzyl) amine; TMS=SiMe3; L=OPMe3 1, OPEt3 2) have been synthesized by the reaction of [(Trapen(TMS))Dy(THF)] and OPMe3, OPEt3, respectively. The molecular structures of 1 and 2 were fully established by single crystal X-ray diffraction. The centre Dy-III ions of both complexes exhibit distorted triangular bipyramid geometries, varying with different neutral ligands L on the apex. The Trapen(TMS) ligand forms the pyramid base of the trigonal bipyramid. Subtle structural modifications of the axial O-ligand terminal substituent result in different dynamic magnetic behaviors. Magnetic data analysis reveals that 1 and 2 exhibit the characteristic behavior of SMM without a dc field, accompanying an unambiguous quantum tunneling of the magnetization. Field-induced slow magnetic relaxation was observed for 1 and 2 under an applied bias dc field of 1000 Oe, the relaxation energy barriers are 107 K and 88 K for 1 and 2, respectively. We have investigated the differences in magnetism and magnetic anisotropy between the two complexes by ab initio calculations.