Axial Ligand as a Critical Factor for High-Performance Pentagonal Bipyramidal Dy(III) Single-Ion Magnets

The choice of axial ligands is of great importance for the construction of high-performance Dy-based single-molecule magnets (SMMs). Here, combining axial ligands Ph3SiO- (anion of triphenylsilanol) and 2,6-dichloro-4-nitro-PhO- (the anion of 2,6-dichloro-4-nitrophenol) with a neutral macrocyclic ligand 2,14dimethyl-3, 6,10,13,19-pentaazabicyclo [13.3.1 ] nonadeca1(19),2,13,15,17-pentaene (L2N5) generates two new pentagonal bipyramidal Dy(III) complexes [DyIII(L2N5) (X)2](BPh4) (X = Ph3SiO-, 1; 2,6-dichloro-4-nitro-PhO-, 2) with strong axial ligand fields. Magnetic characterizations show that 1 possesses a large energy barrier above 1000 K and a magnetic hysteresis up to 9 K, whereas 2 only displays field-induced peaks of alternating-current susceptibilities without the hysteresis loop, even though 2 has a similar coordination geometry with 1. Detailed Ab initio calculations indicate an apparent difference in the axial negative charge between both complexes, which is caused by the diverse electron donating properties of the axial ligands. The present work provides an efficient strategy to enhance the SMMs' properties, which highlights that the electron-donating property of the axial ligands is especially important for constructing the high-performance Dybased SMMs.