Impact of Bulky Counteranions on the Magnetization Dynamics of Cationic D5h-Symmetrized Dy(III) Single-Ion Magnets

Abstract

The most prominent lanthanide ion is Dy(III), which has been used to design low-coordinate and highly symmetrical single-molecule magnets (SMMs) since it possesses larger magnetic moments and strong spin–orbit coupling. These Dy(III)-based SMMs exhibit remarkable blocking temperatures (TB) and large effective energy barriers (Ueff). The well-known highly symmetrical mononuclear Dy(III)-SMMs are the pseudo-D5h-symmetrized pentagonal bipyramidal (PBP) complexes. These pseudo-D5h-Dy(III) complexes tend to show variable Ueff ranging from 58 to 1261 cm–1 and TB ranging from 11 to 30 K, depending on the axial and equatorial ligands that help to maintain the high symmetry. However, when the [pseudo-D5h-Dy(III)]+ cation is isolated with different bulky counteranions, how their magnetization dynamics are going to be affected has not been explored yet. To explore this effect in detail, highly anisotropic [pseudo-D4h-Dy(III)]−, isotropic [FeCl4]−, and diamagnetic [NbCl4(THF)(O)]− anions were chosen. Using these anions, a homometallic [Dy(THF)5Cl2][DyCl2(THF)2] (1) complex and two heterometallic [Dy(THF)5Cl2][FeCl4] (2) and [Dy(THF)5Cl2][NbCl4(THF)(O)] (3) complexes were isolated by varying the above-mentioned counteranions surrounding the D5h-symmetric[Dy(THF)5Cl2]+ cation. Magnetic studies revealed that 1 does not show any SMM behavior, whereas 2 and 3 show zero-field SMM behavior with a Ueff of 21.3 K and 29.4 K, respectively. Ab initio CASSCF/RASSCF/RASSI-SO/SINGLE_ANISO calculations were performed further to investigate the magnetic behavior of 1–3. Computational results backed up the experimental observations and indicate that the misalignment of magnetic axes between two Dy centers could weaken the SMM behavior of 1, whereas the presence of isotropic and diamagnetic counteranions in 2 and 3 helps in achieving the SMM behavior.