Deciphering the Magnetostructural Criteria in DyIII and HoIII Macrocycle-Based Single-Molecule Magnets with Pseudo-D5h Symmetry: A Combined Single-Crystal Hysteresis and Theoretical Study

Abstract

The employment of the metal-ion-assisted [1+1] Schiff-base condensation has led to complexes [Ln(L^N5)(Ph₃SiO)₂](PF₆), where Ln^III = Dy^III (1-Dy) and Ho^III (1-Ho), with close-to-ideal D_5h local symmetry. For the Kramers Dy^III system, slow magnetization relaxation was observed up to 64 K yielding a sizable U_eff value of 963 K, whereas the non-Kramers Ho^III compound exhibited no out-of-phase signals at T > 2 K. Single-crystal magnetic hysteresis measurements uncovered the magnetization blockage of 1-Dy at T < 4 K with typical butterfly shaped loops, while open rectangular-shaped hysteresis loops were observed for 1-Ho below 0.2 K. Doping of the 1-Ho compound into a diamagnetic Y^III matrix unveiled the hyperfine-driven QTM steps reflected by staircase-like hysteresis loops for 1-Ho@Y. Detailed analysis through ab initio calculations has shed light on the low-lying energy levels of both compounds leading to well-isolated and pure ground states of m_J = ±15/2 and m_J = ±8 for 1-Dy and 1-Ho, respectively. The magnetization relaxation for 1-Dy advances through the third excited state, whereas the significant tunnel splitting (Δ_tun) of ∼0.15 cm^–1 in the ground state of 1-Ho has fostered the onset of fast tunneling relaxation.