Deciphering the Magnetostructural Criteria in Dy$^{III}$ and Ho$^{III}$ Macrocycle-Based Single-Molecule Magnets with Pseudo- D$_{5h}$ Symmetry: A Combined Single-Crystal Hysteresis and Theoretical Study

The employment of the metal-ion-assisted [1+1] Schiff-base condensation has led to complexes [Ln(LN5)(Ph3SiO)2](PF6), where LnIII = DyIII (1-Dy) and HoIII (1-Ho), with close-to-ideal D5h local symmetry. For the Kramers DyIII system, slow magnetization relaxation was observed up to 64 K yielding a sizable Ueff value of 963 K, whereas the non-Kramers HoIII 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 YIII 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 mJ = ±15/2 and mJ = ±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 (Δtun) of ∼0.15 cm–1 in the ground state of 1-Ho has fostered the onset of fast tunneling relaxation.