Abstract
The employment of the metal-ion-assisted [1+1] Schiff-base condensation has led to complexes [Ln(L(N5))(Ph(3)SiO)(2)](PF(6)), 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.