Crystal packing effects on the magnetic slow relaxation of Tb(III)-nitronyl nitroxide radical cyclic dinuclear...

Four lanthanide-based nitronyl nitroxide radical cyclic molecular clusters of formula [Ln(hfac)3(NITPhPO(OEt)2)]2 (Ln(III) = Gd (1), Tb (2A and 2B), and Dy (3) and NITPhPO(OEt)2 = 4'-[2-(1-oxyl-3-4,4,5,5-tetramethylimidazoline)phenyl]diethoxylphosphine oxide) have been synthesized. Their X-ray structures have been solved and highlight two different crystal packings. For the particular case of the Tb(III) derivative, both of them can be obtained. In 2A, the molecules are well-isolated, while 2B shows short contacts between N-O radical groups. Static magnetic studies on the Gd(III) derivative (1) demonstrate that lanthanides and radicals are ferromagnetically coupled (J = 3.46 ± 0.04 cm(-1)). Dynamic magnetic studies show that both compounds 2A and 2B exhibit single molecule magnet behavior. A comparison of their magnetic behaviors highlights that the crystal packing has a crucial influence on the temperature range in which the SMM behavior is observed. In the case of the well-insulated Tb(III)-based derivative (2A), the SMM behavior is observed at higher temperatures and lower frequencies than for the one that presents close-packing between the molecules (2B). Comparisons are then possible only under an applied external magnetic field (0.2 T) with Δ = 27.5(6) and 21.0(5) K and τ0 = 2.64(25) × 10(-9) and 1.76(20) × 10(-9) s for 2A and 2B, respectively.