Crystal Packing Effects on the Magnetic Slow Relaxation of Tb(III)-Nitronyl Nitroxide Radical Cyclic Dinuclear Clusters

Four lanthanide-based nitronyl nitroxide radical cyclic molecular clusters of formula [Ln­(hfac)₃(NITPhPO­(OEt)₂)]₂ (Ln^III = Gd (<b>1</b>), Tb (<b>2A</b> and <b>2B</b>), and Dy (<b>3</b>) 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 <b>2A</b>, the molecules are well-isolated, while <b>2B</b> shows short contacts between N–O radical groups. Static magnetic studies on the Gd^III derivative (<b>1</b>) demonstrate that lanthanides and radicals are ferromagnetically coupled (<i>J</i> = 3.46 ± 0.04 cm^–1). Dynamic magnetic studies show that both compounds <b>2A</b> and <b>2B</b> 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 (<b>2A</b>), the SMM behavior is observed at higher temperatures and lower frequencies than for the one that presents close-packing between the molecules (<b>2B</b>). 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 <b>2A</b> and <b>2B</b>, respectively