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

Article abstract of DOI:10.1021/ic301394x

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.