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 (LnIII = 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 TbIII 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 GdIII 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 TbIII-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.