Modulating the magnetization dynamics in Ln–Cu-Rad hetero-tri-spin complexes through cis/trans coordination of nitronyl nitroxide radicals around the metal center

Abstract

Self-assembling the novel nitronyl nitroxide radical NIT-3Py-5-Ph (2-(5-phenyl-3-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) with Ln(hfac)₃·2H₂O and Cu(hfac)₂ (hfac = hexafluoroacetylacetonate) resulted in two heterometallic complexes with formula [LnCu(hfac)₅(NIT-3Py-5-Ph)₂] (Ln = Gd 1, Dy 2), in which two NIT-3Py-5-Ph radicals are coordinated with the Ln^III ion via their nitroxide units in the cis-arrangement manner and the Cu^II ion is ligated by the pyridyl N donors of the radicals. Interestingly, when the phenyl group of NIT-3Py-5-Ph was replaced with a p-pyridyl group, a new family of 2D networks, namely, {[Ln(hfac)₃][Cu(hfac)₂]₂(NIT-3Py-5-4Py)₂}_n (Ln = Gd 3, Tb 4, Dy 5; NIT-3Py-5-4Py = 2-(5-(4-pyridyl)-3-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) was obtained. In the 2D sheet, each NIT-3Py-5-4Py ligand serves as a μ₃-bridge to bind one Ln^III center by the aminoxyl moiety and two Cu^II ions through two pyridine groups to form a 2D structure. The Ln^III ion is coordinated by two NO units of two radicals in a trans configuration. DC magnetic measurements indicate that ferromagnetic Ln^III–NO exchange occurs in 1–5. AC studies reveal that 2 displays slow relaxation of the magnetization while no such magnetic relaxation is found in complex 5. The observed different magnetic relaxation behaviors of two Dy analogues could be attributed to the different coordination modes of NO groups of the radicals, and the coordination geometry of the Dy center is from C_2v in 2 to D_2d in 5.