Syntheses, structures and magnetic properties of the lanthanide complexes of the pyrimidyl-substituted nitronyl nitroxide radical

Abstract

Four 2p-4f Ln-radical complexes, [(NIT-2-Pm)Ln(hfac)]·0.5CH (NIT-2-Pm = 2-pyrimidyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide, hfac = hexafluoroacetylacetonato, Ln = Tb (1), Dy (2), Ho (3) and Er (4)), and four 4f-2p-4f Ln-radical-Ln complexes, [(μ-NIT-2-Pm)Ln(hfac)(HO)]·0.5CH (Ln = Tb (5), Dy (6), Ho (7) and Er (8)) have been synthesized and characterized structurally and magnetically. These compounds can be selectively obtained by controlling the reaction ratio of Ln(hfac)·2HO to the radical ligand NIT-2-Pm. The crystal structures show that in the former four complexes 1-4, the NIT-2-Pm radical acts as a terminal bidentate ligand chelating to one Ln ion, while in 5-8, the NIT-2-Pm acts as a bridging ligand linking two Ln ions to form a binuclear three-spin system. Magnetic studies revealed that complexes 1-4 and 6 show frequency-dependent ac magnetic susceptibilities, suggesting a possible single-molecule magnet behavior. To the best of our knowledge, complexes 3 and 4 are the first Ho-NIT and Er-NIT compounds showing slow magnetic relaxation. Compounds 5-8 represent a rare family of compounds showing the NIT bridged 4f-2p-4f three-spin motif, while complex 6 is a rare NIT bridged multinuclear lanthanide compound possessing SMM-like behaviour. Ab initio calculations were performed on all these complexes. The fitting of the magnetic susceptibilities of these compounds suggests weak antiferromagnetic coupling between the Ln and NIT radical in 1-8 and weak ferromagnetic Ln-Ln interactions in 5-8.

Lanthanide Complexes with a Tripodal Nitroxyl Radical Showing Strong Magnetic Coupling

“…For example, for hexafluoroacetylacetonate gadolinium complexes containing one acyclic nitroxyl radical, 47,73,74 the value of |j| varies between 9.6 and 12.5 cm −1 . For the analogous compounds comprising one nitronyl nitroxyl radical, 49,[53][54][55]60,61 exchange coupling is also lower (|j| = 0.77 ÷ 8.35 cm −1 ). For the Gd complex of the closest relative of Rad TEMPO (TEMPO = 2,2,6,6tetramethylpiperidin-1-oxyl), the metal−radical interaction is very small (j = 2.43 cm −1 ).…”

“…Purposeful control of the structural parameters of complexes is impossible without ascertainment the influence of the geometry of the coordination polyhedron on the nature and magnitude of the parameters of the magnetic exchange interaction between a lanthanide ion and a radical center. In the last years, a series of Ln single radical complexes of the mono-, [46][47][48] bi- [49][50][51][52][53][54][55][56] and tricoordinated 57,58 nitroxides have been reported. Practically, all of them comprise three diamagnetic once negatively charged ligands -hexafluoroacetylacetonates (hfac).…”

“…The use of a radical ligand introduces another important component: the magnetic interaction. In recent years, a series of Ln single-radical complexes of mono-, − bi-, − and tricoordinated , nitroxides have been reported. Practically, all of them comprise three diamagnetic anionic ligandshexafluoroacetylacetonates (hfac).…”

Lanthanide–Nitronyl Nitroxide Chains Derived from Multidentate Nitronyl Nitroxides

“…Using these multidentate nitronyl nitroxides, unique one-dimensional (1D) loop chains, {[Ln­(hfac) 3 ] 2 (Nit-Ph-3,5-btrz)} n [Ln III = Gd ( 1 ) and Dy ( 2 ); hfac – = hexafluoroacetylacetonate] and {[Dy­(hfac) 3 ] 2 (Nit-Ph-3,5-bPy)} n ( 3 ), have been obtained. These involve a rare 4f–2p–4f spin arrangement, ,, Both dysprosium complexes exhibit slow magnetic relaxation behavior.…”

“…Remarkable results have been obtained using this approach; for example, a simple binuclear terbium complex bridged by a N 2 • 3– radical exhibited magnetization blocking for a temperature ( T B ) as high as 20 K . Many Ln-radical single-molecule magnets (SMMs)/single-chain magnets (SCMs) take advantage of the nitronyl nitroxide radical because of its high stability and ease of chemical modification. , Of interest are nitronyl nitroxide ligands functionalized with additional donor groups of coordination preferences different from those of the aminoxyl O atoms, which may be assembled around metal ions in various arrangements with novel structural features. − The first Ln-radical SMM was a cyclic four-spin dysprosium­(III) dimer bridged by pyridyl-substituted nitronyl nitroxide . However, to date, in most reported nitronyl nitroxide–metal compounds, the functionalized radical ligand includes only one functional group such as pyridyl, imidazolyl, etc.…”