Unprecedented ferromagnetic Gdnitronyl nitroxide coupling through a hydrogen bonding bridge

Unusual zig-zag-shaped Ln-radical coordination networks derived from a nitronyl nitroxide with two imidazole groups: single-crystal-to-single-crystal transformations and magnetic properties

Two unusual two-dimensional coordination networks, namely, {[Gd(hfac)3]3(NIT-Ph-3,5-bIm)2}·2.25C6H14 (1) and {[Dy(hfac)3]3(NIT-Ph-3,5-bIm)2}·2.5C6H14 (2), were obtained using the multidentate nitronyl nitroxide ligand containing two additional imidazole groups, NIT-Ph-3,5-bIm (NIT-Ph-3,5-bIm = 2-[3,5-bis(1-imidazole)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; hfac = hexafluoroacetylacetonate). In these Ln-radical complexes, the multidentate nitronyl nitroxide radical ligand served as a tridentate linker, binding three LnIII ions via one NO group and two imidazole units, resulting in unique zig-zag-shaped 2D networks with KIa topology. These coordination networks underwent a single-crystal-to-single-crystal transformation from a low-temperature non-centrosymmetric space group (P21) to a room-temperature centrosymmetric space group (P21/c) accompanied by the loss of helicity in chiral chains and an order-disorder transition of the LnIII ions. Direct-current magnetic susceptibility studies of the Gd derivative showed dominant ferromagnetic Gd-NO exchange. Dynamic magnetic studies of the Dy analogue displayed temperature-dependent nonzero out-of-phase signals, indicating slow magnetic relaxation behavior.

Syntheses, structures, and magnetic properties of the lanthanide complexes of imidazole-substituted nitronyl nitroxide biradicals

Two new bis-bidentate imidazole-substituted nitronyl nitroxide biradicals, BNITIm-C2 (BNITIm-C2 = 1,1'-(1,2-ethanediyl)bis(1H-imidazole-2-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide)) and BNITIm-C4 (BNITIm-C4 = 1,1'-(1,4-butanediyl)bis(1H-imidazole-2-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide)), and two series of lanthanide complexes, namely [(BNITIm-C2)Ln(NO₃)₃](MeOH) (Ln = Gd (1Gd) and Tb (2Tb)), (BNITIm-C2)Dy(NO₃)₃ (3Dy) and (BNITIm-C4)[Ln(hfac)₃]₂(C₇H₈)₂ (Ln = Gd (4Gd), Tb (5Tb) and Dy (6Dy), hfac = hexafluoroacetylacetonate), have been prepared and characterized structurally and magnetically. Single crystal X-ray crystallographic analyses revealed that complexes 1Gd-3Dy exhibit 1D chain structures where the Ln(NO₃)₃ units are bridged by the BNITIm-C2 bis-bidentate biradical, while complexes 4Gd-6Dy exhibit binuclear structures with two Ln(hfac)₃ units bridged by the BNITIm-C4 biradical. The bulky hfac anions prohibit the further coordination of Ln^III to another NIT ligand and the formation of a similar 1D chain structure. Due to the very long intra- and intermolecular distances of the spin centers, complexes 1Gd-3Dy can be magnetically regarded as an isolated 2p-4f-2p tri-spin system while complex 4Gd-6Dy can be regarded as an isolated 2p-4f bi-spin system. Magnetic analyses on the two Gd^III compounds revealed the ferromagnetic Gd^III-NIT interactions and antiferromagnetic NIT-NIT interactions through the Gd^III ion in 1Gd. Alternating-current (ac) magnetic susceptibility investigations revealed that complex 5Tb exhibits the typical SMM behavior under a zero dc field while complex 6Dy was proved to be a field-induced SMM. Ab initio calculations were performed on complexes 2Tb and 5Tb to understand their magnetic anisotropy together with their different magnetic dynamics.

Slow relaxation of magnetization in lanthanide-biradical complexes based on a functionalized nitronyl nitroxide biradical

Three novel lanthanide-biradical complexes {[Ln(hfac)3]2(mbisNITPyPh)(H2O)}{[Ln(hfac)3](mbisNITPyPh)}·CHCl3 (1-Gd; 2-Tb; 3-Dy) were successfully achieved by reacting the biradical mbisNITPyPh (5-(3-pyridyl)-1,3-bis(1-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene) with Ln(hfac)3·2H2O (hfac = hexafluoroacetylacetonate). These Ln-biradical complexes consist of two kinds of spin moieties, namely, dinuclear {[Ln(hfac)3]2(mbisNITPyPh)(H2O)} and mononuclear {[Ln(hfac)3](mbisNITPyPh)}, in which two adjacent dinuclear units are linked by intermolecular hydrogen bonds involving the uncoordinated nitroxide units and the coordinated water molecules of Ln ions, forming a cyclic tetranuclear structure unit. The magnetization study reveals that intramolecular Ln(iii)-coordinated NO ferromagnetic interactions are dominant in the present system. Moreover, the clear frequency dependence of ac magnetic susceptibilities of complex 3-Dy is indicative of slow relaxation of magnetization behavior, indicating its single-molecule magnet nature.

A loop chain and a three-dimensional network assembled from a multi-dentate nitronyl nitroxide radical and M(hfac)2 (M = CoII, CuII)

A multi-dentate nitronyl nitroxide radical Nit-Ph-3,5-bIm (Nit-Ph-3,5-bIm = 2-[3,5-bis(1-imidazole)-phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) reacted with Co(hfac)2·2H2O or Cu(hfac)2 to yield two novel 3d-radical complexes {[Co(hfac)2]2(Nit-Ph-3,5-bIm)}n1 and {[Cu(hfac)2]7(Nit-Ph-3,5-bIm)3}n2 (hfac- = hexafluoroacetylacetonate). In both compounds, the Nit-Ph-3,5-bIm radical ligand behaves as a tetradentate ligand to link four M(hfac)2 units in a μ4-η1:η1:η1:η1-coordination mode via its two NO groups and two N atoms of imidazole, generating a unique loop chain for 1 and a three-dimensional framework for 2. The magnetic studies revealed a strong antiferromagnetic Co-ON exchange interaction in 1 while a ferromagnetic Cu-ON interaction is observed in 2. The dc magnetic behaviors of two complexes are analyzed by means of appropriate magnetic models. Furthermore, ac magnetic susceptibilities of the Co complex reveal slow relaxation of magnetization.