Multitopic organic linkers can provide a means to organize metal cluster nodes in a regular three-dimensional array. Herein, we show that isonicotinic acid N-oxide (HINO) serves as the linker in the formation of a metal–organic framework featuring Dy₂ single-molecule magnets as nodes. Importantly, guest solvent exchange induces a reversible single-crystal to single-crystal transformation between the phases Dy₂(INO)₄(NO₃)₂⋅2 solvent (solvent=DMF (Dy₂-DMF), CH₃CN (Dy₂-CH₃CN)), thereby switching the effective magnetic relaxation barrier (determined by ac magnetic susceptibility measurements) between a negligible value for Dy₂-DMF and 76 cm⁻¹ for Dy₂-CH₃CN. Ab initio calculations indicate that this difference arises not from a significant change in the intrinsic relaxation barrier of the Dy₂ nodes, but rather from a slowing of the relaxation rate of incoherent quantum tunneling of the magnetization by two orders of magnitude.

Multitopic organic linkers can provide a means to organize metal cluster nodes in a regular three-dimensional array. Herein, we show that isonicotinic acid N-oxide (HINO) serves as the linker in the formation of a metal–organic framework featuring Dy₂ single-molecule magnets as nodes. Importantly, guest solvent exchange induces a reversible single-crystal to single-crystal transformation between the phases Dy₂(INO)₄(NO₃)₂⋅2 solvent (solvent=DMF (Dy₂-DMF), CH₃CN (Dy₂-CH₃CN)), thereby switching the effective magnetic relaxation barrier (determined by ac magnetic susceptibility measurements) between a negligible value for Dy₂-DMF and 76 cm⁻¹ for Dy₂-CH₃CN. Ab initio calculations indicate that this difference arises not from a significant change in the intrinsic relaxation barrier of the Dy₂ nodes, but rather from a slowing of the relaxation rate of incoherent quantum tunneling of the magnetization by two orders of magnitude.

Four isostructural cobalt(II)–lanthanide(III) heterometallic metal–organic frameworks (MOFs), formulated as {[(CH₃)₂NH₂]₃[Co₃Ln(tda)₃(HCOO)₃] 2H₂O 0.75dmf}_n [Ln = Eu (Co-Eu), Gd (Co-Gd), Tb (Co-Tb), and Dy (Co-Dy); H₃tda = 1H-1,2,3-triazole-4,5-dicarboxylic acid] have been successfully synthesized and structurally characterized in detail. In these Co-Ln heterometallic MOFs, three neighboring Co^II ions are connected by HCOO^– anions to form triangular [Co₃(HCO₂)₃]³⁺ clusters. The equivalent triangular [Co₃(HCO₂)₃]³⁺ clusters are connected by six tda^3– anions to generate a 1D trigonal prismatic chain, which are further interconnected through Ln^III ions to give unique 3D (6,6)-connected nia nets with the Schläfli symbol of (4¹² 6³) (4⁹ 6⁶). The magnetic properties of the four isostructural Co-Ln heterometallic MOFs have also been studied.

A series of six-coordinate lanthanide complexes {(H₃O)[Ln(NA)₂]⋅H₂O}_n (H₂NA=5-hydroxynicotinic acid; Ln=Gd^III (1⋅Gd); Tb^III (2⋅Tb); Dy^III (3⋅Dy); Ho^III (4⋅Ho)) have been synthesized from aqueous solution and fully characterized. Slow relaxation of the magnetization was observed in 3⋅Dy. To suppress the quantum tunneling of the magnetization, 3⋅Dy diluted by diamagnetic Y^III ions was also synthesized and magnetically studied. Interesting butterfly-like hysteresis loops and an enhanced energy barrier for the slow relaxation of magnetization were observed in diluted 3⋅Dy. The energy barrier (Δ_τ) and pre-exponential factor (τ₀) of the diluted 3⋅Dy are 75 K and 4.21×10⁻⁵ s, respectively. This work illustrates a successful way to obtain low-coordination-number lanthanide complexes by a framework approach to show single-ion-magnet-like behavior.

A rare, robust microporous lanthanide metal–organic framework with 1D honeycomb-type channels is presented. Excellent adsorption capabilities for N₂, H₂, and CO₂ and significant selective sorption of CO₂ over N₂ and CH₄ were observed. Moreover, the guest-dependent luminescent behavior of these lanthanide materials shows a potential use for the sensing of small-molecule pollutants such as benzene and acetone.

The combination of the anisotropic Dy^III ion and organic radicals as spin carriers results in discrete and one-dimensional lanthanide–radical magnetic materials, namely, [Dy(hfac)₃(NITThienPh)₂] (1) and [Dy₂(hfac)₆(NITThienPh)₂]_n (2; hfac=hexafluoroacetylacetonate, NITThienPh=2-(5-phenyl-2-thienyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide). Linking monomeric 1 with the Dy^III ion leads to the formation of polymeric 2, and the transformation between them is chemically controllable and reversible. The characterization of both static and dynamic magnetic properties shows that the dominant intrachain exchange interaction is important to observe magnetic bistability in 2 rather than that in 1. Monomeric 1 exhibits paramagnetic behavior, whereas polymeric 2 shows the unusual coexistence of superparamagnetic and two-step field-induced metamagnetic behaviors. The antiferromagnetic ground state of 2 does not prevent the dynamic relaxation of the magnetization with the finite-sized effect in the lanthanide–radical system. Energy barriers to thermally activated relaxation for 2 are 53 and 98 K in the low- and high-temperature regimes, respectively. A hysteresis loop is observed with the coercive field of 99 Oe at 2 K.

Two new 3D lanthanide(III)copper(II) organic frameworks based on unusual {OLn₆} clusters have been successfully synthesized and fully characterized. Crystallographic studies showed that the {OLn₆} clusters acted as 12-connected nodes that were linked together by [CuL₂] (H₂L=3-hydroxypyrazine-2-carboxylic acid) moieties to construct an interesting 4,12-c net with the point symbol {4³⁶.6³⁰}{4⁴.6²}₃. Magnetic studies revealed that these two isostructural heterometallic frameworks exhibited different magnetic properties, depending on the different anisotropies of the lanthanide spin carriers: Gd-Cu showed a large magnetocaloric effect, with an entropy change (−ΔS_m) of 35.76 J kg⁻¹ K⁻¹, which is one of the largest values in high-dimensional complexes, whilst Dy-Cu exhibited slow relaxation of the magnetization at low temperatures.

A new porous metal-organic framework (MOF) {[CuL]·DMF·2H₂O}_n [1a, H₂L = 5-(4H-1,2,4-triazol-4-yl)benzene-1,3-dicarboxylic acid; DMF = N,N-dimethylformamide] with zigzag-shaped, narrow channels was prepared by solvothermal synthesis and structurally characterized. 1a has zigzag-shaped, open channels with micropore sizes of approximately 5.6 × 4.6 Ų. The (3,6)-connected network of 1a leads to an α-PbO₂ (apo) topology. The adsorption properties (N₂, H₂, and CO₂) of the desolvated sample 1c have been studied. The results show that 1c has a highly porous structure and a high adsorption capacity: the total capture of CO₂ at 1 bar and 298 K is 22.01 wt.-%, and the total H₂ uptake at 1 bar and 77 K is 1.46 wt.-%. This high adsorption capacity can be attributed to strong interactions between the small gas molecules and the irregularly open channels with their zigzag shape and their narrow pore size.

A robust porous metal–organic framework (MOF), [Co₃(ndc)(HCOO)₃(μ₃-OH)(H₂O)]_n (1) (H₂ndc=5-(4-pyridyl)-isophthalic acid), was synthesized with pronounced porosity. MOF 1 contained two different types of nanotubular channels, which exhibited a new topology with the Schlafli symbol of {4².6⁵.8³}{4².6}. MOF 1 showed high-efficiency for the selective sorption of small molecules, including the energy-correlated gases of H₂, CH₄, and CO₂, and environment-correlated steams of alcohols, acetone, and pyridine. Gas-sorption experiments indicated that MOF 1 exhibited not only a high CO₂-uptake (25.1 wt % at 273 K/1 bar) but also the impressive selective sorption of CO₂ over N₂ and CH₄. High H₂-uptake (2.04 wt % at 77 K/1 bar) was also observed. Moreover, systematic studies on the sorption of steams of organic molecules displayed excellent capacity for the sorption of the homologous series of alcohols (C₁–C₅), acetone, pyridine, as well as water.