Lanthanide‐Based Metal‐Organic‐Frameworks for Proton Conduction and Magnetic Properties

High proton conduction in a series of three-dimensional lanthanide(III)-organic frameworks constructed by 2,5-dihydroxyterephthalic acid

In designing and preparing new proton-conductive materials, using cheap and easily available raw materials to efficiently prepare metal-organic frameworks (MOFs) with high stability and excellent proton conductivity is still a huge challenge. Herein, six lanthanide(III)-MOFs, {[Ln2(DHBDC)3(DMF)4](DMF)2}n [(Ln III = Pr III (1), Nd III (2), Sm III (3), Eu III (4), Gd III (5), Tb III (6))] with high stability were solvothermally synthesized utilizing 2,5-dihydroxy-1,4-benzenedicarboxylic acid (H4-DHBDC) as a bridging ligand. These isostructural MOFs all possess a three-dimensional framework and a dense H-bond network formed by the carbonyl groups in the framework, the non-coordinated hydroxyl groups, and the coordinated and free DMF molecules, which ensure efficient proton conduction. Their good water and thermal stability were verified using various characterization techniques (powder X-ray diffraction, thermogravimetric analysis, and infrared). Then, their proton conductivity was investigated in detail concerning temperature and relative humidity (RH). At 100 °C and 97 % RH, their optimum proton conductivity can reach up to 0.96 × 10-2, 0.67 × 10-2, 0.85 × 10-2, 1.03 × 10-2, 0.53 × 10-2, and 0.93 × 10-2 S/cm for 1-6, respectively. Finally, their proton-transport processes were thoroughly examined through detailed structural analyses, adsorption-property determinations, and activation energy values. Notably, these MOF materials have the advantages of easy preparation and relatively low cost, which paves the way for their practical applications.

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.

Proton -conducting lanthanide metal-organic frameworks: a multifunctional platform

Lanthanide metal-organic frameworks (LMOFs) have established themselves as promising proton-conducting materials among all types of porous coordination polymers and covalent organic frameworks. The structural diversity of LMOFs and high oxophilicity with a high coordination number of lanthanide ions make LMOFs a standout material for proton conduction. In the last few years, ample research efforts have been devoted to designing and developing proton-conducting lanthanide metal-organic frameworks (PCLMOFs). Some of the PCLMOFs have shown great potential with proton conductivity comparable to that of commercially used perfluorosulfonic acid (PFSA) polymers for proton-exchange membranes (PEMs) in fuel cells. At present, it is apparent that PCLMOFs are becoming a potential platform to explore other functional properties (e.g. fluorescence sensing, gas adsorption, molecular magnetism, impedance sensing, ferroelectricity, and nonlinear optics). The intrinsic structural features of PCLMOFs inevitably bring the opportunity to introduce the multifunctional character of such materials. Therefore, any scope for additional functional properties must be investigated for this class of material. In this article, we concisely discuss the design strategy and structural features of some multifunctional PCLMOFs. Furthermore, multifunctional properties of some excellent PCLMOFs are reviewed. In addition, the prospect of PCLMOFs is briefly discussed in the context of real-world material applications.

Impact of Synthetic Variables on the Structural Diversity of TbIII-Carboxylate Frameworks: Gas Adsorption, Magnetism, and Organocatalysis Investigations

In this work, three unique TbIII-carboxylate frameworks with the formula {[Tb2(OH)2(H2O)2(abtc)]·2H2O}n (1), {[Tb2(abtc)1.5(H2O)3(DMA)]·H2O}n (2) and {[Tb3(abtc)2.5(H2O)4]·H3O}n (3), each displaying structural variations, have been successfully synthesized by the solvothermal reactions of Tb(NO3)3·6H2O with the azo-containing ligand 3,3',5,5'-azobenzene tetracarboxylic acid (H4abtc) under varying conditions. Detailed single-crystal X-ray diffraction (SC-XRD) analysis manifested a remarkable diversity in these structures, demonstrating various coordination patterns of TbIII-metal nodes with the carboxylate groups of the organic linker, which contributed to the generation of intricate three-dimensional (3D) coordination networks with remarkable chemical resistance. Furthermore, frameworks 2 and 3, characterized by porous networks containing two and three independent TbIII-metal nodes, respectively, were both demonstrated to be efficient heterogeneous catalysts toward the cyanosilylation of imines under mild conditions with excellent reusability. In addition, direct current (Dc) magnetic susceptibility measurements conducted on frameworks 1, 2, and 3 indicated that there were obvious antiferromagnetic interactions among the TbIII-metal nodes, which suggests the involvement of intricate intra- and intertrimer exchange channels, adding another fascinating dimension to their physical properties.

Base-Directed Formation of Isostructural Lanthanide-Sulfate-Glutarate Coordination Polymers with Photoluminescence

A series of five isostructural 3D lanthanide-based coordination polymers [LnIII2(H2O)6(glu)(SO4)2]n [Ln = Pr(1), Nd(2), Sm(3), Eu(4), and Gd(5)] was effortlessly obtained within a few minutes via the microwave-heating method. The employment of auxiliary bases, that is, sodium hydroxide, 4,4'-bipyridine, and 1,4-diazabicyclo[2.2.2]octane, led to the formation of the title complex, whereas base-free synthesis yielded a three-dimensional inorganic coordination polymer, [Ln2(H2O)4(SO4)3]n·nH2O, Ln = Nd (2a). The robustness of the synthetic method was illustrated as both microwave-heating and conventional hydrothermal techniques also enabled the formation of a high-crystalline phase-pure complex 1-5. In the structure of 1-5, glutarato (glu2-) and sulfato ligands link dinuclear Ln(III) building units into three-dimensional frames. The glu2- ligands act as tethering linkers, expanding the structure into a neutral 3D coordination network. Hydrogen bonds were found to be the predominant intermolecular interactions in the crystal structures. Photoluminescence of the complex 1-5 was studied.

Variable Dimensionality of Europium(III) and Terbium(III) Coordination Compounds with a Flexible Hexacarboxylate Ligand

A reaction between 4,4',4″-(benzene-1,3,5-triyltris(oxy))triphthalic acid (H₆L) and lanthanide(III) nitrates (Ln = Eu³⁺, Tb³⁺) in water under the same conditions gave a molecular coordination compound [Tb(H_4.5L)₂(H₂O)₅]∙6H₂O in the case of terbium(III) and a one-dimensional linear coordination polymer {[Eu₂(H₃L)₂(H₂O)₆]∙8H₂O}_n in the case of europium(III). The crystal structures of both compounds were established by single-crystal X-ray diffraction, and they were further characterized by powder X-ray diffraction, thermogravimetric analysis and infrared spectroscopy. The compounds demonstrated characteristic lanthanide-centered photoluminescence. The lanthanide-dependent dimensionality of the synthesized compounds, which are the first examples of the coordination compounds of hexacarboxylic acid H₆L demonstrates its potential as a linker for new coordination polymers.

A new mode of luminescence in lanthanide oxalates metal–organic frameworks

Two lanthanide metal–organic frameworks [Ln-MOFs, Ln = Eu(III), Tb(III)] composed of oxalic acid and Ln building units were hydrothermally synthesized and fully characterized by powder X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscope, and energy-dispersive X-ray spectroscopy. Furthermore, their magnetic susceptibility measurements were obtained using SQUID based vibrating sample magnetometer (MPMS 3, Quantum Design). Both Ln-MOFs exhibited highly efficient luminescent property. Solid-state photoluminescence (PL) measurements revealed phosphorescence emission bands of Eu-MOF and Tb-MOF centered at 618 nm (red emission) and 550 nm (green emission) upon excitation at 396 nm and 285 nm, respectively. Eu-MOF and Tb-MOF displayed a phosphorescence quantum yield of 53% and 40%, respectively. Time-resolved PL analyses showed very long lifetime values, at 600 and 1065 ± 1 µs for Eu-MOF and Tb-MOF, respectively. Calculations performed by density functional theory indicated a charge transfer form metal centres to the ligand which was in good agreement with the experimental studies. Therefore, this new mode of highly photoluminescent MOF materials is studied for the first time which paves the way for better understanding of these systems for potential applications.

Dysprosium(III) Metal-Organic Framework Demonstrating Ratiometric Luminescent Detection of pH, Magnetism, and Proton Conduction

A multifunctional metal-organic framework, (Hdmbpy)[Dy(H₂dobdc)₂(H₂O)]·3H₂O (Dy-MOF, H₄dobdc = 2,5-dihydroxyterephthalic acid, dmbpy = 4,4'-dimethyl-2,2'-bipyridine), was synthesized and structurally characterized. The metal center Dy^III is connected by four carboxyl groups to form the [Dy₂(CO₂)₄] binuclear nodes, which are further interconnected by eight separate H₂dobdc^2- ligands to form a three-dimensional (3D) framework including hydrophilic triangular channels and abundant hydrogen-bonding networks. Dy-MOF has good stability in aqueous solution as well as in harsh acidic or alkaline solutions (pH range: 2.0-12.0). Furthermore, the luminescence signal of Dy-MOF undergoes a visualized color change as the acidity of the solution alters, which is the typical behavior of pH ratiometric probe. At a 100% relative humidity, Dy-MOF exhibits a high proton conductivity σ (1.70 × 10^-4 S cm^-1 at 303 K; 1.20 × 10^-3 S cm^-1 at 343 K) based on the proton hopping mechanism, which can be classified as a superionic conductor with σ exceeding 10^-4 S cm^-1. Additionally, the ferromagnetic interaction and magnetic relaxation behavior are simultaneously achieved in Dy-MOF. Herein, the combination of luminescence sensing, magnetism, and proton conduction in a single-phase 3D MOF may offer great potential applications in smart multitasking devices.