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

Lanthanide-Based Metal-Organic-Frameworks for Proton Conduction and Magnetic Properties Soumava Biswas*[a] and Petr Neugebauer*[a] The coexistence of proton conductivity and magnetic properties in metal organic frameworks (MOFs) is simply an attractive prospect to design and create multifunctional molecular materials. However, the simultaneity of interesting magnetic properties (e.g. magnetocaloric effect, slow magnetic relaxation or magnetic ordering) and proton conduction in a MOF is very implausible to be observed, as chemical and structural necessities are absolutely poles apart for either of the properties. In this context, lanthanide-based metal-organic frameworks (LMOFs) have emerged as an intriguing class of materials to observe such coexistence in few cases. In this perspective, the advantages of combining lanthanide ions and organic acids as ligands to construct a proton-conductive magnetic metal organic framework are briefly discussed. The current status of this specific research area regarding some fascinating LMOFs exhibiting proton conductivity and magnetic properties is reviewed in detail. Also, a future perspective of such materials is discussed. 1. Introduction Universal effort has been dedicated to the field of MOF research for the last two decades. The versatility of this material has been justified unanimously in diverse research areas (eg. magnetic properties, gas storage, transport phenomenon, device fabrications, sensing, catalysis etc.).[1] In recent years, researchers are focusing more on introducing multifunctionality in the MOF material by modulating their structural features with different design strategies.[2] So in this regard, optimizing magnetic metal organic framework for proton conduction could become an attractive strategy to explore the multifunctional behavior of MOF materials. It has been established that MOFs are the versatile platform for studying the proton conduction behavior.[3] There are ranges of MOFs showing unique protonconducting behavior that gains pervasive attention due to their potential applications in fuel cell technologies. Simply having high structural integrity and tunable chemical functionality make the difference for MOF as a proton conductor in a longterm race. On the other hand, investigating magnetic properties for MOFs is always a tempting task as exchange coupled metal centers can induce long-range magnetic ordering. Also, magnetically anisotropic metal centers along with the inherent porosity of a MOF can create porous magnets. There are number of magnetic MOFs in the literatures showing single molecule magnet (SMM) behavior, slow magnetic relaxation, spin glass behavior, magnetocaloric effect, spin canting etc.[4] Now putting together two dissimilar properties (proton conductivity and magnetic properties) in a single framework structure must be a challenging job to perform. The combination of molecular magnetic properties and proton conduction in the same MOF offers the prospects to create the next generation of multifunctional magnetic molecular materials for molecular electronics and spin-protonics applications.[5] There are few worthy attempts was made in the recent past in this regard. The concurrence of interesting magnetic properties and proton conduction in a single molecular system is unlikely to be observed, as they have different chemical and structural requirements. Nevertheless, the introduction of lanthanide ions as a metal node in a framework might decipher the issue to some extent. The superlative magneto-chemical nature of lanthanide ions enforces predominant interest in the design and creation of lanthanide-based molecular magnets. The unquenched orbital angular momentum, crystal field effect and single-ion anisotropy of lanthanide ions significantly contribute to the energy barrier and relaxation time of the lanthanide based molecular magnets.[6] Gadolinium based magnetic frameworks are known for magnetocaloric materials which are explored for their potential applications in magnetic refrigeration.[7] Lanthanide ions are also useful as metal nodes for creating proton conducting MOFs.[8] The rich oxophilic nature and high coordination number of lanthanide ions not only facilitate the release of acidic proton from coordinated water but also induce the formation of water filled nanochannels for proton transport in the framework.[8] In this article, we have reviewed the present status and future prospect of lanthanide-based MOFs with proton conduction behavior and magnetic properties. [a] Dr. S. Biswas, Dr. P. Neugebauer CEITEC BUT, Brno University of Technology Purkyňova 123, Brno 61200, Czech Republic E-mail: [email protected] [email protected] http://spectroscopy.ceitec.cz/ © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Minireviews doi.org/10.1002/ejic.202100686 4610Eur. J. Inorg. Chem. 2021, 4610 –4618 © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Montag, 29.11.2021 2145 / 222798 [S. 4610/4618] 1 2. Methodologies for Combining Proton Conduction and Magnetic Properties Usage of carboxylic acid based ligands for the construction of proton conductive LMOFs must be a beneficial one. Because of the various coordination mode of such ligands, the resultant framework could have stability and robustness with desired architecture favorable for proton conductivity. Also, the carboxylate moiety can surely act as a building unit for the formation of hydrogen bonded networks in nanochannels (or cavity) of the frameworks.[9] Lattice water or other polar guest molecules could easily interact with the carboxylate moiety via hydrogen bonding to build the proton conducting pathways in the framework. In addition, free carboxylic groups will be the potential proton source for conducting pathways.[9] Similar arguments are also applicable for phosphonate-based ligands in terms of their significant chemical and structural features. There are numerous metal phosphonate frameworks with exciting structures and functionalities, documented in the literatures.[10] Also, it has been established that the metal phosphonate frameworks are one of the most promising candidates for exploring proton conductivity. Robustness and stability of metal phosphonate are manifested again and again through the extensive measurement condition for proton conductivity. In addition, phosphonates can bind to the metal ions in a higher number compared to carboxylates. In this scenario, introducing lanthanides for constructing proton conductive magnetic MOF must be a rational approach to combining proton conduction and magnetic properties. The higher coordination number for lanthanide ions enables the accumulation of coordinated water molecules and forming a hydrogen-bonded network. Also, the oxophilic nature of lanthanide ions could ease the release of the proton from coordinated acidic water as well.[8] Because of their exceptional magneto-chemical natures, lanthanides are also predominantly used to construct attractive molecule-based magnets in recent times. The preeminence of lanthanide-based SMMs are not only proved in discrete molecular systems but also in higher dimensional space (i.e. MOFs and coordination polymers).[11] Among the lanthanides, dysprosium and terbium are used every so often for their intrinsic magnetic anisotropy and the increased number of unpaired f electrons of respective ions, leading to the high anisotropic energy barrier and long spin relaxation time.[12] The magnetically isotropic nature of gadolinium ions makes them an absolute choice for fabricating magnetocaloric materials for ultra-low temperature refrigeration.[7,13] Magnetic refrigeration (or magnetocaloric effect) is a thermodynamic phenomenon generally characterized by the isothermal magnetic entropy change followed by an adiabatic decrease in temperature for an applied magnetic field.[14] In this regard, a number of LMOFs with rousing magnetic properties has been reported so far.[11] However, surprisingly little efforts have been devoted to study proton conductivity and magnetism together in LMOFs. It is worth mentioning that only two proton conductive lanthanidebased magnetic MOFs with pure phosphonate ligand is presented, whereas very few reports are there for sulfonatecarboxylate and phosphonate-carboxylate based ligand to date. The basic design strategy of such molecular material involves creating extended lanthanide coordination networks along with dense hydrogen-bonded water (or polar solvent) filled nanochannels for proton migrating pathways. (Scheme 1). The choice of ligands is crucial to governing the design and synthetic strategy for LMOFs with proton conductivity and magnetic properties. Chosen ligands should offer acidic hydrogen and take part in the hydrogen bonding within the resultant framework. Organic ligands with multiple acidic groups, hydroxyl groups and amines are the potentially good selection. Other hands, the choice of the lanthanides will depend on the desired magnetic properties for the resultant framework. Surprisingly, most of the reported proton conductive LMOFs are having diamagnetic metal nodes of lanthanum (III).[3,10] Some of the ligands are presented in Scheme 2. In this review, the discussed LMOFs are classified according to the types of ligands Soumava Biswas obtained his PhD in chemistry from IISER Bhopal, India under the supervision of Professor Sanjit Konar in 2017. Then he joined the research group of Prof. Masahiro Yamashita, Tohoku University, Japan as a JSPS postdoctoral fellow. At present, he is working as a postdoctoral researcher with Prof. Petr Neugebauer at CEITEC BUT, Czech Republic. His research interest is focused on the studies of single molecule magnets and spin crossover complexes. Petr Neugebauer obtained his PhD degree with Marie Curie fellowship from Physics of Condensed Matter and Radiation at Grenoble High Magnetic Field Laboratory (GHMFL) and Grenoble University, France, in 2010. After his two years postdoctoral stay at the Center for Biomolecular Magnetic Resonance, Goethe University, Germany, he joined the research group of Prof. Joris van Slageren at University of Stuttgart, Germany. He is the group leader and founder of the Magneto-Optical and THz Spectroscopy (MOTeS) group at the CEITEC BUT, Czech Republic. The group focuses on the development of High Frequency Electron Paramagnetic Resonance (HFEPR) spectroscopy, especially frequency rapid scan above 100 GHz (ERC starting grant), HFEPR applications to molecular magnetism, thin films and molecular materials. Minireviews doi.org/10.1002/ejic.202100686 4611Eur. J. Inorg. Chem. 2021, 4610 –4618 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Montag, 29.11.2021 2145 / 222798 [S. 4611/4618] 1 (e.g. carboxylate, phosphonate, phosphonate-carboxylate etc.) (Table 1). 3. Carboxylate-based LMOFs The first heterometallic magnetic MOF, [Gd6Cu24Na12(ANMA)12(μ3-OH)24(μ5-O)6Cl(H2O)42](NO3)13Cl4(H2O)48 having proton conductivity was reported by Kong and coworkers in 2011.[15] This 3D MOF, featuring a Gd6Cu24Na12 core was obtained via microwave irradiation by using L-alanine-Nmonoacetic acid (H2ANMA) as a ligand (Figure 1). The direct current (dc) magnetic measurements reveal the presence of ferromagnetic interaction within the heterometallic core at low temperature region (below 100 K). The conductance value (σ) of 1.36×106Scm1was obtained from alternating current (ac) impedance measurements at 318 K (Figure 1). The authors proposed that the conductivity originates from the proton transfer from the guest water molecules which is acting as suppliers of protons also. The conductance value decreases gradually with increase in temperatures and reaches 0.04× 106Scm1at 358 K. This decrease in conductivity is due to the removal of guest water molecules from the framework. Konar and coworker presented the first example of lanthanide-based coordination polymers, with the coexistence of cryogenic magnetic refrigeration or slow magnetic relaxation and proton conduction for a molecular system.[16] Three isostructural 2D layered coordination polymers {[Ln2(L1)3(H2O)2]n·2nCH3OH)·2nH2O} (Ln=Gd, Tb and Dy) have been synthesized by using cyclobutane-1,1-dicarboxylic acid (H2L1). In the structures, lattice water and methanol molecules form hydrogen bonding with metal coordinated water through the interlayer space. Helical water channel between two adjacent 1D chains was observed in the structures (Figure 2). Investigation of magnetic property confirm the presences of Scheme 1. Representation of lanthanide MOF with hydrogen bonding for proton conduction. Scheme 2. Some of the ligands to prepare lanthanide based MOFs with proton conductivity and magnetic properties. Figure 1. (a) Crystal structure of [Gd6Cu24Na12] cation. (b) Polyhedral view of the 3D framework and (c) Nyquist plots for the MOF over the temperature range of 318–368 K. Reproduced from Ref. [15]. Copyright (2011), with permission from American Chemical Society. Minireviews doi.org/10.1002/ejic.202100686 4612Eur. J. Inorg. Chem. 2021, 4610 – 4618 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Montag, 29.11.2021 2145 / 222798 [S. 4612/4618] 1 weak ferromagnetic interactions and significant magnetocaloric effect for the gadolinium analogue whereas the dysprosium one shows slow magnetic relaxation (Figure 3). The Impedance analyses revealed that all the coordination polymers show humidity-dependent proton conductivity at higher temperatures. For the dysprosium analogue, the highest conductivity was found to be 1.1×103Scm1at 353 K and 95% RH (Figure 3). Several factors have been contributed to the higher temperature conductivity such as 1) strong and continuous Hbonding interaction;[17] 2) acidty of metal coordinated water molecules; 3) thermal activation of the coordinated water molecules, which facilitates the proton transfer through the vehicle mechanism.[18] Consequently, another two 3D lanthanide (gadolinium and dysprosium) MOFs are explored by Konar and coworker for magnetic properties and proton conductivity.[19] The 3D structure of gadolinium based MOF, {[Gd(L2)(Ox)(H2O)]n·3H2O} (H2L2=mucic acid; H2Ox=oxalic acid) was made up of rectangular metallocycles SBU. There are two types of 1D nanochannels present in the framework that are filled with lattice Table 1. Summary of lanthanide-based MOFs with proton conductivity and magnetic properties.[a] MOFs Structure σ [S cm1] Ea [eV] Magnetic interactions Energy barrier [K] and τ0[s] ΔSM [J kg1K1] Ref. [Gd6Cu24Na12(ANMA)12(μ3-OH)24 (μ5-O)6Cl(H2O)42](NO3)13Cl4(H2O)48 3D 1.36×106 (45°C) Ferromagnetic 15 {[Gd2(L1)3(H2O)2]n·2nCH3OH)·2nH2O} 2D 1.5×105 (75°C, 95% RH) 0.52 Ferromagnetic 32.8 (ΔH=7 T at 4 K) 16 {[Tb2(L1)3(H2O)2]n·2nCH3OH)· 2nH2O} 2D 2.07×104 (80°C, 95% RH) 0.86 Antiferromagnetic 16 {[Dy2(L1)3(H2O)2]n·2nCH3OH)·2nH2O} 2D 1.1×103 (80°C, 95% RH) 0.84 Antiferromagnetic 19.2 and 2.5×106 16 {[Gd(L2)(Ox)(H2O)]n·3H2O} 3D 4.7×104 (80°C, 95% RH) 0.88 Antiferromagnetic 30.6 (ΔH=7 T at 3 K) 19 {[Dy(L2)(Ox)(H2O)]n·1.5H2O} 3D 9.06×105 (80°C, 95% RH) 0.70 Antiferromagnetic 36.5 and 73× 106 19 {[Gd2(CO3)(Ox)2(H2O)2]·3H2O}n3D 1.98×103 (150°C) 0.27 Antiferromagnetic 58.5 (ΔH=7 T at 2 K) 20 [H4BAPEN]1.5 ·[Gd2(HEDP)3]· 5H2O 1D 6.47×104 (90°C, 100% RH) Antiferromagnetic 21 [H4BAPEN]0.5 ·[GdLi-(HHEDP)2(H2O)2]·2H2O 2D 3.75×104 (95°C, 100% RH) Antiferromagnetic 21 (Dy(H2O)3[Ru2(Hamdp)2][Ru2(Hamdp)2(H2O)2]- [Ru2(Hamdp)(amdp)]0.5 ·12H2O 2D 1.44×106 (55°C, 95% RH) 0.77 Antiferromagnetic 22 (Yb(H2O)3[Ru2(Hamdp)2][Ru2(Hamdp)2(H2O)2]- [Ru2(Hamdp)(amdp)]0.5 ·15H2O 3D 0.93×105 (55°C, 95% RH) 0.64 Antiferromagnetic 22 [Dy (L3)(H2O)2]n2D 1.13×106 (80°C, 95% RH) 0.38 Ferromagnetic 72 and 3.05×107 23 [Er (L3)(H2O)2]n2D 2.73×103 (80°C, 95% RH) 0.42 Antiferromagnetic 38.42 and 4.60×108 23 [{Gd2(L3)2(H2O)3}·H2O]n2D 6.27 ×106 (80°C, 95% RH) 0.40 Antiferromagnetic 49.29 (ΔH=7 T at 3 K) 23 {[Gd4(OH)4(L4)2(H2O)8]·4.6H2O·1.4 CH3CN}n3D 2.02×106 (80°C, 95% RH) 0.27 Antiferromagnetic 34.6 (ΔH=7 T at 3 K) 24 {[Dy4(OH)4(L4)2(H2O)8]·4.6H2O·1.4 CH3CN}n3D 2.96× 106 (80°C, 95% RH) 0.25 Antiferromagnetic 30.40 and 2.47×107 24 {[Ho4(OH)4(L4)2(H2O)8]·4.6H2O·1.4 CH3CN}n3D 4.56×103 (80°C, 95% RH) 0.38 Antiferromagnetic 24 {[Er4(OH)4(L4)2(H2O)8]·4.6H2O ·1.4 CH3CN}n3D 6.59×103 (80°C, 95% RH) 0.32 Antiferromagnetic 24 [a] Abbreviations for the ligand: H2ANMA: L-alanine-N-monoacetic acid; H2L1: cyclobutane-1,1-dicarboxylic acid; H2L2: mucic acid; H2Ox: oxalic acid; BAPEN: 1,2-bis(3-aminopropylamino)ethane; H4HEDP: 1-hydroxyethylidenediphosphonic acid; H5amdp: 1-ammoniummethylenediphosphonic acid; H3L3: phosphonoacetic acid; Na2H2L4: disodium-2,2’-disulfonate-4,4’-oxydibenzoic acid. Figure 2. Illustration of the continuous helical water channel between two adjacent 1D chains in the layered framework. Reproduced from Ref. [16]. Minireviews doi.org/10.1002/ejic.202100686 4613Eur. J. Inorg. Chem. 2021, 4610 – 4618 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Montag, 29.11.2021 2145 / 222798 [S. 4613/4618] 1 water. The prominent hydrogen bonding interaction between lattice water and metal coordinated water in the channels leads to a feasible proton-conducting pathway in this MOF (Figure 4). The dysprosium based 3D framework, {[Dy- (L2)(Ox)(H2O)]n·1.5H2O} was originated from carboxylate bridged dimeric criss-cross-type building blocks. A significant magnetic entropy change (ΔSM=30.6 Jkg1K1for ΔH=7 T at 3 K) was observed for the gadolinium-based framework (Figure 4). SMM like behavior was found for the dysprosiumbased framework. Impedance analysis shows that the proton conductivity of both frameworks increase with a rise in temperature and reaches up to the maximum value of 4.7 ×104Scm1 for gadolinium one and 9.06×105Scm1for dysprosium one at high temperature (>75°C) and relative humidity (RH; 95%) (Figure 4). Another worth mentioning material is a gadolinium carbonate cluster-based MOF, {[Gd2(CO3)(Ox)2(H2O)2]·3H2O}nfor high proton transport and magnetic refrigeration, reported by Zheng and coworkers.[20] This MOF has been made up with the self-assembly of gadolinium carbonate and oxalate under hydrothermal conditions. Metal coordinated water molecules and the oxalate produce the hydrogen-bonded conducting pathway in a one-dimensional pore channel along the a-axis in the framework (Figure 5). Strong hydrogen-bonding and good thermal stability of the framework leads to the highest proton conductivity of 1.98×103Scm1at T =150°C for anhydrous condition (Figure 5). Magnetic investigations reveal an excellent magnetocaloric effect with a magnetic entropy change of 58.5 Jkg1K1at 2 K for a field change of 7 T (Figure 5). This magnetocaloric effect is the largest one reported so far for any proton-conducting MOF. 4. Phosphonate -based LMOFs Recently, Wang et al. reported two gadolinium-based proton conducting MOFs, [H4BAPEN]1.5 ·[Gd2(HEDP)3]·5H2O and [H4BAPEN]0.5 ·[GdLi-(HHEDP)2(H2O)2]·2H2O with a diphosphonate ligand (HEDP=1-hydroxyethylidenediphosphonate).[21] In the structures, protonated aliphatic organic amine 1,2-bis(3-aminopropylamino) ethane (BAPEN) acts as counter cation. 1 D chains of lanthanide centers, connected with diphosphonate are observed. Those 1 D chains are connected by the linkage of lithium ions to form 2D layered heterometallic diphosphonate networks in the other framework (Figure 6). Several extensive hydrogen bonding interactions have been established among H4BAPEN, phosphonate moieties, and between the adsorbed water molecules (or lattice water) in the frameworks. For these frameworks, the proton conductivities increase with an increase in temperature in saturated water vapour to reach the maximum values of 6.47×104and 3.75×104Scm1at 90°C and 95°C, respectively (Figure 6). Direct current (dc) magnetic Figure 3. (a) Magnetic entropy change (ΔSM) calculated by using the magnetization data of gadolinium analogue. (b) Temperature dependence of the out-of-phase AC susceptibility component for dysprosium analogue and nyquist plot for proton conductivity of gadolinium analogue and dysprosium analogue (c and d respectively) (inset: Arrhenius plot for the activation energy). Reproduced from Ref. [16]. Figure 4. (a) Representation of lattice water molecules in the channel (along a-axis) of {[Gd(L2)(Ox)(H2O)]n·3H2O}, (b) ΔSMcalculated by using the magnetization data at different fields and temperatures. (c) Nyquist plot for proton conduction for {[Gd(L2)(Ox)(H2O)]n·3H2O} at 95 % RH and 80°C. Reproduced from Ref. [19]. Copyright (2017), with permission from American Chemical Society. Minireviews doi.org/10.1002/ejic.202100686 4614Eur. J. Inorg. Chem. 2021, 4610 – 4618 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Montag, 29.11.2021 2145 / 222798 [S. 4614/4618] 1 measurement reveals antiferromagnetic interactions between intra-chain gadolinium centres (Figure 6). Zheng and coworkers reported two exciting example of heterometallic phosphonate coordination networks, (Dy- (H2O)3[Ru2(Hamdp)2][Ru2(Hamdp)2(H2O)2]-[Ru2(Hamdp) (amdp)]0.5 ·12H2O and (Yb(H2O)3[Ru2(Hamdp)2] [Ru2(Hamdp)2(H2O)2]-[Ru2(Hamdp)(amdp)]0.5 ·15H2O.[22] Both the structures are having mixed-valent Ru2 II/III units. The dysprosium one has a pillared-bilayer structure, whereas a three-dimensional open framework is observed for the ytterbium-based framework (Figure 7). Magnetically anisotropic lanthanide ions and the Ru2dimer make the materials exceptional. AC Magnetic studies for both the coordination networks reveal field-induced slow magnetic relaxation (Figure 7). Proton conductivities of these complexes are studied via impedance measurements (Figure 7). A large number of lattice water molecules and protonated amino groups facilitate proton conduction in the frameworks. At an optimized condition (95% RH and 55°C), moderate conductivities of 1.44×106(for Dy) and 0.93×105Scm1(for Yb) were observed. 5. Phosphonate carboxylate-based LMOFs There is only one report of proton conductive magnetic LMOFs with phosphonic and carboxylic groups in the same ligand. Konar and coworkers reported three LMOFs (Ln =Dy, Er and Gd) using phosphonoacetic acid.[23] All of the three MOFs have been explored in terms of their magnetic properties and proton conductivity. The gadolinium analogue, [{Gd2(L3)2(H2O)3}·H2O]n Figure 5. (a) Crystal structure for {[Gd2(CO3)(Ox)2(H2O)2]·3H2O}nshowing the 1D channels along the a-axis. (b) Impedance plots at 100 and 150°C without additional humidity. (c) ΔSMcalculated by using the magnetization data at various fields and temperatures. Reproduced from Ref. [20]. Copyright (2018) with permission from American Chemical Society. Figure 6. (a) View of the 3D supramolecule of the gadolinium MOF (b) perspective view of the 3D supramolecular framework of the gadolinium lithium MOF; Nyquist plots of compounds gadolinium MOF (c) and gadolinium lithium MOF (d) at different temperatures under 100% relative humidity. Reproduced from Ref. [21]. Copyright (2019) with permission from American Chemical Society. Figure 7. (a) Packing diagram of dysprosium based framework b) Packing diagram of ytterbium based framework. Out of phase AC susceptibility for the (c) dysprosium framework and (d) ytterbium framework (d) Plots of log (σ(Scm1)) vs RH for both frameworks. Reproduced from Ref. [22]. Copyright (2019) with permission from American Chemical Society. Minireviews doi.org/10.1002/ejic.202100686 4615Eur. J. Inorg. Chem. 2021, 4610 – 4618 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Montag, 29.11.2021 2145 / 222798 [S. 4615/4618] 1 shows antiferromagnetic interaction with a high entropy change (ΔSM=49.29 Jkg1K1). Whereas dysprosium analogue, [Dy(L3)(H2O)2]nexhibits ferromagnetic interaction with fieldinduced slow magnetic relaxation mechanism (Figure 8). The absence of zero-field SMM behaviour is due to large quantum tunnelling among the ground Kramer’s state, confirmed by computational studies. Regarding the proton conductivity, the presence of a rich hydrogen bonding network throughout the framework and hydrophilic interlayer space for water absorption enhance the potentiality of these MOFs as proton conductors (Figure 8). The highest proton conductivity of all these MOFs are found to be 1.13×106Scm1for dysprosium MOF, 2.73 ×103Scm1for erbium MOF and 6.27×106Scm1for gadolinium MOF at 80°C and 95% RH (Figure 8). 6. Sulfonate Carboxylate-based LMOFs A series of LMOFs with a sulfonate carboxylate-based ligand are documented by Konar et al.[24] In this work, four isostructural 3D LMOFs, {[Ln4(OH)4(L4)2(H2O)8]·4.6H2O·1.4 CH3CN}n((Ln=Gd (1), Dy(2), Ho (3) and Er (4)), are investigated for their molecular magnetic properties and proton conductivity. Disodium-2,2’- disulfonate-4,4’-oxydibenzoic acid (L4) was used as a ligand to construct all these MOFs. Structural analyses reveal the presence of cubane-shaped [Ln4(μ3-OH)4] secondary building blocks connected by the ligands to form the 3D networks. In addition, 1D hydrophilic channels are present along the crystallographic c direction (Figure 9). Regarding the magnetic properties, a substantial magnetic entropy change was observed for gadolinium analogue, whereas dysprosium analogue shows a field-induced slow magnetic relaxation process with an anisotropic energy barrier of 30.40 K and relaxation time (τ0) of 2.47×107s (Figure 9). From a structural point of view, it is quite evident that the uncoordiFigure 8. (a) Packing view and proton-conducting channel of [Dy (L3)(H2O)2]n (Green and deep green dot lines represent H-bonding between the interlayer and intralayer of the framework) b) The out-of-phase signals of the temperature dependence of AC susceptibility signals at given frequencies and under a 2000 Oe DC field for [Dy(L3)(H2O)2]n. (c) Nyquist plots for proton conduction for [Er(L3)(H2O)2]nat different temperatures and 95% RH. Reproduced from Ref. [23]. Figure 9. (a) A packing view of {[Ln4(OH)4(L4)2(H2O)8]·4.6H2O·1.4 CH3CN}n along the crystallographic c axis; guest solvates are omitted for clarity. b) ΔSMvalues calculated from the magnetization data of gadolinium analogue at various fields and temperatures (c) Nyquist plots for proton conduction for erbium analogue. Reproduced from Ref. [24]. Copyright (2018) with permission from The Royal Society of Chemistry. Minireviews doi.org/10.1002/ejic.202100686 4616Eur. J. Inorg. Chem. 2021, 4610 – 4618 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Montag, 29.11.2021 2145 / 222798 [S. 4616/4618] 1 nated oxygen of sulfonate, coordinated water, and guest solvent molecules are taking part in strong hydrogen bonding interaction to create 1D hydrophilic channels. AC impedance analyses show the increase in proton conductivities with increasing temperature for high humid conditions for all MOFs. A maximum value of 6.59×103Scm1was found for erbium analogue at 80°C and 95% RH (Figure 9). The prominent variation of conductivities in these MOFs are explained in terms of ionic radius of the respective lanthanide ions. 7. Conclusions and Perspectives This review summarizes the relevant research approach and results on the LMOFs for proton conduction and magnetic properties. As the introduction of multifunctionality in a molecular material is a current concern, it is evident that the LMOFs can comprehensively exhibit their dual properties: magnetism and proton conduction. The synthetic strategy involves combining lanthanide salts and organic acids to prepare such a class of materials. Here, the lanthanide ions deliver the hard coordination ability and higher coordination number with coordinated water molecules for the organic acids. To build the effectual proton transfer pathways, aqueous medium has been used for most cases, so a large number of lattice water molecules can take part in strong H-bonded networks with coordinated water molecules and doublebonded oxygen (of organic acid) in the frameworks. It is observed that the presence of a continuous H-bond network through the hydrophilic nanochannels in the farmework plays a more critical role compared to proton carrier concentration for such types of molecular materials. Importantly, preserving the strong and continuous H-bonding interaction among the lattice water molecules over the experimental conditions offers higher proton conductivities at a higher temperature (~ 80°C) than at a lower temperature. The maximum conductivity values are observed in the order of 103Scm1for few cases in high humidity (95%RH). From the direct current (DC) magnetic measurements, either ferromagnetic or antiferromagnetic exchange interactions between the adjacent lanthanide ions are observed for most of the LMOFs. For a few of them, the magnetic interactions are originated from the intracluster magnetic exchange. However, none of the proton conductive LMOFs is showing long-range magnetic ordering. Among the frameworks, gadolinium analogues show a significant magnetocaloric effect, whereas field-induced slow magnetic relaxation is observed for dysprosium ones. The challenge is the accurate optimization of lanthanide-based MOFs for magnetic properties and proton conductivity. The significance of this challenge is designated to understand the synergy between single-molecule magnetism and proton conductivity. Due to the difference in working temperature for both of these properties, the simultaneous presence of proton conductivity and magnetic properties is not explored for molecular materials. Nevertheless, indirectly, mutual influence could be introduced for scheming such lanthanide-based metal-organic frameworks. Specifically, the water absorbed (or loaded) framework could show altered magnetic properties because of the formation of extensive hydrogen bonds after the humidity-dependent conductivity measurements. So, this kind of material can offer enormous opportunities for future research. From the present perspective, it can be proposed that the interplay between proton migration and magnetism can also be explored on single crystals because of higher dimensional structural ordering in MOFs. Besides having huge potential applicability in hydrogen fuel cell-related technologies, polyelectrolytic (nafion like) membranes fused with LMOFs can offer diverse magnetic properties for nanofabrications. Also, chemical sensor-based molecular switches can be created with such LMOFs for molecular electronic devices,[3a] as one of the most recent reports shows the simultaneous implementation of resistive switching and rectifying effects in a metal-organic framework with a switched hydrogen-bond pathway.[25] Acknowledgements This work was supported by the ESF under the project CZ.02.2.69/ 0.0/0.0/20_079/0017436. SB is thankful to MSMT for funding. PN wants to acknowledge funding through the ERC, under the European Union’s Horizon 2020 research and innovation programme (GA No. 714850). 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