RESEARCH ARTICLE www.afm-journal.de A Metal-Organic Framework Incorporating Eight Different Size Rare-Earth Metal Elements: Toward Multifunctionality À La Carte Zhen Li, Xiao-Bao Li, Mark E. Light, Anna E. Carrillo, Ana Arauzo, Manuel Valvidares, Claudio Roscini, Francesc Teixidor, Clara Viñas, Felipe Gándara, Elena Bartolomé,* and José Giner Planas* 1. Introduction Multi-metallic multivariate (MTV) rare earth (RE) metal−organic frameworks (MOFs) are of interest for the development of multifunctional materials, Across various domains, the presence of materials incorporating multiple metal ele- however examples with more than three RE cations are rare and obstructed by ments is essential. This is because the in- compositional segregation during synthesis. Herein, this work demonstrates tegration of different metal cations often the synthesis of a multi-metallic MTV RE MOF incorporating two, four, six, or leads to the emergence of novel or improved eight different RE ions with different sizes and in nearly equimolar amounts properties that cannot be achieved by em- and no compositional segregation. The MOFs are formed by a combination of ploying a single metal alone. In the field of alloys, the advent of a new alloy design con- RE cations (La, Ce, Eu, Gd, Tb, Dy, Y, and Yb) and a 1,7-di(4-carboxyphenyl) cept based on multi-components (5 or more -1,7-dicarba-closo-dodecaborane (mCB-L) linker. The steric bulkiness and transition metals)[1,2] has opened a flurry of acidity of mCB-L is crucial for the incorporation of different size RE ions into research interest from all over the world.[3,4] the MOF structure. Demonstration of the incorporation of all RE cations is None of these research would have been performed via compositional and structural characterization. The more possible without the ability to synthesize complex MTV MOF, including all eight RE ions (mCB-8RE), are also “compositionally complex materials” in the first place. In this sense, the addressable characterized using optical, thermal, and magnetic techniques. Element- combination of multiple metal elements selective X-ray absorption spectroscopy and X-ray Magnetic Circular remains a major synthetic challenge in Dichroism measurements allow us to characterize spectroscopically each of the field of metal-organic frameworks.[5–7] the eight RE ions and determine their magnetic moments. This work paves MOFs are a type of materials constructed the way for the investigation of MTV MOFs with the possibility to combine RE through the combination of organic link- ers and metal ions into periodic, ex- ions à la carte for diverse applications. tended structures. In the particular case of Z. Li, X.-B. Li, A. E. Carrillo, F. Teixidor, C. Viñas, J. G. Planas M. E. Light Inorganic Materials and Catalysis Laboratory (LMI) Department of Chemistry Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) University of Southampton Campus UAB, Bellaterra 08193, Spain Highfield, Southampton SO17 1BJ, UK E-mail: jginerplanas@icmab.es A. Arauzo Z. Li Instituto de Nanociencia y Materiales de Aragón (INMA) Shandong Provincial Key Laboratory of Monocrystalline Silicon Semicon- Departamento de Física de la Materia Condensada ductor Materials and Technology CSIC-Universidad de Zaragoza College of Chemistry and Chemical Engineering Zaragoza 50009, Spain Dezhou University M. Valvidares Dezhou 253023, China ALBA Synchrotron Light Source Cerdanyola del Vallès The ORCID identification number(s) for the author(s) of this article Barcelona 08290, Spain can be found under https://doi.org/10.1002/adfm.202307369 C. Roscini © 2023 The Authors. Advanced Functional Materials published by Catalan Institute of Nanoscience and Nanotechnology (ICN2) Wiley-VCH GmbH. This is an open access article under the terms of the CSIC Creative Commons Attribution License, which permits use, distribution and Barcelona Institute of Science and Technology (BIST) and reproduction in any medium, provided the original work is properly Bellaterra, Barcelona 08193, Spain cited. DOI: 10.1002/adfm.202307369 Adv. Funct. Mater. 2023, 33, 2307369 2307369 (1 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de multi-metal multivariate (MTV) MOFs, different metal elements occupy topologically equivalent positions in the secondary build- ing units (SBUs), without altering the network connectivity as compared to that of their single-metal counterparts.[5,8–10] All these materials manifest an increased complexity, which can provide a new generation of materials with diverse properties. Thus, the combination of different metal cations in MOF struc- tures introduces “heterogeneity within order,”[11,12] as the spa- tial arrangement of metal atoms might vary within an ordered and symmetric backbone (Sequencing in Figure 1). Such MTV MOF materials show photo-[13] electro-[14,15] catalytic[16,17] or lu- minescence properties[18,19] and are emerging as supercapaci- tor candidates[20,21] among other applications.[22] However, the greater the number of elements to be combined, the more challenging and difficult is their judicious incorporation and characterization of the MOF structure.[5] Nevertheless, such complexity has been achieved in the case of transition met- als, and multi-metal MTV MOFs, where structures incorporat- ing up to ten different transition metals elements have been synthesized.[6,9,14,15,23–28] Interestingly, for the case of the 10 metal MOF-74 it was already recognized that the preferential incorpo- ration of some elements over others leads to an inhomogeneous metal distribution and prevents precise control of the metal ra- tios. More recently, Yaghi and coworkers have reported the pre- cise sequence of two metals within the MOF-74 structure by atom Figure 1. Outcomes in the competitive formation of crystals with various probe tomography,[29] finding different types of metal sequenc- bimetallic combinations and rod-shape SBU examples. a) Combination of different size rare earth metal cations in rod-shaped SBUs constructed ing depending on the reaction temperature, and demonstrating from dicarboxylated linkers results in preferential partial segregation. b) that the spatial distribution of the different metal elements is One rod SBU of RPF-4 composed of mono-metallic LnO9 polyhedra shar- governed by the different sets of synthetic conditions. Moreover, ing faces (Ln = Yb or La) by H2 hfipdd linker. c) one rod SBU of mCB-Tb their ratio cannot be precisely controlled due to the more re- composed of LnOx (x = 8–9) polyhedra sharing vertexes by mCB-L linker. stricted coordination environment of these d-metals.[5,30] These results emphasize the lack of uniformity in the distribution of metals within transition metals MTV MOFs, highlighting the in- single-molecule magnets (SMMs).[44–46] At present, great efforts homogeneity of metal mixing. are being made to develop molecular systems coordinating dif- In the case of rare-earth (RE) MOFs, high homogeneity can be ferent RE ions, as their combination can be used in many ap- found in the corresponding MTV MOFs when mixing two metal plications, for example, for anticounterfeiting,[47] as thermomet- cations of similar size. The quite similar coordination numbers ric probes,[48,49] to improve up-conversion efficiency,[50] to modify of RE ions has facilitated the incorporation at various ratios (e.g., the color or brightness of emission in luminescent systems,[51,52] 1% increments) of two or three RE metals of close sizes.[30,31] to produce contrast agents,[53] or in the field of quantum comput- However, at present there are no existing MTV MOFs with a high ing, for the realization of qubits and qugates,[54–56] spin valves,[57] number of RE metals.[32] RE elements refer to the 15 lanthanide or to perform quantum error correction protocols.[58,59] In some elements, scandium and yttrium. Due to their particular elec- of the above applications, the coexistence of RE cations of signifi- tron configuration (gradual filling of their 4f orbitals from La3+ cantly varying sizes is of outmost importance. This achievement to Lu3+ ), RE element-based materials display unique properties remains elusive for MOFs, and has so far been limited to discrete with interest in diverse fields such as energy conversion,[33–35] complexes, wherein linkers with distinct coordination pockets— catalysis,[34,36] luminescence,[37,38] or magnetism.[39] In particu- a smaller one and a larger one—are employed.[55,56] lar, the electronic properties of trivalent Ln(III) lanthanides make Although RE MOFs are in principle particularly favorable for them ideal for the design of both luminescent materials[40–43] and the preparation of MTV MOFs due to their similar reactivity and coordination environments,[10] the majority of the reported materials comprise only two metals (e.g., Eu3+ and Tb3+ ),[21] with a few exceptions having three.[10,18,19,60,61] In general, cations F. Gándara Fronteras en Química de Materiales of similar size can replace one another in inorganic materials Materials Science Institute of Madrid-Spanish National Research Council and therefore can be incorporated simultaneously within the (ICMM-CSIC) crystal.[31] However, incorporation of different size RE cations is Madrid 28049, Spain more challenging. In this regard, research results, including our E. Bartolomé own, have revealed that despite their similar coordination chem- EUSS Research Group Escola Universitària Salesiana de Sarrià (EUSS) istry behavior, when it comes to MOF synthesis, reactions with Passeig Sant Joan Bosco, 74, Barcelona 08017, Spain different lanthanide elements might lead to different crystalliza- E-mail: ebartolome@euss.es tion pathways even to produce the same structure type known for Adv. Funct. Mater. 2023, 33, 2307369 2307369 (2 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 2. Comparison of molecular structures of icosahedral dicarba-closo-dodecaboranes (carboranes) with benzene and acidities of carboxylic deriva- tives. the individual, single-metal RE-MOFs (Figure 1a).[62] Thus, the 3D “aromatic” analogs of arenes.[65–67] It is therefore common to synthetic and crystallization study carried out with the rare-earth compare carboranes with benzene.[90,91] The former are signifi- polymeric framework (RPF-4) family, which consists of a rod cantly larger (≈40%) than the benzene ring rotation envelop.[69] SBU and a V-shaped fluorinated organic linker, demonstrated Carboranes behave as strong electron-withdrawing groups (sim- that some binary RE combinations result in compositional seg- ilar to a fluorinated aryl) on a substituent at one of the cluster regation of crystals, while others allow the formation of RE MTV carbons.[69,92–94] Thus, the carboxylic derivatives of carboranes MOFs with controllable metal arrangements. Particularly, the (Figure 2; pKa values o-carborane, 2.64; m-carborane 3.34, p- inorganic SBU in the RPF-4 family comprises REO9 polyhedra carborane 3.64) are more acidic than the corresponding benzoic that share faces via three 𝜇-O atoms, resulting in short distances acid (pKa 5.76).[95] Our mCB-Ln MOF family was originally re- between RE elements, ranging from 3.6 to 3.9 Å (Figure 1b). ported with the use of Eu and Tb, and also with their multi- While rod-shaped SBUs are commonly encountered in metal Eu/Tb MTV-combination.[64] In all cases, single-crystal MOFs,[63] the distance between metal elements in the rods de- X-ray diffraction (SCXRD) showed a structure formed by rod- pends on the different coordination modes of the carboxylate shaped SBUs, which are formed by vertex-sharing REOx (x = 8–9) groups, which in turns is imposed by the employed organic polyhedra. When compared to the RPF-4 series (Figure 1b,c),[96] linker. In this sense, we have recently reported a RE-MOF fam- a close inspection to the SBUs shows a considerably larger metal ily with the carborane-based linker 1,7-di(4-carboxyphenyl)−1,7- – metal distance along the rod SBU in the mCB-Tb MOF (Tb– dicarba-closo-dodecaborane (mCB-L).[64] Icosahedral carboranes Tb: 4.640, 4.583, and 5.255 Å) than in RPF-4 family (La–La: 3.896 1,n-C2 B10 H12 (n = 2, 7 or 12; Figure 2), are a class of com- Å; Yb–Yb: 3.661 Å). We reasoned that this much larger RE–RE mercially available and exceptionally stable 3D-aromatic boron- distance in our carborane MOF could provide a more favorable rich clusters[65–67] that possess material-favorable properties such scenario to accommodate a number of different RE cations with as thermal/chemical stability and high hydrophobicity.[68–74] The dissimilar radii, and therefore facilitate the preparation of a MTV spherical nature of the closo-carboranes, with slightly polarized MOFs with a large number of different metal elements. hydrogen atoms and the presence of the hydride-like hydro- Thus, in the present work, we report a linker-directed synthe- gens at the B-H vertexes, make the carboranes very hydropho- sis approach that enables the direct integration of two to eight bic. Carborane-based MOFs are known[75–82] and research, in- rare-earth metal elements into a single structure. This method cluding our own work, has shown that the incorporation of hy- enables control over the number and size of metal cations (La3+ , drophobic carborane linkers into the MOF frameworks increases 1.250 Å; Ce3+ , 1.220 Å; Eu3+ , 1.120 Å; Gd3+ , 1.105 Å; Tb3+ , 1.090 their hydrolytic stabilities.[64,83–88] Beyond stability, the delocal- Å; Dy3+ , 1.075 Å; Y3+ , 1.011 Å and Yb3+ , 1.010 Å),[97,98] and in a ized electron density is not uniform through the icosahedral closo- nearly equimolar ratio within the resulting material. The choice carborane cages, giving rise to extraordinary differences in the of linker and initial molar ratio of the metal elements employed electronic effects of the cluster.[89] This unusual electronic struc- governs the incorporation of the metal components within the ture is often highlighted by considering carboranes as inorganic MOF structure. The synthesis, structural, optical, magnetic and Adv. Funct. Mater. 2023, 33, 2307369 2307369 (3 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 3. Crystal structures of mCB-Ce, mCB-8RE (La, Ce, Eu, Gd, Tb, Dy, Y, and Yb) and illustration of homogeneous metal mixing in mCB-8RE. a–d) Crystal structure of single-metal mCB-Ce, showing the construction of the 3D structure from the rod-shaped SBUs and the carborane linker mCB-L (a). Blue and yellow polyhedra represents the Ce coordination spheres and icosahedral closo-carboranes, respectively. H atoms are omitted for clarity. Color code: B, orange; C, gray; O, red; N, dark blue; Ce, blue. Three independent metal atoms (SBU) are repeated along the structure to provide 1D inorganic rod-shaped chains that are bridged by the carborane linker to form the observed 3D structures. b) PXRD patterns of experimental mCB-Ce (blue) and mCB-8RE (red) and simulated mCB-Ce (black), demonstrating phase purity and isostructurality. c) SEM image of mCB-8RE and EDX analysis of point scans of the RE metals on the indicated positions (A and B) in one crystal and on multiple crystals shown in (d). SEM images (secondary (i) and backscattered (ii) electrons) and EDX mapping of the same sample area, demonstrating the presence of each of the 8 RE metals in mCB-8RE. thermal characterization of the more complex multivariate MOF mCB-Ce which, as expected, is isostructural with that of mCB- containing eight different rare earths is presented. We discuss Tb (Figure S2, Supporting Information). These two MOFs crys- the possible reasons for the formation and stability of this MTV tallize in the monoclinic Pn space group and analysis of the MOF and how we could modulate the intermetallic distances by structure revealed the formation of a 3D framework based on choosing the appropriate ligand steric bulkiness. The possibility a [(RE)3 (-COO)8 (NO3 )(ODMF )x ] (x = 4 (Tb), 5 (Ce); DMF = N,N- of introducing multiple numbers of different RE3+ cations, each Dymethylformamide) SBU (Figures 1, 3 and Figure S2, Support- having distinct luminescence or magnetic properties, and with ing Information). As in the Tb structure, the SBU of mCB-Ce different sizes, paves the way for the investigation of the funda- is composed of 3 non-equivalent crystallographic metal atoms, mental properties of multi-metallic MOFs and the preparation of which are linearly arranged and capped by bridging or chelate- multifunctional MOFs with customized properties. bridging mCB-L, chelate NO3 − and DMF molecules. Whereas one of the Ce atoms in the SBU is in a CeO9 coordination envi- 2. Results and Discussion ronment, the other two are in a CeO8 environment (Figure 3a and Figure S1, Supporting Information). Similarly, as observed in the 2.1. Synthesis and structural characterization Tb structure, the Ce─Ce distances in mCB-Ce (4.63–5.25 Å) are considerably larger than those in other rod-shaped Ce structures We first synthesized and characterized the single-metal mCB- described in the literature.[99,100] A decrease of the cell parame- RE samples for a wide variety of ionic radii RE elements (RE = ters from the mCB-Ce (Table S1, Supporting Information) to the La, Ce, Gd, Dy, Y, or Yb), that together with the previously re- mCB-Tb[64] structures is observed, being consistent with the cor- ported Eu and Tb MOFs (mCB-Eu and -Tb),[64] covers most pos- responding decrease in ionic radii.[101] Powder X-ray diffraction sible ionic radii sizes in the RE series. The solid-state structure (PXRD) (Figure S3, Supporting Information) and Fourier Trans- for single-metal mCB-Tb[64] and mCB-Ce (Table S1 and Figure form Infrared (FT-IR) spectroscopy (Figure S4, Supporting In- S1, Supporting Information) have been determined by SCXRD. formation) confirmed that all eight single-metal mCB-RE com- Figure 3 shows a representation of the solid-state structure for pounds (RE = La, Ce, Eu, Gd, Tb, Dy, Y, or Yb) are structurally Adv. Funct. Mater. 2023, 33, 2307369 2307369 (4 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 4. SEM images of mCB-nRE-MOF (n = 2 (La and Yb), 2′ (Ce and Yb), 4 (La, Ce, Gd and Yb), and 6 (La, Ce, Gd, Tb, Y, and Yb) and illustration of their homogeneous metal mixing and cations relative sizes. a–d) SEM images of crystal samples for mCB-2RE-MOF (a), mCB-2′RE-MOF (b), mCB-4RE- MOF (c), and mCB-6RE-MOF (d), showing the regions on which EDX analyses were taken (A and B) and the plot of ratio of metals (%; insets), found in each region, demonstrating the presence of each metal in nearly equimolar ratio. identical. The coincidence between their experimental PXRD counterparts {[(RE)3 (mCB-L)4 (NO3 )(DMF)x ]n ·Solv} (mCB-RE, pattern (Figure S3, Supporting Information) and the one calcu- where RE = Eu, Tb, or Eux Tb1−x ) (Figure S5, Supporting lated for the single-crystal structure of mCB-Tb[64] and mCB-Ce Information).[64] Powder X-ray diffraction data for mCB-8RE (Figure 3b) proves that they present the same crystal structure. crystals (Figure 3b and Figure S3, Supporting Information) Once confirmed that all single-metal mCB-RE materials dis- revealed that the material is also isostructural with all above play the same crystal structure consisting of rod-shaped REOn mCB-RE MOF materials. The chemical composition of the (n = 8–9) polyhedra sharing vertexes, with significantly larger mCB-8RE bulk sample was determined by means of inductively RE–RE distances than those of the related RPF-4 materials coupled plasma (ICP) measurements and elemental analysis. (Figure 1b),[96] we continued our study into the possibility of The ICP results reveal a nearly equimolar ratio of Ce, Eu, Gd, preparing a multi-metal RE MTV MOFs. Solvothermal synthe- Tb, Dy, and Yb, whereas that for the largest metal La is slightly sis of mCB-L linker with a mixture of eight rare earth metal smaller and that for Y is slightly larger. These data confirm that salts (RE(NO3 )3· xH2 O, RE = La, Ce, Eu, Gd, Tb, Dy, Y, Yb; all eight RE metals are effectively incorporated into the MOF Methods and Supporting Information) provided colorless crys- structure by one-pot solvothermal synthesis. Element selective tals of {[(La0.07 Ce0.11 Eu0.13 Gd0.11 Tb0.11 Dy0.13 Y0.20 Yb0.14 )3 (mCB- X-ray absorption spectroscopy (XAS) also confirmed the pres- L)4 (NO3 )(DMF)x ]n ·Solv} (mCB-8RE). This RE MTV MOF shows ence of all metals in the crystals (vide infra). Scanning electron a crystal habit similar to that of the individual and bimetal microscopy (SEM)/Energy dispersive X-ray (EDX) spectroscopy Adv. Funct. Mater. 2023, 33, 2307369 2307369 (5 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 5. Comparison of rod-shaped SBUs for TPDC-Ce, mCB-Ce and illustration of carboxylates binding. a) TPDC linker and the 1D inorganic rod- shaped chains that are bridged by the linker in the SCXRD for TPDC-Ce. b) 1D inorganic rod-shaped chains that are bridged by the mCB-L linker in the SCXRD for mCB -Ce. Blue polyhedra represents the Ce coordination spheres. Color code: C, gray; O, red; N, dark blue; Ce, blue. c) Dicarboxylates bringing modes found in TPDC-Ce and mCB-Ce. Figure 6. Optical and photophysical properties for mCB-8RE (La, Ce, Eu, Gd, Tb, Dy, Y, Yb). a) Emission spectrum of mCB-8RE (𝜆ex = 280 nm); b) Color coordinates drawn onto the 1931 CIE chromaticity diagram for mCB-Tb, mCB-Eu, and mCB-8RE. Adv. Funct. Mater. 2023, 33, 2307369 2307369 (6 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de investigated via thermogravimetric analysis (TGA) in the wide temperature range from 25 to 800 °C (Figure S6, Supporting Information). The TGA analysis indicates that all materials un- dergo two processes during the heating: loss of the solvent (co- ordinated to metal sites and/or distributed in the pores) in the range 200–350 °C and decomposition of the framework above 500 °C. Decomposition temperatures for all single-metal mate- rials (525–538 °C) and multi-metal material (532 °C) are rather uniform, showing a quite similar thermodynamic stability. All the above data confirmed the chemical composition of the single- metal, mCB-RE, and the multi-metal RE MTV MOF, mCB-8RE. The amount of both coordinated and uncoordinated DMF and H2 O vary in the family of compounds. The high variability in the solvation is consistent with a dynamic solvation structure of lanthanides[102] that is further complicated with the inclusion of solvent in the framework voids. Sorption measurements af- ter thermal activation (removal of solvent in the voids by thermal treatment under high vacuum) revealed that all compounds are non-porous to N2 but porous to CO2 with Dubini–Radushkevich surface areas ranging from 147 to 331 m2 g−1 (Figure S7, Sup- porting Information). As mentioned above, Gándara and coworkers found that for certain binary combinations of RE metals, particularly those in- volving the combination of lanthanum with elements other than cerium, compositional segregation was observed (Figure 1a), while for other cases the binary RE MTV systems were success- fully obtained. Differences in the crystallization mechanism, as well as thermodynamic effects were proposed to be responsible for the differently observed behavior.[62] So the question arises as to how is it possible that we could now incorporate eight dif- ferent RE metals with nearly equimolar concentration and cov- ering all possible ionic radii sizes? A systematic crystallization study is discarded in the present case considering the large, un- manageable amount of experimental data required for all possi- Figure 7. Dc magnetometry of mCB-8RE. a) Field-dependence of the mag- ble metal combinations involving eight different metal elements. netization, M(H), at different temperatures between T = 1.8–30 K (Mw However, we have selected a few cation combinations, keeping = 2297.43 g mol−1 ); b) temperature dependence of the susceptibility- temperature, 𝜒T(T), at 𝜇 0 H = 1 T. The dashed line signals the high-T con- the largest and smallest size cations in each combination, in or- stant saturation value expected for the combination of Tb3+ , Dy3+ , Gd3+ , der to confirm the generality of the proposed synthetic strategy. Yb3+ , Ce3+ , and Eu2+ ions, in the proportions found from ICP and XAS; Consequently, we have prepared the following series of RE MTV the increasing 𝜒T above ≈190 K is owed to the non-linear Eu3+ contribu- MOFs, mCB-nRe-MOF (n = 2 (La and Yb), 2′ (Ce and Yb), 4 tion (the grey shaded region is the error bar). The star symbols represent (La, Ce, Gd, and Yb), and 6 (La, Ce, Gd, Tb, Y, and Yb), with the 𝜒T values obtained at 3.4 and 21 K from XMCD/H data. Inset, right: all cations in equimolar ratio. We have structurally characterized inverse of the susceptibility, 1/𝜒. all new compounds by PXRD and the crystals analyzed by SEM- EDX (Experimental, Figure 4, Figures S8 and S9, Supporting In- was used to further investigate the distribution of the different formation). The results confirmed the isostructurality of all com- metals in the material. Figure 3c,d shows the SEM/EDX map- pounds and proved that all metals in each material are introduced ping of crystals of the as-synthesized mCB-8RE, showing that in a nearly equimolar ratio, even for the binary La-Yb and Ce- the different RE metals are uniformly distributed within the area Yb combination (Figure 4a,b and Figure S9, Supporting Infor- of the crystals and are not a physical mixture of different metal mation). The observation of the same crystal habit for all single- compounds. Moreover, all analyzed crystals displayed similar metal and multi-metal materials suggests that a similar crystal- metal ratios, demonstrating that the sample has a homoge- lization mechanism is followed for all elements, and therefore it neously distributed composition, ruling out the compositional does not play a crucial role in this case. segregation scenario (Figure 1a). A similar brightness in the It is important to remark that the coordination chemistry EDX mapping for all metals suggests their nearly equal contents of RE-metals is very diverse, with only small energetic differ- in the MOF, being consistent with the ICP results. The overall ences between different coordination numbers and geometries, data confirms that the eight RE metals are dispersed in the MOF and where coordination geometry is dictated primarily by lig- lattice. and steric effects, as interactions are electrostatic in nature.[101,103] The thermal decomposition processes of the single-metal This is a consequence of the shielding of the 4f electrons by mCB-RE and the multi-metallic mCB-8RE were comparatively the 5s[2] and 5p[6] orbitals, which makes the electrons poorly Adv. Funct. Mater. 2023, 33, 2307369 2307369 (7 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 8. Heat capacity and magnetocaloric effect of mCB-8RE. a) Temperature dependence of the heat capacity at different applied magnetic fields, C(T, H), normalized to the ideal gas constant R = 8.314 J mol−1 K−1 . The line corresponds to the lattice contribution, CL /R(T) = ATn , with A/R = 0.014 K−2.57 , n = 2.57; b) magnetic heat capacity contribution, Cm (T, H), and high-temperature dependence, Cm /R(T, H = 0) = DT−2 , with D/R = 0.26 K2 ; c) temperature dependence of the magnetic entropy change, −ΔSm , obtained from the Cm (T) data for the indicated magnetic field changes ΔB (left axis, units in R; right axis, units in mJ.cm−3 K−1 ). The dashed line represents the full entropy content for the mCB-8RE containing 8 RE ions; d) temperature dependence of the adiabatic temperature change ΔTad (T) for different field changes ΔB. available for covalent interactions. Thus, in contrast to transi- RE cations than the corresponding phenyl carboxylates. All these tion metals, covalence plays a minor role in RE-ligand bonds and carborane features could provide more flexibility to the linkers the nature of the coordination sphere is controlled by a tenuous to accommodate different size RE cations in a linear rod-shaped balance between Coulombic interaction and steric hindrance.[97] SBU. Due to their high charge density, RE cations behave as hard In order to test the above hypothesis, we have synthesized and Pearson acids and bind strongly with hard bases such as oxy- characterized a Ce3+ MOF with the m-terphenyl-4,4-dicarboxylic gen, and there is a linear relationship between the basicity of acid (TPDC) ligand, which is the result of replacing the carbo- the carboxylates and the stability of the RE carboxylates.[104] Aro- rane fragment in mCB-L by a phenyl ring (Figure 5). Although matic acids are among the most widely used to prepare such the 3D structure of TPDC-Ce (Figure S10, Supporting Informa- derivatives.[104,105] The aromatic fragments are rigid and facili- tion) is different to that of our mCB-Ce one, it also consists of tate the formation of ordered single crystals.[106] As mentioned in rod-shaped LnO9 polyhedra providing the [(Ce)2 (COO)6 (ODMF )] the introduction, icosahedral closo-carborane clusters are bulkier SBU (Figure 5a). Contrary to mCB-Ce, no nitrate anions from and more electron-withdrawing than a phenyl ring. The experi- the starting Ce salt are incorporated in the TPDC-Ce structure. mentally observed higher acidity of carboxylic acids of carboranes The absence of the nitrate anion in this new structure can be ex- results in a lower basicity of the corresponding carboxylates, as plained by the higher basicity of the TPDC-carboxylate, whereas compared with the benzoate anion. Thus, a combination of the a less basic m-carborane-carboxylate cannot substitute all nitrate weaker basicity and bulkier nature of the closo-carboranyl car- anions from the starting Ce(NO3 )2 . Quite remarkably, the Ce─Ce boxylate derivatives may explain the observed SBUs and longer distances in the flat aromatic MOF are significantly shorter than metal-to-metal distances of our rod-shaped units (elongation ef- those found in the structure for mCB-Ce (4.137, 3.850 Å and fect) when compared with conventional flat aromatic carboxy- 4.630, 4.720, 5.251 Å, respectively; See Table S2, Supporting In- lates. On one hand, the intermolecular interaction (e.g., 𝜋─𝜋 formation, for full comparison of bonds and distances), and com- and/or C─H─𝜋) between aromatic rings of aromatic carboxylates parable to those observed in the RPF-4 family. Quite interest- in the vicinity of a polymeric structure will not be present when ingly, whereas mCB-L carboxylates bond to the Ce atoms in a replacing the aromatic rings by the aromatic 3D closo-carborane bridging mode (mostly type I in Figure 5c; Table S2, Support- moieties. This can certainly increase the distances of the car- ing Information), those for the non-carborane carboxylate are all boxylates in the rod-shaped structures and therefore the metal– in a chelate bridging mode (III in Figure 5c). The higher num- metal distances. In addition, a somewhat weaker basicity of the ber of chelate bridging mode coordination in the phenyl deriva- carboranyl carboxylates could provide a looser bonding to the tive result in shorter Ce···Ce distances. Conversely, the higher Adv. Funct. Mater. 2023, 33, 2307369 2307369 (8 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 9. XAS and XMCD of mCB-8RE. Background-subtracted X-ray Absorption Spectroscopy (XAS), and X-Ray Magnetic Circular Dichroism (XMCD) spectra measured at 3.4 K and 6 T, at the L2,3 edge of Y, and M4,5 edges of La, Ce, Eu, Gd, Tb, Dy, and Yb. Spectra are displayed in order of decreasing ion size. number of bridging mode coordination in the carborane deriva- coordinate (0.40, 0.48). The emitted color is due to the combina- tive result in larger cations’ distances. All these data suggest tion of the two extreme colors (green and red) for the individual that the use of bulky linkers such as carboranes for the synthe- Tb3+ and Eu3+ lanthanides, respectively (Figure 6b). The lumi- sis of multi-metal RE MTVMOFs directs the formation of SBUs nescence decay curves for mCB-8RE (Figure S12, Supporting In- with longer metal–metal distances, allowing the introduction of formation) exhibit the typical monoexponential decay functions multiple cations with different size, and it may become a ver- with calculated lifetimes (850.8 and 741.9 μs for the 543 and 614 satile tool for sequencing RE cations in this type of complex nm bands, respectively) which are nearly identical to those of the MOFs. individual mCB-Tb (849.7 μs) and mCB-Eu (739.0 μs) MOFs.[64] Thus, the luminescence properties of Tb3+ and Eu3+ are additive in mCB-8RE. The increased dilution of these two cations within 2.2. Optical Characterization the material prevents the energy transfer that was observed in the Eu/Tb binary MOFs.[64] The absorption spectrum of mCB-8RE displays a slight broaden- ing of the UV band in comparison to that for the free ligand mCB-L and similar to the Eu and Tb-MOFs (Figure S11, Sup- 2.3. Static Magnetic Characterization porting Information). The emission spectrum in the visible re- gion for a powder sample of mCB-8RE (𝜆ex = 280 nm; air at- Figure 7a shows the magnetization of mCB-8RE as a function of mosphere; room temperature (RT)) is shown in Figure 6a. The the applied field, M(H), measured at different temperatures be- spectrum shows the characteristic emission for Eu3+ (strongest tween T = 1.8 and 30 K. The magnetization of mCB-8RE at 1.8 peak at 614 nm) and Tb3+ (strongest peak at 543 nm). The first K reaches 7.1 𝜇 B /f.u. at the maximum field applied (6 T), without triplet state (T1) energy for mCB-L, calculated by time-dependent reaching saturation. The temperature dependence of the suscep- density functional theory (TDDFT), showed that our carborane tibility, plotted as 𝜒T(T) is shown in Figure 7b. The 𝜒T data tends linker is a good sensitizer for these two cations.[64] Overall the at190 K to the expected constant value of 13.9 emu.K mol−1 cal- mCB-8RE shows a yellow-orange emission, with a 1931 CIE color culated considering the contribution of Tb3+ , Dy3+ , Gd3+ , Tb3+ , Adv. Funct. Mater. 2023, 33, 2307369 2307369 (9 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Table 1. Number of holes (nh ), orbital (mL ), spin (mS ) and total (mTOT = 2.4. Heat Capacity and Magnetocaloric Effect mL + mS ) magnetic moment per ion calculated from the sum rules at T = 3.4 K and 𝜇 0 H = 6 T. We have performed heat capacity (HC) measurements to charac- terize the thermal and magnetocaloric properties of our multi- RE nh mL [𝜇 B ] mS [𝜇 B ] mTOT [𝜇 B ] metallic MOF material. We measured the heat capacity of mCB- Dy3+ 5 2.37 2.09 4.46 ± 0.05 8RE as a function of the temperature in the range 0.3–300 K Tb3+ 6 0.82 3.28 4.09 ± 0.08 and at different applied magnetic fields, 𝜇 0 H = 0, 0.5, 1, 2, and Yb3+ 1 0.95 0.38 1.26 ± 0.21 3 T. Figure 8a shows the low-temperature region of the mea- Ce3+ 13 1.23 −0.42 0.81 ± 0.09 sured C(T) curves. The magnetic contribution to the heat ca- Gd3+ 7 0.81 5.84 6.65 ± 0.05 pacitance, Cm (T) was obtained after subtracting the lattice con- Eu2+ /Eu 3+ (0.2/0.8) 7.8 0.01 2.13 2.14 ± 0.35 tribution, which follows a power-law dependence, CL = ATn (Figure 8b). The Cm (T) curves exhibit a broad Schottky anomaly, which is shifted toward higher temperatures with increasing field. The magnetocaloric effect (MCE) performance parameters Ce3+ , and Eu2+ ions, with the ion proportions given by ICP, and of the mCB-8RE material were determined from the HC data, fol- the Ce3+ /Ce and Eu2+ /Eu ratios obtained from XAS-XMCD mea- lowing the procedure described elsewhere.[108] Figure 8c presents surements (vide infra). The theoretical contribution to the satura- the magnetic entropy change ΔSm (T, ΔB) obtained for selected tion of each ion, given by 𝜒Tfree ion = gJ 2 J(J + 1)/8 (where gJ is the changes of the magnetic field ΔB = 𝜇 0 (Hf − Hi ), where 𝜇 0 Hf Landé constant and J the total angular momentum), is 11.8 for = 0.5, 1, 2, 3 T and 𝜇 0 Hi = 0. To facilitate comparison with ex- Tb3+ , 14.1 for Dy3+ , 7.9 for Gd3+ , 2.6 for Yb3+ , 0.8 for Ce3+ and 7.9 isting literature, we discuss the results obtained for 𝜇 0 Hf = 2 emu K mol−1 for Eu2+ . Above this temperature, the 𝜒T(T) curve T, which is considered the standard as it can be easily attained increases more abruptly owing to the non-saturated linear con- with permanent magnets in magnetic refrigerator applications. tribution of Eu3+ ions, reaching a maximum value of 15.2 emu For ΔB = 2 T the maximum entropy change value (−ΔSm max K mol−1 at 300 K. This limit is close to the expected value, esti- = 7.34 mJ cm−3 K−1 ) is observed at Tmax = 0.71 K (Figure 8c, mated considering the Eu3+ /Eu ratio of ions present in the sam- green curve). The full width at half maximum of the −ΔSm curve ple and their temperature-dependent contribution, which is also is 𝛿TFWHM = 3.0 K, and the relative cooling power is RCP = at the origin of the deviation from the linear behavior in the 1/𝜒 −ΔSm max × 𝛿TFWHM = 22.0 mJ cm−3 . Besides, the adiabatic tem- data at high temperatures (see Figure 7b, inset). By decreasing perature change ΔTad (T) = Tf − Ti obtained from the HC data is the temperature, the 𝜒T value decreases smoothly till reaching shown in Figure 7d. For ΔB = 2 T the maximum temperature de- 10.8 emu K mol−1 at 2 K. Unlike in the mono-metallic Tb and crease is ΔTad max = 5.05 K at T = 0.76 K (Figure 8d, green curve). Dy MOFs,[107] the 𝜒T(T) in mCB-8RE does not show an abrupt Although these figures of merit are lower than those obtained for increase at low temperatures, indicating that the ferromagnetic denser mono-metallic Gd compounds proposed as refrigerants interactions that were present in the former are diluted in the RE at cryogenic temperatures,[108,109] the multi.metallic approach of- MTV MOF. fers the possibility of combining Gd3+ ion (favorable for MCE ef- fect) with other lanthanides to obtain self-refrigerated multifunc- tional compounds for different applications, for example, {Gd- Dy} SMM units, {Gd-Tb}, {Gd-Er} qubits, {Gd-Eu/Tb}, {Gd-Tb- Eu} luminescent units, etc. We are currently working in this di- rection. 2.5. X-ray Absorption Spectroscopy and X-ray Magnetic Circular Dichroism Element selective X-ray absorption spectroscopy (XAS) and X- ray magnetic circular dichroism (XMCD) experiments were per- formed to investigate the magnetism of each of the individual ions within the structure of the mCB-8RE material. Figure 9 sum- marizes the XAS and XMCD spectra measured at a magnetic field of 6 T and 3.4 K at the L2,3 edge of Y and the M4,5 edges of La, Ce, Eu, Gd, Tb, Dy, and Yb. Spectra are displayed in order of decreasing ion size. Figure 10. M(H) of mCB-8RE and ion contributions measured by For yttrium, the XAS spectrum exhibits two peaks at the L3 XMCD(H). (Left scale) Data in color: mTOT (H)/ion cycles measured for (2082.4 eV) and L2 (2138.8 eV) edges expected for Y3+ ion, and the Dy, Tb, Gd, Ce, Yb, and Eu ions; (Right scale) Black bold symbols: mag- XMCD signal is zero. Likewise, the XAS spectrum of lanthanum netic moment for mCB-8RE calculated as the linear combination (LC) of the previous cycles, considering the elemental composition per formula presents two peaks at the predicted positions for La3+ ion at the unit. Open symbols: M(H) measured by SQUID at 3.4 K. The error bar ac- M5 (835.7 eV) and M4 (851.8 eV) edges and, as in the case of Y, counts for the uncertainty in the molecular weight, Mw (±1 DMF of solvent there is no dichroic signal. Thus, Y and La ions do not contribute per f.u.). to the overall magnetization of the sample. Adv. Funct. Mater. 2023, 33, 2307369 2307369 (10 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 11. Ac susceptibility. Top: out-of-phase 𝜒″(f) components of the susceptibility as a function of the frequency measured at a) 𝜇 0 H = 0 T and b) 𝜇 0 H = 0.3 T at different temperatures, and c) at T = 2.0 K and different applied magnetic fields. Bottom: dependence of the relaxation time with d) the inverse temperature and e) with the applied magnetic field. For gadolinium, the XAS at the M5 edge presents a pre-peak the spectral features of the XAS across the M4,5 edges reveal the shoulder 1184.4 eV, main peak at 1186.8 and two satellites at presence of Ce3+ and a minor fraction of Ce4+ . By comparing 1191.5 and 1194.2 eV, whereas the M4 edge exhibits two peaks at the data against the experimental XAS of pure Ce2 O3 (Ce3+ ) and 1216.6, 1218.4 eV. These spectral features show that Gd ion is in CeO2 (Ce4+ ) from reference,[117] we were able to quantify the frac- the common trivalent state.[110] In the case of terbium, the XAS tion of Ce3+ -Ce4+ to be 80%–20% (see Figure S14, Supporting spectra is related to the transition from 3d10 4f[8] to 3d9 4f[9] The 3d Information). The XMCD arises mainly from the Ce3+ contri- – 4f absorption is characterized by a M5 with double-peak struc- bution, as observed in other systems with mixed Ce3+ /Ce4+ va- ture (1240.7, 1241.5 eV) and M4 at 1273.9 eV, clearly separated in lence states.[118] In the case of europium, the XAS-XMCD spec- energy. The XMCD signal is strong, as expected for a lanthanide tra at the M4,5 edges evidence also the coexistence of two va- having a high spin magnetic moment (J = 6 for Tb3+ ). The high lence states, Eu3+ and Eu2+ . The experimental spectral charac- intensity of the XMCD at the M5 edge compared to the M4 edge teristics could be explained considering a linear combination of indicates a strong contribution from the orbital moment of the Tb the XAS-XMCD spectra calculated using atomic multiplet theory ion.[111] The XAS and XMCD line shapes resemble those earlier for magnetic Eu2+ (f7 , J = 7/2), Eu3+ (f6 , J = 1), and non-magnetic reported for Tb3+ at the M4,5 edge in different materials.[112–114] Eu3+ (f6 , J = 0), with weight percentages of 20%–20%–60%, re- The absorption spectrum of dysprosium at the M5 edge exhibits spectively (see Figure S16, Supporting Information). The coexis- a three-peak structure (1293.6, 1296.0, 1297.7 eV) and a single tence of Eu2+ /Eu3+ or Ce3+ /Ce4+ ions were already found in the peak at the M4 edge (1331.7 eV). The XMCD has strong intensity Eu(NO3 )3 or Ce(NO3 )3 salt precursors, respectively, and the syn- at the position of the main M5 peak. The XAS and XMCD thesized mCB-8RE, as confirmed by XPS (See Figures S13 and spectra correspond to Dy ion in trivalent state, as measured in S14, Supporting Information). other systems.[114,115] In ytterbium the XAS consists of a sharp, The orbital moment (mL ), spin moment (ms ) and total structureless peak at the M5 edge (1522.7 eV), due to the only al- magnetic moment (mTOT = mL + ms ) for Dy3+ , Tb3+ , Yb3+ , 13 9 lowed dipolar electronic 3d10 4f7∕2 to 3d5∕2 4f 14 transition, and the Ce3+ , and Gd3+ ions were determined from the XAS-XMCD corresponding XMCD is a symmetric single-peak as expected for spectra across the M4,5 edges using the corrected sum rules Yb3+ .[116] for lanthanides,[111,119] following the procedure described by XAS and XMCD results for Ce and Eu deserve special atten- Tripathi[120] (see Supporting Information S4 for details). The cor- tion, as they show the coexistence of valence states. For cerium, rection is needed to take into account the large jj mixing between Adv. Funct. Mater. 2023, 33, 2307369 2307369 (11 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de the 3d5/2 and 3d3/2 core levels, on the one hand, and the contri- 2.6. Dynamic Spin Magnetic Relaxation bution of the magnetic dipole term, which is not negligi- ble for RE3+ ions, on the other. To obtain the magnetic moment AC susceptibility experiments as a function of the frequency, of Cerium from the sum rules, the Ce3+ XAS was separated out magnetic field and temperature were performed to investigate by subtracting the Ce4+ contribution (see Section 3). In the case the spin magnetic relaxation properties of the mCB-8RE material of Europium, the dichroic signal mainly arises from the Eu2+ (Figure 11). At zero applied field, the out-of-phase component ions, for which the ≈ 0,[121] and has a contribution from of the susceptibility measured at different temperatures shows the Eu3+ (f6 , J = 1) fraction of ions, whereas the absorption spec- no frequency dependence (Figure 11a). The relaxation time as a trum is owed to all Eu ions in the sample. Hence, the “averaged” function of the inverse temperature, 𝜏(1/T), determined from the magnetic moment per Eu ion was calculated applying the sum 𝜒″(f) peak is 𝜏 HF ≈ 3 × 10−5 s, and corresponds to a mechanism rules with nh = 7.8, taking into account the experimentally deter- of spin relaxation through Quantum Tunneling of the Magnetiza- mined fraction of ions, Eu2+ (Eu3+ ) = 0.2(0.8), and the number of tion (QTM). Under the application of a constant magnetic field, holes of each one, nh = 7 (Eu2+ ), 8 (Eu3+ ). Table 1 summarizes 𝜇 0 H = 0.3 T, the 𝜒″(f, T) curves exhibit a peak at very low frequen- the obtained orbital, spin and total magnetic moments for all RE cies and a tail at higher frequencies (Figure 11b). The relaxation magnetic ions in mCB-8RE. time as a function of the temperature for this second process was The field-dependence of the dichroic signal, XMCD(H), was estimated as:[42] independently measured for Gd, Tb, Dy, Yb, and Eu by following 𝜒 ′′ the intensity of the XMCD peak at the M5 edge (the M4 edge for 𝜏 (T ) = ( ) (2) Ce), between −6 T and 6 T (Figure 10). The mTOT (H) curve for 2𝜋f 𝜒 ′ − 𝜒s each ion was found scaling the XMCD(H) with the value found from the sum rules at 6 T. The total magnetic moment per for- where 𝜒 s is the adiabatic susceptibility (i.e., in the limit of f → mula unit of the mCB-8RE compound was obtained considering ∞), which was approximated to 𝜒 s ≈ 0. The determined 𝜏(1/T, 0.3 the element composition per formula unit and the magnetic mo- T) dependence (Figure 11d) reflects the existence of a thermally- ment determined for each ion as: activated spin relaxation mechanism at higher temperatures (al- though the energy barrier cannot be accurately determined from the existing data) which is replaced by QTM with 𝜏 HF ≈ 3.6 × 10−6 mmCB−8RE (𝜇B ∕fu) = (Ce∕fu) × (Ce3+ ∕Ce4+ ) × mCe3+ s at low temperatures, and a very slow relaxation mechanism with 𝜏 LF ≈0.12 s associated to a direct process, affected by bottleneck. + (Gd3+ ∕fu) × mGd3+ + (Tb3+ ∕fu) × mTb3+ This later type of process has been often observed in lanthanide- + (Dy3+ ∕fu) × mDy3+ + (Yb3+ ∕fu) × mYb3+ based coordination polymers.[46,127] The 𝜒″(f, H) measurements at constant T = 2 K shown in Figure 11c allowed to determine the + (Eu∕fu) × mEu (1) magnetic field dependence of the relaxation time, 𝜏(H), for the three observed processes Figure 11e, and confirm the assigned Figure 10 shows that the field-dependence mmCB-8RE (H) deter- relaxation mechanisms. mined from the linear combination of elemental mTOT (H) curves is in very good agreement with the magnetization curve M(H) 3. Conclusions measured by SQUID for mCB-8RE at T = 3.4 K, within experi- mental error. This multidisciplinary study reveals that designing rare-earth Additional XAS-XMCD measurements were conducted at a (RE) MOFs with 3D bulky carborane-based linkers offers a higher temperature of T≈21 K (see Supporting Information, sec- unique opportunity to achieve highly multi-metal RE multivari- tion 5), which revealed qualitatively similar spectral characteris- ate (MTV) MOFs. With the present method, scientists can select, tics (Figure S17, Supporting Information). The obtained mTOT (H) based on intuition, calculations or machine learning methods, curves at this temperature were obviously noisier, and showed the desired combination of metals and their ratio for a potential a more rounded, quasi-linear behavior, as expected for RE para- property or combination of various. A proof of concept is pro- magnetic ions. However, their weighted linear combination was vided with the MTV MOF (mCB-8RE), the first-ever MOF struc- again in good agreement with the magnetization curve of the ture that incorporates eight different sizes of RE cations (La, Ce, compound determined by SQUID at 21 K (Figure S18, Sup- Eu, Gd, Tb, Dy, Y, and Yb) that are homogeneously distributed porting Information). Moreover, the 𝜒T values obtained through in the MOF network. This design strategy has been further val- XMCD at 3.4 and 21 K (using the initial M/H slope as 𝜒) idated by the synthesis of the corresponding MTV MOFs con- were also consistent with the SQUID-determined-values at 0.1 taining two (La andYb, or Ce and Yb), four (La, Ce, Gd, and T (Figure 6b, star symbols). Yb) or six (La, Ce, Gd, Tb, Y, and Yb) RE cations in equimolar The above analysis of mCB-8RE is a beautiful example showing amounts. The comparison of the mono- and multi-metallic RE the power of element-selective XMCD technique to investigate MOF structures with a cerium(III) TPDC structure, where the the magnetic response of individual ions within a very complex 3D purely inorganic carborane moiety has been replaced by a 2D material. In previous works in the literature, XMCD was used to phenyl group, and with the RPF-4 family, provides solid evidence study the magnetism of 2 or 3 three ions in intermetallics,[122,123] for the role of the carborane linker in the achievement of these 3d-4f[124,125] or 4f-4f[126] clusters. In this work, it was possible to new family of RE MTV MOFs. We demonstrate the co-existence determine the spectroscopic and magnetic properties of up to of the eight RE metals in mCB-8RE, and showcase the poten- eight different RE ions in mCB-8RE. tially tunable functionalities provided by the RE cations. Those Adv. Funct. Mater. 2023, 33, 2307369 2307369 (12 of 15) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202307369 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de provide optical, magnetocaloric and magnetic properties to the Keywords new mCB-8RE material. The foregoing results show, for example, an interesting magnetocaloric effect (MCE) that would permit carboranes, lanthanide molecular magnets, multivariate metal-organic frameworks, rare-earth materials preparing self-refrigerated {Gd-nRE} materials for different ap- plications (e.g., SMM memories, qubits, luminescent units etc), Received: June 28, 2023 and revealed that mCB-8RE exhibits Quantum Tunneling of the Published online: July 11, 2023 Magnetization (QTM) under H = 0 and field-induced slow relax- ation of the magnetization through, simultaneously, a thermally- activated mechanism and a direct mechanism. This work paves the way for the synthesis of MTV MOFs with different combinations and ratios of RE cations, for fundamen- tal research and the development of multifunctional materials [1] J.-W. Yeh, S.-K. Chen, S.-J. Lin, J.-Y. Gan, T.-S. Chin, T.-T. Shun, C.-H. with customized magnetic, optical and thermal properties. We Tsau, S.-Y. Chang, Adv. Eng. Mater. 2004, 6, 299. [2] B. Cantor, I. T. H. Chang, P. Knight, A. J. B. Vincent, Mater. Sci. Eng., highlight that our strategy for the preparation of MTV materi- A 2004, 375–377, 213. als could be used to develop “QuMOFs” with high dimension- [3] A. Amiri, R. Shahbazian-Yassar, J. Mater. Chem. A 2021, 9, 782. ality of qubits (either MOFs with large number of qubits, or d- [4] E. P. George, D. Raabe, R. O. Ritchie, Nat. Rev. Mater. 2019, 4, 515. state Qudits, making use of the three different coordination sites [5] J. Castells-Gil, N. Almora-Barrios, B. Lerma-Berlanga, N. M. Padial, present in the SBU), to advance in the scaling of molecular sys- C. Martí-Gastaldo, Chem. Sci. 2023, 14, 6826. tems for Quantum Computing, a very attractive possibility that [6] C.-B. Celia, V. A. de la Pena-O’Shea, I. Puente-Orench, J. R. de Paz, we are prospecting now. R. Saez-Puche, E. Gutierrez-Puebla, F. Gandara, A. Monge, Sci. Adv. 2022, 3, e1700773. [7] C. López-García, S. Canossa, J. Hadermann, G. Gorni, F. E. 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Zhang, H. Deng, Q. Li, Nat. Rev. Mater. 2020, 5, 764. Project RASMIA E12-23R. Zhen Li and Xiao-Bao Li acknowledge the [13] S. Patial, P. Raizada, V. Hasija, P. Singh, V. K. Thakur, V.-H. Nguyen, China Scholarship Council (CSC) for their PhD grants (201808310071 and Mater. Today Energy 2021, 19, 100589. 202106650003, respectively). Xiao-Bao Li is enrolled in the UAB Ph.D. pro- [14] M. Zhang, W. Xu, T. Li, H. Zhu, Y. Zheng, Inorg. Chem. 2020, 59, gram. XAS and XMCD experiments were performed at the BOREAS beam- 15467. line of the ALBA Synchrotron Light Source via official proposal 2022085925 [15] Y. Wang, Y.-Y. Xue, L.-T. Yan, H.-P. Li, Y.-P. Li, E.-H. Yuan, M. Li, S.-N. and in-house beamtime. Li, Q.-G. Zhai, ACS Appl. Mater. Interfaces 2020, 12, 24786. [16] M. Giménez-Marqués, A. Santiago-Portillo, S. Navalón, M. Álvaro, V. Briois, F. Nouar, H. Garcia, C. Serre, J. Mater. Chem. A 2019, 7, Conflict of Interest 20285. The authors declare no conflict of interest. [17] E. López-Maya, N. M. Padial, J. Castells-Gil, C. R. Ganivet, A. Rubio- Gaspar, F. G. Cirujano, N. Almora-Barrios, S. Tatay, S. Navalón, C. Martí-Gastaldo, Angew. Chem., Int. Ed. 2021, 60, 11868. Author Contributions [18] J. I. Deneff, K. S. Butler, L. E. S. Rohwer, C. J. Pearce, N. R. Valdez, M. A. Rodriguez, T. S. Luk, D. F. Sava Gallis, Angew. Chem., Int. Ed. J.G.P. conceived the project concept. J.G.P., E. B., and Z.L. designed the 2021, 60, 1203. detailed project scope and the experiments. Z.L. performed the sample [19] K. A. White, D. A. Chengelis, K. A. Gogick, J. Stehman, N. L. Rosi, S. preparation and conducted sample optical measurements under the su- Petoud, J. Am. Chem. Soc. 2009, 131, 18069. pervision of C. R. and D. R. Z.L conducted basic characterizations. X-B. L. [20] R. Rajak, R. Kumar, S. N. Ansari, M. Saraf, S. M. Mobin, Dalton Trans. performed the sample preparation and characterization for the 2, 4, and 2020, 49, 11792. 6 RE MOFs. M.E.L conducted the X-ray diffraction measurements. A.A., [21] K. M. Choi, H. M. Jeong, J. H. Park, Y.-B. Zhang, J. K. Kang, O. M. M.V., and E.B. conducted the thermal and magnetic measurements. A. E. Yaghi, ACS Nano 2014, 8, 7451. C. conducted the SEM -EDX measurements. J.G.P., E. B., and Z.L analyzed [22] N. Hanikel, X. Pei, S. Chheda, H. Lyu, W. Jeong, J. Sauer, L. Gagliardi, the data and wrote the paper. A.A., C. R., F.T., C.V., and F.G. discussed O. M. Yaghi, Science (80-.) 2021, 374, 454. the results and commented on the manuscript. All authors revised the [23] L. J. Wang, H. Deng, H. Furukawa, F. Gándara, K. E. Cordova, D. Peri, manuscript. O. M. Yaghi, Inorg. Chem. 2014, 53, 5881. [24] C. Castillo-Blas, N. López-Salas, M. C. Gutiérrez, I. Puente-Orench, Data Availability Statement E. Gutiérrez-Puebla, M. L. Ferrer, M. Á. Monge, F. Gándara, J. Am. Chem. Soc. 2019, 141, 1766. The data that support the findings of this study are available in the sup- [25] X. Zhao, Z. Xue, W. Chen, X. Bai, R. Shi, T. Mu, J. Mater. Chem. 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