Stepwise Construction of Multivariate Metal–Organic Frameworks from a Predesigned Zr ₁₆ Cluster

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Dec 2021Stepwise Construction of Multivariate Metal–Organic Frameworks from a Predesigned Zr16 Cluster Baoshan Hou, Chao Qin, Chunyi Sun, Xinlong Wang and Zhongmin Su Baoshan Hou Key Lab of Polyoxometalate Science of Ministry of Education, National and Local United Engineering Laboratory for Power Battery, Northeast Normal University, Changchun, Jilin 130024 , Chao Qin Key Lab of Polyoxometalate Science of Ministry of Education, National and Local United Engineering Laboratory for Power Battery, Northeast Normal University, Changchun, Jilin 130024 , Chunyi Sun Key Lab of Polyoxometalate Science of Ministry of Education, National and Local United Engineering Laboratory for Power Battery, Northeast Normal University, Changchun, Jilin 130024 , Xinlong Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Lab of Polyoxometalate Science of Ministry of Education, National and Local United Engineering Laboratory for Power Battery, Northeast Normal University, Changchun, Jilin 130024 and Zhongmin Su *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Lab of Polyoxometalate Science of Ministry of Education, National and Local United Engineering Laboratory for Power Battery, Northeast Normal University, Changchun, Jilin 130024 https://doi.org/10.31635/ccschem.020.202000630 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail A series of multivariate metal–organic frameworks (MOFs) with predictable topologies are stepwise constructed from a predesigned versatile hexadec-nuclear zirconium cluster. The [Zr16] cluster is constructed from a center classic [Zr6(μ3-OH)4(μ3-O)4(COO)4] cluster and two capped [Zr5(μ3-OH)4(μ2-O)4(HSQA)4)(DMF)4] (SQA = squaric acid, DMF = dimethylformamide) clusters which have 24 potential coordination sites to link metal ions and four formate groups to be substituted by other organic ligands. Based on this predesigned cluster, one two-dimensional (2D) MOF [Zr16-BPDC] (BPDC = 4,4′-biphenyl dicarboxylic acid) and two three-dimensional (3D) extended multinary MOFs [Zr16-BPDC-Ln] (Ln = Eu and Tb) have been obtained through a kinetic-controlled step-by-step synthesis, based on single-crystal-to-single-crystal structural transformation. Moreover, by means of the lanthanides codoping strategy, the Eu/Tb mixed multinary MOF [Zr16-BPDC-Eu0.8Tb0.2] with temperature-dependent luminescent behavior can serve as a luminescent thermometer from 80 to 280 K. This work provides new insight into the discovery of new zirconium cluster-based molecular building blocks and the design of functional multivariate MOFs. Download figure Download PowerPoint Introduction Crystal engineering is the design of functional crystalline materials with specific physical and chemical properties based on the understanding of the intermolecular interactions during the crystal growth and packing.1 The greatest challenge of crystal engineering is that the final crystal structures cannot be predicted easily from the structures and geometries of reactants.2 To solve this problem, the concept of supramolecular synthon was introduced to implement the process of supramolecular assembly and crystallization.3 In the field of coordination compounds and metal–organic materials, crystal engineering is utilized as a strategy to target assembly of three-dimensional (3D) structures based on the extension of building blocks.4–8 As an evolving subject, the molecular building blocks (MBBs) approach was developed by Wuest9 and Hosseini,10,11 which are special molecules with multiple peripheral sites that have strong directional interaction. Followed by the concept of secondary building units (SBUs) devised by Yaghi and co-workers,4,12,13 metal–organic frameworks (MOFs) with predictive topologies were designed based on the extension of specific SBUs. Recently, a new stepwise synthetic approach has been developed, which involves a presynthesized inorganic precursor and a subsequent substitution process that occurs between the precursor and the organic ligand.14,15 These strategies undeniably shed light on the prediction, design, and synthesis of the resulting structures with desired functionality. In a recent review, Helal et al.16 proposed that the future of MOFs will take inspiration from biological systems and have multiple building units and functionality within one crystal structure with specific arrangement sequences. Multicomponent MOFs, including multimetallic MOFs and multilinker MOFs, with desired multifunctionalities are particularly interesting because of their potential applications in gas storage and separation, drug delivery, and molecular sensors. How to introduce heterogeneity by designing and arranging their order is still a challenge for crystal engineering because of the lack of rational control over the self-assembly in a one-pot reaction.17,18 In 2010, Yaghi and co-workers19 incorporated as many as eight functional groups into multivariate MOFs (MTV-MOFs), showing outstanding features distinct from their parent frameworks. Later, Liu and Telfer20 demonstrated a new strategy to design mix-linker MOFs by incorporating multifunctional linkers with various geometries and connectivities. Zhou et al.21,22 developed linker installation strategies to precisely anchor suitable linkers into zirconium-based MOFs (Zr-MOFs) via single-crystal-to-single-crystal (SCSC) structural transformation. All these strategies make the crystal engineering and design of MTV-MOFs more reasonable and predictable. However, these reported MTV-MOFs are synthesized either by a one-pot reaction using multifunctional linkers or by postsynthetic modification (PSM) from MOF to MOF. Inspired by the synthon theory from crystal engineering, we try to design and construct a multicomponent MOF similar to that in organic synthesis. The whole assembly process is controlled by kinetics, which means the MOF should be synthesized to bottom-up and stepwise obtain a predictable topology structure from a predesigned molecule. To pursue a specific molecule that could be used as a precursor for the subsequent incorporation of organic and inorganic components, three key requirements need to be addressed: (1) the precursory synthon molecule needs to be stable and remains structurally intact through the modification process; (2) the precursor should have an intrinsic geometry as well as a specific coordination direction to guarantee the predictability of stepwise modification; and (3) the precursor must contain potential coordination sites for the incorporation of hetero organic and inorganic components. Among the various MOF systems, zirconium-based MOFs (Zr-MOFs) have been intensively studied, since the first Zr-MOF UIO (University of Oslo)-66 was found in 2008, owing to their high hydrolytic and chemical stabilities, as well as basic and acidic stabilities, that stem from the strong ZrIV–carboxylate bond.23 Most Zr-MOFs are constructed from a [Zr6(μ3-O)4(μ3-OH)4(COO)12] cluster-based building block (Figure 1a), including the UIO-series,24 the Northwestern University-series,25 the Porous Coordination Network-series,26 and so on, which show 4–12 coordination numbers with carboxylate ligands. Other zirconium cluster building blocks, such as [Zr8(μ2-O)8(μ2-OH)4(COO)12] (Figure 1b) and [Zr12O8(μ3-OH)8(μ2-OH)6(CO2)18] (Figure 1c) clusters, have also been reported.27,28 Herein, a new versatile zirconium cluster [Zr16(SQA)8(HSQA)8(μ3-OH)12(μ3-O)4(μ2-O)8(COO)4(DMF)8] ([Zr16], SQA = squaric acid, DMF = dimethylformamide; Figure 1d) was synthesized under a mild condition; Figure 1d) was synthesized under a mild condition. The [Zr16] cluster has good stability owing to the strong affinity between Zr(IV) and O atoms. Meanwhile, there are 24 potential coordination sites to link metal ions and 4 formate groups to be substituted by other organic ligands. Such a [Zr16] cluster can stepwise extend into a two-dimensional (2D) multilinker MOF by ligand substitution and further extend into 3D multinary MOFs, incorporating two kinds of linkers and metal ions.29 Impressively, all these structural transformations are performed in a single-crystal-to-single-crystal (SCSC) structural transformation manner under kinetic controls. Furthermore, the mixed Eu/Tb multinary MOF [Zr16-BPDC-Eu0.8Tb0.2] exhibits good temperature sensing behavior in which the intensity ratio of the 5D0 → 7F2 (Eu3+) to 5D4 → 7F5 (Tb3+) transition has a good linear relationship within the temperature from 160 to 280 K. Figure 1 | Discrete SBUs of Zr MOFs. (a) [Zr6(μ3-O)4(μ3-OH)4(COO)12]. (b) [Zr8(μ2-O)8(μ2-OH)4(COO)12]. (c) [Zr12O8(μ3-OH)8(μ2-OH)6(COO)18]. (d) [Zr16(SQA)8(HSQA)8(μ3-OH)12(μ3-O)4(μ2-O)8(COO)4(DMF)8]. Download figure Download PowerPoint Results and Discussion The [Zr16] cluster was synthesized from Cp2ZrCl2 and H2SQA in DMF at 60 °C. Single-crystal X-ray diffraction analysis reveals that [Zr16] cluster crystallizes in the tetragonal, space group P4/mnc ( Supporting Information Table S2). Specifically, the structure of [Zr16] cluster is composed of one [Zr6(μ3-OH)4(μ3-O)4(SQA)8(COO)4] in the center and two capped [Zr5(μ3-OH)4(μ2-O)4(HSQA)4)(DMF)4] clusters, which are bridged by SQA ligands, as shown in Figure 2a. The central [Zr6] cluster is coordinated with 12 organic ligands, including 8 SQA ligands and 4 formate groups, from the hydrolysis of DMF and exhibits a cuboctahedral geometry.30,31 While the capped [Zr5] cluster with a quadrangular pyramid geometry is linked with four SQA, four HSQA ligands, and four DMF molecules, the [Zr16] cluster has the tetragonal prismatic configuration with a size of 12.5 × 12.5 × 25.8 Å3 ( Supporting Information Figure S1). The most interesting feature of the [Zr16] cluster is that it contains multiple potential coordination sites. Specifically, there are eight SQA ligands with one potential binding site in the middle and eight HSQA ligands with two coordination sites hanging on both sides of the [Zr16] cluster ( Supporting Information Figure S2). Moreover, there are also four formate groups at the equatorial position, which can be substituted by other carboxylate ligands on account of the instability of this monocoordination mode. These distinctive features qualify the [Zr16] cluster as a potential MBB to construct multicomponent MOFs. Figure 2 | (a) Crystal structure and potential coordination sites of [Zr16] cluster. (b and c) Magnified parts in the [Zr16-BPDC] and [Zr16-BPDC-Ln] displaying the connectivity between ligands, metal ions, and [Zr16] clusters. (d and e) The 2D framework of [Zr16-BPDC] and the 3D framework of [Zr16-BPDC-Ln]. Download figure Download PowerPoint After carefully examining the space packing of the [Zr16] cluster, we found that the distance between two opposite formate groups of adjacent clusters is 11.81 Å with a short parallel misalignment distance 2.25 Å ( Supporting Information Figure S3), which is close to the length of the 4,4′-biphenyl dicarboxylic acid (H2BPDC) ligand (11.24 Å). So, we select the H2BPDC ligand to substitute the formate groups and connect the adjacent [Zr16] clusters ( Supporting Information Figure S4). As we speculated, when the presynthesized [Zr16] single crystals were immersed in the DMF solution of H2BPDC at 80 °C overnight, the BPDC ligands substituted formate groups successfully and interlinked [Zr16] clusters into a 2D grid layer, [Zr16(SQA)8(HSQA)8(μ3-OH)12(μ3-O)4(μ2-O)8(BPDC)2(DMF)8] ([Zr16-BPDC]; Figures 2b and 2d). Compared with the crystal data of [Zr16] cluster, [Zr16-BPDC] still crystallized in the tetragonal system but with a different space group I422. A slight decrease of cell parameter on the c-axis (from 34.23 to 32.31 Å) and the a/b-axis (from 19.52 to 18.97 Å) was observed because the distance between [Zr16] clusters is shortened by the formation of coordination bonds in BPDC and Zr atoms (from 11.81 to 11.25 Å). Another effect of this ligand substitution is that the cluster undergoes a slight rotation (∼6.6°) in the space packing, resulting in the change of space group. After H2BPDC ligand substitution, each [Zr16] MBB still has 16 coordination unsaturated SQA ligands with the ability to bind metal atoms. Structural analysis revealed that a cavity with a regular tetrahedral configuration was found between two adjacent layers ( Supporting Information Figure S5). The cavity was composed of four uncoordinated oxygen atoms of HSQA ligands from capped [Zr5] cluster, and the distance from the oxygen atom to the center was about 3.08 Å, which can match some metal–oxygen coordination bond lengths. Considering the affinity between the lanthanide metal ions and oxygen atoms, coupled with their rich and unique functionalities, we chose Eu and Tb cations to install in this cavity. The single crystals of [Zr16-BPDC] were put into the Ln(NO3)3/DMF solution at 80 °C overnight. Crystallographic analysis clearly manifests the existence and coordination environment of the subsequently incorporated Ln3+ in the center of the tetrahedral cavity. As shown in Supporting Information Figure S6, each [Zr16] cluster can catch eight Ln atoms by uncoordinated oxygen atoms of HSQA ligands, and each Ln center is eight atoms coordinated with a distorted dodecahedral geometry. The adjacent [Zr16-BPDC] layers are further interconnected by Ln cations and extended into 3D frameworks [Zr16Ln(SQA)11(HSQA)5(μ3-OH)12(μ3-O)4(μ2-O)8(BPDC)2(DMF)8(H2O)4] ([Zr16-BPDC-Ln], Ln = Eu and Tb; Figures 2c and 2e and Supporting Information Figures S7 and S8). Interestingly, such a structural transformation was also performed in an SCSC manner, and [Zr16-BPDC-Ln] crystallizes in the same space group with [Zr16-BPDC] except for the little change in the cell parameter. It is worth mentioning that we failed to install Co2+, Ni2+, Sr2+, Zr4+, Cs+, Ba2+, In3+, Bi3+, and so forth in this cavity to form other multinary MOFs with a similar synthetic procedure. Two possible reasons can explain this selectivity. First, the surrounding oxygen atoms from the SQA ligands formed a cavity with tetrahedral geometry. However, the coordination bond length of metal ions with four-coordinate configurations such as Co2+ and Zn2+ ions, and other metal ions like Ni2+, Zr4+, In3+, and Bi3+ cannot meet this condition. Second, alkali metal and alkaline earth metal ions, such as Sr2+, Cs+, and Ba2+, cannot be installed in this cavity due to the large ion radius even if the bond length is appropriate. Only Ln ions with their flexible coordination mode and sufficient coordination numbers can be successfully installed in this cavity. All of these stepwise SCSC structural transformations are under kinetic control, including the ligand substitution and inorganic components installations, which do not change during their transformation as confirmed by crystal size or shape ( Supporting Information Figure S9). When we tried to synthesize multinary MOFs through a one-pot approach and react them with Cp2ZrCl2, H2SQA, and H2BPDC or Cp2ZrCl2, H2SQA, H2BPDC, and Ln(NO3)3, only opaque white precipitates were obtained. However, the pure [Zr16-BPDC-Ln] can be obtained when the [Zr16] single crystals were introduced to the DMF solution with H2BPDC and Ln(NO3)3, which also showed the high selectivity and accurate regional distributivity in this system. The basic characterization of these compounds is shown in Supporting Information Figures S15–S20 Inspired by the pioneering work of Qian's group32 and Carlos's group33,34 on luminescent ratiometric thermometers based on mixed-lanthanide framework materials, [Zr16-BPDC-Ln] was investigated as a luminescent thermometer owing to its unique configuration distinct from the conventional SBU-ligand-type MOFs. The isostructural mixed-lanthanide MOF [Zr16-BPDC-Eu0.8Tb0.2] was synthesized with a similar synthetic procedure. The ration of Eu/Tb was matched well with the initial inputs and verified by inductively coupled plasma (ICP) spectroscopy. The solid-state photoluminescence spectra of [Zr16-BPDC-Ln] (Ln = Eu and Tb) were examined at room temperature. [Zr16-BPDC-Eu] displays the characteristic emissions at 592, 613, 652, and 701 nm with a red luminescence, which can be attributed to the 5D0 → 7FJ (J = 1–4) transitions of Eu3+ ( Supporting Information Figure S10), while [Zr16-BPDC-Tb] exhibits emission peaks at 488, 544, 587, and 621 nm with a green luminescence, which steps from the 5D4 → 7FJ (J = 6–3) transitions of Tb3+ ( Supporting Information Figure S11). Interestingly, [Zr16-BPDC-Eu0.8Tb0.2] exhibits the characteristic emissions of both Tb3+ and Eu3+ ( Supporting Information Figure S12). To evaluate their potentials as ratiometric thermometers, the temperature-dependent photoluminescent behaviors of [Zr16-BPDC-Eu], [Zr16-BPDC-Tb], and [Zr16-BPDC-Eu0.8Tb0.2] were investigated. As shown in Figures 3a and 3b, the luminescent intensities of both [Zr16-BPDC-Eu] and [Zr16-BPDC-Tb] are decreased gradually with the temperature increases from 80 to 280 K. For [Zr16-BPDC-Eu0.8Tb0.2], the temperature-dependent emissions of Eu3+ (613 nm, 5D0 → 7F2) decays faster than that of Tb3+ (544 nm, 5D0 → 7F2) as the temperature increases, which creates the possibility of being a self-referencing luminescent thermometer (Figures 3c and 3d). Temperature is readily related to the emission intensity ratio of 5D0 → 7F2 (Eu3+, 613 nm) to 5D4 → 7F5 (Tb3+, 544 nm) transition (IEu/ITb), which does not require any additional calibration of luminescence intensity. As depicted in Figure 4a, the temperature can be linearly correlated to IEu/ITb by eq 1 from 160 to 280 K: T = 428.97 − 127.06 I Eu / I T b (1) Figure 3 | Emission spectra of (a) Zr16-BPDC-Eu, (b) Zr16-BPDC-Tb, and (c) Zr16-BPDC-Eu0.8Tb0.2 recorded between 80 and 280 K (excited at 347 nm). (d) Integrated area values for Zr16-BPDC-Eu0.8Tb0.2 with 613 and 544 nm emissions. Download figure Download PowerPoint The correlation coefficient is 0.995, and the maximum relative thermal sensitivity (Sr) is calculated to be 0.66%·K−1 at 280 K ( Supporting Information Figure S13 and Table S1). Temperature cycling between 160 and 280 K exhibits repeatability better than 99.2% ( Supporting Information Figure S14). Temperature changes also give rise to luminescence color changes, which can be directly read out according to the Commission Internacionale d'Eclairage (CIE) chromaticity diagram. The corresponding CIE coordinates change from orange (0.522, 0.443) at 80 K to yellow (0.455,0.497) at 280 K (Figure 4b). Figure 4 | (a) Temperature-dependent intensity ratio of Eu3+ (613 nm) to Tb3+ (544 nm) and the fitted curve for Zr16-BPDC-Eu0.8Tb0.2. (b) CIE chromaticity diagram showing the luminescence color of [Zr16-BPDC-Eu0.8Tb0.2] at different temperatures. Download figure Download PowerPoint Conclusion A stable and versatile zirconium cluster [Zr16] with tetragonal prismatic geometry has been synthesized under a mild condition. Crystallographic analysis reveals that there are 24 unsaturated coordination sites capable of coordinating with metal ions and four formate groups that can be substituted by other organic ligands in this cluster. Such an unprecedented feature makes the cluster an ideal MBB to construct multivariate Zr-MOFs. Under kinetic controls and SCSC transformations, one 2D and two 3D Zr-MOF with different organic ligands and metal ions have been constructed through a step-by-step synthesis. These multinary MOFs have the potential as collaborative luminescence systems, to be luminescence ratiometric thermometers. With this versatile cluster, we believe that more multinary MOFs with expected functionalities will be synthesized in the future. Supporting Information Supporting Information is available and includes single-crystal X-ray diffraction data, IR spectra, TGA curves, PXRD patterns, and photoluminescence spectra. CCDC numbers: 2040513–2040516. Conflict of Interest The authors declare that they have no conflict of interest. Acknowledgments This work was supported financially by the NSFC of China (nos. 21671034 and 21771035). References 1. Desiraju G. R.Crystal Engineering: A Holistic View.Angew. Chem. Int. Ed.2007, 46, 8342–8356. Google Scholar 2. Desiraju G. R.Crystal Engineering: From Molecule to Crystal.J. Am. Chem. Soc.2011, 135, 9952–9967. Google Scholar 3. Desiraju G. R.Supramolecular Synthons in Crystal Engineering—A New Organic Synthesis.Angew. Chem. Int. Ed.1995, 34, 2311–2327. Google Scholar 4. Eddaoudi M.; Moler D. B.; Li H.; Chen B.; Reineke T. M.; O'Keeffe M.; Yaghi O. 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