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A Process-Tracing Study on the Post-Assembly Modification of Highly Stable Zirconium Metal-Organic Cages Guoliang Liu, Yi Di Yuan, Jian Wang, Youdong Cheng, Shing Bo Peh, Yuxiang Wang, Yuhong Qian, Jinqiao Dong, Daqiang Yuan, and Dan Zhao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03517 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018
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Journal of the American Chemical Society
A Process-Tracing Study on the Post-Assembly Modification of Highly Stable Zirconium Metal-Organic Cages Guoliang Liu,† Yi Di Yuan,† Jian Wang,† Youdong Cheng,† Shing Bo Peh,† Yuxiang Wang,† Yuhong Qian,† Jinqiao Dong,† Daqiang Yuan,*,‡ and Dan Zhao*,† † Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002 Fujian, P. R. China
Supporting Information Placeholder ABSTRACT: Metal-organic cages (MOCs) are discrete molecular assemblies formed by coordination bonds between metal nodes and organic ligands. The application of MOCs has been greatly limited due to their poor stability, especially in aqueous solutions. In this work, we thoroughly investigate the stability of several Zr-MOCs and reveal their excellent stability in aqueous solutions with acidic, neutral, and weak basic conditions. In addition, we present for the first time a process-tracing study on the post-assembly modification of one MOC, ZrT-1-NH2, highlighting the excellent stability and versatility of Zr-MOCs as a new type of molecular platform for various applications.
Discrete metal-organic cages (MOCs), which are constructed by coordination-driven self-assembly of metal ions or clusters with organic ligands, have been extensively studied owing to their fascinating structures with various topologies and potential applications,1 such as substrate for hierarchical structures,2 nanoreactors,3 host-guest chemistry,4 and gas sorption.5 Unlike extended porous materials, the solubility of MOCs enables solution-based processing, facilitating their incorporation as additives into more complex systems such as membranes.6 However, most of the reported MOCs are based on early transition metal ions (e.g., Cu2+)7 with poor water stability. As a result, the application of these MOCs is mainly limited in non-aqueous environment. On the other hand, MOCs that can survive in aqueous environment with broad pH values are highly desirable due to the vast applications in aqueous conditions such as drug delivery and bioimaging.8 Recently, zirconium-based metal-organic frameworks (ZrMOFs) have attracted wide interests owing to their high stability in aqueous solutions with wide pH ranges (typically from 1.0 to 11.0).9 Their high stability can be attributed to the relatively strong Zr-O bonds (Zr-O bond energy: 776 kJ mol-1). Inspired by these materials, we have firstly reported the synthesis of zirconium-based metal-organic cages (Zr-MOCs) which consist of trinuclear zirconium clusters and carboxylate ligands of different geometries.10 Those cages exhibit potential applications in gas sorption and separation. Herein, we present a systematic study on their stability in aqueous environment with different pH values. In addition, the step-wise post-assembly modification of one MOC is elucidated for the first time. Our finding can dramatically extend the applications of these hybrid cage compounds, especially in aqueous solutions.
Figure 1. The crystal structures of four Zr-MOCs in this study. Color code: Zr turquoise, O red, N blue, C black. Yellow balls represent cavity. Counter ions (Cl-), solvent molecules, and H atoms are omitted for clarity. We focus on the four previously reported Zr-MOCs for the stability study: ZrT-1, ZrT-1-NH2, ZrT-3, and ZrT-4 (Figure 1 and S1-2).10a,c,11 These MOCs have different topology (ZrT-1, ZrT-1NH2, and ZrT-3 have V4E6 topology, while ZrT-4 has V4F4 topology; V: vertex, E: edge, F: face), aperture size (4.01, 3.73, 7.01, and 3.69 Å for ZrT-1, ZrT-1-NH2, ZrT-3, and ZrT-4, respectively), and cavity size (7.05, 7.05, 9.87, and 8.99 Å for ZrT-1, ZrT-1NH2, ZrT-3, and ZrT-4, respectively). In addition, ZrT-1-NH2, which has been previously reported as UMOP-1-NH2,11 will be used as a platform MOC to explore the post-assembly modification based on its reactive amino groups.12 Water vapor sorption tests were used as a preliminary evaluation on the water stability of MOCs.13. The maximum water uptake at 25 °C is 156, 149, 132, and 171 cm3 g-1 for ZrT-1, ZrT-1NH2, ZrT-3, and ZrT-4, respectively, highlighting their stability under wet conditions (Figure S3). The hysteresis observed in the desorption branches of the isotherms reflects slow desorption kinetics of water, which can be attributed to strong interactions between water and the ionic cages. This finding encourages us to further study the water stability of Zr-MOCs in solution-based conditions. ZrT-3 and ZrT-4 exhibit relatively low solubility in
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common solvents (Table S1). Interestingly, adding water helped to dissolve ZrT-1, when methanol or ethanol was used as the cosolvent. In the case of ZrT-1-NH2, it displays enhanced solubility compared to other MOCs. To be specific, ZrT-1-NH2 is soluble in aqueous solutions containing other organic co-solvents, such as N, N-dimethylformamide (DMF), N, N-dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), methanol, acetone, and acetonitrile. The fact that adding water can significantly enhance the solubility of ZrT-1-NH2 is probably due to the ionic nature of this cage compound. Similar behavior has been observed in other ionic cages.14 ZrT-1-NH2 dissolved in acetonitrile/water solution was further evaluated using high resolution electrospray ionization mass spectrometry (ESI-TOF-MS).15 The ESI-TOF-MS spectra clearly demonstrate the presence of intact tetrahedral cages in solutions (Figure S5). We observed one dominant set of peaks with continuous charge states from +1 to +4, which is due to the loss of counter ions (Cl-) and successive loss of hydrogen from µ2-OH. Specifically, the observed ion peaks at m/z of 804.3700, 1072.4854, and 1607.7103 correspond to [M-4Cl]4+, [M-4Cl-H]3+, and [M4Cl-2H]2+ ions, respectively. After deconvolution of the m/z, the average measured molecular mass of the assembly is 3359 Da, which exactly agrees with the molecular composition of ZrT-1NH2 {[(C5H5)3Zr3µ3-O(µ2-OH)3]4(NH2-C6H5)6}∙Cl4. In addition, the experimental isotopic distribution patterns of each charge state are in good agreement with the theoretically calculated values based on the chemical composition of ZrT-1-NH2.
Figure 2. ESI-TOF-MS spectra of ZrT-1-NH2 in acetonitrile/water solutions with various pH values. During the synthesis of ZrT-1-NH2, the reagent Cp2ZrCl2 transformed to tri-nuclear zirconium clusters via hydrolysis with hydrochloric acid being released as the by-product. Therefore, ZrT1-NH2 crystals were actually obtained under acidic conditions (the mother solution after crystallization has a pH value of ~4.0), suggesting good stability of ZrT-1-NH2 under acidic conditions. These results encouraged us to further investigate the stability of ZrT-1-NH2 dissolved in acetonitrile/water solutions with various pH values. ZrT-1-NH2 can be fully dissolved when the solution pH value is between 2.0 and 10.0. After dissolving for 1 day, the solutions were subjected to ESI-TOF-MS analysis, confirming the intactness of ZrT-1-NH2 in solutions with this pH range (Figure 2). We noticed a poor solubility of ZrT-1-NH2 when the solution pH value was less than 2.0, which is probably due to the common-ion effect.16 To be specific, the chloride ions from hydrogen chloride can inhibit the dissociation of ZrT-1-NH2 to cationic cages and chloride ions. The cage starts to decompose when the solution pH value is higher than 10.0 (Figure 2), indicating its low stability in basic conditions which is identical to Zr-MOFs.17 Besides ZrT-1-NH2, other Zr-MOCs (ZrT-1, ZrT-3, and ZrT-4) were tested similarly. They exhibit similar stability in aqueous solutions as that of ZrT-1-NH2 (Figure S4, 6-10), confirming the gen-
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erally high stability of Zr-MOCs in aqueous solutions with wide pH ranges (2.0-10.0). The superior stability of Zr-MOCs, which is comparable to that of their MOF counterparts, stems from the strong Zr-O bonds and makes them suitable molecular platform for various applications, especially those solution-based ones. Covalent post-assembly modification (PAM) has been used to alter the properties of MOCs to reveal the full potential of these supramolecular assemblies.18 Those reactions that can be used for PAM studies should meet the prerequisites of proceeding with near quantitative yields under mild reaction conditions without destroying the cages. However, very few reactions have been used to modify MOCs in aqueous conditions mainly because of the poor stability of cages. In addition, the characterization of PAM products in the reported studies is mainly focused on final products without much knowledge of the possible intermediate products.19 Considering that MOCs containing several reactive sites should possess a series of intermediates during PAM, the processtracing study of these intermediates can expand our understanding of MOC-based chemistry. To the best of our knowledge, such study has not been reported previously.
Figure 3. (a) Scheme for the post-assembly modification (PAM) of ZrT-1-NH2 up to 6 sites per cage. (b) ESI-TOF-MS-based process-tracing of the PAM products of ZrT-1-NH2 over various intervals: (1) 1 min; (2) 20 min; (3) 50 min; (4) 120 min; (5) 600 min; (6) 1400 min. Herein, we choose the Mannich reaction to conduct the process-tracing study of PAM on ZrT-1-NH2 due to its high efficiency,20 which was confirmed by the quantitative yield of the Mannich reaction product by reacting dimethyl 5-aminoisophthalate with formaldehyde and methanol (Figure S11). Covalent PAM of ZrT-1-NH2 was achieved by treating ZrT-1-NH2 with formaldehyde in methanol and water solution at 25 oC. (Figure 3). The entire process was initially monitored by 1H NMR.21 The appearance of new NMR peaks indicates the occurrence of PAM (Figure S12). Since each ZrT-1-NH2 cage possesses six amino groups as the active reaction sites, theoretically we should be able to obtain six PAM products. However, each intermediate does not possess feature peaks owing to peak broadening and similar chemical environment. Therefore, 1H NMR can only indicate the overall degrees of the reaction without giving detailed information of intermediates. On the other hand, ESI-TOF-MS is a powerful mean to unambiguously determine the molecular formula of nano-assemblies. It has been used to successfully trace the growth of gold nanoclusters at molecular level to reveal the growth mechanism.22 In order to trace the possible intermediates, the Mannich reaction of ZrT-1-NH2 was analyzed by ESI-TOF-MS at various time intervals during the process of PAM (Figure 3). At t = 20 min after mixing, some new peaks at 815.3535, 826.3588,
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Journal of the American Chemical Society 837.3635, and 848.3675 with normal distribution appeared in ESI-TOF-MS spectrum. These new peaks indicate that the amino groups start to participate in the PAM. These peaks gradually shifted to higher m/z ratio because more amino groups were functionalized. Peak deconvolution indicates that both formaldehyde and methanol take part in the covalent PAM process (Figure S13). After a reaction time of 1400 min, the ESI-TOF-MS peaks were dominated by those contributed from ZrT-1-NH2 being decorated with five or six methoxy groups (peaks at 859.6443 and 870.3993). The possibility of forming modified cages with various degrees-of-modification was confirmed by the structural model of PAM products, which were constructed and geometrically optimized (Figure S14). In a comparable experiment where H2-NH2-BDC was treated with formaldehyde in methanol/water solution, extra NMR peaks in the aromatic region can be found, indicating side reactions that were absent during the PAM of ZrT1-NH2 (Figure S15). This finding suggests that the carboxylic acid groups of the ligand may interact with amino groups, giving rise to side reactions during Mannich reaction. Such interference, however, is absent in ZrT-1-NH2 possibly due to the formation of coordination bonds between carboxylate groups and Zr cations. In summary, we have for the first time thoroughly explored the stability of Zr-MOCs. Because of the strong Zr-O bonds, these cages exhibit high stability in neutral, acidic, and even weak basic aqueous environment. The PAM of one Zr-MOC, ZrT-1-NH2, was demonstrated using the Mannich reaction. In addition, a process-tracing study of the PAM of MOC was unprecedentedly demonstrated based on ESI-TOF-MS, indicating a sequential manner of the PAM process. The high stability and potential functionality of Zr-MOCs demonstrated in this study make these cages promising candidates for solution-based processes, an ongoing project in our lab.
ASSOCIATED CONTENT Supporting Information Experimental procedures, TGA, PXRD, water sorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
[email protected];
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT D.Z. thanks the support from the National University of Singapore (CENGas R-261-508-001-646), Ministry of Education Singapore (MOE AcRF Tier 1 R-279-000-472-112, R-279-000540-114), and Agency for Science, Technology and Research (PSF R-279-000-475-305, IRG R-279-000-510-305). D.Y. thanks the support from National Natural Science Foundation of China (Grant Nos. 21771177 and 21390392), the Strategic Priority Research Program of CAS (XDB20000000), and the Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-SLH019).
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