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Tracking the Superefficient Anion Exchange of a Dynamic Porous Material Constructed by Ag(I) Nitrate and Tripyridyltriazole via Multi-Step Single-Crystal-To-Single-Crystal Transformations Cheng-Peng Li, Bo-Lan Liu, Lei Wang, Yue Liu, Jia-Yue Tian, Chun-Sen Liu, and Miao Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16757 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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ACS Applied Materials & Interfaces

Tracking the Superefficient Anion Exchange of a Dynamic Porous Material Constructed by Ag(I) Nitrate and Tripyridyltriazole via Multi-Step Single-Crystal-To-Single-Crystal Transformations Cheng-Peng Li,† Bo-Lan Liu,† Lei Wang,† Yue Liu,† Jia-Yue Tian,‡ Chun-Sen Liu,*,‡ and Miao Du*,†,‡ †

College of Chemistry, Tianjin Normal University, Tianjin 300387, P. R. China

‡

Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, P. R. China ABSTRACT: To avoid the instability and inefficiency for anion exchange resins and layered double hydroxides materials, we present herein a flexible coordination network [Ag(L243)](NO3)(H2O)(CH3CN) (L243 = 3-(2-pyridyl)-4-(4-pyridyl)-5-(3-pyridyl)-1,2,4-triazole) with superefficient trapping capacity for permanganate, as a group-7 oxoanion model for radiotoxic pertechnetate pollutant. Further, a high-throughput screening strategy has been developed based on concentration-gradient design principle to ascertain the process and mechanism for anion exchange. Significantly, a series of intermediates can be successfully isolated as the qualified crystals for single-crystal X-ray diffraction. The result evidently indicates that such a dynamic material will show remarkable breathing effect of the 3D host framework upon anion exchange, which mostly facilitates the anion trapping process. This established methodology will provide a general strategy to discover the internal secrets of complicated solid state reactions in crystals at the molecular level. KEYWORDS: dynamic porous material, anion exchange, high-throughput screening, capture mechanism, crystal transformation

chemical stability, which have shown the potential applications in a wide range of fields.22–25 Remarkably, such coordination supramolecular systems may display dynamic response upon the external stimuli.26–28 As a role, the cationic coordination networks can be constructed by the linkages of neutral ligands with metal centers, in which the charge-balancing anions are included with the aid of host-guest interactions. Recently, these materials have been applied to exchange and trap oxometal anions (MnO4–, ReO4–, CrO42–, and Cr2O72–) to provide feasible and extensive applications.13,14,29–32 The anion-exchange equilibrium will dominate the sorption process and mostly determine the trapping capacity and efficiency for such adsorbents with stuffy host frameworks, which however could be significantly enhanced when strong interactions between the host matrixes and guest oxometal anions exist. In this condition, singlecrystal X-ray diffraction (SC-XRD) of the oxoanion-loaded coordination networks will provide the mostly straightforward and convincing evidence for such host-guest interactions, despite the great difficulty in achieving the qualified single-crystal samples after anion exchange.

INTRODUCTION The ubiquitous presence of oxometal anionic pollutants in soil and water has raised increasing concerns on the environmental contamination and human health.1,2 In particular, pertechnetate is a problematic monomeric anionic contaminant in vitrification of nuclear wastes.3,4 Seeking high-efficiency anion-exchange materials for pertechnetate, as one major radiotoxicity contributor during the longterm waste storage, represents a great challenge at current stage.5–11 In this context, the anion-exchange study on permanganate, as the group-7 oxoanion model for pertechnetate, can provide more available ways to efficiently remove pertechnetate from the waste solutions.12–16 Up to date, the potential materials for capture of oxometal anions are ion exchange resins17 and layered double hydroxides (LDHs).18 The conventional organic resins with both cationic groups and exchangeable anions are usually of limited thermal and chemical stability, and thus, poor longevity.19 While most LDHs materials can only exchange the extra-framework ions by inorganic anions with similar or smaller sizes, and their exchange capacity and selectivity will be hindered by weak electrostatic interactions between the cationic hosts and anionic guests as well as the equilibrium of process.20 In addition, LDHs have the poor exchange selectivity in water owing to the presence of CO32– anion generated from CO2 gas in atmosphere, which shows the strongest affinity to LDHs materials.21

In this work, to develop new oxoanions exchangeable materials, we have designed and prepared a unique 3D coordination polymer with cationic porous network for nitrate inclusion. Remarkably, this material shows the quick exchange and high trapping capacity for MnO4– (almost 100% after ca. 1.5 h), setting a new record over all known materials. The presence of coordination bonds between the host framework and guest MnO4– anion has been further confirmed by SC-XRD of the anion exchange product. More importantly, this

Porous coordination polymers are a rising class of crystalline materials with ordered frameworks, tunable pores, and good physico1

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considered as the 4-connected net nodes, and the 3D framework could be simplified into a uninodal sra topology (Figure 1c).34

exchange process could be well tracked from beginning to end, via multi-step single-crystal-to-single-crystal (SC-SC) transformations, by using a newly developed high-throughput screening technology, which clearly reveals the breathing effect for such flexible crystalline materials.

RESULTS AND DISCUSSION Structural Characterization and Permanganate Adsorption. The targeted material was designed based on a versatile ligand 3(2-pyridyl)-4-(4-pyridyl)-5-(3-pyridyl)-1,2,4-triazole (L243), which can be easily synthesized from the industrial raw materials in good yield. The L243 ligand has multiple binding sites and may take diverse conformations caused by the rotation of pyridyl groups. As a result, such a flexible tecton will facilitate the construction of cationic coordination network with AgI centers, which possess the favorable binding tendency to the N-donors of both pyridyl and triazolyl functional groups. Colorless block crystals of [Ag(L243)](NO3)(H2O)(CH3CN) ( NO3) were obtained in high yield by layering an acetonitrile solution of the L243 ligand onto a AgNO3 water solution. SC-XRD analysis (Table S1) for NO3 reveals that each AgI is four-coordinated by four types of nitrogen donors (N1, N4, N5, and N6) coming from different L243 ligands and each L243 acts as a tetradentate ligand to connect four AgI ions (Figure 1a), where the 2- and 3-pyridyl groups are twisted from the central triazolyl ring by 43.5 and 56.8o, respectively (Table S2). As a result, the AgI ions are extended by L243 ligands to form a cationic 3D coordination network. Uniform 1D channels are found along a axis, with the pore dimensions of 14.55 × 15.36 Å2 (Table S3), which can be accessible for valid anion exchange (Figure 1b). Calculation of the porosity for 3D host framework by PLATON33 indicates a value of 702.4 Å3 (33.6% per unit cell volume) for the free voids. In this structure, both AgI centers and L243 ligands can be

Figure 1. Crystal structure of NO3. (a) Coordination environment of AgI and binding fashion of L243. Symmetry codes: A = –1 + x, y, z; B = 1 – x, 2 – y, –z; C = 3/2 – x, 1/2 + y, 1/2 – z. (b) 3D cationic coordination network with 1D open channels for inclusion of nitrate. (c) Topological view of the 3D framework showing the inner channels.

Figure 2. UV-Vis spectra of KMnO4 solutions exchanged with NO3 (diluted with deionized water) at different time intervals: (a) NO3 : MnO4– = 1 : 1 at 298 K; (b) NO3 : MnO4– = 2 : 1 at 298 K; (c) NO3 : MnO4– = 4 : 1 at 298 K; (d) NO3 : MnO4– = 1 : 1 at 353 K; (e) NO3 : MnO4– = 2 : 1 at 353 K; (f) NO3 : MnO4– = 4 : 1 at 353 K. 2


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Anion exchange of NO3 was monitored by UV-Vis based on intensity variation of the maximum absorption for MnO4– anion in solution. Immersing the microcrystalline sample of NO3 into a water solution of equimolar KMnO4 at ambient conditions leads to a quick concentration decrease of MnO4– anions by 47 and 95% after 264 and 344 min, respectively. These values correspond to the exchange capacities (mol/mol) of 0.47 and 0.95 and also, the trapping capacities (mg/g) of 140.65 and 283.40 (Figure 2a and Table S4), which agree well with the results from inductively coupled plasma mass spectrometry (ICP-MS) with the exchange capacities of 0.49 and 0.96 mol/mol. After that, a slow concentration decrease of MnO4– is found, and the exchange ratio reaches up to 100% after 432 min with MnO4– trapping capacity of 297.89 mg/g, revealing a complete anion exchange. Oliver and co-workers have reported a novel MnO4– trapping material, namely [Ag2(4,4’-bipyridine)2(O3SCH2CH2SO3 4H2O] (SLUG-21), in which the [Ag(4,4’bipyridine)]n chains are sustained by the interchain stacking to construct 2D cationic supramolecular layers.13 Notably, the high sorption capacity of SLUG-21 (irreversible uptake of 65 and 97% in 24 and 48 h) is presumed as a result of crystal transformation during the anion exchange course. In comparison, NO3 can finish the anion exchange in only ca. 7 h under similar conditions. Whereas for the uncalcined and calcined magnesium aluminum hydroxycarbonate, as the commercially available LDH materials, the sorption capacities are significantly lower under similar conditions (7.92 and 41.60 mg/g after 48 h).13 This is due to the only existence of electrostatic interactions between the MnO4– anions and cationic layers, with the adsorption mechanism of typical equilibrium-driven anion exchange. The MnO4 exchange capacity of NO3 was also explored at different NO3 : MnO4– molar ratios and temperatures (Figure 2 and Table S4). As expected, in all the cases, the 100% exchange ratio can be realized in different periods (Table S5). At 298 K, the completion time for MnO4– exchange with NO3 : MnO4– of 1 : 1 is 432 min, which can be reduced to 336 min by doubled amount of NO3. Further increasing the initial molar ratio to 4 : 1, a complete removal of MnO4– is achieved in only 208 min. The ICP tests indicate the adsorption capacities (mol/mol) of 0.99, 0.50, and 0.25, for the anion-exchange products with different initial molar ratios, which are consistent with the UV-Vis results. At 353 K, the completion time of MnO4– exchange could be greatly shortened to 152, 130, and 96 min, with the initial molar ratios for NO3 : MnO4– of 1 : 1, 2 : 1, and 4 : 1. Accordingly, the maximum trapping capacities (mg/g) of NO3 are 298.68 (297.33), 149.34 (148.80), and 74.67 (74.19) based on the UV-Vis (ICP) tests. The exchange kinetics can be derived by the plot of MnO4– concentration as the function of reaction time with different MnO4– : NO3 molar ratios (Figure 3). At 298 K, in 1 : 1 exchange reaction, the anion concentration smoothly decreases in 200 min, after which a sharp decrement is found in the period of 200–350 min. With regard to the 2 : 1 and 4 : 1 reactions, the anion trapping rates are obviously faster within 50 min. However, all the cases with different NO3 : MnO4– molar ratios show the similar sorption behaviors for MnO4– at 353 K, which indicate the temperature-dependent nature for the anion exchange. –

reaction. The partial rate law for such anion exchange reaction can be described as r = dc(MnO4–) / dt = kc(MnO4–)2, in which r (reaction rate), c (concentration), and t (time) have their usual meanings. The rate constant (k) is evaluated to be 0.00149 and 0.056 s–1·M–1 at 298 and 353 K. Notably, the exchange reaction beyond such specific periods is far away from the linear distribution at both temperatures, probably suggesting a complicated reaction mechanism for anion exchange as illustrated below.

Figure 3. Kinetics curves for anion exchange reactions and photos for the diluted upper clear solutions of KMnO4 used for UV-Vis measurements, with the molar ratios of NO3 : MnO4– in 1 : 1, 2 : 1, and 4 : 1 at (a) 298 K and (b) 353 K. The exceptionally high adsorption capacity and rapid uptake velocity of NO3 for MnO4– anion may result from the formation of stronger interactions between the 3D host network and captured MnO4– anion. That is, the structural transformation may occur to provide the additional driving force, instead of a typical equilibrium-driven exchange. To further confirm this proposal, anion exchange was studied with the well-shaped NO3 crystals and fortunately, minute quantity of black block crystals of [Ag(L243)(MnO4)] ( MnO4) can be selected from the anion exchange product for SC-XRD analysis (Figure S3a). The crystal structure clearly reveals that the adsorbed MnO4– ions are accommodated in the 1D channels along a axis with the presence of host-guest interactions. In this case (Figure 4a), each AgI center is penta-coordinated by four nitrogen donors (N1, N2, N4, and N6) from distinct L243 ligands and an oxygen atom from MnO4–. L243 also serves as a tetradentate ligand to bridge four AgI ions, in which the 2- and 3-pyridyl rings are twisted from the central triazolyl ring by 83.0 and 62.3o, respectively. In the resulting 3D network, 1D channels are also observed (Figure 4b) and different to that in NO3, the coordination interaction of Ag–O (2.722 Å) is found between the host framework and guest anion in MnO4. A calculation of the available voids for this structure indicates a value of 278.8 Å3 (15.2% per unit cell volume). Topologically, each AgI ion and L243 ligand serve as the 4connected nodes to construct a uninodal sra topological network (Figure 4c).34

The nearly linear fits for the plots of MnO4– concentration as the function of reaction time (Figure S2) reveal that the exchange reaction at 298 or 353 K within 200 or 50 min is a second-order 3



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the capture of MnO4– by NO3 can be viewed as an anion diffusion process from exterior into interior of crystals. The diffusion degree will change at different time intervals, and at the different stages of crystals, the anion exchange reactions with different NO3 : MnO4– molar ratios can occur. In fact, the anion exchange rate will be evidently decreased for NO3, when larger crystals are used as the starting materials instead of the crushed powders, which may reveal more stages during the anion exchange. This strategy can also be roughly regarded as a derivative of ''Plate Theory'' in terms of crystallography. That is to say, the shorter period of crystal transformations could be extended by high-throughput changes on the molar ratios of NO3 : MnO4–. After repeating attempts, the molar weight of NO3 crystals and KMnO4 could be properly adjusted, which are distributed in 100 glass tubes for the anion exchange reactions within one day (Table S6). 1) At very low molar ratios of MnO4– to NO3 (e.g. 0.01 to 0.09 mmol in No. 9 tube), the surface of crystals seems purple but the interior of crystals is clearly transparent and colorless (Figure 5). SC-XRD reveals that the crystals for NO3 remain unchanged. Figure 4. Crystal structure of MnO4. (a) Coordination environment of AgI and binding mode of L243. Symmetry codes: A = –1 + x, –1/2 – y, –1/2 + z; B = –1 + x, y, z; C = –x, –1 – y, 1 – z; D = –x, –1/2 + y, 1/2 – z. (b) 3D cationic coordination network with 1D open channels for holding the interacted MnO4–. (c) Topological view of the 3D network showing the inner channels.

2) With a gradual increment of the molar ratio for MnO4– to NO3 (e.g. 0.02 to 0.04 mmol in No. 14 tube), the purple block crystals will be given. However, the purple dreg on the crystal surface can be easily removed by steel needle to afford transparent light purple crystals (Figure S3b). SC-XRD reveals that [Ag(L243)](NO3) ( NO3) is formed as a supramolecular isomer37,38 of NO3 (Table S1). NO3 has a different 3D host framework to that in NO3, the 1D open channels of which are more distorted from square to rhombic (Table S3) for the inclusion of lattice nitrate anion (Figure 5).

NO3 and MnO4 crystallize in the same space group P21/n but with distinct unit cell parameters (Table S1). Notably, the L243 ligands take different binding modes with discriminative N-donor groups (three pyridyl plus one monodentate triazolyl in NO3 while two pyridyl plus the bidendate triazolyl in MnO4 with a free 2-pyridyl). Moreover, the captured MnO4– is tightly immobilized in the host framework of MnO4 by coordination bonding, which is responsible for the superior anion exchange performance of NO3. Fascinatingly, the 3D host network also exhibits a striking breathing effect in anion exchange, involving rearrangement of molecular components in crystalline lattices and considerable structural deformation of the overall frameworks (Figure 1 and Figure 4). High-Throughput Screening Strategy. The clarification of process and mechanism for ion exchange is facing huge difficulty due to the lack of available approaches. Clearly, the capture and isolation of intermediates during this course can be quite helpful to solve this problem. Furthermore, the characterization of such intermediates is also pivotal, for which SC-XRD will be most powerful to provide detailed information on their structures. However, it is almost impossible to achieve the qualified single crystals for metastable intermediate states during such dynamic processes, because the single crystallinity of materials could hardly be reserved throughout the reactions from exterior to interior of the crystals27,35,36 and moreover, the existence of intermediate states may be too momentary to be trapped. To address this issue, we have developed herein a new high-throughput screening approach based on concentration-gradient design of the starting reagents. The dynamic variable time during anion exchange can be translated to static variable concentration gradient. Thus, the traces of molecular motions may be achieved to discover the processes and mechanisms in such high-throughput reactors. In this connection,

Figure 5. The anion exchange in a sequential process of NO3 NO3 NO3 MnO4 MnO4 MnO4, highlighting the breathing effect of host coordination networks during this course (inset: photos of the single crystals for each state). 3) Further increasing the MnO4– to NO3 molar ratio leads to black solids, most of which are not suitable for SC-XRD analysis. Actually, the FT-IR spectra indicate that the characteristic peak at 4


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ca. 1380 cm–1 for NO3– ion turns weaker and that at ca. 890 cm–1 for MnO4– ion is stronger. This may suggest that the MnO4– anions in solution are trapped by NO3 with a partial of NO3– anions being exchanged. However, this clue is insufficient to confirm if the two peaks for MnO4– and NO3– arise from the co-existence of two anions in the same crystal or from a mixture of two different crystals. Nevertheless, a sampling inspection of the crystals in different tubes with specific molar ratios of MnO4– to NO3 (e.g. 0.12 to 0.05 mmol in No. 45 tube) can hit good-quality crystals (Figure 5) for SC-XRD tests. Notably, such a reliable success will heavily depend on the quality of original crystals and cannot be accurately controlled. That is to say, the collection of qualified crystals will be random from those tubes with proper starting ratio of MnO4– to NO3. As a result, black block single crystals for [Ag(L243)](NO3)0.16(MnO4)0.84(CH3CN)0.5 ( NO3 MnO4) and [Ag(L243)](MnO4)(CH3CN)0.5 ( MnO4) can be determined and the tests for all available samples suggest that the two crystals are concomitant and hardly distinguished with eyes for their similar shapes and colors. Further tries to isolate the two products by tuning the concentration-gradient in more refined molar ratios also fail, probably due to the very quick conversion from NO3 MnO4 to MnO4. Notably, NO3 MnO4 and MnO4 are isostructural, in which co-existence of MnO4– and NO3– is found in NO3 MnO4, while only MnO4– is located at the same position of lattice for MnO4. Thus, the concomitant occurrence of such two types of crystals can be attributed to their isostructural nature with similar lattice energy. The 1D channels within NO3 MnO4 and MnO4 are more expansive in contrast to NO3, while an evident distortion from rhombic to square is observed compared with that of NO3 (Figure 5 and Table S3), indicating a dynamic process for anion exchange. 4) Very high molar ratio of MnO4– to NO3 (e.g. 0.28 to 0.08 mmol in No. 88 tube) results in a mass of black solids, among which single crystals can be isolated for SC-XRD to confirm the formation of anion exchange product MnO4. Accordingly, the sharp FT-IR band for NO3– ion at ca. 1380 cm–1 disappears and that for MnO4– at ca. 890 cm–1 can be clearly observed (Figure S6). Recyclability, Low-Concentration Sorption, and Selectivity. Reversibility and reusability are also critical to the materials for practical application. In this case, the microcrystalline sample for MnO4 can be completely transformed to NO3 within several hours at 353 K, when soaked in a saturated water solution of KNO3 (Figure S7). Accordingly, the colorless solution gradually becomes purple due to the release of MnO4– anion from MnO4 (Figure S8). However, the transformation from NO3 to NO3 is not found when further immersing the sample at 353 K for as long as three days. This may originate from the similar 3D networks for NO3 and MnO4 (Tables S1 and S3). That is, more energy or rigorous condition will be required to achieve the transformation from NO3 to NO3, which yet is unnecessary considering the aim of this study. Nevertheless, reversible anion exchange between NO3 and MnO4 is readily, and the NO3 crystals can efficiently work even after six cycles, with the final trapping and releasing efficiency of ca. 91% and 84% (Figure S9a and Figure S9b), confirming its great potential in practical application as a high-performance anion-exchange material. In addition, the loss in solid mass of NO3 and MnO4 upon anion exchange and regeneration is very small (± 2%, Figure S9c).

Also, the minimum capture time, detection limit, and selective capture of targeted anions show great relevance and are more challenging. When the powders of NO3 were immersed into the 100-fold molar excess of KMnO4 solution, the shortest time of adsorption saturation is less than 5 min. The resulting product MnO4 can be confirmed by FT-IR and PXRD patterns (Figure S10). The detection limit experiment was taken by dipping NO3 into 1 ppm KMnO4 solution (Figure S11), where MnO4– ion can be effectively detected and fully trapped within only two minutes, with the color of sample turning from white to pink. The ICP-MS test reveals that 98.7% MnO4– in the solution can be trapped into the material with the residual concentration of only 13 ppb. To explore its selectivity for MnO4– ion over other anions, NO3 was immersed into a mixture solution of CrO42–, SO42–, CF3SO3–, ClO4–, BF4– and Cr2O72– anions of each in water. FT-IR spectra (Figure S12a) show that MnO4– will be preferentially captured by NO3 over other competing anions with different ratios. The characteristic band for MnO4– could be observed in all anionexchanged products, with absence of the bands from the competing anions. Notably, NO3 shows superior selectivity to CrO42– and SO42–, even in the presence of 600- and 500-fold competing anions, with a final product of MnO4 as confirmed by PXRD patterns. In addition, when NO3 was immersed into a mixture solution of KMnO4 and all other competing anions (CrO42–, 100-fold; SO42–, 100-fold; CF3SO3–, 5-fold; ClO4–, 1-fold; BF4–, 1-fold; Cr2O72–, 1fold) for 48 h, the sorption of MnO4– can still be at a high efficiency of 86% (Figure S12b). Benefiting from the isolation and characterization of intermediates, we can deduce that anion exchange and capture for this fascinating system will experience a sequential process of NO3 NO3 NO3 MnO4 MnO4 MnO4. Further structural analysis shows that the mechanism of such an anion exchange reaction can be considered as a result of the diverse conformations of L243 ligand and flexible host networks in crystal transformation, showing the self-adjustable nature to adapt the release of nitrate and also the uptake of permanganate (Video S1). In fact, the dihedral angles between the pyridyl rings and the central triazolyl group in L243 ligand can be drastically modulated during the anion exchange (Table S2), with the change of coordination geometries of Ag(I) from tetrahedral to trigonal bipyramid (Figure S1). Regarding to the adsorption preference of permanganate over nitrate, it can be properly attributed to the bulkier volume of permanganate, which will induce the framework distortion to promote the formation of coordination bonding between Ag(I) and permanganate in the final product. In detail, the transformation from NO3 to NO3 with the unchanged crystal composition results in a change of the binding mode and conformation of L243 (Table S2) and accordingly, an obvious deformation of the 3D framework (Table S3). After that, partial NO3– anions are replaced with MnO4– in the same site of crystalline lattice (crystallographic disorder with different occupation factors) to form the intermediate NO3 MnO4, during which deformation of the host framework is also found. The conversion from NO3 MnO4 to MnO4 proceeds without structural change, which thus is easy and rapid. However, this phase is indeed an intermediate state, which is finally converted to MnO4 along with the distortion of host framework and formation of metal–anion bonding to drive the anion exchange progress.

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Actually, reactions of crystals in solid state, especially SC-SC transformations are not effortless due to the frustrating effect of molecular stacking in crystal lattices, with not only less freedom of the molecular movements but also the difficulty in the retainment of single crystallinity throughout.39 In this point, SC-SC transformations trigged by anion exchange, involving the anion movement in crystals and sometimes the drastic change of lattices are rather rare.40–42 Moreover, the reported results on SC-SC transformations mostly concern the original or terminal stages of crystals, while the process and mechanism are always presumed with the aid of some indirect evidences, such as FT-IR and PXRD. However, to clearly demonstrate the mechanism of anion exchange reaction that is still mysterious, the reaction process must be monitored by providing the definite structural information of intermediates, especially for those with dynamic structural transformations of the materials, just as observed in this case.

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Technology Innovative Research Team in University of Henan Province (15IRTSTHN002), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (152101510003), and Plan for Scientific Innovation Talent of Henan Province (154200510011).

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CONCLUSION This work presents a unique flexible crystalline material NO3 with high-efficiency anion exchange and capture performance. Although the loaded MnO4– anions are fixed in the 1D channels of the anion exchange product MnO4 by coordination interactions to promote the anion exchange, both reversibility and reusability are available. This promising material is anticipated to be practically used, also considering the short capture time, low detection limit and high selectivity. Of most importance, a high-throughput screening method has been developed, based on which a series of intermediates can be successfully isolated and structurally determined. This progress can evidently reveal what happens in the confined coordination space during anion exchange and why the anions could be readily trapped by such dynamic crystal materials, as shown by multi-step SC-SC transformations. We hope this new strategy can be broadly applied to illustrate the mechanisms of different solid state reactions, which is also of great significance for crystal design and material synthesis toward targeted applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Experimental, supplementary structural figures, kinetic curves, crystal photographs, PXRD patterns, IR spectra, crystallographic parameters, structural features during anion exchange, UV-Vis data, video for the anion exchange. CCDC 1049524–1049528.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21471134, 21541002 & 21571158), Program for Science & 6


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