Highly efficient Cr2O72− removal of a 3D metal-organic framework fabricated by tandem single-crystal to single-crystal transformations from a 1D coordination array

Cheng-Peng Li a, Hang Zhou a, Si Wang a, Jing Chen a, Zhong-Liang Wang *b and Miao Du *a
aCollege of Chemistry, Tianjin Normal University, Tianjin 300387, China. E-mail: dumiao@public.tpt.tj.cn
bTianjin Key Laboratory of Water Environment and Resources, Tianjin Normal University, Tianjin 300387, China. E-mail: zhongliang_wang@163.com

Received 11th June 2017 , Accepted 10th July 2017

First published on 10th July 2017

The single-crystal to single-crystal (SC–SC) transformation from a 1D coordination chain to a 3D coordination network, which is triggered by both solvent and anion exchanges, has been revealed to suffer from a tandem mechanism as proved by isolation of the intermediate state. The resulting porous crystalline material shows a high efficiency for the capture of dichromates (207 mg g−1) via the SC–SC anion-exchange.

Dynamic structural transformations for coordination polymers (CPs) or metal-organic frameworks (MOFs), in particular single-crystal to single-crystal (SC–SC) transformations, have attracted considerable attention to modify the functionality of such crystalline materials.1 These transformations may occur as a result of the cooperative action of organic and inorganic components upon exposure to various exogenous stimuli, including concentration, light, temperature, mechanical force, etc. and their synergistic effect.2–4 In general, the robust frameworks show network stability upon exchange of lattice guests, while the flexible frameworks suffer from topochemical reactions during transformations, which are thus of more interest. Currently, different topochemical structural transformations between MOFs are widely observed in a SC–SC fashion, including those between discrete and polymeric arrays,5 between lower and higher dimensional networks,6 and between interpenetrating frameworks of different degrees.7 In these instances, the coordination geometries of the metal ions are usually changed upon exposure to exogenous stimuli, and some coordinating tectons (such as solvents or anions) escape via bond breakage, facilitating the reformation of new bonding in a concerted way. On the other hand, low-dimensional (1D or 2D) secondary building blocks (SBUs) can show more structural freedom than their 3D prototypes and thus, these structural transformations between hetero-dimensional MOFs can be properly viewed as the movement of such SBUs, including their translation, migration, inversion etc.8 For instance, the hetero-dimensional SC–SC transformations driven by a photo-induced cross-linking reaction of MOFs in the solid state were explored by Vittal and co-workers.9 Kitagawa et al. reported the reversible solvent-responsive topochemical transformation from a 2D sheet to a 3D network.10 Wu et al. revealed the nitrite-induced structural transformation from a 1D helix to a 0D loop.11 However, most of the examples show ordinary mutual sliding of the coordination arrays, which are triggered by a single stimulus, such as light, solvent or anion. Otherwise, the rotation of polymeric SBUs during the SC–SC transformation, which obviously requires more energy, has not been observed so far.

Recently, we found a 1D-to-3D topochemical transformation from [Ag(L243)(NO2)](CHCl3) (1) to [Ag(L243)](CF3CO2)(H2O) (3) in a SC–SC way, triggered by both solvent and anion exchanges (Fig. 1). In this transformation, the rotation of partial 1D chains in 1 will facilitate the lattice rearrangement and then, formation of a 3D framework, in which the coordinated anions in 1 move to the channels in 3 with the breakage of metal–anion bonds and the CHCl3 guests in 1 are exchanged by H2O molecules. A possible reaction mechanism for this unique SC–SC transformation has been proposed as either a synergistic or a tandem pathway. Evidently, isolation and characterization of the intermediates are highly desired for elucidation on the mechanism details of such hetero-dimensional SC–SC transformations.

image file: c7cc04527a-f1.tif
Fig. 1 Schematic representation of the hetero-dimensional SC–SC transformation from 1D chains to a 3D framework triggered by solvent and anion exchanges.

Reaction of L243 with AgNO2 in CHCl3/CH3CN leads to the formation of a 1D CP [Ag(L243)(NO2)](CHCl3) (1). Single-crystal X-ray diffraction (SC-XRD) analysis shows that 1 crystallizes in the triclinic space group P[1 with combining macron] (Table S1, ESI) with a 1D coordination chain structure. Each AgI is chelated to two O atoms from one nitrite anion and coordinated to three N atoms from three different L243 ligands (Fig. 2a), resulting in a distorted tetragonal pyramid (τ = 0.43).12 Each L243 ligand links three AgI centers via 2-pyridyl, 3-pyridyl, and triazole groups as a μ3-bridge, forming a 1D polymeric array (Fig. 2b). In this motif, the AgI centers are locked by nitrite anions, and all the 1D arrays are arranged in a parallel fashion with a separation of 9.355 Å along the b axis (Fig. 2c). Notably, the voids between these arrays provide sufficient space for the free rotation of 1D SBUs. In the TG curve of 1, the first weight loss of 19.89% in the 40–126 °C temperature range is consistent with the removal (20.83%) of one lattice CHCl3 molecule (Fig. S2, ESI). The subsequent sharp weight loss at ca. 250 °C indicates its thermal decomposition.

image file: c7cc04527a-f2.tif
Fig. 2 (a) Coordination environment of AgI in 1 (A = 1 − x, 2 − y, 1 − z; B = 2 − x, 1 − y, 1 − z). (b) 1D chain array in 1. (c) Parallel packing of the 1D chains in 1. (d) Coordination environment of AgI in 3 (A = −1 + x, y, z; B = −x, 1/2 + y, 3/2 − z; C = −x, 1 − y, 1 − z). (e) View of a 3D cationic framework of 3 with lattice CF3COO anions in the channels, in which the 1D SBUs are highlighted.

The spontaneous SC–SC transformation of 1 can be achieved, when the crystalline sample was soaked in the aqua solution of CF3COONa. Interestingly, the crystallinity can be kept intact for at least one week. Although no visible change in the color, morphology, and size was observed for the crystals, the PXRD pattern clearly reveals the formation of a new phase (Fig. 3a). The SC-XRD study reveals that a 3D MOF [Ag(L243)](CF3CO2)(H2O) (3) is generated via the SC–SC structural transformation. It crystallizes in monoclinic space group P21/c (Table S1, ESI). Each AgI is coordinated to four N atoms from four L243 ligands (Fig. 2d), resulting in a distorted tetrahedral geometry. Each L243 ligand links four AgI ions via the 2-pyridyl, 3-pyridyl, 4-pyridyl, and triazole groups, affording a 3D cationic coordination framework. In this structure, 1D channels are observed along the a axis with pore dimensions of ca. 10.70 × 10.70 Å2, which can be accessible for the accommodation of trifluoroacetate anions and aqua molecules (Fig. 2e). The calculation of the porosity for a 3D host framework using PLATON13 reveals a value of 668.0 Å3 for the voids (32.5% per unit cell volume). From the viewpoint of topological simplification, this 3D structure can be considered as a uninodal 4-connected sra network (Fig. S3, ESI) with the point symbol of (42.63.8). The TG analysis shows a weight loss of 2.86% in the 120–142 °C temperature range, which can be attributed to the loss of the lattice water molecule (calculated 3.33%) (Fig. S2, ESI). A sharp weight loss at ca. 215 °C indicates the thermal decomposition of the framework.

image file: c7cc04527a-f3.tif
Fig. 3 (a) PXRD patterns for the transformation from 1 to 2 and then, to 3 with their single crystal photos. (b) Schematic representation of the mechanism for the SC–SC transformation, showing two-step reactions with the isolation of intermediate 2.

To further understand such a SC–SC transformation, it will be helpful to compare the structural features of 1 and 3 that consist of similar 1D polymeric chains SBUs (Fig. 2b and e left). In these 1D arrays, the Ag⋯Ag distances are 3.259/8.772 Å in 1 and 3.163/8.873 Å in 3, indicating a very small deformation of the chains. However, a significant difference in the coordination geometry of Ag(I) is observed, where the removal of the coordinating NO2 anion modifies its coordination sphere from the tetragonal pyramid in 1 to a tetrahedron in 3 (Fig. 2a and d). As a result, the transformation changes the framework from neutral to cationic, and the CF3COO anions in 3 are located within the channels to maintain the charge balance. Moreover, the 4-pyridyl ring of the L243 ligand is ligating to the Ag(I) ion in 3, rather than uncovering in 1.

In flexible MOFs, the coordinative unsaturated or open metal sites can be obtained by removing the terminal coordinated components, which are energetically unstable and are required to be coordinatively fulfilled. Nevertheless, to reach the minimum of the local energy, the whole network and the local coordination geometry of a flexible MOF tend to distort, to facilitate the bonding formation between the open metal site and the free donor site from an adjacent SBU. This may lead to a drastic movement of the molecular fragments and even a structural transformation, in view of the dimensionality change of the network. At this stage, the mechanism for this topochemical SC–SC transformation can be appropriately interpreted by migration of the 1D SBUs (Fig. S4, ESI). In 1, the SBUs are separately distributed in a parallel mode. The width of the SBU chain is ca. 15.29 Å, while the distance of two alternate chains is ca. 17.67 Å (Fig. S5, ESI). Therefore, there is enough space to facilitate the rotation of the SBU chains. Upon exposure to the stimuli of solvent and anion exchanges, these alternate 1D SBUs rotate with an angle of ca. 42° in the resulting 3D crystalline lattice (Fig. S5, ESI), promoting the generation of the coordination interactions of Ag–N4-pyridyl (2.312 Å) with the adjacent SBUs. In this way, the distance between two alternate 1D chains is enlarged to ca. 20.72 Å in 3. As a result, a 3D cationic framework is afforded, where the lattice trifluoroacetate anions and aqua molecules are located in the voids. Such a topochemical SC–SC transformation from a 1D to a 3D network involving the rotation of SBUs and migration of anions has never been reported. However, a structural transformation from 3 to 1 cannot be realized, which may be due to the requirement of more energy for breakage of the coordination bonds and also the rigidity of a 3D framework for 3.

Considering that two parameters for both anions and solvents are changed in this reaction, we are interested in whether they work in a synergistic or tandem mechanism. In order to elucidate this issue, two such parameters can be introduced in a tandem manner.14 First, the effect of solvent exchange on the reaction was explored. Single crystals of 1 were immersed into the aqua medium for ca. two days or exposed to water vapor for ca. one week, which will be kept intact and were selected for SC-XRD and PXRD characterization (Fig. 3a). As a consequence, a new crystalline phase [Ag(L243)](NO2)(H2O)2.25 (2) was isolated. The crystallographic analysis indicates that upon the solvent exchange of CHCl3 by H2O, the hetero-dimensional SC–SC transformation from a 1D neutral chain to a 3D cationic framework occurs (Fig. S6, ESI). Then, immersing 2 into a water solution of CF3COONa (0.1 mol L−1) for a short period (ca. 8 hours) will afford 3, revealing a SC–SC conversion induced by anion-exchange (Fig. 3b). Thus, 2 can be properly considered as an intermediate for the SC–SC transformation from 1 to 3. The TG analysis curve (Fig. S2, ESI) of 2 reveals a thermal stability for the host framework upon heating to 215 °C, which is similar to that of 3. The solid-state fluorescence properties of 1–3 and the L243 ligand were explored at room temperature (Fig. S7, ESI). The L243 ligand shows a maximum emission peak at 472 nm (λex = 335 nm), which can be attributed to the π → π* or n → π* transitions. Excitation of the microcrystalline samples of 1–3 at 335 nm leads to the generation of different fluorescence emissions, with the peak maxima at 471 nm for 1, and 494 nm for 2 and 3. The obvious red-shift emission for 2 and 3λ = 23 nm), compared with L243 and 1, can be ascribed to the reduced flexibility of the L243 ligands when bound to the AgI ions in their 3D polymeric frameworks.

Nowadays, water pollution becomes a global concern and in particular, dichromate (Cr2O72−) is on the top-priority list of the toxic pollutants as defined by the US Environmental Protection Agency (EPA).15 Thus, several approaches have been developed to effectively remove Cr2O72− anions from water streams, such as anion-exchange, adsorption, membrane separation, chemical reduction, and electrolytic methods.16 In general, ion-exchange is a particularly attractive option, owing to the cost, sensitivity, simplicity and selectivity considerations. Very recently, cationic MOFs have emerged as promising ion-exchange materials to remove such oxoanion pollutants.17

The anion exchange was performed under ambient conditions by simply dipping dried crystals of 3 into a dichromate solution, with a molar ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1. Notably, most single crystals retain the crystallinity integrity during this process and yellow crystals of [Ag(L243)](Cr2O7)0.5 (solvent) (4) can be produced. As a result, the CF3COO ions in 3 are replaced with Cr2O72− anions in 4 (Fig. S8, ESI). The included dichromate anions are located in the 1D channels of the cationic framework (Fig. 4a), which form multiple weak C–H⋯O interactions with the host framework (Table S3 and Fig. S9, ESI). The 1D channels in 4 show some contraction with obvious distortion of the organic ligands, in comparison with those for 3 (Table S4, ESI). The solution of the anion exchange can be monitored with UV-vis spectroscopy at intervals (Fig. 3b). The intensity of the characteristic adsorption peak of dichromate (352 nm) quickly decreases by 49.5% within only 20 minutes, and then 96.8% of the dichromate ion can be removed after 320 minutes. Meanwhile, the color of the crystals evidently changes from colorless to yellow, and the change in the color of the solution corresponds well with the time-dependent UV-vis absorption spectra. As a result, the capacity of 3 to capture the dichromate anion is 207 mg g−1, which is one of the highest values among all known materials (Table S5, ESI),17 and the distribution coefficient (Kd) of 3 is estimated to be 2.8 × 103 mL g−1.18 Inspired by a high Cr2O72− trapping capability and water stability, a chromatographic column with 3 as a stationary phase was fabricated. Notably, a water solution of the K2Cr2O7 salt can be successfully purified (91.15%) by passing it through the column (Fig. S10, ESI). Furthermore, the selectivity and recyclability are also critical for real use of the materials. Dried crystals of 3 (100 mg) were immersed into a solution (10 mL) of mixed anions (F, Cl, Br, OAc, SO42−, NO3, BF4, CH3SO3, CF3SO3, ClO4 and Cr2O72− with 0.1 mmol of each anion) for two days. The UV-vis and ICP-MS results suggest that the Cr2O72− trapping capability of 3 will not be disturbed by the other anions (95.4%, Fig. S11a, ESI). When a mixture of 10-fold disturbing anions was used, the capability of Cr2O72− sorption is still at a high efficiency (92.1%, Fig. S11b, ESI). The release experiments were performed with 4 in a saturated KNO3 aqueous solution (ca. 120-fold excess). The UV-vis spectra show that the release efficiency for Cr2O72− is high up to 86.8% after one day (Fig. S12a, ESI). Although the material after the release of the Cr2O72− anion loses its single-crystallinity, its PXRD pattern reveals that the 3D framework remains intact (Fig. S12b, ESI). Notably, the parent material 1 shows no capacity for the capture of dichromates, which suggests the critical role of porosity for such applications considering the same SBUs in 1 and 3.

image file: c7cc04527a-f4.tif
Fig. 4 (a) Schematic representation of the dichromate capture by 3 with concurrent loss of the CF3COO ions. (b) (Left) UV-vis absorption spectra of the dichromate aqueous solution during exchange with a molar ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1. (Right) Photographs of color changes in the aqueous solution and single crystals before and after trapping of Cr2O72−.

In conclusion, we have successfully revealed the mechanism of the unique hetero-dimensional SC–SC transformation from a 1D neutral chain to a 3D cationic framework, triggered by both solvent and anion exchanges. Such a conversion is followed by a tandem pathway, as confirmed by isolation of intermediate 2. Furthermore, 3 shows a highly efficient capture capability for the dichromate ion (207 mg g−1) via the anion-exchanged SC–SC transformation. This work provides a new strategy to elucidate the nature of such hetero-dimensional SC–SC transformations, and also a feasible approach to fabricate novel porous crystalline materials for the capture of anionic pollutant species.

This research was supported by the National Natural Science Foundation of China (21031002 and 21541002), the Tianjin Natural Science Foundation (17JCYBJC22800), and the Innovation Foundation of Tianjin Normal University (52XC1402).

Notes and references

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Electronic supplementary information (ESI) available: Experimental details, comparison on dichromate capture for the reported materials, structural figures, PXRD patterns, TG curves, fluorescent and UV-vis spectra, and tables for crystallographic data. CCDC 1506803–1506806. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc04527a

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