Solvent-Induced Topological Diversity of Two Zn(II) Metal–Organic

Article pubs.acs.org/crystal

Solvent-Induced Topological Diversity of Two Zn(II) Metal−Organic Frameworks and High Sensitivity in Recyclable Detection of Nitrobenzene Di-Ming Chen, Xiao-Zhou Ma, Wei Shi,* and Peng Cheng Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, P. R. China S Supporting Information *

ABSTRACT: The solvent-induced topological diversity and different framework stabilities were studied in two Zn(II) metal−organic frameworks (MOFs), {[Zn(L)]·DMA}n (1) and {[Zn(L)]·MeOH·0.5NMP}n (2) (H2L = 5-(4H-1,2,4triazol-4-yl)isophthalic acid). Despite the fact that both of these MOFs are (3,6)-connected networks based on the same coordination environment of Zn(II) ions and the same coordination mode of the ligands, they adopt distinct nets with apo and rtl topologies, respectively, which originate from the template effects of the solvents and the conformational flexibility of the ligand. The two Zn(II) MOFs show remarkably different framework stability upon guest exchange due to their different topological structures. The luminescent properties of MOF 1 with different solvents have been studied in detail, which shows high sensitivity in the detection of nitrobenzene (NB). In addition, MOF 1 can be reused by simply washing with fresh solvent.

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INTRODUCTION The field of metal−organic framework (MOF) material has undergone flourishing development in the past few years. The unique features of this type of materials, such as tunable structures, diversiform topologies, as well as potential applications in many useful areas, have made it a hot research topic for the scientist worldwide, especially for those who work in the field of crystal engineering.1−8 In general, the applications of MOFs are directly related to their structural features. Therefore, the development of new synthetic strategies to achieve MOFs with targeted structures and properties has become a great challenge. Although porous MOFs can be synthesized using multidentate ligands, their final structural topologies are highly influenced by several factors, including metal−ligand ratio, pH, solvent, temperature, as well as the oxidation state of the metal ion.9 In particular, the nature of the solvent is an important factor for its template effect, which could govern the process of crystallization and the quality of the final products. Different MOFs could be selectively synthesized from the same raw materials but using different solvents.10−13 The conformational flexibility of the ligand is another factor that should be considered in the construction of the targeted MOFs, because the linker could adopt different conformations in the crystallization and further leads to structural isomerism.14 As a result, predicting the final topologies of MOFs, especially when using ligands with a flexible nature, is much more difficult due to the possibility of structural isomers.15 Although many MOFs with topological © XXXX American Chemical Society

diversity have been achieved, few examples are reported based on the same metal nodes and ligand connection mode; in particular, the study of the relationship between the topological structure and framework stability is rather rare.16,17 On the other hand, ligand-centered luminescence of MOFs is one of the promising applications.18−25 The rich delocalized πelectron framework nature of MOFs allows them to act as fast, reversible, and highly sensitive chemical sensors, especially in the detection of small molecules. For example, significant quenching of luminescent intensity has been found in a series of electron deficient molecules based on transition metal or lanthanide MOFs.26−31 Herein we demonstrate a facile approach based on varying solvents to control the topology of MOFs, and we further study their framework stability and luminescent properties. A bifunctional ligand 5-(4H-1,2,4triazol-4-yl)isophthalic acid (H2L) was selected, and two Zn(II)-MOFs, namely, {[Zn(L)]·DMA}n (1) (DMA = N,Ndimethylacetamide) and {[Zn(L)]·MeOH·0.5NMP}n (2) (NMP = 1-methyl-2-pyrrolidinone), with 3D (three-dimensional) frameworks but different stabilities were synthesized (Scheme 1). 1 is a (3,6)-connected net showing a apo topology, and 2 is also a (3,6)-connected net but with a rtl topology. The stability of 1 is much higher than that of 2 due to the different topology. The luminescent properties of 1 Received: May 5, 2015 Revised: June 21, 2015

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DOI: 10.1021/acs.cgd.5b00614 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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X-ray Crystallography. The X-ray single crystal data of 1 and 2 were collected on an Oxford Supernova TM diffractometer using Mo Kα radiation (λ = 0.71073 Å) at low temperature. The structures were solved by direct methods and refined by the full matrix least-squares technique (SHELXL-97).33 Anisotropic thermal parameters were used in the refinement of all non-H atoms. The hydrogen atoms for the organic ligands were placed in idealized positions with a riding model. The SQUEEZE option of PLATON34 was used to calculate the solvents disordered area and remove their contribution to the overall intensity data. The final chemical formulas of 1 and 2 were obtained from crystal data combined with the results of elemental and thermogravimetric analysis. The crystal data of 1 and 2 are listed in Table 1. Selected bond lengths and bond angles are given in Tables S1 and S2. CCDC 1030137 and 1030138 correspond to 1 and 2, respectively.

Scheme 1. Synthetic Routes of the Two MOFs

Table 1. Crystallographic Data and Structure Refinement Details for 1 and 2

dispersed in different solvents have been studied systematically, which demonstrates distinct solvent-dependent luminescent spectra with significant emission intensity quenching toward nitrobenzene. In addition, 1 could be conveniently synthesized and reused by simply washing.

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EXPERIMENTAL SECTION

Materials and Methods. All the materials and reagents are commercially available and were used without further purification. The ligand 5-(4H-1,2,4-triazol-4-yl)isophthalic acid (H2L) was synthesized by following the published method.32 Powder X-ray diffraction measurements were performed using a D/Max-2500 X-ray diffractometer equipped with Cu Kα radiation. Thermogravimetry analyses (TGA) data were recorded on a Labsys NETZSCH TG 209 Setaram apparatus under N2 atmosphere from room temperature to 800 °C at a heating rate of 10 °C min−1. FT-IR spectra were measured on a Bruker Tensor 27 spectrometer on KBr disks in the 4000−400 cm−1 region. Elemental analyses for C, H, and N were carried out by using a PerkinElmer analyzer. Solid state luminescent spectra were obtained on a Varian Cary Eclipse fluorescence spectrophotometer. Synthesis of {[Zn(L)]·DMA}n (1), Method 1. A mixture of Zn(NO3)2·6H2O (0.030 g, 0.1 mmol), H2L (0.017 g, 0.075 mmol), DMA (2 mL), and MeOH (1 mL) was added to a 5 mL glass vessel and heated to 80 °C for 72 h under autogenous pressure. The vessel was then cooled down to room temperature. About 23 mg of colorless block crystals were obtained. The yield was 80% for 1 (based on H2L). IR (KBr, cm−1) for 1: 3422 (br), 3111 (m), 3098 (m), 1631 (s), 1597 (s), 1431 (s), 1369 (w), 1298 (w), 1215 (w), 1171 (w), 1096 (m), 835 (m), 795 (m), 619 (s); elemental analysis calcd (%) for 1 (C14H14N4O5Zn): C 43.83, H 3.68, N 14.60; found: C 43.69, H 3.87, N 14.73. Synthesis of {[Zn(L)]·DMA}n (1), Method 2. A mixture of Zn(NO3)2·6H2O (0.75 g, 2.5 mmol), H2L (0.35 g, 1.5 mmol), and DMA 35 mL was added to a beaker and stirred for 10 min to give a clear solution, and then the solution was transferred into a 100 mL sealed tube, and 15 mL of MeOH was added. The tube was heated in the oil bath for 2 days at 85 °C. 420 mg of pure crystals was harvested after cooling to room temperature. The yield was 73% based on H2L. {[Zn(L)]·MeOH·0.5NMP}n (2). A mixture of Zn(NO3)2·6H2O (0.030 g, 0.1 mmol), H2L (0.017 g, 0.075 mmol), NMP (2 mL), and MeOH (1 mL) was sealed in a 5 mL glass vessel and heated to 80 °C for 72 h under autogenous pressure. About 5 mg of colorless crystals with blocky shape were obtained after cooling the reaction mixture to room temperature. The yield was 17% for 2 (based on H2L). IR (KBr, cm−1) for 2: 3421 (br), 3089 (m), 2957 (s), 2930 (s), 1653 (s), 1596 (s), 1520 (m), 1390 (s), 1346 (m), 1296 (m), 1107 (s), 785 (m), 739 (m), 720 (s); elemental analysis calcd (%) for 2 (C13.5H13.5ZnN3.5O5.5): C 42.88, H 3.60, N 12.96; found: C 42.95, H 3.52, N 12.80.

compound

1

2

formula fw temp, K crystal syst space group a (Å) b (Å) c (Å) α (deg) ß (deg) γ (deg) V (Å3) Z Dc (g/cm3) μ (mm−1) crystal size/mm3 Rint reflections collected/unique GOF on F2 R1, wR2 [I > 2σ(I)] R1,wR2 (all data) largest peak, hole (e Å−3)

C14H14N4O5Zn 383.66 125.6(1) orthorhombic Pbcn 90 90 90 20.4409(14) 11.9176(14) 14.9473(11) 3641.2(6) 8 1.082 1.355 0.2 × 0.2 × 0.2 0.0709 9925/3203 1.042 0.0549, 0.1715 0.0797, 0.1866 0.71/−0.47

C13.5H13.5ZnN3.5O5.5 378.15 127.5(1) monoclinic P21/c 90.00 109.791(3) 90.00 10.9484(3) 12.1060(4) 14.7728(5) 1842.35(10) 4 1.069 1.339 0.3 × 0.1 × 0.1 0.0439 6734/3222 1.149 0.0550, 0.1400 0.0596, 0.1421 0.64/−0.78

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RESULTS AND DISCUSSION Synthesis and Structures. Pure colorless crystals of 1 could be afforded in the DMA−MeOH (2:1, v/v) solvents system. A single-crystal X-ray diffraction study reveals that 1 possesses a similar 3D network as the Cu-MOF we reported before.35 However, their synthesis conditions are quite different: 1 was prepared at a low temperature and can be conveniently synthesized under mild conditions (Figure S1), and this is not the case for the Cu-MOF. Single-crystal X-ray study confirms that 1 belongs to the orthogonal space group Pnca, and its asymmetric unit consists of one Zn(II) ion and one L2− ligand. As shown in Figure 1a, two five-coordinate Zn(II) ions are connected by four carboxylate groups to give a paddle-wheel secondary building unit with a Zn···Zn distance of 3.012(2) Å, and the axial sites of the paddle-wheel are occupied by triazolyl N atoms (Figure 1). The Zn−O bond distances are in the range of 2.027(3) Å to 2.061(3) Å, and the Zn−N bond length is 2.010(4) Å. The whole framework could be simplified to a (3,6)-connected net with an alpha-PbO2 (apo) topology by considering the paddle-wheel SBU as a 6-connected octahedral B

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Figure 1. Structural features of MOF 1 (a) and 2 (b).

Figure 2. Topological representations of 1 and 2.

Figure 3. Natural tiling of 1 and 2.

node and the ligand as a 3-connected trigonal linker (Figures 2 and 3). To the best our knowledge, there are many MOFs with a (3,6)-connected net showing rtl topology, but the 3,6connected alpha-PbO2 (apo) net is rarely reported.35−42 PLATON analysis reveals the 3D structure is composed of a solvent accessible volume of 1931.7 Å3, 53.1% of the crystal cell volume.34 1 exhibits 1D zigzag channels with a diameter of ca. 3.9 Å along the a axis (Figure S2).43 We speculated that a

different solvent system might give rise to different structures of 1. With this in mind, NMP was used instead of DMA, and MOF 2 was obtained under similar reaction conditions as those for 1. 2 crystallizes in the monoclinic space group P21/c, and its asymmetric unit also contains one Zn(II) ion and one L2− ligand. In the crystal structure of 2, similar Zn paddle-wheel SBU and solvent accessible volume (53.3%) as those for 1 were observed. The Zn−O bond distances are in the range of C

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with that of the free H2L might be attributed to the increased degree of π electron overlap of the organic linkers.47−49 The changeable relative intensities of the emission peaks of the two MOFs compared with the free H2L could be tentatively assigned to the influence of coordination interactions between the metal atom and the ligands.50 The different luminescent intensities between MOFs 1 and 2 might be ascribed to the more rigid and compact topological structure of MOF 1 (Figure 2), which increases the ligand conformational rigidity and reduces the nonradiative decay of the intraligand.51 The high stability of 1 (Figure S3) as well as its easy preparation prompts us to consider its potential application in luminescence sensing of small molecules. Nitrobenzene (NB) was widely used as an important industrial material in plastic processing, inorganic synthesis, and pesticides, especially the production of aniline, which has attracted intensive attention for its high toxicity.31 In order to investigate the potential application of 1 in detecting NB, five milligrams of finely ground 1 was immersed in different solvents, such as MeOH, EtOH, hexane, DMF (N,N-dimethylformamide), CH3CN, THF, DMA, dioxane, and nitrobenzene, and then treated by ultrasonication for 30 min. The significant luminescent intensity quenching was observed only for nitrobenzene (Figure 5). Other solvents did not strongly affect the luminescence intensity of 1. These results demonstrate that 1 has high selectivity for NB compared to other solvent molecules. The sensing properties of 1 for nitrobenzene were further investigated by monitoring a series of emissions of 1 in DMF with gradually increased nitrobenzene concentration (Figure 6). Before measurements, the emission spectra of the suspended solution of 1 in DMF along with the increasing time was monitored (Figure S6), which did not change much after 96 h, indicating 1 could be stable in DMF for a long time. The luminescent intensity of the suspended solution of 1 in DMF gradually decreases with increasing concentration of nitrobenzene (Figure 6). At a concentration of 750 ppm, the luminescence intensity is nearly completely quenched with a high quenching efficiency of 93.0% (quenching percentage = (I0 − I)/I0 × 100%, where I0 and I are the fluorescent intensities of 1 before and after exposure to the NB). This value is higher than those of the Cd-MOF and Zn-MOF reported recently.52,53 As the encapsulation of NB into the channels of 1 can be ruled out for its small pore diameter (3.9 Å) compared with the dynamics diameter of an NB molecule (6.0 Å), the fluorescent quenching observed may be attributed to the photoinduced electron transfer from the excited MOF to the electron deficient NB molecule which is adsorbed on the surface of the MOF particles.54−56 It has been confirmed by molecular orbital theory that nitrobenzene with an electrondeficient functional group can capture an electron from the ligand.57 We have calculated the molecular orbitals of the ligand and nitrobenzene at the level of B3LYP (Figure 7). The LUMO of nitrobenzene is much lower than the LUMO of the ligand, which makes the transformation of electrons from the MOF to nitrobenzene possible, leading to a quenching effect to luminescence.52 In addition, the finely grounded 1 can be dispersed very well in the solution, which would endow nitrobenzene with close attachment to the surface of the MOF and facilitate possible host−guest interactions.58,59 1 could be regenerated and reused for at least four cycles by simply filtering and washing several times with DMF. It is noteworthy that the initial fluorescence intensity and the quenching efficiency did not change much over four repeated cycles,

2.027(4) Å to 2.044(4) Å, and the Zn−N bond length is 2.018(4) Å. 2 also shows a 3D (3,6)-connected framework but with a rutile (rtl) topology upon considering the paddle-wheel SBU as a 6-connected octahedral node and the ligand as a 3connected trigonal linker. However, in the structure of 2, the twist angle between the triazole ring and the plane of the benzene ring of the ligand (42°) is lower than that of 1 (55°) (Figure 1), which leads to different angles among the center of the Zn-paddle-wheel, the center of the benzene ring, and the N atom connecting with the Zn atom. And this further leads to topological diversities between 1 and 2 (apo for 1 and rtl for 2). 2 possesses 1D channels with the maximum diameter of ca. 7.8 Å. It is noted that the channel shape of 2 differs from that of 1 (Figure S2). Power XRD and TGA Data. In order to evaluate the phase purity of 1 and 2, powder X-ray diffraction experiments have been performed at ambient conditions (Figure S3). Their high purity solid state phases were confirmed by the good match of the patterns for the as-synthesized samples and simulated ones from the single-crystal data. Thermogravimetric analyses were also investigated for 1 and 2 (Figure S4). 1 shows a weight loss of 22.2% from 133 to 307 °C, corresponding to the loss of one lattice DMA molecule (calcd 22.7%). The framework collapses upon further heating. 2 displays a weight loss of 10.8% from 65 to 153 °C, corresponding to the loss of one lattice MeOH molecule (calcd 10.5%), and then a weight loss of 13.2% occurred from 182 to 325 °C, corresponding to the release of half a lattice NMP molecule (calcd 13.1%). The framework collapses upon further heating. The final decomposition products of both 1 and 2 should be ZnO (calcd 21.3% for 1 and 22.4% for 2), but there are still some organic compositions left even at 800 °C (found 30.1% for 1 and 27.3% for 2). Luminescent Properties. The room temperature luminescent properties of MOFs 1 and 2 as well as free H2L ligand were studied in solid state (Figure 4). A maximum emission

Figure 4. Solid-state emission spectra of H2L and 1 and 2 at room temperature.

peak at 405 nm was observed for the free ligand H2L upon excitation at 319 nm (Figure S5), which may be ascribed to the intraligand π*−π and π*−n transitions.44 Intense emission bands were found at 409 and 416 nm for 1 and 2, respectively. As the Zn(II) ion is difficult to oxidize or to reduce due to its d10 electronic configuration, the emission bands of 1 and 2 are neither metal-to-ligand charge transfer (MLCT) nor ligand-tometal charge transfer (LMCT) but originate from the intraligand charge transfer because similar emission was observed for the free H2L ligand.45,46 The observed red shift of the maximum emission bonds of the two MOFs as compared D

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Figure 5. Emission spectra and intensity of 1 in different solvents.

Figure 6. Emission spectra of 1 with different NB concentrations (left). Reproducibility of the quenching ability of 1 in DMF and in the presence of 750 ppm of NB.

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CONCLUSION In summary, two 3D (3,6)-connected MOFs with interesting solvent-dependent topological diversities have been constructed under solvothermal reactions. Interestingly, although their frameworks are based on the same SBU as well as the same coordination mode of the ligand, the resulting (3,6)-connected topologies are distinctly different (apo for 1 and rtl for 2). This might originate from the different template effects of the solvents and flexibility of the ligand. Luminescent studies of 1 show significant selectivity and sensitivity to nitrobenzene. Furthermore, 1 can be conveniently reused by simply washing with fresh solvent. These features make 1 a promising candidate for the development of low-cost and recyclable NB luminescence sensors in the future. Our investigation also gives new insights into the exploration of luminescent MOFs from not only the aspects of topology but also functions such as for NB detection.

Figure 7. Calculated HOMO (black) and LOMO (red) for NB and ligand.

indicating a high efficiency of 1 for its long time NB detection application (Figure 6, right). The structure of 1 after four cycles of NB detection was still stable, as confirmed by powder X-ray diffraction. To the best of our knowledge, luminescent MOFs with both scaled-up synthesis and selective detection of NB features have not been reported. The relationships between the luminescent intensities of 1 and the concentration of nitrobenzene have also been studied, which show good agreement with the first-order exponential equation, indicative of the diffusion-controlled quenching process for the nitrobenzene molecules (Figure S7).60−62

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Additional figures and structures (Figures S1−S2), PXRD patterns (Figures S3), TGA curves (Figure S4), and X-ray crystallographic files (CIF) for 1−2. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00614. Corresponding Author

*E-mail: [email protected]. E

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the “973 program” (2012CB821702), the MOE (NCET-13-0305 and IRT-13R30), and 111 Project (B12015) for financial support.

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DOI: 10.1021/acs.cgd.5b00614 Cryst. Growth Des. XXXX, XXX, XXX−XXX