Ligand Symmetry Modulation for Designing Mixed-Ligand Metal

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Ligand Symmetry Modulation for Designing Mixed-Ligand Metal− Organic Frameworks: Gas Sorption and Luminescence Sensing Properties Di-Ming Chen, Jia-Yue Tian, and Chun-Sen Liu* Henan Provincial Key Lab of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, Henan, China S Supporting Information *

ABSTRACT: Herein, we report the synthesis of a new mixed-linker Zn(II)-based metal−organic framework (MOF), {[Zn2(atz)2(bpydb)](DMA)8}n (1) (atz = deprotonated 3-amino-1,2,4-triazole, bpydb = deprotonated 4,4′-(4,4′-bipyridine-2,6-diyl) dibenzoic acid, DMA = N,Ndimethylacetamide), through symmetry modulation of a triazole ligand. The desymmetrized triazole linkers not only bond to the Zn(II) ions to result in a new helical Zn-triazolate chain building unit but also lead to the formation of a highly porous framework (N2 uptake: 617 cm3/g; BET surface area: 2393 m2/g) with 1D helical channels. The adsorption properties of desolved 1 were investigated by H2, C2H2, CO2, and CH4 sorption experiments, which showed that 1 exhibited high uptake capacity for H2 at 77 K and C2H2 around room temperature. More importantly, the high C2H2 uptake capacity but low binding energy makes this MOF a promising candidate for effective C2H2 capture from C2H2/CO2 and C2H2/CH4 mixed gases with low regenerative energy cost. In addition, 1 shows potential application for the luminescence sensing of small aromatic molecules picric acid (PA) and p-xylene (PX).

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enhanced tunability.11−20 The successful combination of acid− acid and acid−base ligands has created many MOFs with fantastic structures and outstanding properties. Although there are abundant reports on the structural variations of mixedligand MOFs by selecting different carboxylic or N-donor ligands, seldom of them pay attention to the symmetry of the ligands selected.19,21−24 In the previous investigation, a structural transformation from a 2D layered framework to a pillar-layered MOF via utilizing 4,4′-bpy (4,4′-bipyridine) as pillar was reported.25 The substitution of 4,4′-bpy with Hdatz (3,5-diamino-1,2,4-triazole) can afford a doubly interpenetrated pillar-layered MOF with an enhanced CO2 uptake capacity and separation ability.26 As mentioned above, symmetry of the ligands could greatly affect the structures of final products; if we replace C2 symmetric Hdatz with C1 symmetric Hatz, such an asymmetric structural element might be inherited into the resulting MOF. Bearing this point in mind, we choose 3-amino-1,2,4-triazole (Hatz) as the auxiliary ligand for its asymmetric structural feature (only one C1 symmetric axis existing), and thus was a new noninterpenetrated porous metal−organic framework {[Zn2(atz)2(bpydb)](DMA)8}n (1) with a novel helical Zntriazolate chain building unit constructed (see Scheme 1). Remarkably, this MOF exhibits high C2H2 uptake capacity, moderately high C2H2/CO2 and C2H2/CH4 selectivities, as well as low C2H2 binding energy around room temperature, which

INTRODUCTION As a promising kind of crystalline materials, metal−organic frameworks (MOFs) have undergone flourishing development in the past few decades and are emerging as a hot research topic in crystal engineering, chemistry, and the materials science community.1,2 MOFs can be readily self-assembled from inorganic (metal ions/clusters) and organic (ligands) building blocks, and such an assembly mode makes them inherit the merits of both constituents.3,4 Compared with the in situ generated inorganic building blocks, the effects of organic ligands are more designable and predictable. Chemists could design various organic ligands with different donor groups or functionalize them by means of organic synthesis.5−7 Meanwhile, the functional groups/donor groups on the organic ligands could not only bring new functionalities into MOFs but also guide the formation of the resulting frameworks through their effects on the molecular symmetry. For instance, Zhang et al. reported the effect of functional group positions in a BTB linker on the topology of MOF-177 frameworks, which could result in a 25% enhancement of hydrogen uptake. Matzger and co-workers have proposed a linker-directed vertex desymmetrization strategy for constructing MOFs using desymmetrized ligands, and the obtained MOFs show different structures compared with using the higher symmetry ones. Zhou and coworkers have prepared four isostructural MOFs with various functionalized pore surfaces by employing a series of desymmetrized diisophthalate ligands.8−10 On the other hand, the mixed-ligand strategy has been proved to be a feasible method to obtain MOFs with low cost, easy manipulation, and © XXXX American Chemical Society

Received: June 13, 2016

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DOI: 10.1021/acs.inorgchem.6b01419 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Comparison of Crystal Structures and SBUs of the Noninterpenetrated MOF (This Work) and Doubly Interpenetrated MOF (Previous Work) Viewed Along c Direction

indicates that it can be potentially used in the selective C2H2 separation with low regenerative energy cost. In addition, 1 could also be used as luminescent sensor for the detection of small aromatic molecules such as picric acid (PA) and p-xylene (PX).

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Table 1. Crystal Data and Refinement Results for 1 empirical formula formula weight T (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z ρcalcg/cm3 μ/mm−1 radiation goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] largest diff. peak/hole/e Å−3 Flack parameter

EXPERIMENTAL SECTION

Materials and Methods. The bpydbH2 ligand was prepared according to the literature procedure.27 The Labsys NETZSCH TG 209 Setaram apparatus was used to record the thermal analyses in the temperature range of 25−800 °C under a N2 atmosphere. Elemental analyses for C, H, and N were collected on a Vario EL III Elementar analyzer. Powder X-ray diffraction patterns were acquired from a Rigaku model Ultima IV diffractometer with Cu Kα radiation. N2, H2, C2H2, CO2, and CH4 sorption isotherms were collected on the Belsorp MAX volumetric adsorption equipment. A Varian Cary Eclipse fluorescence spectrophotometer was used to record the luminescent spectra. GCMC Simulation for the N2 Adsorption Isotherm. Simulation of N2 uptake for 1a at 77 K was performed with the Adsorption Isotherm task in the Sorption module embedded in Material Studio 6.0. Equilibration steps were set to 2 500 000, production steps were set to 100 000, fugacity steps were set to 40, and the sorption isotherm was set to logarithmic. The Metropolis method and universal force field (UFF) were applied during the simulation. Synthesis of {[Zn2(atz)2(bpydb)](DMA)8}n (1). A mixture of Zn(NO3)2·6H2O (0.120 g, 0.4 mmol), bpydbH2 (0.040 g, 0.1 mmol), Hatz (0.15 g, 1 mmol), and DMA (4 mL) was sealed in a 25 mL Teflon-lined stainless steel container and heated at 120 °C for 72 h under autogenous pressure. After the mixture was cooled to room temperature, a large amount of yellow block crystals were obtained. The yield was 75% for 1 (on the basis of the bpydbH2 ligand). Single-Crystal Structure Determination. An Oxford Xcalibur Gemini Eos diffractometer using the ω-scan technique was used to collect the single-crystal data of 1 at room temperature. The SHELXTL crystallographic software package was used to solve and refine the structure.28 All non-hydrogen atoms were refined anisotropically, and all H atoms of the organic linker were generated geometrically. The SQUEEZE option embedded in PLATON was used to remove the contribution of the highly disordered solvents.29 The chemical formula of solvated 1 was derived from the combination of the crystallographic data, elemental analysis, and TGA curve. Details for the structural parameters of 1 are listed in Table 1, and the selected bond lengths and angles are listed in Tables S1 and S2.

C84H106N20O16Zn3 1848.02 293(2) trigonal P3121 21.027(2) 21.027(2) 20.5274(10) 90 90 120 7859.6(16) 3 0.730 0.714 Mo Kα (λ = 0.71073) 0.991 R1 = 0.0834, wR2 = 0.1981 R1 = 0.1276, wR2 = 0.2330 1.05/−0.43 0.01(2)

layered MOF. The replacement of 4,4′-bpy with Hdatz could also afford a doubly interpenetrated pillar-layered MOF featuring a novel Zn-datz “paddle-wheel” as the pillar. Such a case was not found for the Hatz ligand-based MOF, in which the Zn-atz “paddle-wheel” connects with the additional Zn(II) ions along the c direction to result in a novel helical Zn-triazole chain building unit. Crystal Structure and Topological Analysis. The crystal data analysis shows that compound 1 crystallizes in the chiral trigonal space group P3121 and exhibits a three-dimensional porous framework with two different types of 1D helical channels along the c axis. There are one and a half crystallographically independent Zn(II) centers and one bpydb2− and one atz− ligand in one asymmetric unit. Atom Zn1 (occupancy: 0.5) is four-coordinated by two triazole N atoms and two carboxylic O atoms from two different bpydb2− ligands, creating a tetrahedral geometry (see Figure 1a). Zn2 is five-coordinated by two triazole N atoms and one pyridine N atom on the bpydb2− ligand, and the remaining sites are finished by two carboxylic O atoms from another bpydb2− ligand. The Zn−O bond lengths fall in the range of 1.954(1)− 2.380(2) Å, and the Zn−N bond lengths range from 2.010(7) to 2.056(2) Å. Zn2 and its symmetry-related atom Zn2A are ligated by two atz− ligands to afford a Zn−atz “paddle-wheel” cluster which is further connected by the Zn1 atom along the c

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RESULTS AND DISCUSSION Synthesis. In our previous study, we found the solvothermal reactions of Zn(NO3)2·6H2O with the bpydbH2 ligand in DMA tend to form the {[Zn2(bpydb)2(H2O)2](DMA)3(H2O)}n 2D layered structure with an ···ABAB··· packing fashion. Utilizing this characteristic, the 4,4′-bpy ligand was introduced to produce a doubly interpenetrated pillarB

DOI: 10.1021/acs.inorgchem.6b01419 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Compared with the MOF constructed from the same bpydb2− ligand and datz− pillar, we can speculate that the asymmetry substituent group on the atz− ligand might be responsible for the formation of the helical chain. The Flack parameter of 1 is 0.01, which further demonstrates the validity of the absolute configuration. Three such 1D helical SBUs are connected with each other via bpydb2− ligands to give rise to a large trigonal channel with a sphere diameter ca. 6.8 Å (Figure 1b and Figure S2 in the Supporting Information). It should be noted that the free NH2 groups are mostly located in the smaller channels formed by the 1D SBU chains. By considering the ligand as a three-connected node, Zn1 as a four-connected node, and the Zn-atz “paddle-wheel” as a six-connected node, the whole network of 1 can be viewed as a novel 3,4,6-connected net with the Schläfli symbol of (52.62.72)(52.6)2(53.62.77.82.9), which has not been reported in MOF chemistry (Figure 1c).33 PLATON calculation reveals that 1 contains 63.5% solvent accessible voids (Figure 1d), which is higher than that of the 4,4′-bpy pillared (51.5%) and datz− pillared (50%) MOFs reported earlier.25,26,35,36 PXRD and TGA Experiments. The phase purity of 1 was confirmed by the comparison of the simulated PXRD patterns from the crystal data with the experimental one (Figure S3 in the Supporting Information). The TGA curve of 1 exhibits a weight loss of 38.1% from 50 to 284 °C (calcd: 37.7% for eight lattice guest DMA molecules; see Figure S4 in the Supporting Information). The guest-free phase 1a can be readily activated by exchanging the DMA with CH2Cl2 and then pumped under high vacuum at room temperature. Its framework integrity and full activation were further confirmed by PXRD measurements and TGA, respectively (Figures S3 and S4).

Figure 1. (a) The coordination surroundings for the Zn2+ ions in 1; symmetry code A: −x, −x + y, 4/3 − z. (b) 1D helical SBU chain and channel in 1. (c) 3,4,6-connected net for 1. (d) Connolly surface diagram displays the two different types of channels in 1.

axis to give rise to an interesting 1D right-handed helical SBU chain with a diagonal distance of 2.8 Å (Figure 1a and Figure S1 in the Supporting Infomation). It should be noted that the two amino groups on the atz− ligands point to the opposite direction, which might be attributed to the minimization of the interaction energy in the “paddle-wheel” cluster. To our knowledge, the combination of the Hatz ligands and Zn(II) ions tends to give Zn-triazolate layers, and 1 represents the first MOF based on the helical Zn-triazolate chain-like SBUs.30−34

Figure 2. (a) N2 sorption isotherm at 77 K for 1a (inserted: PSD calculated by the Horvath−Kawazoe method). (b) The 77 K H2 sorption isotherms of 1a. (c) C2H2, CO2, and CH4 sorption isotherms around room temperature. (d) The C2H2 and CO2 Qst for 1a derived from the virial equation. C

DOI: 10.1021/acs.inorgchem.6b01419 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Guest-dependent emission intensity of 1. (b) Luminescent spectrum of 1 dispersed in DMA solution by addition of PA and PX. (c) The calculated HOMO and LUMO energies for the analytes and bpydbH2 ligand based on density functional theory at the B3LYP level.

on the organic ligand.43 The present H2 sorption behavior for 1a might be attributed to its unique 1D helical channels. The unique helical channel-type structure and permanent microporosity in 1a motivated us to study its potential applications for the gas storage and separation around room temperature. The single-component adsorption isotherms for C2H2, CO2, and CH4 were collected and are plotted in Figure 2c. Due to its large solvent accessible void, 1a could take up a high amount of C2H2 (218 cm3/g at 273 K and 120 cm3/g at 298 K) under 1 bar around room temperature. The C2H2 uptake value at ambient temperature is much higher than that of some promising MOFs with open metal sites and Lewis basic pyridyl sites used for selective C2H2 capture, such as UTSA-50 (91 cm3/g), UTSA-100a (95.6 cm3/g), M’MOF-3a (20 cm3/g), and UTSA-60a (70 cm3/g). As far as we know, 1a represents the best C2H2 adsorption property among the mixed-ligand MOFs.44−50 1a shows moderate CO2 uptake with the values of 109 cm3/g at 273 K and 52 cm3/g at 298 K. It should be pointed out that the CO2 sorption isotherms show a linear increase without reaching saturation even at 1 bar (which could not), which indicates a lack of strong binding sites in the helical channels of 1a. This speculation is further supported by the isosteric heat of adsorption of CO2 calculated by using the virial equation, whose values are about 20 kJ/mol during the loading process (Figure 2d). In addition, the Qst value at zero loading for C2H2 is 25 kJ/mol, and then decreases to 24 kJ/mol at higher loading, suggesting that there exist weak interaction sites for the C2H2 molecule in 1a. The Qst value of 1a for C2H2 at zero loading is much lower than that of many MOFs with open metal sites or polar donor atoms.44−50 These results show that

Gas Sorption Properties. To evaluate the permanent porosity of 1a, its N2 sorption isotherm was measured at 77 K (Figure 2a). 1a shows a typical type-I sorption isotherm without hysteresis, giving a BET (Brunauer−Emmett−Teller) surface area of 2340 m2/g and Langmuir surface area of 2393 m2/g; the BET surface area is similar to the one calculated from the Poreblazer_v3.0.2 (2388 m2/g).37 The saturated N2 uptake is 617 cm3/g, corresponding to a pore volume of 0.955 cm3/g. The GCMC simulated N2 sorption isotherm matches well with the one from the experimental result with a saturated N2 uptake of 626 cm3/g, indicating its full activation. Analysis of the pore size distribution from the N2 sorption data reveals a narrow distribution of micropores from 5.5 to 6.2 Å. The low-pressure H2 adsorption isotherms for 1a were collected at 77 and 87 K. 1a adsorbs 1.93 wt % (214 cm3/g) of H2 at 77 K, and 1.17 wt % (130 cm3/g) of H2 at 87 K (Figure 2b). Significantly, the H2 sorption data of 1a lie in the upper region of mixed-ligand MOFs.38−40 The Clausius−Clapeyron equation was used to evaluate the H2 isosteric heat, and its value at zero loading is 6.75 kJ/mol and decreases slowly with increasing H2 loading (Figure S6 in the Supporting Information). The Qst value of 1a at zero loading is comparable to those MOFs with open metal sites such as HKUST-1 (6.83 kJ/mol) and NOTT-122 (6.0 kJ/ mol), indicating a moderately strong framework−H2 interaction.41,42 The storage capacities of 1a for H2 are still not saturated, which means that their uptake can be further maximized at higher pressure. According to the literature, the H2 adsorption of MOFs under low pressure is mainly influenced by pore size, open metal site, and functional groups D

DOI: 10.1021/acs.inorgchem.6b01419 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the energy required for the regeneration of 1a will be lower than that of other MOFs for C2H2 capture, resulting in a significant energy savings and high working capacity for adsorption−desorption cycles. In addition, the CH4 sorption isotherms were also collected around room temperature, which showed the least uptake values (23 cm3/g at 273 K and 14 cm3/ g at 298 K) among the gases studied. The high C2H2 uptake capacity and low C2H2 binding energy encouraged us to examine its potential application in C2H2/ CO2 and C2H2/CH4 separation at room temperature. Using the initial slopes of the C2H2, CO2, and CH4 isotherms, the Henry’s law C2H2/CO2 and C2H2/CH4 selectivities at 298 K and 1 bar were calculated to be 3 and 11 (Figure S8 in the Supporting Information). Although these values are not comparable with some MOFs with abundant OMSs and polar donor sites, they are still comparable with many famous MOFs reported for selective C2H2 capture.51−54 The high C2H2 uptake capacity and low regenerative energy of 1a make this MOF a promising candidate for the practical use in C2H2/CO2 and C2H2/CH4 separation with low regenerative energy cost. Luminescent Properties. The solid-state luminescent spectra of 1 at room temperature exhibit strong emission at 409 nm upon excitation at 330 nm, which is consistent with the compounds reported related to the bpydbH2 ligand, indicative of a ligand-based luminescence.25,54,55 The high luminescent intensity and unique structural feature of 1 prompt us to explore its application as a luminescent sensor for the detection of harmful substituted benzene derivatives. The sensing ability was investigated by adding 200 ppm of different substituted benzene compounds into 5 mg of 1 dispersed in 3 mL of DMA solution. As shown in Figure 3a, the luminescent intensity of 1 largely depends on the type of substituted groups on benzene, with the luminescent intensity significantly quenched in picric acid (PA) and evidently enhanced in p-xylene (PX). Notably, the fluorescent intensity of 1 rapidly decreased to 51% at 10 ppm and 99% at 80 ppm PA (Figure 3b). Meanwhile, 1 exhibited a fluorescent increase of 56% at 200 ppm PX. In contrast, other substituted benzene compounds such as methylbenzene, chlorobenzene, nitrobenzene, 4-nitrotoluene, 2,6-dinitrotoluene, and 1,3-dinitrobenzene only show little effect on the fluorescence of 1 under the similar conditions (Figure S10 in the Supporting Information). These results demonstrate that 1 has high sensitivity for PX and PA among the substituted benzene derivatives studied. The photoinduced electron-transfer (PET) mechanism could be used for the explanation of the observed experimental results.56−59 As displayed in Figure 3c, the LUMO energy of PX is higher than that of the bypdbH2 ligand and other analytes, so electron transfer from PX to the LUMO of 1 occurs more easily and leads to a luminescent enhancement upon excitation. The LUMO energy of PA is much smaller than that of the bypdbH2 ligand and other analytes, and thus electrons can be transferred from the LUMO of 1 to PA, thereby leading to a quenching effect. This mechanism is consistent with the one previously proposed by other groups.56−59

selectivities. More importantly, the high C2H2 uptake but low C2H2 binding energy of 1a makes it a promising candidate for effective C2H2 capture with low regenerative energy cost. In addition, 1 could be applied as a luminescent sensor for the detection of small aromatic molecules such as PA and PX. This work also demonstrates the power of ligand symmetry modulation in the creation of new mixed-ligand MOFs and further modulation of their interpenetration and pore sizes/ curvatures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01419. Additional structural pictures, PXRD patterns, thermogravimetric analysis, fitting of the adsorption isotherms, emission spectra, and tables for selected bond lengths and angles (PDF) Crystallographic data for 1 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21471134), the Plan for Scientific Innovation Talent of Henan Province (154200510011), and the Program for Science & Technology Innovative Research Team in University of Henan Province (15IRTSTHN-002). D.M.C. thanks the Startup Fund for PhDs of Natural Scientific Research of Zhengzhou University of Light Industry (2015BSJJ042).

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REFERENCES

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CONCLUSION In conclusion, we have achieved a new porous metal−organic framework with an unusual helical Zn-triazolate chain building unit and 1D helical channels through symmetry modulation of a triazole ligand. This MOF exhibits not only significantly high H2 and C2H2 uptake capacities among the mixed-ligand MOFs but also moderately high C 2 H 2 /CO 2 and C 2 H 2 /CH 4 E

DOI: 10.1021/acs.inorgchem.6b01419 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.6b01419 Inorg. Chem. XXXX, XXX, XXX−XXX