Partially Fluorinated Cu(I) Triazolate Frameworks with High

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Partially Fluorinated Cu(I) Triazolate Frameworks with High Hydrophobicity, Porosity, and Luminescence Sensitivity Chao Wang, Jin Huang, Rui-Kang Huang, Zi-Ming Ye, Zong-Wen Mo, Si-Yang Liu, Jia-Wen Ye, Dong-Dong Zhou,* Wei-Xiong Zhang, Xiao-Ming Chen, and Jie-Peng Zhang* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China

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S Supporting Information *

ABSTRACT: Solvothermal reactions of 3-methyl-5-trifluoromethyl-1,2,4-triazole (Hfmtz) with Cu(CH3COO)2 at 120 °C in the presence of Cl− generate two partially fluorinated coordination polymers: i.e., [Cu4Cl(fmtz)3] (1 or MAF-51) and [Cu7Cl(fmtz)6] (2 or MAF52). Single-crystal X-ray diffraction revealed 1 to have a three-dimensional (3D) nonporous structure with pcu topology consisting of 6-connected Cu4(μ4-Cl) clusters and 2 to possess a highly porous (void ratio 48%) 3D bnn network consisting of 5-connected Cu5(μ5-Cl) clusters. Benefiting from the hydrophobic pendant groups, complete coordination of the ligand N atoms, and strong M−N coordination bonds, 1 and 2 possess high water stability (exposed to water for at least 1 year) and hydrophobicity (water contact angles of 141° and 148°, respectively). The N2 sorption isotherm of activated 2 gave Langmuir/BET surface areas of 1023/848 m2 g−1 and a pore volume of 0.365 cm3 g−1. Moreover, 2 can adsorb large amounts of benzene and methanol but barely adsorb water. Both 1 and 2 show phosphorescence of Cu(I) complexes, but only that of porous 2 is sensitive to O2, showing a linear Stern−Volmer response below 1 mbar with an ultrahigh Ksv value of 5234 bar−1 and ultralow limit of detection of 1.9 ppm.

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2-methylimidazole),33 ANA-[Zn(eim)2] (MAF-5, Heim = 2ethylimidazole),33,34 and RHO-[Zn(eim)2] (MAF-6),33,35 all the azolate N atoms are involved in coordination, and the open faces of the hydrophilic Zn(II) ions are covered by either methyl or ethyl groups.36 Due to the high electronegativity and small radius/ polarizability of fluorine, fluorinated organic molecules have special properties of low surface free energy and surface tension.37,38 MOFs consisting of fluorinated organic ligands usually exhibit high chemical stability and hydrophobicity. [Ag(bftz)] (FMOF-1, Hbftz = 3,5-bis(trifluoromethyl)-1,2,4triazole) is a representative example, which can exclude water molecules completely and adsorb large amounts (ca. 2.0 mmol g−1) of aromatic molecules such as benzene, p-xylene, o-xylene, and m-xylene.39,40 However, fluorinated organic ligands are generally difficult to synthesize, restricting their application. For example, Hbftz, the ligand of FMOF-1, needs to be synthesized by a complicated method with six steps.41 3-Methyl-5-trifluoromethyl-1,2,4-triazole (Hfmtz) is a partially fluorinated organic molecule, which can be synthesized easily by a one-step reaction of acetamidine hydrochloride and trifluoroacetohydrazide on a large scale.42 To explore the potential of Hfmtz for the construction of hydrophobic MOFs, we studied the reactions of Hfmtz and copper salts and obtained two new coordination polymers, showing notable porosity, hydrophobicity, and luminescence properties.

INTRODUCTION Porous materials can be applied to a wide variety of applications such as gas separation/storage, catalysis, water treatment, fluorescence sensing, optical devices, nanomedicine, and so on.1−8 There are many types of porous materials showing different properties.9,10 For example, open metal sites and micropores are suitable for catalysis and gas separation, respectively.3,11,12 The wettability of solid surfaces can vary between hydrophilicity and hydrophobicity. Because hydrophilic polar sites generally exist on the pore and particle surfaces, most porous materials are hydrophilic and have strong adsorption affinity for polar molecules.13−15 Without a polar functional group on the pore surface, porous materials can be hydrophobic, showing higher affinity to molecules with low polarity, which is useful for separating organic molecules from water and air.16−19 BPL, a typical hydrophobic commercial activated carbon which has been widely used for adsorption of volatile organic compounds, can readily adsorb a large amount of water at a relative humidity (RH) of 40%.20 The most hydrophobic zeolite is the pure-silica Silicate-1, which has a water uptake of over 2 mmol g−1 at a RH of 90%.21 Porous coordination polymers or metal−organic frameworks (MOFs) are new types of adsorbents with high crystallinity and tunable structures.22−28 Hydrophobic MOFs can be rationally designed by introducing hydrophobic functional groups to shield the metal ions and uncoordinated ligand donors.29−31 For instance, in the typical hydrophobic MOFs [Cu(detz)] (MAF-2, Hdetz = 3,5-diethyl-1,2,4-triazole),32 SOD-[Zn(mim)2] (MAF-4, also known as ZIF-8, Hmim = © XXXX American Chemical Society

Received: January 2, 2019

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DOI: 10.1021/acs.inorgchem.9b00006 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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with a continuous Xe900 xenon lamp and an F900 μs flash lamp. All instrument parameters such as excitation split, emission split, and scanning speed were fixed during the in situ measurements. O2 responses of photoluminescence were measured by placing the sample inside an ARS Optical Cryostat with a four-way valve which connects the chamber to a vacuum pump, an O2 cylinder, and a vacuometer.

EXPERIMENTAL SECTION Materials, Measurement, and Characterization. Commercially available reagents and solvents were used as received without further purification. The synthesis of the Hfmtz ligand was carried out by the literature method.42 Elemental analyses (C, H, N) were performed with a Vario El elemental analyzer. Powder X-ray diffraction (PXRD) patterns were collected on a Bruker D8 Advance diffractometer with Cu Kα radiation. Thermogravimetry (TG) analyses were performed on a TA Q50 thermogravimetric analyzer under nitrogen at a heating rate of 10 °C min−1. The water contact angle was measured on the contact angle system OCA 20 (Dataphysics, Germany). Gas and vapor isotherms were measured with an automatic volumetric adsorption apparatus (ASAP 2020M or BELSORPmax). The temperature was controlled by a liquid-nitrogen bath (77 K) or water bath (298 K). The as-synthesized sample was placed in the sample tube and dried for 5 h under vacuum at 130 °C to remove the remaining solvent molecules prior to measurement. Synthesis of [Cu4Cl(fmtz)3] (1 or MAF-51). A mixture of Cu(CH3COO)2 (54.3 mg, 0.3 mmol), Hfmtz (181.2 mg, 1.2 mmol), and NaCl (17.4 mg, 0.3 mmol) in a solvent mixture of N,N-dimethylformamide (DMF, 4.5 mL) and H2O (4.5 mL) was sealed in a glass tube and heated at 120 °C for 5 days and then cooled to room temperature at a rate of 5 °C h−1. Colorless cubelike crystals of 1 were collected by filtration, washed three times with DMF and H2O (1/1 v/v, 30 mL), and dried under vacuum (yield: 85% based on Cu salt). Anal. Calcd for [Cu4Cl(fmtz)3] (Cu4ClC12N9F9H9): C, 19.48; H, 1.23; N, 17.04. Found: C, 19.93; H, 1.29; N, 16.85. Synthesis of [Cu7Cl(fmtz)6] (2 or MAF-52). A mixture of Cu(CH3COO)2 (54.3 mg, 0.3 mmol), Hfmtz (181.2 mg, 1.2 mmol), and NaCl (17.4 mg, 0.3 mmol) in a solvent mixture of DMF (2.25 mL), H2O (2.25 mL), and toluene (4.5 mL) was sealed in a glass tube and heated at 120 °C for 5 days and then cooled to room temperature at a rate of 5 °C h−1. Colorless rodlike crystals of 2 were collected by filtration, washed three times with DMF and H2O (1/1 v/v, 30 mL), and dried under vacuum (yield: 75% based on Cu salt). Anal. Calcd for [Cu7Cl(fmtz)6] (Cu7ClC24N18F18H18): C, 20.88; H, 1.30; N, 18.26. Found: C, 21.09; H, 1.30; N, 18.18. Crystal Structure Determination. Single-crystal X-ray diffraction data of 1 were collected on a Pilatus XtaLAB P300DS single-crystal diffractometer by using graphitemonochromated Cu Kα radiation (λ = 1.54187 Å). Absorption corrections were applied by using the multiscan program REQAB. Single-crystal X-ray diffraction data of 2 were collected at BL17B of the National Center for Protein Sciences Shanghai and Shanghai Synchrotron Radiation Facility (λ = 0.7025 Å). The data processing was carried out by using the APEX3 program. The structures were solved by direct methods and refined by full-matrix least-squares techniques on F2 using the SHELXTL software package. Anisotropic thermal parameters were applied to all non-hydrogen atoms. The hydrogen atoms were generated geometrically. The PLATON SQUEEZE treatment was applied because the guest solvent molecules in 2 are extremely disordered and could not be modeled. Crystal data as well as details of data collection and refinements for the complexes are summarized in Table S1. Photoluminescence Measurement. Steady-state photoluminescence spectra and lifetime measurements were performed using an Edinburgh FLS980 spectrometer equipped

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RESULTS AND DISCUSSION Syntheses. The solvothermal reaction of Cu(CH3COO)2, Hfmtz, and NaCl in a mixed solvent of DMF and H2O at 120 °C for 5 days afforded colorless cubic crystals of [Cu4Cl(fmtz)3] (1 or MAF-51). If benzene, toluene, xylene, mesitylene, or ethylbenzene was added to the reactant, colorless rodlike crystals of [Cu7Cl(fmtz)6] (2 or MAF-52) could be obtained. Note that both structures contain Cu(I) rather than the Cu(II) of the starting materials, which has been observed in many other examples using solvothermal reactions.43 Excess Hfmtz and NaCl, likely serving as the reductant, are required for the syntheses of pure products; otherwise, CuO would be obtained as an impurity. When CuCl was used as the Cu(I) source, we always obtained 1 rather than 2, regardless of whether the aromatic additive was added or not. Without introduction of Cl−, white powders with unknown structure were obtained (Figure S1). Crystal Structures. Compound 1 crystallizes in the cubic I23 space group. The asymmetric unit contains two Cu(I) ions (Cu1 at general positions, Cu2 at a 3-fold rotation axis), one fmtz− ligand, and a chloride at a 3-fold rotation axis position (Figure S2). Cu1 is trigonally coordinated by two N atoms from two fmtz− ligands and one Cl−, and Cu2 is tetrahedrally coordinated by three N atoms from three fmtz− ligands and one Cl−. Each fmtz− ligand coordinates to three Cu(I) ions (Cu−N 1.90−1.99 Å). Each Cl− coordinates to four Cu(I) ions (Cu−Cl 2.41−2.84 Å) in a distorted-tetrahedral geometry to form a Cu4(μ4-Cl) cluster (Figure 1a), which is terminated by the N4 atoms of three triazolate ligands and three pairs of pyrazolate fragments of triazolate ligands. Each Cu4(μ4-Cl) cluster is linked with six adjacent clusters by asymmetric fmtz− ligands to form a three-dimensional (3D) framework with the

Figure 1. (a) Cu4(μ4-Cl) cluster of 1. (b) Cu5(μ5-Cl) cluster of 2. (c) Coordination network structure of 1. (d) Coordination network and pore-surface structures of 2. B

DOI: 10.1021/acs.inorgchem.9b00006 Inorg. Chem. XXXX, XXX, XXX−XXX

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Porosity and Hydrophobicity. The N2 sorption isotherm of activated 2 measured at 77 K exhibits type I character with a saturated uptake of 10.5 mmol g−1, corresponding to a pore volume of 0.365 cm3 g−1, well consistent with the value calculated from its crystal structure (0.370 cm3 g−1), which demonstrated the high purity and crystallinity of the sample. The Brunauer−Emmett−Teller (BET) and Langmuir surface areas were calculated as 848 and 1023 m2 g−1, respectively (Figure 2). The pore size distribution of 2 calculated with the

pcu topology (Figure 1c and Figure S3). Due to the short linkers and large pendant groups, there is no accessible solventfree volume in 1. Compound 2 crystallizes in the monoclinic C2/m space group. The asymmetric unit contains five Cu(I) atoms (Cu1 and Cu2 at mirror planes, Cu3 and Cu4 at general positions, Cu5 at a 2-fold rotation axis), four fmtz− ligands (two at mirror planes and two at general positions) and a chloride at the mirror plane (Figure S4). Cu1 and Cu5 are linearly coordinated by two N atoms from two fmtz− ligands (Cu−N 1.91−2.05 Å). Cu2 adopts a distorted T-shaped coordination geometry with two N atoms from two fmtz− ligands and one Cl−. Cu3 and Cu4 are tetrahedrally coordinated by three N atoms from three triazolate ligands and Cl−. All fmtz− ligands exhibit the normal μ3 coordination mode. The Cl− coordinates to one Cu2, two Cu3, and two Cu4 ions, forming a pentanuclear Cu5(μ5-Cl) cluster (Cu−Cl 2.52−2.62 Å), which is terminated by two fmtz− ligands with the N4 atoms and six fmtz− ligands with the pyrazolate fragments (Figure 1b). Each Cu5(μ5-Cl) cluster is connected with five adjacent clusters in a complicated way to form a 3D framework with 5connected bnn topology (Figure S5). The framework possesses 3D channels with a total solvent-accessible volume of 48.3% (calculated by PLATON), and hydrophobic CF3 and CH3 groups of the ligands are exposed on the surface (Figure 1d and Figure S6). As shown in Figure S6, the 3D pore system of 2 consists of cavities with a diameter of 10.6 Å with large apertures along the b-axis (7.2 Å × 8.2 Å) and the c-axis (7.4 Å × 8.3 Å). Structural Stability. Powder X-ray diffraction (PXRD) patterns of as-synthesized 1 and 2 matched well with their simulated patterns, demonstrating their phase purity (Figures S7 and S8). The thermogravimetry (TG) curve of 1 showed a sharp weight loss above 300 °C, corresponding to decomposition (Figure S9a). TG curve of as-synthesized 2 showed a continuous weight loss of 18% below 100 °C corresponding to the guest removal and then a plateau from 100 to 280 °C until decomposition was observed (Figure S9b). The PXRD pattern of activated 2 is slightly different from that of as-synthesized 2. However, the PXRD pattern of as-synthesized 2 can be recovered by exposing activated 2 to toluene vapor at room temperature (Figure S8), meaning that the structural transformation is guest-dependent and reversible. The color of 1 and 2 remained white even on exposure to air and water, and their PXRD patterns showed no change (Figures S10−S12). The high chemical stability of porous 2 is rare because Cu(I) complexes are usually air/water sensitive and unstable.44,45 Further, PXRD patterns of 2 remained unchanged after the samples were immersed in HCl solution (pH 2) and NaOH (pH 13) solution for 24 h and for at least 1 year under a RH of 100% in air at room temperature (Figures S11 and S12). These properties can be attributed to the strong metal−triazolate and Cu(I)−Cl bonds, as well as the hydrophobic side groups, which hinder the access of water molecules to the metal sites.17,18 Nevertheless, in boiling water compound 2 decomposed to form CuO (Figure S12). Moreover, compound 2 can transform to 1 in a boiling mixture of methanol and water (Figure S12), indicating that 1 is thermodynamically more stable. Water contact angle measurements performed on pressed powders of 1 and 2 gave values of 141(2)° and 148(2)°, respectively (Figure S13), confirming their high hydrophobicity on the particle surfaces.

Figure 2. N2 (77 K), methanol, benzene, and water vapor (298 K) adsorption (solid) and desorption (open) isotherms of 2.

N2 adsorption isotherm using the Horvath−Kawazoe (HK) method was mainly distributed in the range 6.5−11.0 Å, with a peak at 8.4 Å (Figure S14), consistent with those measured from the crystal structure. For investigation of the hydrophobicity of the inner pore of 2, water, methanol, and benzene vapor sorption isotherms were measured at 298 K (Figure 2). The saturated uptakes of benzene and methanol reached 3.7 and 7.4 mmol g−1, corresponding to pore volumes of 0.35 and 0.30 cm3 g−1, considering that the liquid densities of benzene and methanol are 0.88 and 0.79 g cm−3, respectively. These pore volumes are smaller than the theoretical volumes calculated from the crystal structures (0.365 cm3 g−1), because the vapor molecules cannot utilize all the pore spaces. The sorption isotherms of benzene and methanol can be described as type V, meaning that the host−guest interactions are weaker than the guest−guest interactions (π−π stacking and hydrogen bonding), consistent with the hydrophobic nature of compound 2. The initial slope of the benzene isotherm is much larger than that for methanol, and the inflection point of the benzene isotherm (P/P0 = 0.09) is much lower than for methanol (P/P0 = 0.26), meaning that the host can bind benzene much more strongly than is the case for methanol. In contrast, 2 can barely adsorb water with only 0.045 mmol g−1 uptake at P/P0 = 0.95. The water and organic solvent adsorption behaviors of 2 are similar to those of typical hydrophobic MOFs: i.e., MAF-2, MAF-4, MAF-5, MAF-6, and FMOF-1.34,35,39 More accurately, the adsorption capabilities of organic solvents of 2 are larger than for those of MAF-2 and FMOF-1, similar to those for MAF-4/MAF-5, and smaller than that for MAF-6. Furthermore, 2 can adsorb various larger hydrocarbons such as n-alkanes, xylene, ethylbenzene, mesitylene, etc. (Figure S15). Luminescence Properties. Oxygen detection/sensing is very important in the chemical industry, metallurgy, biological engineering, and many other places.46 In comparison with C

DOI: 10.1021/acs.inorgchem.9b00006 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry other detection (electrochemical and chemical) methods, luminescence sensing has many advantages such as high sensitivity, no oxygen consumption, noninvasive and fast measurements, and so on. Phosphorescent complexes consisting of precious-metal ions are classical oxygen-sensing luminescence probes. However, they not only are expensive but also must be dispersed/loaded on porous matrix/substrates to allow oxygen diffusion. Luminescent MOFs have attracted increasing attention in recent years as promising singlecomponent oxygen-sensing materials.47 Cu(I)-based luminescent MOFs, especially MAF-2 and its derivatives, have achieved ultrahigh O2 sensitivity, long luminescence lifetime, and ultrahigh O2 sensitivity without using precious metals.48−50 Photoluminescence spectra of microcrystalline 1 and 2 were measured at room temperature under vacuum, which showed broad emission spectra with excitation/emission maxima at 320/525 and 342/536 nm (Figure S16), respectively. Considering the small π conjugating system of the ligands and the large Stokes shifts, the emissions can be assigned to originate from the metal to ligand charge transfer (MLCT) excited states. Moreover, the luminescence lifetimes of 1 and 2 under vacuum were measured to be 21.5 and 14.4 μs, respectively (Figure S17), which further demonstrated that the luminescence of 1 and 2 is phosphorescence originating from the triplet excited states. Interestingly, in air, the luminescence intensity of 1 was basically maintained (Figure S18) but that of 2 decreased obviously, indicating luminescence quenching by O2 because of its porous structure as in the handful of Cu(I) MOFs.48 The photoluminescence of 2 at different O2 pressures (0.001 mbar to 1 bar) was measured at room temperature (Figure 3a). The luminescence intensity of 2 at 1 bar of O2 can be quenched 99.6% (corresponding to I0/I = 265). The oxygen pressure dependent luminescence intensity of 2 was plotted according to the Stern−Volmer (SV) equation (eq S1), which exhibited a nonlinear shape, indicating the existence of more than one luminescence component (Figure 3b). The nonlinear Stern−Volmer plots can be fitted by the two-site SV equation (eq S2), giving the fitting parameters f1 = 0.997, KSV1 = 2464 bar−1, f 2 = 0.003, and KSV2 = 0 bar−1 (Figure 3b). This result indicated the presence of a trace oxygen-insensitive luminescent component in the sample, which might be defects inside the crystal or on the crystal surface.51 Although the SV curve is not linear, it shows good linearity below 1 mbar, meaning that it is suitable for sensing low-pressure oxygen. Linear fitting of the quenching data below 1 mbar gave a very large Stern− Volmer equation constant of 5234 bar−1 and a very low limit of detection (LOD) of 1.9 ppm (Figure 3b). The O 2 luminescence sensitivity of 2 is higher than those of all other MOFs except for a few constructed by the solid-solution strategy.49 Moreover, as shown in Figure S19, the luminescence intensity of microcrystalline 2 in an alternating 1 bar O2 atmosphere and vacuum demonstrated high stability, indicating that the trace oxygen-insensitive luminescent component in the sample was not generated from continuous decomposition or structural transformation of the material in air. After O2 pressure is altered, the luminescence intensity of 2 reaches equilibrium less than 1 s, and there was no obvious luminescence decreasing during eight cycles.

Figure 3. (a) Emission spectra under various O2 partial pressures and (b) Stern−Volmer plot (points for data and red line for fitting using eq S2) of the O2 pressure-dependent luminescence intensity (inset: low-pressure region fitted by eq S1) of 2 (λex 342 nm, λem 536 nm).

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CONCLUSION In summary, we have studied the self-assembly of a partially fluorinated 1,2,4-triazole ligand and copper ions and obtained two unique coordination polymers containing different Cu(I) chloride clusters and very different porosities. The photoluminescence, adsorption, and hydrophobicity properties of the new compounds were studied systematically, in which the porous compound exhibits not only high hydrophobic and organic solvent adsorption similar to that of the classic hydrophobic MOFs but also extremely high phosphorescence O2 sensitivity.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00006. Experimental procedures, TG curves, PXRD, water contact angle images, and photoluminescence spectra (PDF) Accession Codes

CCDC 1888098−1888099 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. D

DOI: 10.1021/acs.inorgchem.9b00006 Inorg. Chem. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for D.-D.Z.: [email protected]. *E-mail for J.-P.Z.: [email protected]. ORCID

Dong-Dong Zhou: 0000-0003-1105-8702 Wei-Xiong Zhang: 0000-0003-0797-3465 Xiao-Ming Chen: 0000-0002-3353-7918 Jie-Peng Zhang: 0000-0002-2614-2774 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21731007, 21821003, 21701191, and 91622109) and Guangdong Pearl River Talents Program (2017BT01C161). We thank the staffs of the BL17B/ BL18U/BL19U1/BL19U2/BL01B beamlines at the National Center for Protein Sciences Shanghai and Shanghai Synchrotron Radiation Facility for assistance in collecting the singlecrystal diffraction data of 2.

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DOI: 10.1021/acs.inorgchem.9b00006 Inorg. Chem. XXXX, XXX, XXX−XXX