Enhancing Proton Conductivity in a 3D Metal–Organic Framework by

May 8, 2017 - JLU-Liu44 exhibits high proton conductivity of 8.4 × 10–3 S cm–1 at ambient temperature (27 °C) and 98% relative humidity; it is c...
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Enhancing Proton Conductivity in a 3D Metal-Organic Framework by the Cooperation of Guest [Me2NH2]+ Cations, Water Molecules and Host Carboxylates Lifei Zou, Shuo Yao, Jun Zhao, Dongsheng Li, Guanghua Li, Qisheng Huo, and Yunling Liu Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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Enhancing Proton Conductivity in a 3D Metal-Organic Framework by the Cooperation of Guest [Me2NH2]+ Cations, Water Molecules and Host Carboxylates Lifei Zou,a Shuo Yao,a Jun Zhao,b Dong-Sheng Li,*,b Guanghua Li,a QishengHuo a and Yunling Liu*a a

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, P. R. China b

College of Materials and Chemical Engineering, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang 443002, China

*To whom correspondence should be addressed. Professor Yunling Liu, E-mail: [email protected] Professor Dong-Sheng Li, E-mail: [email protected]

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ABSTRACT A novel proton-conducting MOF material has been synthesized by using 3,3’,4,4’biphenyltetracarboxylic acid (H4bptc) as organic ligands, which linked to four [Zn(COO)4]2secondary building units and K ions through four carboxyl groups, and generated a 3D framework [Zn3K2(bptc)3(DMF)2][Me2NH2]4 (JLU-Liu44). The proton-conducting properties of JLU-Liu44 were investigated by impedance spectroscopy. JLU-Liu44 exhibits a low conductivity under anhydrous conditions, but the proton conductivity under humidified conditions significantly enhanced due to the cooperation of guest [Me2NH2]+ cations, water molecules and host carboxylates. JLU-Liu44 exhibits high proton conductivity of 8.4×10−3 S cm−1 at ambient temperature (27 oC) and 98% relative humidity, it is comparable to the most reported proton-conducting MOFs materials under the same condition for powdered sample. The activation energy (Ea) obtained from an Arrhenius plot was 0.25 eV, indicating that the proton conduction

behaviors

occur

through

a

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INTRODUCTION In the last two decades, Metal-organic frameworks (MOFs) have attracted much attention because of their designable architectures and potential applications in gas storage, selective separation, catalysis, and magnetism properties.1-7 With the increasing interests for related applications in fuel cells, MOFs recently have emerged as new promising candidates for novel proton-conducting materials.8-16 MOFs can offer precisely designed frameworks with pores and channels, in which not only accommodate guest molecules as proton carriers such as water, ammonium ion, imidazole, triazoles and so on, but also involve the space for the formation of hydrogen bonds which may be served as efficient proton-conducting pathways.17-26 MOFs exhibit higher proton conductivity in a wider temperature range. Two different types of protonconducting MOFs are widely known. Water-mediated proton-conducting MOFs work efficiently at low temperatures (20-80 oC) and depend on the presence of water molecules and/or hydrogenbonding interactions with water, whereas anhydrous proton-conducting MOFs operate above 100 o

C by replacing water with amphiprotic organic molecules.27-37 Metal carboxylates are appealing candidates for proton conduction materials.28, 38 Aromatic

carboxylates possess high density of oxygen atoms and rich variety of coordination modes, so that widespread utilized to bind metal centers into stably multidimensional frameworks. Furthermore, these oxygen atoms could contribute to the formation of extended H-bonding network that could provide an efficient pathway for proton conduction. Proton conductivity requires proton carriers belonging to acid groups or networks of hydrogen bonds.22 To date, [Me2NH2]+ cations as proton carriers in the pores of frameworks among the proton-conducting MOFs materials have been rarely reported. Feng et al. reported three InMOFs, one of them with [Me2NH2]+ cations displayed a conductivity of 6.7×10−6 S cm−1 at 22.5

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°C and 99.5% relative humidity (RH) ([NH2(CH3)2]·[In(FDA)2]).39 Banerjee et al. reported two isostructural metal-organic nanotubes (In-5TIA and Cd-5TIA) and two isophthalic acid-based InIA-2D isomeric MOFs, which all containing [Me2NH2]+ cations as proton carriers. They show proton conductivity in the range of 10-3-10-5 S cm-1 at ambient temperature and 98% RH.40-41 Ghosh et al. reported a 3D MOF {[(Me2NH2)3(SO4)]2[Zn2(ox)3]}n which exhibited higher conductivity under humid (4.2×10-2 S cm-1) and anhydrous conditions (1×10-4 S cm-1), respectively. The overall structure can be visualized as an anionic net of [Zn2(ox)3]2-n and a supramolecular cationic net of [(Me2NH2)3SO4]+n which interpenetrate with each other.27 Very recently, Fe-CAT-5 exhibits ultrahigh proton conductivity (5.0×10-2 S cm-1) at 98% RH and 25 o

C, which shows bound sulfate ligands with DMA guests residing in the pores as counterions.42

Therefore, it is desirable as well as challenging to develop MOF-related proton-conducting materials, which contain guest [Me2NH2]+ cations as proton carriers and give rise to high proton conductivity

at

ambient

temperature.

Herein,

a

novel

proton-conducting

material

[Zn3K2(bptc)3(DMF)2][Me2NH2]4 (JLU-Liu44) was successfully synthesized by a solvothermal method with Zn(NO3)2·6H2O, KCl and 3,3’,4,4’-biphenyltetracarboxylic acid (H4bptc) in DMF/H2O solvents. JLU-Liu44 exhibits high proton conductivity of 8.4×10−3 S cm−1 at ambient temperature (27 oC) and 98% relative humidity, which is comparable to the most reported proton-conducting MOFs materials under the same condition for powdered sample. EXPERIMENTAL SECTION Materials and physical characterizations All the reagents were obtained from commercial sources and used without further purification. Powder X-ray diffraction (PXRD) data were collected on a Rigaku D/max-2550 diffractometer with Cu-Kα radiation (λ = 1.5418 Å) with a tube voltage of 40 kV and current of 150 mA in 4 to

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40° in 2θ range. Elemental analyses (C, H, and N) were achieved by using a vario MICRO (Elementar, Germany). The thermal gravimetric analyses (TGA) were performed on a TGA Q500 thermogravimetric analyzer in the temperature range 28-800 oC under air with a heating rate of 5

o

C min-1. Water adsorption isotherms were measured using a Quantachrome

instruments, and the sample was dried at 80 °C under vacuum overnight before measurement. Synthesis of [Zn3K2(bptc)3(DMF)2][Me2NH2]4 (JLU-Liu44) 3,3′,4,4′-biphenyltetracarboxylic acid (H4bptc, 5 mg, 0.015 mmol), Zn(NO3)2·6H2O (12 mg, 0.04 mmol), KCl (3 mg, 0.04 mmol), N,N-dimethylformamide (DMF, 1 mL) and H2O (0.5 mL) were added respectively to a 20 mL vial, which was sealed and heated to 105 °C for 2 days, then cooled to room temperature. Colorless block-shaped crystals were washed with DMF and collected (45% yield based on H4bptc). Elemental analysis calcd. for C62H64O26N6K2Zn3: C, 47.0; N, 5.3; H, 4.0%. Found: C, 46.9; N, 5.9; H, 4.3%. Synthesis of [Zn3K2(bptc)3(DMF)2][Me2NH2]4·0.5H2O (JLU-Liu44·0.5H2O) The Single-crystal X-ray diffraction (SCXRD) measurement of JLU-Liu44·0.5H2O was performed after setting the samples of JLU-Liu44 under a humid atmosphere of 98% for one day and then immersing in DMF/H2O (2:1) solvents for one day. Elemental analysis calcd. for C62H65O26.5N6K2Zn3: C, 46.8; N, 5.3; H, 4.1%. Found: C, 46.9; N, 5.7; H, 4.4%. X-ray Structure Determinations Single-crystal X-ray data of compounds JLU-Liu44 and JLU-Liu44·0.5H2O were collected on a Bruker Apex II CCD diffractometer using graphite-monochromated Mo-Kα (λ = 0.71073 Å) radiation at room temperature. The structures were solved by direct methods using the SHELXS

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program of the SHELXTL package and refined by full-matrix least-squares analyses on F2 (SHELXTL-97).43 All the metal atoms were located first, and then the oxygen and carbon atoms of the compounds were subsequently found in difference Fourier maps. All non-hydrogen atoms were refined anisotropically. The final formula of JLU-Liu44 was derived from crystallographic data combined with elemental and thermogravimetric analysis data. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, CCDC 1438187 for JLULiu44 and 1471868 for JLU-Liu44·0.5H2O. Data can be obtained free of charge upon request at www.ccdc.cam.ac.uk/data_request/cif. Crystal parameters and structure refinement are summarized in Table S1. Proton Conductivity Measurement The alternating current (AC) impedance spectroscopy measurements were performed using a Solartron 1260 impedance/gain-phase analyzer connected to a Solartron 1287 electrochemistry interface. Zplot 3.1 was used as the control software and ZView 3.1 was used as the analysis software. A typical measurement was made over a frequency range between 106 Hz to 1 Hz with an input voltage amplitude of 200 mV (the sample was resistive under anhydrous conditions and the applied voltage was increased to 1500 mV). Variable impedance spectra were collected in different humidity and temperature which was obtained by saturated salt solutions. The conductivity measurements were performed after setting each sample at different humidity for approximately two days. Proton conductivity (S cm-1) was calculated using the formula σ = L/AR, where L is the pellet thickness, A is the pellet area (~9 mm in diameter and ~0.8 mm in thickness), R is the compound impedance obtained from Nyquist plot. RESULTS AND DISCUSSION

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Structure descriptions Single-crystal X-ray analysis of JLU-Liu44 revealed that it crystallizes in the Monoclinic system and C2/c space group. The asymmetric unit of JLU-Liu44 consists of half of Zn1 ion, one Zn2 ion, one K ion, one and half of bptc4- ligand, one DMF and two [Me2NH2]+ cations for charge balance. The bptc4- ligands exhibit two different coordination modes in JLU-Liu44: For one ligand, both aromatic rings are in the same plane, which coordinated to four Zn and two K ions; for the other distorted ligand, a dihedral angle between aromatic rings is 132°, linked to four Zn and three K ions (Supporting Information Fig. S1-2). In JLU-Liu44, each ligand is linked to four [Zn(COO)4]2- secondary building units (SBUs) and K ions through four carboxyl groups, and generated a 3D framework (Figure 1). K ion is coordinated by seven oxygen atoms from five ligands and one DMF molecule. Interestingly, the whole framework is also a 3D carboxylatesbased anionic network when omitting the K ion, and we don’t consider the connectivity to K when determining the framework topology. From a topological point of view, [Zn(COO)4]2SBUs can be simplified to a 4-connected node with tetrahedron geometry, and bptc4- ligand linked to four [Zn(COO)4]2- SBUs, which can be viewed as a 4-connected square planar node. Therefore, the structure of JLU-Liu44 can be described as a (4, 4)-connected network, which gives rise to pts topology with a Schläfli symbol of {42.84}. For another point of topology view, the 4-connected nodes of bptc4- ligand can be simplified as two 3-connected branch nodes, the network of JLU-Liu44 should be assigned to tfi topology with Schläfli symbol of {62.84}{62.8}2 (Figure 2).44 In order to neutralize these anionic structures, [Me2NH2]+ cations are located within the space and around [Zn(COO)4]2- anions along the y-axis. Three [Me2NH2]+ cations and six carboxyl groups of three ligands per asymmetric unit are involved in structural hydrogen bonds with a

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range from 1.887(4) to 2.498(4) Å (NH···OC) (Figure 3). It is well-know that there are three types of concepts to achieve high proton conductivity to MOFs: the first is to introduce guest molecules as proton carriers into the pores (type I); the second is to place acid groups on the framework (type II); the third is to incorporate charge-neutral proton-conducting species within the pores (type III).21,

31-32

JLU-Liu44 simultaneously contains both type I and II structural

features, which indicated that it should exhibit high proton conductivity. Stability analysis of JLU-Liu44 Thermal gravimetric analysis was carried out to assess the thermal stability of JLU-Liu44 under an atmospheric environment. The TGA curve shows a weight loss of 20% between 28-280 o

C, which corresponds to the loss of coordinated DMF molecules and guest [Me2NH2]+ cations.

Upon further heating, a major weight loss of 56% occurs between 280 and 630 oC should be attributed to the release of organic ligands (Fig. S3). In order to further confirm the structural stability of the sample, variable-temperature PXRD experiments were also carried out on assynthesized JLU-Liu44. The results show that the framework of JLU-Liu44 can be stable up to 280 oC (Fig. S4). Proton conductivity properties The proton conductivity of JLU-Liu44 was determined using alternating-current (AC) impedance measurements on compacted pellet samples under anhydrous conditions (Table S2). Below 60 oC, the proton conductivity is negligibly low. The conductivity increases with increasing temperature, 4.3×10−8 S cm−1 at 110 oC and reaches 8.1×10−7 S cm−1 up to 190 oC (Fig. 4a). The enhanced conductivity is attributed to the increased temperature, which may

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accelerate proton transfer within the framework.45 The pellet sample starts to deform above 190 o

C and we could not measure the proton conductivity at a higher temperature. In order to check the proton-conducting ability of JLU-Liu44, the proton conductivity under

humid conditions was also investigated. The proton conductivity of JLU-Liu44 was measured to be 8.4×10−3 S cm−1 at ambient temperature (27 oC) and 98% RH (Fig. 4b). The value is very high and is comparable to the most reported proton-conducting MOFs under the same conditions to date for powder sample (Table S3). The proton conductivity of JLU-Liu44 related with the relative humidity and decreased with decreasing humidification (Fig. S6). The conductivity value slightly decreased from 2.3×10−3 S cm−1 at 82% RH to 1.7×10−4 S cm−1 at 75% RH. With the decrease of humidity, the conductivity drops rapidly by three orders of magnitude reaching 2.4×10−7 S cm−1 at 53% RH. The highly humidity dependence of conductivity suggests that water molecules play a crucial role in the framework for the proton conduction.46 To assess the hydrophilicity and the amount of included water molecules, water adsorption isotherm was measured using dehydrated sample. The amount of water in JLU-Liu44 is determined to be around 1.4 molecules per formula unit at P/Po = 0.9 (Fig. S7). To gain an insight into the proton conduction mechanism, a single crystal structure of JLULiu44·0.5H2O was successfully obtained. The structure of JLU-Liu44·0.5H2O was determined using the same space group of JLU-Liu44, indicating that the both framework structures are the same except for the absence of water molecules (Fig. S8). Water molecule disordered over different positions in the pores, which may interact through hydrogen bonding with [Me2NH2]+ cations. The temperature dependence of the proton conductivity was measured at 98% RH to further probe the mechanism of proton conduction. At the ambient temperature (27 oC), JLU-

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Liu44 exhibits a proton conductivity of 8.4×10−3 S cm−1 at 98% RH. As the temperature increasing, a drop in conductivity is observed (1.6×10−5 S cm−1 up to 120 oC, Fig. S9-10 and Table S4). It is a rare phenomenon among the proton conducting MOFs materials.40-41 JLULiu44 shows higher proton conductivity than In-IA-2D-1 (3.4×10−3 S cm−1) at ambient temperature and 98% RH, because of JLU-Liu44 contains three [Me2NH2]+ cations in the cavity of the framework as proton carriers within 1000 Å3, while In-IA-2D-1 contains two [Me2NH2]+ cations as proton carriers in the same unit cell volume. This result confirms that the concentration of proton carrier ([Me2NH2]+ cations) is important for the proton conductivity.30, 47 Moreover, adsorbed water molecules play a key role in proton conductivity as mediators, which can facilitate the proton transferring to neighboring [Me2NH2]+ cations through reforming hydrogen-bonding networks (Grotthuss mechanism) or as the vehicles in the form of hydronium ions (vehicle mechanism).27, 48-50 The proposed pathway for the proton transport of JLU-Liu44 upon adsorbed water molecules, [Me2NH2]+ cations and host free carboxylate groups was shown in Fig. S11-S12. The results indicate that the high proton conductivity of JLU-Liu44 can be attributed to the host-guest cooperation between carboxylate groups, [Me2NH2]+ cations and adsorbed water molecules under humid conditions.51-52 Although the water uptake capacity of JLU-Liu44 is not higher compared to other MOFs materials, it exhibits good proton conductivity under high humid conditions. It implies that the proton conductivity is not simply influenced by the water concentration, but rather by other ionic species within the pores (i.e., [Me2NH2]+ cations).42 To ensure constant water content and the conductivity variation only depended on the temperature, we derived the activation energy by low temperature range.53 As shown in Figure 5, the Arrhenius plot was linearly approximated well. At 98% RH, the conductivity increases with

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the temperature increasing from 4 to 27 oC. Low temperature proton conductivity measurement shows activation energy of 0.25 eV at 98% RH. It indicates that the proton conduction process of JLU-Liu44 follows the Grotthuss mechanism (0.1-0.4 eV), which is comparable to the most known proton-conducting MOFs materials.54-58 CONCLUSION In conclusion, we have successfully synthesized a new 3D proton-conducting MOF JLULiu44 with 8.4×10−3 S cm−1 at ambient temperature (27 oC) and 98% RH. Notably, the conductivity of JLU-Liu44 is comparable to the most reported proton-conducting MOFs materials under ambient temperature and high humidity (above 90% RH) for powdered sample. To gain an understanding of the proton conduction mechanism, we successfully obtained a single crystal structure of JLU-Liu44·0.5H2O and the possible conduction pathway is proposed. The results indicate that the high proton conductivity of JLU-Liu44 can be attributed to high proton carrier concentration ([Me2NH2]+ cations) and adsorbed water molecules. The activation energy (Ea) obtained from an Arrhenius plot was 0.25 eV, indicating that the proton conduction behaviors occur through a Grotthuss mechanism. These results are important for understanding proton conducting and provide new perspective to further explore new proton-conducting MOF materials. ASSOCIATED CONTENT Supporting Information Crystallographic data in CIF format for JLU-Liu44 and JLU-Liu44·0.5H2O, powder X-ray diffraction patterns for simulated and as-synthesized samples, thermogravimetric analysis and

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proton conduction properties for JLU-Liu44. These materials are available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *(Y.L.) E-mail: [email protected] *(D.-S.L.) E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 21373095, 21371067, and 21621001). REFERENCES (1) Chaemchuen, S.; Kabir, N. A.; Zhou, K.; Verpoort, F. Metal-organic frameworks for upgrading biogas via CO2 adsorption to biogas green energy. Chem. Soc. Rev. 2013, 42, 93049332. (2) Murray, L. J.; Dincǎ, M.; Long, J. R. Hydrogen storage in metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1294-1314. (3) Li, J. R.; Kuppler, R. J.; Zhou, H. C. Selective gas adsorption and separation in metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1477-1504. (4) Li, J. R.; Sculley, J.; Zhou, H.-C. Metal-organic frameworks for separations. Chem. Rev. 2012, 112, 869-932.

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(5) Lee, J. Y.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal-organic framework materials as catalysts. Chem. Soc. Rev. 2009, 38, 1450-1459. (6) Yoon, M.; Srirambalaji, R.; Kim, K. Homochiral metal-organic frameworks for asymmetric heterogeneous catalysis. Chem. Rev. 2012, 112, 1196-1231. (7) Kurmoo, M. Magnetic metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1353-1379. (8) Yoon, M.; Suh, K.; Natarajan, S.; Kim, K. Proton conduction in metal-organic frameworks and related modularly built porous solids. Angew. Chem. Int. Ed. 2013, 52, 2688-2700. (9) Li, S. L.; Xu, Q. Metal-organic frameworks as platforms for clean energy. Energy Environ. Sci. 2013, 6, 1656-1683. (10) Yamada, T.; Otsubo, K.; Makiura, R.; Kitagawa, H. Designer coordination polymers: dimensional crossover architectures and proton conduction. Chem. Soc. Rev. 2013, 42, 66556669. (11) Kreuer, K. D.; Paddison, S. J.; Spohr, E.; Schuster, M. Transport in proton conductors for fuel-cell applications: simulations, elementary reactions, and phenomenology. Chem. Rev. 2004, 104, 4637-4678. (12) Laberty-Robert, C.; Vallé, K.; Pereira, F.; Sanchez, C. Design and properties of functional hybrid organic–inorganic membranes for fuel cells. Chem. Soc. Rev. 2011, 40, 961-1005. (13) Zhao, X.; Mao, C.; Bu, X.; Feng, P. Direct observation of two types of proton conduction tunnels coexisting in a new porous indium-organic framework. Chem. Mater. 2014, 26, 24922495. (14) Jeong, N. C.; Samanta, B.; Lee, C. Y.; Farha, O. K.; Hupp, J. T. Coordination-chemistry control of proton conductivity in the iconic metal-organic framework material HKUST-1. J. Am. Chem. Soc. 2012, 134, 51-54.

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(15) Kim, S.; Dawson, K. W.; Gelfand, B. S.; Taylor, J. M.; Shimizu, G. K. H. Enhancing proton conduction in a metal-organic framework by isomorphous ligand replacement. J. Am. Chem. Soc. 2013, 135, 963-966. (16) Ramaswamy, P.; Wong N. E.; Shimizu, G. K. H. MOFs as proton conductors-challenges and opportunities. Chem. Soc. Rev. 2014, 43, 5913-5932. (17) Yamada, T.; Sadakiyo, M.; Kitagawa, H. High proton conductivity of one-dimensional ferrous oxalate dihydrate. J. Am. Chem. Soc. 2009, 131, 3144-3145. (18) Taylor, J. M.; Mah, R. K.; Moudrakovski, I. L.; Ratcliffe, C. I.; Vaidhyanathan, R.; Shimizu, G. K. H. Facile proton conduction via ordered water molecules in a phosphonate metal-organic framework. J. Am. Chem. Soc. 2010, 132, 14055-14057. (19) Ohkoshi, S.-I.; Nakagawa, K.; Tomono, K.; Imoto, K.; Tsunobuchi, Y.; Tokoro, H. High proton conductivity in prussian blue analogues and the interference effect by magnetic ordering. J. Am. Chem. Soc. 2010, 132, 6620-6621. (20) Sahoo, S. C.; Kundu, T.; Banerjee, R. Helical water chain mediated proton conductivity in homochiral metal-organic frameworks with unprecedented zeolitic unh-topology. J. Am. Chem. Soc. 2011, 133, 17950-17958. (21) Sadakiyo, M.; Yamada, T.; Kitagawa, H. Rational designs for highly proton-conductive metal-organic frameworks. J. Am. Chem. Soc. 2009, 131, 9906-9907. (22) Pardo, E.; Train, C.; Gontard, G.; Boubekeur, K.; Fabelo, O.; Liu, H.; Dkhil, B.; Lloret, F.; Nakagawa, K.; Tokoro, H.; Ohkoshi, S.; Verdaguer, M. High proton conduction in a chiral ferromagnetic metal-organic quartz-like framework. J. Am. Chem. Soc. 2011, 133, 15328-15331. (23) Matoga, D.; Oszajca, M.; Molenda, M. Ground to conduct: mechanochemical synthesis of a metal-organic framework with high proton conductivity. Chem. Commun. 2015, 51, 7637-7640.

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(24) Umeyama, D.; Horike, S.; Inukai, M.; Itakura, T.; Kitagawa, S. Inherent proton conduction in a 2D coordination framework. J. Am. Chem. Soc. 2012, 134, 12780-12785. (25) Zhou, Z.; Li, S.; Zhang, Y.; Liu, M.; Li, W. Promotion of proton conduction in polymer electrolyte membranes by 1H-1,2,3-triazole. J. Am. Chem. Soc. 2005, 127, 10824-10825. (26) Pili S.; Argent S. P.; Morris C. G.; Rought P.; Garcia S. V.; Silverwood I. P.; Easun T. L.; Li M.; Warren M. R.; Murray C. A.; Tang C. C.; Yang S.; Schroder M. Proton conduction in a phosphonate-based metal-organic framework mediated by intrinsic “free diffusion inside a sphere”. J. Am. Chem. Soc. 2016, 138, 6362-6355. (27) Nagarkar, S. S.; Unni, S. M.; Sharma, A.; Kurungot, S.; Ghosh, S. K. Two-in-One: Inherent anhydrous and water-assisted high proton conduction in a 3D metal-organic framework. Angew. Chem. Int. Ed. 2014, 53, 2638-2642. (28) Okawa, H.; Shigematsu, A.; Sadakiyo, M.; Miyagawa, T.; Yoneda, K.; Obha, M.; Kitagawa, H. Oxalate-bridged bimetallic complexes {NH(prol)3}[MCr(ox)3](M = MnII, FeII, CoII; NH(prol)3+ = Tri(3-hydroxypropyl)ammonium) exhibiting coexistent ferromagnetism and proton conduction. J. Am. Chem. Soc. 2009, 131, 13516-13522. (29) Shigematsu, A.; Yamada, T.; Kitagawa, H. Wide control of proton conductivity in porous coordination polymers. J. Am. Chem. Soc. 2011, 133, 2034-2036. (30) Sen, S.; Nair, N. N.; Yamada, T.; Kitagawa, H.; Bharadwaj, P. K. High proton conductivity by a metal-organic framework incorporating Zn8O clusters with aligned imidazolium groups decorating the channels. J. Am. Chem. Soc. 2012, 134, 19432-19437. (31) Sadakiyo, M.; Yamada, T.; Honda, K.; Matsui, H.; Kitagawa, H. Control of crystalline proton-conducting pathways by water induced transformations of hydrogen-bonding networks in a metal-organic framework. J. Am. Chem. Soc. 2014, 136, 7701-7707.

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(32) Sadakiyo, M.; Yamada, T.; Kitagawa, H. Proton conductivity control by ion substitution in a highly proton conductive metal-organic framework. J. Am. Chem. Soc. 2014, 136, 13166-13169. (33) Umeyama, D.; Horike, S.; Inukai, M.; Hijikata, Y.; Kitagawa, S. Confinement of mobile histamine in coordination nanochannels for fast proton transfer. Angew. Chem. Int. Ed. 2011, 50, 11706-11709. (34) Zhai, Q. G.; Mao, C.; Zhao, X.; Lin, Q.; Bu, F.; Chen, X.; Bu, X.; Feng, P. Cooperative crystallization of heterometallic indium-chromium metal-organic polyhedra and their fast proton conductivity. Angew. Chem. Int. Ed. 2015, 54, 7886-7890. (35) Hurd, J. A.; Vaidhyanathan, R.; Thangadurai, V.; Ratcliffe, C. I.; Moudrakovski, I. L.; Shimizu, G. K. H. Anhydrous proton conduction at 150oC in a crystalline metal-organic framework. Nat. Chem. 2009, 1, 705-710. (36) Horike, S.; Umeyama, D.; Kitagawa, S. Ion conductivity and transport by porous coordination polymers and metal-organic frameworks. Accounts Chem. Res. 2013, 46, 23762384. (37) Ramaswamy, P.; Wong, N. E.; Gelfand, B. S.; Shimizu, G. K. H. A Water stable magnesium MOF that conducts protons over 10−2 S cm−1. J. Am. Chem. Soc. 2015, 137, 76407643. (38) Chen, W. X.; Xu, H. R.; Zhuang, G. L.; Long, L. S.; Huang, R. B.; Zheng, L. S. Temperature-dependent conductivity of Emim+ (Emim+ = 1-ethyl-3-methyl imidazolium) confined in channels of a metal-organic framework. Chem. Commun. 2011, 47, 11933-11935. (39) Bu F.; Lin Q.; Zhai Q. G.; Bu X.; Feng P. Charge-tunable indium–organic frameworks built from cationic, anionic, and neutral building blocks. Dalton Trans., 2015, 44, 16671-16674.

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(48) Okawa, H.; Sadakiyo, M.; Yamada, T.; Maesato, M.; Ohba, M.; Kitagawa, H. Protonconductive magnetic metal-organic frameworks, {NR3(CH2COOH)}[MaIIMbIII(ox)3]: effect of carboxyl residue upon proton conduction. J. Am. Chem. Soc. 2013, 135, 2256-2262. (49) Sadakiyo, M.; Okawa, H.; Shigematsu, A.; Ohba, M.; Yamada, T.; Kitagawa, H. Promotion of low-humidity proton conduction by controlling hydrophilicity in layered metal-organic frameworks. J. Am. Chem. Soc. 2012, 134, 5472-5475. (50) Shen, Y.; Yang, X. F.; Zhu, H. B.; Zhao, Y.; Li, W. S. A unique 3D metal-organic framework based on a 12-connected pentanuclear Cd(II) cluster exhibiting proton conduction. Dalton Trans. 2015, 44, 14741-14746. (51) Khatua S.; Bar A. K.; Konar S. Tuning proton conductivity by interstitial guest change in size-adjustable nanopores of a CuI-MOF: a potential platform for versatile proton carriers. Chem. Eur. J. 2016, 22, 16277-16285. (52) Zhang F. M.; Dong L. Z.; Qin J. S.; Guan W.; Liu J.; Li S. L.; Lu M.; Lan Y. Q.; Su Z. M.; Zhou H. C. The effect of imidazole arrangements on proton-conductivity in metal-organic frameworks. J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b01559. (53) Colodrero, R. M. P.; Pastor, P. O.; Losilla, E. R.; Alonso, D. H.; Aranda, M. A. G.; Reina, L. L.; Rius, J.; Demadis, K. D.; Moreau, B.; Villemin, D.; Palomino, M.; Rey, F.; Cabeza, A. High proton conductivity in a flexible, cross-linked, ultramicroporous magnesium tetraphosphonate hybrid framework. Inorg. Chem. 2012, 51, 7689-7698. (54) Colomban, P.; Novak, A. Proton transfer and superionic conductivity in solids and gels. J. Mol. Struct. 1988, 177, 277-308. (55) Kreuer, K. D. Proton conductivity: materials and applications. Chem. Mater. 1996, 8, 610641.

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(56) Mu, Y.; Wang, Y.; Li, Y.; Li, J.; Yu, J. Organotemplate-free synthesis of an openframework magnesium aluminophosphate with proton conduction properties. Chem. Commun. 2015, 51, 2149-2151. (57) Xu, G.; Otsubo, K.; Yamada, T.; Sakaida, S.; Kitagawa, H. Superprotonic conductivity in a highly oriented crystalline metal-organic framework nanofilm. J. Am. Chem. Soc. 2013, 135, 7438-7441. (58) Grancha T,; Soria J. F.; Cano J.; Amoros P.; Seoane B.; Gascon J.; García M. B.; Losilla E. R.; Cabeza A.; Armentano D.; Pardo E. Insights into the dynamics of Grotthuss mechanism in a proton-conducting chiral bioMOF. Chem. Mater. 2016, 28, 4608-4615.

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Figure Captions Figure 1 3D structure of JLU-Liu44. Figure 2 Structure description of JLU-Liu44: (a) Topology simplification of metal core; (b) Topology simplification of ligand; (c) Ball and stick view of the structure; (d) Space-filling view of the structure of JLU-Liu44 along y axis; (e) Polyhedral view of the tfi net; (f) Polyhedral view of the pts net. Color scheme: C, gray; O, red; Zn, green; K, purple. Guest molecules and hydrogen atoms are omitted for clarity. Figure 3 Hydrogen-bonding interactions between carboxylates and [Me2NH2]+ cations along yaxis. Atom colors: C, gray; N, blue; O, red; Zn, green; K, purple. Coordinated DMF molecules and hydrogen atoms are omitted for clarity. Figure 4 (a) Impedance spectrum of JLU-Liu44 under anhydrous conditions at variable temperatures; (b) Impedance spectrum of JLU-Liu44 at 27 oC and 98% RH. Figure 5 (a) Impedance spectrum of JLU-Liu44 at 98% RH and various temperatures; (b) Arrhenius plot of the proton conductivity at 98% RH.

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Figure 1

Figure 2

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Figure 3

Figure 4

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Figure 5

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Table of Contents Enhancing Proton Conductivity in a 3D Metal-Organic Framework by the Cooperation of Guest [Me2NH2]+ Cations, Water Molecules and Host Carboxylates Lifei Zou, a Shuo Yao,a Jun Zhao,b Dong-Sheng Li,*,b Guanghua Li, a QishengHuo a and Yunling Liu*a

A new 3D proton-conducting MOF has been synthesized that contains [Me2NH2]+ cations as proton carriers, which exhibits high proton conductivity of 8.4 × 10−3 S cm−1 at ambient temperature (27oC) and 98% RH.

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