High Proton Conductivity of Zinc Oxalate Coordination Polymers

Aug 23, 2016 - A novel metal–organic framework, (Hpy)2[Zn2(ox)3]·nH2O (n = 0, 1), having a pyridinium cation, was newly synthesized, and the crysta...
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High Proton Conductivity of Zinc Oxalate Coordination Polymers Mediated by a Hydrogen Bond with Pyridinium Teppei Yamada*,†,‡,¶ and Takuya Nankawa*,§ †

Division of Chemistry and Biochemistry, Faculty of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan ‡ Center for Molecular Systems, Faculty of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan ¶ JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Tokai-Mura, Ibaraki 319-1195, Japan S Supporting Information *

the reasons is that these MOF frameworks are anionic when the valence of metal is less than three, and we can introduce the proton donor as counterions. The [M2(ox)3] frameworks consist of large amounts of oxygen atoms that can also make dense hydrogen bonds with adsorbed water inside the frameworks. As a result, (NH4)2[Zn2(ox)3](adp)·3H2O showed extremely high proton conductivity. The highly proton conductive oxalate MOF has a half occupied water site inside the framework and shows phase transition at 86 K, below which the proton motion was frozen.14,22 The disorder of the water site seems to relax the interaction between water molecules and supports the high proton diffusion inside the framework. Usually MOFs are known to have highly ordered structure by design of the metal and ligand pairs, and we can apply such ordered structure for gas sorption and separation. However, in addition to that, the ordered pore would provide the “designed defect.” The instability of the ionic interaction in the frameworks could enhance the ionic conductivity, because ionic conductivity of solid state materials are widely known to infer to that of liquid phase due to the trap of ionic materials at the Coulombic potential minimum. A similar concept has already be proposed by Horike and co-workers.23,24 From the point of view of the “designed defect,” we focused on the pyridinium ion inside MOFs. The pyridinium ion has almost regular hexagonal shape with the acidic proton at the nitrogen site, and it can rotate easily when the hydrogen bond cleaves. Hence, the hydrogen bond could be fluctuated inside the MOFs with some controlled hydrogen bond networks. Here, we report on the structure, hydrogen bond networks, and proton conductivity of (C5H6N)2[Zn2(ox)3]·H2O where the disorder was embedded into the framework. (C5H6N)2[Zn2(ox)3]·H2O (1·H2O) was synthesized by a hydrothermal reaction of zinc oxide, pyridine, and dimethyl oxalate in water with a Teflon-lined autoclave. We intended to grow crystals slowly by using the hydrolysis reaction of dimethyl oxalate. Also, we applied insoluble zinc oxalate and enriched pyridine concentration compared with zinc or oxalate ions, for the sake of avoiding the formation of one-dimensional Zn(ox)·2H2O, which is always a main byproduct in the

ABSTRACT: A novel metal−organic framework, (Hpy)2[Zn2(ox)3]·nH2O (n = 0, 1), having a pyridinium cation, was newly synthesized, and the crystal structures were determined. The hydrated compound shows a high proton conductivity of 2.2 × 10−3 S cm−1 at 298 K and 98% relative humidity. Single crystal XRD analysis revealed a rotational displacement factor for the hydrated pyridinium ring and elongated water site that is thought to cause the high proton conductivity.

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roton conductive metal−organic frameworks (MOFs) have attracted great interest this decade because of their high level of designability that provides a novel study field in proton conductors. Similar to other proton conductors,1−4 proton conductive pathways of the MOFs consist of proton conducting media, mainly water, and acid points that donate protons to these conducting media. MOFs show moderate proton conductivity when compared to other classes of proton conductors; however they are intensely studied because they are advantageous in degrees of freedom in acid points, such as ligands, guests, and counterions introduced inside the framework.5−24 MOFs also provide design in proton conducting pathways consisting of successive hydrogen bond networks of the conducting media, which are widely controlled by the applied humidity of the atmosphere. Various MOFs have already been reported to show high proton conductivity, such as aryl polysulfonate or polyphosphonate MOFs,5 derivatives of MIL-536,7 and MIL-101,7,8 HKUST-1,9 sulfonylated UiO family and lanthanide MOFs,10,25 and some porphyrin MOFs,21,26 which often consist of substituted terephthalate ligands or mineral acid as a guest. Oxalate or phosphate ligands also act as candidates for constructing frameworks of coordination polymers. Although they do not always contain permanent porosity, they construct important components of proton conductive compounds because they sometimes show higher conductivity than those consisting of terephthalate or bipyridyl ligands. Metal oxalate MOFs are widely known to have various kinds of structures, such as 1D straight or zigzag chain structures and 2D honeycomb sheets as well as quartz like 3D structures. Among them, honeycomb [M2(ox)3] showed high proton conductivity and attracted much interest.14,16−18 One of © XXXX American Chemical Society

Received: June 30, 2016

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DOI: 10.1021/acs.inorgchem.6b01534 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

“staggered” structure, due to the tetrahedral or planar structure of the guest ion.17,18 From the crystal structure, it was found that two pyridinium rings locate in the interlayer of the [Zn2(ox)3] sheets (Figure 1a), and interestingly, pyridinium rings have large thermal parameters even at 143 K, as shown in Figure 1c. One of these two nitrogen atoms located in each pyridinium ring should be a carbon atom, which is disordered into two sites, and a nitrogen atom of the pyridinium ring has a hydrogen bond with crystalline water (N1−O4···2.98 Å). Thus, one of the two sites have a hydrogen bond while the other did not strongly have one. The oxygen atom of the water molecule is located at the middle of two adjacent pyridiniums; however the thermal ellipsoids of the oxygen also elongated along the a axis. It could also be disordered to be two sites, and one of them shows a hydrogen bond with one of the nitrogen atoms. These results indicate that the water atom can move easily at the sites, which also gives a perturbation to the acidity of the crystalline water. From the optimization of occupancy, the oxygen atoms of water molecules have occupancies of 0.5, and this crystal contains one water per formula from the optimization. The thermal ellipsoid of the N1 atom is elongated along the rotation of the pyridine ring, which indicates that the swing motion of the pyridine ring is observed in the framework. We also succeeded in determining the crystal structure of anhydrous 1 by exposing a crystalline 1·H2O to nitrogen gas flow at 30 °C as shown in Figure 2. Disorder of the pyridinium

reaction. A detailed reaction scheme is revealed in the Supporting Information. Water content in the MOF was estimated by thermogravimetry (TG), elemental analysis, and water vapor adsorption isotherms. From the thermogravimetry measurement, 1·H2O loses 4.0 wt % of weight until 100 °C, which corresponds to the loss of crystalline water (Figure S2 in SI). Compound 1 loses pyridine at 200 °C, and it decomposes at 300 °C. A 4.0 wt % loss of weight corresponds to the one equivalent of water, which is in good agreement with elemental analysis and water adsorption isotherm measurement (Figure S1). The existence of crystalline water was also detected by IR measurement as shown in Figure S3. Broad peaks can be observed at 2600 and 2200 cm−1, which corresponds to the fast exchange of proton between two pyridinium rings and intermediate water. It clearly indicates that another crystalline water is located inside the MOF. The purity of the sample was supported by the elemental analysis and X-ray powder diffraction measurement, and thus there is a water site inside the MOFs. The XRPD pattern of the 1 after water adsorption was measured and is no different from that of the as-grown sample as shown in Figure S4, and 1 was found to be stable against humid conditions. The crystal structure of 1·H2O was determined by a singlecrystal X-ray diffraction measurement at 143 K, as shown in Figure 1. It forms two-dimensional sheets of honeycomb

Figure 2. Crystal structure of 1 along the c axis (a) and (101) direction (b). Purple, orange, and gray balls corresponds to zinc, oxygen, and carbon atoms, respectively. Figure 1. Crystal structure of 1·H2O along the a (a) and the c (b) axes. Gray, pink, blue, and olive balls correspond to carbon, oxygen, nitrogen and zinc atoms, respectively. Hydrogen atoms are omitted for clarity. (c) Thermal ellipsoid and hydrogen bonding (shown as blue and white colored bars) of water and pyridinium. [Zn2(ox)3]∞ sheet is shown as white balls.

ring cannot be observed, and a nitrogen atom in the pyridinium has a hydrogen bond only to the oxygen atoms of the framework oxalate. It supports the hypothesis that water molecules in hydrated 1 can move inside the framework with evoking the rotation of pyridinium rings. Proton conductivity was evaluated by the AC impedance measurement by a quasi-four-probe method using a compacted pellet, both sides of which were attached to gold wire with gold paste. We estimated the proton conductivity by fitting the semicircle of the Nyquist plots, some of which are cited in the SI (Figure S5). As shown in Figure 3, 1 shows high proton conductivity at 95% RH, and at 98% the proton conductivity reaches 2.2 × 10−3 S cm−1 at 298 K. The conductivity increases drastically between 95 and 98% RH, and from the water

frameworks consisting of zinc and oxalate. Each zinc ion is coordinated by six oxygen atoms from three oxalate ions, and they form Δ and Λ fashion, alternatively, and make achiral compounds. All sheets stack in “eclipse” fashion and overlap along the c axis, similar to the (NH4)2[Zn2(ox)3](adp)·3H2O, where a linear adipic acid molecule trims the hole of the [Zn2(ox)3] sheets.14 On the other hand, {NR3(CH2COOH)}[MCr(ox)3]·nH2O and {NH(prol)3} [MCr(ox)3] are in a B

DOI: 10.1021/acs.inorgchem.6b01534 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

In summary, we have succeeded in introducing a highly disordered water site, the site of which is determined by XRD measurement. The proton conductivity is arguably high, due to the protonation of water by the acidity of pyridinium. Low activation energy was also achieved in the MOF, due to the disordered water sites by design. This result provides us confirmation of the strategy that the fluctuated hydrogen bond in the rigid and periodic framework could enhance the proton motion and proton conductivity.

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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.6b01534. Experimental details, TG, IR and powder XRD patterns (PDF) Crystallographic information (CIF) Crystallographic information (CIF)

Figure 3. Proton conductivity of 1·nH2O at various humidity (T = 298 K).

adsorption isotherm (Figure S1), the water uptake of 1 reaches 1.5 water at 95% RH. It is in good agreement with TG and elemental analysis. The crystal can incorporate additional water in the disordered structure due to the disordered structure, which could enhance the proton conductivity. In our previous result,14 the conductivity was 8 × 10−3 S cm−1, 4 times higher than the present result. This is due to the lower protonation ability of the pyridinium than adipic acid (pKa = 5.25 and 4.42, respectively), which causes a lower concentration of oxonium ions inside MOF. Proton conductivity at low humidity is in the range between 10−9 and 10−5 S cm−1, lower than that in the high humidity region by more than 2 orders of magnitude. The activation energy (Ea) of the proton conduction was evaluated by the Arrhenius plot by measuring proton conductivity at various temperature conditions, as shown in Figure 4.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected], Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to show our appreciation to Ms. Chihoko Fukakusa for experimental supports. The authors acknowledge Grants-in-Aid from MEXT of Japan (Nos. 21108001 and 21108002, area 2107, Coordination Programming).

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REFERENCES

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Figure 4. Arrhenius plot of the proton conductivity of 1 under 95% RH.

Conductivity was measured at 95% RH at all cases, and Ea is estimated to be 0.36 eV. The Ea value is lower than that previously reported, and the low activation energy indicates that the proton conduction of 1 is due to the Grotthuss mechanism, where proton conduction occurs by mixing a proton transport in the hydrogen bond with water to make oxonium and rotation of the proton carrier that causes reformation of the hydrogen bonds. From crystal structure, the disordered water and pyridinium site enhance the jumpdiffusion mode. It may facilitate the hydrogen bond formation and rearrangement, which causes low activation energy in proton conduction. C

DOI: 10.1021/acs.inorgchem.6b01534 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01534 Inorg. Chem. XXXX, XXX, XXX−XXX