Proton conductivity and dielectric anomalies in a chain-based metal–organic framework with a hydrogen-bonded network of [SO₃] groups, pyrazine and water molecules

Abstract

We report a chain-based multifunctional MOF featuring an extensive hydrogen-bonded network composed of coordinated/uncoordinated water, 2,7-NDS, and py molecules. This compound exhibits a proton conductivity of 6.03 × 10⁻⁶ S cm⁻¹ at 60 °C under 95%RH. It also shows two distinct dielectric anomaly behaviors arising from thermally induced guest molecule loss and structural reorganization, respectively.

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Metal–organic frameworks (MOFs), a class of emerging organic–inorganic hybrid materials, have been extensively explored for applications in gas storage,^1–3 separation,^4–7 catalysis,^8–10 and proton conduction.^11–13 Their high crystallinity and structural tunability make them highly promising candidates for advanced proton-conducting materials, as these properties not only facilitate a systematic investigation of the structure–property relationship but also provide an ideal platform for exploring proton conduction pathways. A continuous hydrogen-bonding network serves as a key pathway for rapid proton migration, thereby playing an essential role in achieving considerable proton conductivity through the facilitation of intermolecular proton transfer.^14,15 Hydrogen-bonding networks in MOFs can be constructed by introducing proton carriers (e.g., H₂O, H₃O⁺, imidazole, NH₄⁺, H₂SO₄, HCl) or by grafting functional groups (e.g., –COOH, –SO₃H, –PO₃H₂) into their frameworks.^16–20 While carboxylates readily assemble with metal ions due to their strong coordination, this often immobilizes the groups and hinders their role as hydrogen-bond acceptors in proton transport. Similarly, sulfonates and phosphonates, though weaker in coordination, also see their proton-conducting ability limited once incorporated into the framework. In contrast, the use of proton carriers as free molecules can overcome this limitation. This approach enables the formation of robust hydrogen-bonding networks for proton transport, while the proton carriers within the channels readily undergo an order–disorder phase transition, imparting additional functionalities such as dielectric response.^21–24 Although the Zheng²⁵ and Duan²⁶ groups have reported bifunctional MOFs with both proton conduction and dielectric response by incorporating ionic liquids and DABCO into the channels, respectively, the integration of these two properties within a single MOF material remains relatively scarce. There is a pressing need to develop more such multifunctional platforms to advance the field.

Herein, by leveraging the coordination differences between aromatic acid ligands and N-containing ligands, a chain-based multifunctional MOF, denoted as FUT-3, formulated as {[Cu(py) (H₂O)₄]·(2,7-NDS) (py) (H₂O)₂}_n (py = pyrazine, 2,7-NDS = 2,7-naphthalenedisulfonate) was synthesized, which displayed proton conductivity and dielectric anomalies. The extensive hydrogen-bonding chains inside the 1D interlayer spaces are constructed via coordinated water molecules, uncoordinated water molecules, uncoordinated py and uncoordinated 2,7-NDS units. To the best of our knowledge, this work presents the first chain-based MOF incorporating three types of guest molecules, which exhibits both dielectric anomalies and proton conductivity.

Blue needle-shaped crystals of FUT-3 were obtained by crystallizing Cu(NO₃)₂·3H₂O, pyrazine and a disodium salt of 2,7-NDS under an aqueous medium at 80 °C. Single-crystal X-ray diffraction shows that FUT-3 crystallized in a monoclinic crystal system with the P2/c space group (Table S1, SI), and the asymmetric unit is composed of 0.5 Cu1 ions, 0.5 Cu2 ions, four coordinated water molecules, a one coordinated py ligand and one uncoordinated 2,7-NDS unit and one uncoordinated py unit along with two uncoordinated water molecules (Fig. 1a). Each Cu(II) metal ion is coordinated to two nitrogen atoms from two different py ligands and four oxygen atoms from four axially coordinated water molecules, resulting in a distorted octahedral geometry {CuN₂O₄}. The py molecules function as bidentate bridging ligands, linking two adjacent Cu(II) metal ions and generating an infinite 1D chain (Fig. 1b). While the adjacent 1D chains are identical in composition, they differ in the spatial orientation of the py ligands and coordinated water molecules. This difference creates cavities between the chains, which accommodate and stabilize the uncoordinated 2,7-NDS, H₂O, and py molecules (Fig. 1c). The uncoordinated 2,7-NDS and py molecules are intercalated between two adjacent 1D chains, establishing a hydrogen-bonding network with both coordinated and free water molecules (Fig. 1d and Fig. S4). The Cu2 coordinated water molecules (O4w, O3w) and the uncoordinated water molecules (O6w) form hydrogen-bonded chains through interactions (2.656–2.810 Å) with the O4 and O5 atoms of the sulfonate groups of the 2,7-NDS ligands (Fig. 1e). Due to the symmetrical arrangement of the coordinated water molecules along the Cu2 atom, each metal chain is surrounded by two symmetrical hydrogen-bonded chains. The same situation is also observed in the Cu1 chain. These hydrogen-bonded chains hold great promise for proton conduction in this material. Furthermore, the adjacent 2,7-NDS and py layers exhibit π–π stacking interactions with centroid-to-centroid distances of 3.563 Å (Fig. 1f), which provide additional stability to the framework.

Fig. 1 (a) Asymmetric unit of FUT-3 (symmetry codes: A = 1 − x, y, 1/2 − z; B = x, 1 + y, z; C = x, −1 + y, z; D = −x, y, −1/2−z); (b) 1D polymeric infinite chain; (c) the guest 2,7-NDS and Py molecules between the layer and layer; (d) neighbouring chains are interlinked through H-bond interactions along the b-axis and enlarged view (e); (f) π⋯π stacking interactions.

To check the purity and homogeneity, the as-synthesized samples of FUT-3 were measured by powder X-ray diffraction (PXRD) at room temperature. As shown in Fig. 2, the PXRD pattern of the as-synthesized FUT-3 is in good agreement with the simulated pattern obtained from the single-crystal structure and indicates the phase purity of the as-synthesized sample. FUT-3 retains PXRD diffraction peaks consistent with those of the as-synthesized material after 12 h in common solvents such as EtOH, CH₃CN, and THF, as well as after 12 h in aqueous solutions at pH 1 or 11, revealing its high structural stability. Furthermore, exposure to air for one year or pressing into slices does not compromise the stability of the FUT-3 framework (Fig. S1b, SI). Such high chemical stability can be attributed to the presence of extensive π–π stacking and hydrogen-bonding interactions that permeate the entire structure. Thermogravimetric-differential scanning calorimetry analysis (TG-DSC) under a N₂ atmosphere has been performed to investigate its thermal stability (Fig. S3, SI). The weight loss of 18.26% before 101 °C is attributed to the removal of uncoordinated py and H₂O molecules (calcd: 18.79%), which corresponds to an exothermic process. On further heating, the weight loss of 11.99% was observed at 193 °C, which could be attributed to the expelling of coordinated water molecules (calcd: 11.66%). During this period, the endothermic peaks appear on the DSC curve, suggesting that the loss of coordinated water may induce structural reorganization. Following reorganization, the framework is stable up to 327 °C. Variable-temperature PXRD patterns were measured to further verify their thermal stability (Fig. S1a, SI). The experimental PXRD results match quite well with the simulated pattern below 100 °C. With a continuous increase in temperature, new diffraction peaks appear at 2θ = 7.4, signifying a structural change. This transition results in a new phase with superior thermal stability that remains stable up to 350 °C. The structural transition of FUT-3 during thermal activation provides an opportunity for us to study its dielectric response.

Fig. 2 PXRD patterns of FUT-3 treated with different organic solvents (a) and different pH aqueous solutions (b).

The high chemical stability and the presence of a hydrogen-bonded network composed of coordinated/uncoordinated water molecules, Py and 2,7-NDS units, prompted us to investigate the proton conductivity of FUT-3. The proton conductivities were measured by alternating current (AC) impedance measurements using a compacted pellet of the crystalline powder sample. Proton conduction measurements were performed by varying the temperature (20–60 °C) at 95% RH. At 20 °C, FUT-3 displayed a proton conductivity of 3.55 × 10⁻⁷ S cm⁻¹ under 95% RH. The proton conductivity gradually increases with increasing temperature, reaching a maximum value of 6.03 × 10⁻⁶ S cm⁻¹ at 60 °C, which is comparable to several reported MOF materials [Cd₄(cpip)₂(Hcpip)₂]_n·n(H₂bmib)·n(H₂O) (2.2 × 10⁻⁵ S cm⁻¹, 60 °C, 95% RH),²⁷ {[Zn(bpeH)(5-sip)(H₂O)]·(H₂O)}_n (2.5 × 10⁻⁶ S cm⁻¹, 65 °C, 95% RH),²⁸ and YCu161 (1.45 × 10⁻⁵ S cm⁻¹, 60 °C, 93% RH).²⁹ Although the proton conductivity of FUT-3 is not particularly high, it can nonetheless provide a strategy for designing proton-conducting materials and enrich the diversity of such materials. To examine the mechanism of proton conduction in FUT-3, we calculated the activation energy in terms of the least-square fit of the Arrhenius plot's slope. As shown in Fig. 3b, the activation energy of FUT-3 was 0.64 eV, which indicated that the proton transfer followed the vehicle mechanism.³⁰ Although FUT-3 forms an extensive hydrogen-bonding network through its sulfonic groups and water molecules (Fig. S5, SI), a relatively high activation energy is still required for proton transport, which may be due to the significant energy barrier for proton dissociation from water molecules, the source of protons in this system. Furthermore, the PXRD and IR results show that the peak positions are consistent with those of the as-synthesized sample (Fig. S1b and S2, SI), indicating that the MOFs remained intact and demonstrated good stability after the AC impedance measurement at 95% RH and different temperatures.

Fig. 3 (a) Nyquist plots of FUT-3 at different temperatures under 95% RH, the inset figure shows the enlarged plot at high frequency. (b) Arrhenius plot of the proton conductivity of FUT-3 under 95% RH at various temperatures. The least-squares fitting is shown as a solid line.

The dielectric properties of FUT-3 were also investigated using metal–insulator–metal parallel plate capacitance measurements. Fig. S7 shows the frequency dependences of the imaginary part of the and dielectric loss of FUT-3, respectively, at 293–573 K and at frequencies ranging from 100 kHz to 2 MHz. The and dielectric loss versus frequency plots show indistinct or undetectable relaxation peaks at the measured temperature and frequency range, which is likely due to restricted dipolar motion caused by hydrogen bonding and the increased structural rigidity following the removal of coordinated water. Despite the lack of clear relaxation in the frequency domain, the temperature dependent values clearly exhibited two distinct anomalies at 373 K and 453 K (Fig. 4a). The value is largely stable and shows no obvious frequency dependence between 293 and 353 K. However, at 373 K, it rises abruptly to a maximum of 6.47 at 100 kHz, which is lower than the values of JUC-125³¹ and CdCl(H-TBP)³² but higher than that of [Zn₂(Hbbim)₂(bbim)]_n,³³ indicating it as a low-κ dielectric material. And this peak value shows a slight decline as the frequency increases. Meanwhile, the dielectric loss also shows a sharp decline and decreases with increasing frequency at 373 K (Fig. 4b). These phenomena may be due to a combination of ionic polarization, orientational polarization of water molecules, and hydrogen bonding between molecules.^31,32 When the temperature further increases, the dielectric response initially decreases, exhibits a small response at 453 K, and then levels off, while the dielectric loss shows no significant change. TG-DSC and PXRD indicate that the structural reorganization of FUT-3 at 453 K is induced by the gradual loss of coordinated water molecules (Fig. S1a and S3, SI). Therefore, the second dielectric response is attributed to the thermal-induced loss of polar coordinated water molecules and subsequent structural reorganization. This process provides sufficient energy to enhance the dynamic dipole motion of water molecules and promote the formation of oxidized chains following the loss of coordinated water, resulting in a dielectric constant increase.³⁴ Ultimately, the expulsion of these polar molecules from the framework reduces the possibilities for polarization and intermolecular interactions (hydrogen bonding or ionic), thus lowering both the value and dielectric loss (Fig. 4).

Fig. 4 The relative dielectric constant (a) and the dielectric loss (b) for FUT-3 at different frequencies with temperature.

To summarize, we have presented a multifunctional MOF {[Cu(py) (H₂O)₄]·(2,7-NDS)(py)(H₂O)₂}_n (FUT-3), possessing both proton conduction and dielectric anomalies. The structure of FUT-3 features an extensive hydrogen-bonding network woven by coordinated water in its 1D chains, uncoordinated water between chains, uncoordinated py and 2,7-NDS units. FUT-3 exhibited a proton conductivity of 6.03 × 10⁻⁶ S cm⁻¹ at 60 °C and 95% RH. And anomalous dielectric behaviours at 373 K and 453 K, respectively. These intriguing properties may arise from water molecules, hydrogen-bonded chains, and structural reorganization. Our study establishes a viable route to multifunctional MOFs with tailored dielectric and proton conduction properties by engineering hydrogen-bonding networks.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data are available from the corresponding author on reasonable request.

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: crystal data, experimental details and additional figures. See DOI: https://doi.org/10.1039/d5nj04186d.

CCDC 2497464 contains the supplementary crystallographic data for this paper.³⁵

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22201036), the Guiding Project of the Fujian Provincial Science and Technology Department (2024H0010), and the University Student Innovation and Entrepreneurship Training Program (X202510388056).

Notes and references

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Footnote

† These authors contributed equally to this work.

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