Water-mediated proton conduction in Ni(II) and Co(II) benzenetriphosphonates

Lu Fenga, Zhi-Quan Pana, Hong Zhou*a, Min Zhoub and Hao-Bo Houb
aCollege of Chemistry and Environmental Technology, Wuhan Institute of Technology, Wuhan 430073, Hubei, China. E-mail: hzhouh@126.com
bSchool of Resource and Environmental Science, Wuhan University, Wuhan 430072, Hubei, China

Received 19th July 2019 , Accepted 23rd August 2019

First published on 28th August 2019


Two novel transition metal compounds, [Ni(4,4′-bipyH)2(H2O)4]·2(H4bmt)·9H2O (1) and [Co(4,4′-bipy)(H2O)4][Co(4,4′-bipyH)2(H2O)4]·2(H3bmt)·6H2O (2), have been synthesized. They possess continuous H-bond networks and high-density [PO3] groups, which give conductivity values in the order of 10−3 S cm−1 in a wide temperature range and 98% relative humidity.


Proton-conducting materials have attracted increasing interest owing to their potential applications in electrochemical reactors, electrochromic displays, sensors and fuel cells.1 Currently, the conductivity of commercial Nafion-based polymer membranes can reach 10−1 S cm−1 at 60–80 °C and 98% RH.2 However, the application of Nafion is limited due to its high cost, low thermal stability and restricted operating temperature.3 Moreover, the proton pathway is difficult to investigate at the atomic level due to the amorphous nature of Nafion. Recently, significant efforts have been offered to explore desirable proton-conducting materials with higher proton conductivity and long-term recycling lifetime.4 To achieve this purpose, many materials, including organic polymers, inorganic materials, covalent organic frameworks (COFs), etc., have been studied.5 Among them, metal–organic frameworks/coordination polymers (MOFs/CPs) are considered good candidates because of their exceptionally high crystallinity and well-designed pores for proton-conducting pathways.6 To date, the magnitude of excellent conductivity for several MOFs/CPs is over 10−2 S cm−1,7 indicating the potential application in proton conduction of these hybrid materials. Compared with carboxylate ligands, phosphonic acid ligands exhibit more versatile coordination modes and stronger complexation ability towards metal ions. Furthermore, the P–OH groups are hydrophilic and facilely hydrogen bond with guest molecules, serving as proton conduction sites.8 Thus, the complexes derived from phosphonic acid ligands, treated as potential proton conductors, are based on the above structural characters. Zheng and coworkers have made systematic summaries of the recent progress on the proton-conductive metal phosphonates, revealing that the concentration of proton carriers and continuous H-bond networks are the key factors affecting proton conduction.9 The materials, which include PCMOF-10 (σ = 3.55 × 10−2 S cm−1, 70 °C, 95% RH),10 Al-HPB-NET (σ = 5 × 10−2 S cm−1, 120 °C, 50% RH),11 PCMOF-2½ (σ = 2.1 × 10−2 S cm−1, 85 °C, 90% RH)12 and (H3O)[Y2(H5btp)(H4btp)]·H2O (σ = 2.58 × 10−2 S cm−1, 94 °C, 98% RH),13 give the best proton conduction results, comparable to the commercially available Nafion materials. These research works have provided favorable support for the development of phosphonate-based proton-conducting materials. Benzene-1,3,5-tris(methylenephosphonic acid) (H6bmt) features nine phosphonate oxygen atoms and three flexible –CH2–PO3H2 groups, and it is apt to construct hydrophilic networks. To the best of our knowledge, the proton conduction of H6bmt-derived complexes has not been studied so far. In this work, two water-stable compounds, [Ni(4,4′-bipyH)2(H2O)4]·2(H4bmt)·9H2O (1) and [Co(4,4′-bipy)(H2O)4][Co(4,4′-bipyH)2(H2O)4]·2(H3bmt)·6H2O (2), have been assembled from H6bmt and 4,4′-bipy, and exhibit superprotonic conductivity in the order of 10−3 S cm−1 in a wide temperature range and 98% RH. The values are higher than most of the reported proton conductors of phosphonate-based materials.14

Single-crystal X-ray diffraction study reveals that 1 and 2 crystallize in the triclinic space group P[1 with combining macron] (Table S1). The molecular structure of 1 contains one Ni2+, two H4bmt2−, two 4,4′-bipyH cations, four coordinated water molecules, and nine lattice water molecules. Meanwhile, the asymmetric unit in 2 is constituted by two Co2+, two H3bmt3−, two 4,4′-bipyH cations, one 4,4′-bipy, eight coordinated water molecules and six lattice water molecules. It can be seen that the ligand (H6bmt) exists in both complexes as counter anions (H4bmt2− in 1 and H3bmt3− in 2) located in the outer sphere of the complexes (Fig. S1 and S2). In the mononuclear structure of 1, each Ni(II) ion has a distorted octahedral coordination configuration surrounded by four water molecules and two nitrogen atoms from two 4,4′-bipy molecules (Fig. S1a–1c). Each coordination unit [Ni(4,4′-bipyH)2(H2O)4] has H-bond connections with two H4bmt2−. The coordination environment of Co1(II) in 2 is similar to Ni(II) (Fig. S2a–2b). However, different from 1, two N atoms of each 4,4′-bipy molecule in 2 participate in the coordination interactions with Co2(II), resulting in a 1D chain structure (Fig. S2c). The bond lengths of Ni–O and Ni–N fall in the range of 2.045(2)–2.068(2) Å and 2.106(2)–2.110(3) Å, respectively. The Co–O lengths are between 2.091(3) Å and 2.116(2) Å, and the Co–N lengths are 2.147(2) Å and 2.151(2) Å, respectively (Table S2). The bond lengths between metal ions and O(N) are comparable with those reported in the literature.15 Instead of coordinating with metal ions, all of the phosphonate oxygen atoms in 1 and 2 link with surrounding water molecules through H-bond interactions; some of the water molecules also coordinate with metal ions, leading to 3D supramolecular self-assembly, shown in Fig. S1d and Fig. S2d, respectively. However, there are distinct variations in the connectivity of H-bond networks and the incorporation manner between phosphonate oxygen atoms, nitrogen atoms and water molecules in the two complexes. In complex 1, there are two different forms of water clusters, (H2O)2 (O25, O28) and (H2O)6 (O24, O26, O27, O29, O30, O31), which further connect with three coordinated water molecules (O19, O20, O22), one lattice water molecule (O23), nitrogen atom (N2) as well as phosphonate oxygen atoms. The H-bond interactions lead to a H-bond network (Fig. 1a). On the other hand, there are two (H2O)3 clusters, consisting of O11, O15 and O16 for one and O12, O13 and O14 for the other, which further link with one coordinated water molecule (O10), nitrogen atom (N2) and H3bmt3− to form a H-bond network in complex 2 (Fig. 1b). The hydrogen bonding parameters of 1 and 2 are listed in Tables S3 and S4, respectively.


image file: c9dt02960e-f1.tif
Fig. 1 The H-bond connections of 1 (a) and 2 (b) formed between phosphonate oxygen atoms, nitrogen atoms and water molecules.

The IR spectra of 1 and 2 display strong and broad bands at 3345 cm−1 and 3379 cm−1, respectively, indicating the existence of water molecules in the crystal lattice.16 The characteristic absorption bands in the region of 1475–1300 cm−1 and around 1610 cm−1 are assigned to the bending vibrations of pyridine and stretching vibration of benzene ring, respectively.17 Moreover, the bands located within 1250–941 cm−1 are attributed to the stretching vibrations of C–PO3 groups18 (Fig. S3). The thermogravimetric results show that the loss of water molecules is completed in the range of 100–200 °C (exp 16.61% and calcd 17.69%) for 1 and 100–185 °C (exp 16.24% and calcd 16.22%) for 2, respectively (Fig. S4). The PXRD patterns of 1 and 2 matched well with the simulated data from single-crystal diffraction, indicating the high purity of the as-synthesized samples. Moreover, the water stability of the two complexes has been examined under different experimental conditions, such as immersion in aqueous solution for two weeks and reflux in water for one day. The results indicate that the PXRD pattern of the soaking samples remained almost unchanged when compared to the simulated pattern of the crystals, revealing good tolerance of 1 and 2 to water under different conditions (Fig. S5).

Proton conduction studies of 1 and 2 were carried out by impedance measurements under different temperatures and relative humidity. The conductivity was deduced from the spikes and/or the arcs. The proton conductivity plots of the two complexes at different temperatures are shown in Fig. 2. It can be seen that the Nyquist plots of 1 at the low-temperature stage (287–297 K) contain a partial arc (Fig. 2a), which may be attributed to the grain interior contribution. However, there are only spikes observed at the low-frequency region of 2 (Fig. 2c), which can be attributed to the pile-up of protons at the electrodes.7b The conductivity values (σ) of 1 and 2 are highly temperature-dependent under 98% RH, displaying noticeable increase of conductivity along with the increasing temperature and reaching the higher values of 1.78 × 10−3 S cm−1 and 9.87 × 10−3 S cm−1 at 358 K, respectively (Tables S5 and S6 and Fig. 3a). This phenomenon is due to a thermal activation mechanism.19 Moreover, both complexes show no significant decrease of conductivity at 373 K, suggesting that the two complexes possess excellent conductivity even at high temperature. The Nyquist plots of 1 and 2 show the decreased size of the arc when switching relative humidity from 60% to 98% (Fig. S6). At 297 K, relative humidity dependence measurements show that the conductivity values of 1 and 2 are 9.74 × 10−4 S cm−1 and 2.02 × 10−3 S cm−1 at 98% RH, which then continuously decrease to 3.76 × 10−6 S cm−1 and 7.6 × 10−6 S cm−1 at 60% RH, respectively (Tables S7 and S8 and Fig. 3b), illustrating that the high-moisture atmosphere can accelerate proton transfer.20 The explanation for this is that the increase of water content in the environment promotes water adsorption, which helps the diffusion of protons around the skeleton of the complexes through H-bond interactions. We notice that 2 shows excellent conductivity when compared to 1. It is known that the ionic conduction is directly proportional to the carrier concentration and proton transport efficiency,7a which is highly related to the structures of the complexes. Based on the molecular formula, 1 has higher water percentage than 2, and so, the better proton conduction of 2 is mainly ascribed to the proton transport efficiency. The structure analysis shows that 1 is a mononuclear complex and 2 is a 1D coordination polymer. Although abundant H-bond interactions exist in both, due to the existence of 1D chain structure in 2, the H-bond interaction occurs in between the chains, which is not only convenient for proton directional transfer, but also helps to stabilize the proton transfer pathway (Fig. S7).


image file: c9dt02960e-f2.tif
Fig. 2 Nyquist plots for proton conductivity of 1 (a) and 2 (c) at 287–297 K and 98% RH. Proton conductivity plots of 1 (b) and 2 (d) at higher temperatures and 98% RH.

image file: c9dt02960e-f3.tif
Fig. 3 The relative data of 1 (black) and 2 (red): (a) Plots of log(σ) vs. T from 287 to 373 K. (b) Humidity dependence of the proton conductivity at 297 K. (c) Time-dependent proton conductivity at 373 K and 98% RH. (d) Arrhenius plots at 98% RH.

To further illustrate the stability and repeatability of 1 and 2, time-dependent proton conductivity analyses were conducted at 373 K and 98% RH. The results show that both complexes retain high conduction values with negligible decline even after five cycles (Tables S9 and S10, Fig. S8 and Fig. 3c). The samples of 1 and 2, after measurement for proton conduction, were collected and characterized by XRD. The peaks of the test samples are consistent with those in the simulated XRD pattern derived from the X-ray diffraction of the respective single crystals, indicating good stability under the extreme environments for water-mediated proton conduction (Fig. S5). Therefore, we can conclude that complexes 1 and 2 are potential proton conductors, for their high conductivity and long-term stability. When compared to other representative phosphonate-based materials measured under the same relative humidity, it was found that the conductivity values of 1 and 2 obtained at 287–373 K and 98% RH are significantly higher than those of following complexes: [Li3(HPA)(H2O)4]·H2O (σ = 1.1 × 10−4 S cm−1, 297 K),21 Ca-PiPhtA-I (σ = 5.7 × 10−4 S cm−1, 297 K)22 and Zn(m-H6L) (σ = 1.39 × 10−4 S cm−1, 314 K).23 Furthermore, the proton conduction of 2 is better than that of PCMOF-5 (σ = 2.5 × 10−3 S cm−1, 333 K)8 and K2(HHPA)(H2O)2 (σ = 1.3 × 10−3 S cm−1, 297 K).21 However, the two complexes demonstrate slightly lower proton conductivity at room temperature than the excellent phosphonate-based materials Na2(HHPA)(H2O)4 (σ = 5.6 × 10−3 S cm−1),21 Ca-PiPhtA-NH3 (σ = 6.6 × 10−3 S cm−1)22 and La(H5DTMP) 7H2O (σ = 8 × 10−3 S cm−1).24

In order to illustrate the mechanism of the proton transfer, the proton transport activation energy (Ea) of 1 and 2 were calculated from the Arrhenius equation [σ = σ0[thin space (1/6-em)]exp(−Ea/kBT)] in the temperature range of 287–297 K. Ea values are 0.25 eV for 1 and 0.29 eV for 2, respectively (Fig. 3d), indicating the proton transfer proceeds through the Grotthuss mechanism in these two complexes.8,24,25 The protons in coordinated water molecules are exerted a push–pull by [PO3] groups and adjacent water molecules, which promote proton hopping forward through the extensive H-bond network.

In conclusion, two novel Ni(II) and Co(II) compounds assembled by coordination and hydrogen bond interactions among metal ions (Ni(II) and Co(II)), H6bmt and 4,4′-bipy were obtained. 1 is a mononuclear complex, while 2 is a 1D chain coordination polymer; both extend to 3D supramolecular assemblies through H-bond interactions. Continuous hydrogen bond networks formed by uncoordinated [PO3] groups and abundant water molecules as well as good water stability make these two complexes potential proton conductors. The results indicate that the higher conduction values at 358 K and 98% RH of 1.78 × 10−3 S cm−1 for 1 and 9.87 × 10−3 S cm−1 for 2 demonstrate comparable conductivity to most of the reported phosphonate-based materials. Moreover, the two complexes even demonstrate stable and highly reproducible proton conductivity under high-temperature environment. The higher proton conductivity of 1 and 2, maintained in a wide temperature range, indicates their potential application in portable power devices and fuel cells. To the best of our knowledge, the two complexes assembled from H6bmt are the first of such complexes investigated for proton conduction. We believe that these results will provide more structural information for researchers to explore metal phosphonates with high proton conductivity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by Hubei provincial science and technology department (2018ACA158).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: CCDC 1922400 and 1922402. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt02960e

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