Proton conductive carboxylate-based metal–organic frameworks

Coordination Chemistry Reviews 404 (2020) 213100

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Proton conductive carboxylate-based metal–organic frameworks Xiao-Xin Xie, Ying-Chao Yang, Bao-Heng Dou, Zi-Feng Li, Gang Li ⇑ College of Chemistry, Zhengzhou University, Zhengzhou 450001, Henan, PR China

a r t i c l e

i n f o

Article history: Received 13 July 2019 Accepted 21 October 2019

Keywords: MOFs Carboxylate Proton conduction Mechanism Progress

a b s t r a c t As a significant type of crystalline solid proton conducting materials, metal–organic frameworks (MOFs) have been paid great attention and pursued by researchers. In this review, we will mainly summarize the proton conduction explorations of MOFs based on carboxylate ligands (including aliphatic carboxylatebased and aromatic carboxylate-based MOFs) from the aspects of synthetic strategies, stability, proton conductive properties and mechanism, application, etc. Finally, on the basis of summarization of literature and our own research on proton conduction, development prospects and challenges for such conductive materials in the future are highlighted. Ó 2019 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Proton conductive carboxylate-based MOFs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Abbreviations: HFA, formic acid; H2ox, oxalic acid; H2adp, adipic acid; L-mal, L-malic acid; tart, tartaric acid; L-asp, L-aspartic acid; H2fum, Fumaric acid; H2Mes, methylfumaric acid; H2ma, mucic acid; EDTA, ethylenediaminetetraacetic acid; S,S-H4ama, N,N0 -bis((S)-2-propanoic acid)oxamide; p-H2BDC, benzene-1,4-dicarboxylate; mH2BDC, isophthalic acid; o-H2BDC, 1,2-benzenedicarboxylic acid; H3BETC, 1,3,5-benzenetricarboxylic acid; H4BTC, benzene-1,2,4,5-tetracarboxylic acid; H2BDA, benzene-1,3diacrylic acid; H2OBA, 4,40 -oxybisbenzoic acid; H3cpip, 5-(4-carboxyphenoxy)isophthalic acid; H2BBDC, 4,40 -biphenyldicarboxylic acid; 220 ,6,60 -H4BPTC, 2,20 ,6,60 -tetracarb oxybiphenyl; 3,30 ,4,40 -H4BPTC, 3,30 ,4,40 -biphenyltetracarboxylic acid; 2,20 ,4,40 -H4BPTC, 2,20 ,4,40 -biphenyltetracarboxylic acid; H4TBDP, 3,30 ,5,50 -tetracarboxydiphenylme thane; H4EBTC, 1,10 -ethynebenzene-3,30 ,5,50 -tetracarboxylic acid; H3TPTCA, 1,10 :30 ,100 -terphenyl]-4,400 ,50 -tricarboxylic acid; H3TPT, [1,10 :30 ,10 0 -terphenyl]-20 ,4,40 0 -tricarboxylic acid; H4TPFoCA, [1,10 :30 ,10 0 -terphenyl]-4,40 ,40 0 ,60 -tetracarboxylic acid; H5TPFiCA, [1,10 :30 ,10 0 -terphenyl]-20 ,4,40 ,40 0 ,60 -pentacarboxylic acid; H3TCA, 4,40 ,400 -tricarboxytripheny lamine; H4pmip, 5-(phosphonomethyl)isophthalic acid; H2NIPA, 5-nitroisophthalic acid; H4dobdc, 2,5-dioxido-1,4-benzenedicarboxylate; NaH2CS, 5-sulfoisophthalic acid monosodium salt; H3CS, 5-sulfoisophthalic acid; 4-Hsba, 4-sulfobenzoic acid, H3STA, 2-sulfoterephthalic acid; H4DBCA, 2,5-dimercapto-1,4-benzenedicarboxylic acid; H4DTA, 2,5-Disulfo-terephthalic acid; H4BPDSDC, biphenyl-3,30 -disulfonyl-4,40 -dicarboxylic acid; Na2H2DSO, disodium-2,20 -disulfonate-4,40 -oxydibenzoic acid; Na2H2DSOD, 1,2-bis (sodium-2-sulfonate-4-carboxyphenoxy)ethane; H3BTAA, [3-(4-methyl-benzoyl)-thioureido]-acetic acid; H3BTEA, 2-(3-benzoylthioureido)ethanoic acid; H3BTPA, 2-(3-ben zoylthioureido)propionic acid; H3NTAA, [3-(naphthalene-1-carbonyl)-thioureido] acetic acid; H2SBBA, 440 -sulfobisbenzoic acid; H4Lleu, L-leucine-derived ligand; H4PPhA, 5(dihydroxyphosphoryl)isophthalic acid; H6DBDP, 2,5-dicarboxy-1,4-benzene-diphosphonic acid; H2NDC, naphthalene-2,6-dicarboxylic acid; H4SSCC, 4,8-disulfonaphtha lene-2,6-dicarboxylic acid; H4PTC, 3,4,9,10-perylenetetracarboxylic acid; H3CP, (4-carboxynaphthalen-1-yl)phosphonic acid; H3IDC, 1H-imidazole-4,5-dicarboxylic acid; H3MIDC, 2-methyl-1H-imidazole-4,5-dicarboxylic acid; H3PhIDC, 2-phenyl-4,5-imidazole dicarboxylic acid; o-CPhH4IDC, 2-(2-carboxylphenyl)-1H-imidazole-4,5dicarboxylic acid; H3DMPhIDC, 2-phenyl(3,4-dimethyl)-imidazole-4,5-dicarboxylic acid; p-ClPhH3IDC, 2-(p-chlorophenyl)imidazol-4,5-dicarboxylic acid; m-ClPhH3IDC, 2(m-chlorophenyl)imidazole-4,5-dicarboxylic acid; o-BrPhH2IDC, 2-(o-bromo)phenyl-4,5-imidazole dicarboxylic acid; m-BrPhH2IDC, 2-(m-bromo)phenyl-4,5-imidazole dicarboxylic acid; H2BDP, 1,4-bis(4-pyrazolyl)benzene; p-IPhH3IDC, 2-(p-N-imidazol-1-yl)-phenyl-imidazole-4,5-dicarboxylic acid; p-TIPhH3IDC, 2-p-(1H-1,2,4-triazolyl)ph enyl-4,5-imidazoledicarboxylic acid; H2pdc, pyridine-3,5-dicarboxylate; H2mpca, 5-methyl-2-pyrazinecarboxylic acid; LCl, 3-methyl-2-(pyridin-4-ylmethylamino)-butanoic acid; 5-TIA, 5-triazole isophthalic acid; H2bpdc, 2,20 -bipyridyl-3,30 -dicarboxylic acid; H4L, N-phenyl-N0 -phenyl bicyclo[2,2,2]-oct-7-ene-2,3,5,6-tetracarboxdiimide tetracarboxylic acid; H4PPTTA, 4,40 ,400 ,4000 -(1,4-phenylenbis(pyridine-4,2,6-triyl))-tetrabenzoic acid; H3TTTPCBr3, 1,10 ,100 -(2,4,6-trimethylbenzene-1,3,5-triyl)-trimethylenetris(4-car boxypyr idinium) tribromide; H2DCDPP, 5,15-di(4-carboxylphenyl)-10,20-di(4-pyridyl)porphyrin; FDA, 2,5-furandicarboxylic acid; H4FTA, tetrahydrofuran-2,3,4,5-tetra carboxylic acid; HSA, thiophene-2-carboxylic acid; H2SDA, thiophene-2,5-dicarboxylic acid; BPTA, 4-((E)-3-(pyridin-4-yl)acrylamido)-N-(4-((E)-3-(pyridin-4-yl)acrylamido) phenyl)benzamide; 2-MBIm, 2-methyl benzimidazole; bpg, [4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5-f][1,10]-phenanthroline-6,13-dione]; Htrz, 1H-1,2,4-triazole; 5-mtz, 5-methyltetrazole; PyOH, 4-pyridinol; H2bmib, 1,4-bis(2-methylimidazol-10 -yl)butane; DPDS, 2,20 -dipyridyl disulfide; DMF, N,N0 dimethylformamide; DMA, dimethylammonium; Im, imidazole; 4,40 -bipy, 4,40 -bipyridine; 2,20 -bipy, 2,20 -bipyridine; 2-apy, 2-aminopyridine; 3-apy, 3-aminopyridine; PPh3, triphenyl phosphine; hmt, hexamethylenetetramine; Emim, 1-ethyl-3-methylimidazolium; SC, single crystal; PEMFCs, proton exchange membrane fuel cells; PEM, proton exchange membrane; MD, molecular dynamics; MEA, membrane electrode assembly. ⇑ Corresponding author. E-mail address: [email protected] (G. Li). https://doi.org/10.1016/j.ccr.2019.213100 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.

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2.1.

3.

Aliphatic carboxylate-based MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1.1. Oxalate-based MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1.2. Formate-based MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.1.3. Other aliphatic carboxylate-based MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2. Aromatic carboxylate-based MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.2.1. Phenyl carboxylate-based MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.2.2. MOFs based on phenyl carboxylate containing other functional groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.3. Polycyclic aromatic carboxylate-based MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.2.4. N-heterocyclic carboxylate-based MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.2.5. O-heterocyclic and S-heterocyclic carboxylate-based MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.3. Proton conduction mechanism: Insights from molecular dynamics simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.4. Research progress of other proton conducting coordination systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4.1. Membrane electrode assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4.2. Single crystal proton conduction studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Conclusion and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

1. Introduction In recent years, taking into account the non-renewable and environmental pollution of fossil fuels (coal, oil, etc.), scientists are investing with great urgency in the developing of new sources of clean energy [1–3]. In this context, fuel cells are considered to be an important method to solve current environmental and energy issues. In particular, PEMFCs are considered to be a promising clean energy technology due to the prominent virtues of supernal energy density, lofty energy conversion efficiency and environmental friendliness [4,5]. PEM as the core component of PEMFCs directly determines their performance and service life [6–8]. Accordingly, the design and development of PEM with high proton transfer capability and excellent water, chemical and electrochemical stability are the frontier and hot field. Nafion membrane is a perfluoroalkyl sulfonic acid polymer with high proton transport efficiency and is the most widely used PEM in PEMFCs [9–11]. Nevertheless, the Nafion membrane manufacturing process is complex, the cost is high, and the operating conditions are limited. Additionally, the amorphous nature of Nafion membranes hinders the clear comprehension of the conducting mechanism, so it is momentous to develop the crystalline proton conductive materials that can overcome above defects [12–14]. Previous researches have clearly demonstrated that metal–organic frameworks (MOFs) can act as advanced proton-conductive materials due to their structural controllability and exceptionally high crystallinity [15–18]. Although the earliest report of proton conductive MOFs emerged in 1979 [19], the proton conduction mechanism has not been well revealed due to the needy crystallinity of these MOFs. Until 2009, both H. Kitagawa and G. K. H. Shimizu groups firstly used the crystalline MOFs for proton conduction investigation. Accordingly, they confirmed that the single crystal structures can reasonably reveal the proton conduction mechanism [20,21]. Up to now, a great number of crystalline proton conductive MOFs have been narrated [22–25]. The studies of proton conduction in MOFs mainly include two kinds of situations: one is that under the anhydrous condition, it mainly relies on the inherent water molecules or other minor molecules containing protons (for instance, imidazole, trizole, histamine, etc.) and H-bond network in the structures to carry out the proton transfer [26,27]; another one is that under the aqueous condition, the external H2O units play a vital function on the proton transmission [28–30]. Currently, explorations of conductive MOFs have given priority to the development of conductive materials with superhigh proton

conductivity and elucidation of mechanisms [34–38]. In general, the proton transport mechanism can be mainly proposed by activation energy (Ea), which is obtained in terms of the Arrhenius equation [31]:

rT ¼ r0 expðEa =kT Þ where r, r0, k and T represents the proton conductivity, a constant, the Boltzmann constant, and thermodynamic temperature, respectively. According to the Ea values, the proton conduction mechanisms can be roughly divided into two categories [32,33]: Typical Ea values are 0.1–0.4 eV for the Grotthuss mechanism, which means that the protons jump mainly via the H-bond networks; The Ea values being >0.5 eV pertain to the vehicle mechanism indicating that the complicated hydrated protons migrated in a certain direction by concentration diffusion and solvent molecules. Therefore, the concentration of proton carriers and effective proton transport pathways will have a crucial effect on the properties of proton conductive materials. Hence, the MOFs with high proton conductivity can be prepared in accordance with the following two kinds of strategies: (1) The acidity and hydrophilicity of the organic ligands can be enhanced by introducing other functional groups, such as –COOH, –PO3H, –SO3H, and –OH, which would help the MOFs to form an efficient proton transport pathway [34–37]. (2) The guest molecules (eg, water, imidazole, triazole, histamine, etc.) and counterions or acids are brought into the voids, which will facilitate the formation of complicated H-bonded networks and be advantageous to improve proton conductivity [38–44]. On this basis, a number of MOFs with superhigh r values around 101–102 S/cm have been designed and produced [45–56]. With the deepening of the research and the comprehension of proton conducting MOFs, in recent years, several research groups have made a preliminary summary and reviewed the previous experimental results [33,57–70]. Nevertheless, these reviews are extremely general and rarely summarize a specific class of proton conductive MOFs. As an example, only L. M. Zheng et al. reviewed the proton conductive properties about metal phosphonate frameworks [71]. To the best of our knowledge, the review about proton conductive carboxylate-based MOFs is extremely scarce. We are interested in proton conducting carboxylate-based MOFs for three reasons. First, carboxylate compounds have versatile coordination modes and strong coordination ability, which can be adopted in coordination with metal ions to establish MOFs with regular structures and stable frameworks. These outstanding features will be extremely useful for proton conducting materials.

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For the reason that, in practical applications, taking the influence of humidity and temperature into account, the proton conductive materials must have good water and chemical durability [72–74]. Second, the O atom and OH group of carboxylate unit inside the frameworks can construct complex H-bonded networks with each other or with guest polar small molecules (such as, H2O, NH3 and so on), which will be more than good for proton conduction. Finally, by careful regulation, uncoordinated carboxylate groups (–COOH) in MOFs will not only donate protons but also can constitute plentiful H-bond networks [75–80], which will hugely boost the proton conductivity. Consequently, herein, we systematically expounded the research progress of proton conducting MOFs, including aliphatic carboxylate-based, phenyl carboxylate-based, phenyl carboxylate containing other functional groups-based, N-, S- and Oheterocyclic carboxylate-based, and mixed carboxylate-based MOFs. We summarize these MOFs from the following aspects: the design, stability, proton conduction and proton conduction regulation, proton conduction mechanism and potential applications. The carboxylate ligands mentioned in this review are illustrated in Scheme 1. Tables 1 and 2 give the summaries of the proton conductive carboxylate-based MOFs reported so far.

2. Proton conductive carboxylate-based MOFs 2.1. Aliphatic carboxylate-based MOFs 2.1.1. Oxalate-based MOFs As enumerated in Table 1, the kinds of aliphatic carboxylate ligands used for the construction proton conductive MOFs in the past are intensely limited. Oxalate ligand was mostly adopted due to the simplicity of synthesis process and the high water stability of the relevant MOFs. In 2016, H. Kitagawa and co-workers have narrated in detail the oxalate-bridged 1D, 2D and 3D proton conductive MOFs [60]. Therefore, in this review, we generally describe the oxalate-based MOFs that appeared before 2016, and make a detailed summary of the newly appeared oxalate-linked MOFs. The simplest 1D oxalate-joined MOF, Fe(ox)2H2O was prepared in 2009 [81] by H. Kitagawa group, in which ox2 anions bridge Fe2+ cations to form a 1D framework including 1D water chains for effective proton conduction. This MOF exhibits a high r value of 1.3  103 S/cm at 25 °C and 98% relative humidity (RH) by a polycrystalline pellet. Five years later, S. Tominaka and co-workers employed a single crystal sample of Fe(ox)2H2O to examine its intrinsic and extrinsic proton conductivity [82]. Unexpectedly, along the b-axis, its conductivity is 5.4  109 S/cm at 20 °C and 80% RH, which is much lower than that of polycrystalline sample under similar conditions. They interpreted that the hydrated interparticle phases gave a prominent contribution to proton conduction in quite a few MOFs as measurements made on pellets. This also illustrates the complexity on proton conduction research of MOFs. Anyway, S. Tominaka found that when a polycrystalline pellet was adopted again, the intensely similar r value of 1.5  103 S/cm at 20 °C and 100% RH to H. Kitagawa reported in 2009 could be found. In 2009, H. Kitagawa group perceived that NH+4 ions play an essential role on the construction of efficient H-bond networks with oxalate ions and H2adp units in a 2D MOF (NH4)2(H2adp) [Zn2(ox)3]3H2O [20]. To confirm this judgment, they also synthesized a similar structural 2D MOF, K2(H2adp)[Zn2(ox)3]3H2O [83] without NH+4 cations, and compared their proton conductive properties and mechanisms. As to be expected, the r value of K2(H2adp) [Zn2(ox)3]3H2O is 1.2  104 S/cm under 25 °C and 98% RH, which is two orders of magnitude smaller than that of (NH4)2(H2adp)

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[Zn2(ox)3]3H2O (0.8  102 S/cm under 25 °C and 98% RH). The Ea values of the two MOFs are larger than 0.4 eV but slightly different (for (NH4)2(H2adp)[Zn2(ox)3]3H2O, Ea = 0.63 eV; for K2(H2adp) [Zn2(ox)3]3H2O, Ea = 0.45 eV). In 2016, they introduced Rb+ ion, having a much close ionic radius (1.52 Å) to NH+4 (1.61 Å) ion than K+ (1.38 Å), to MOF, Rb2(H2adp)[Zn2(ox)3]3H2O to accurately study the effect of cations on the proton conduction [84]. By systematic comparisons the crystal structures, r and Ea values of Rb2(H2adp) [Zn2(ox)3]3H2O with (NH4)2(H2adp)[Zn2(ox)3]3H2O and K2(H2adp)[Zn2(ox)3]3H2O, they pointed out that the difference of metal cation radius has a major influence on the Ea values entailed in proton conduction. The effect of different cations is achieved by affecting the distance between disordered O10 sites, which is related the Ea values. Moreover, they verified again that the ammonium ion gives a great contribution on the enhanced proton conductivity. Subsequently, by means of single crystal structural analysis, H. Kitagawa group has studied the effect of crystallization water molecule on proton conduction of MOFs, (NH4)2(H2adp)[Zn2(ox)3]nH2O (n = 0, 2, 3)) [78]. As denoted in Fig. 1, all NH+4 cations, ox2 anions, H2adp units, and crystallization H2O units involve the complicated H-bond networks in (NH4)2(H2adp)[Zn2(ox)3]3H2O (Fig. 1a) and (NH4)2(H2adp)[Zn2(ox)3]2H2O (Fig. 1b). Note that in (NH4)2(H2adp)[Zn2(ox)3]2H2O, the hydrogen bonds are longer than those in (NH4)2(H2adp)[Zn2(ox)3]3H2O, and the number of hydrogen bond decreases. Although the H-bonding distances in (NH4)2(H2adp)[Zn2(ox)3] are close to those in (NH4)2(H2adp) [Zn2(ox)3]2H2O, a complete H-bond network was not formed due to the lack of crystallization H2O molecules (Fig. 1c). Naturally, the differences in hydrogen bond systems within the frameworks lead to differences in proton conductivities. The r value for (NH4)2(H2adp)[Zn2(ox)3]3H2O is 0.8  102 S/cm under 25 °C and 98% RH, which is 100 times larger than that of (NH4)2(H2adp) [Zn2(ox)3]2H2O (7  105 S/cm), and greatly larger than that of (NH4)2(H2adp)[Zn2(ox)3] (1012 S/cm) under similar conditions. M. Verdaguer group has described another NH+4-containing high proton conductive 2D chiral ferromagnetic quartz-like MOF, (NH4)4[MnCr2(ox)6]4H2O in 2011 [85], which was synthesized by the reaction of (NH4)3Cr(ox)3 with MgCl2 in H2O and slow diffusion of EtOH. In this compound, NH+4 ions as counter-cations located in the channels and acted as proton carriers. At the same time, the H-bond networks inside the framework and the crystallization and absorbed water molecules result in the high proton conduction (1.7  103 S/cm under 40 °C and 96% RH)). The lower Ea value of 0.23 eV suggests that proton conduction obeys a Grotthuss mechanism. Six years later, E. Pardo synthesized a similar structural MOF, (C3N2H5)4[MnCr2(ox)6]5H2O [86] with imidazolium cation (C3N2H+5) instead of NH+4 ion to inspect the effect of the acidity of the proton donating guest units on proton transfer (Fig. 2). The results denote that the more acidic the cation is, the higher the proton conductivity of the corresponding MOF will be. For example, the imidazolium cation (pKa = 7.05) is more acidic than NH+4 ion (pKa = 9.25), so (C3N2H5)4[MnCr2(ox)6]5H2O has a higher r value (1.86  103 S/cm under 22 °C and 88% RH) than that of (NH4)4[MnCr2(ox)6]4H2O (4.64  104 S/cm under 25 °C and 88% RH). The comparable Ea value of (C3N2H5)4[MnCr2(ox)6]5H2O (0.28 eV) as (NH4)4[MnCr2(ox)6]4H2O (0.23 eV) could be observed. However, the authors did not give a detailed explanation of the effect of both cations on the conductivities due to the limitations of the structure determinations. We speculate that the more acidic cations may have the stronger ability to supply protons and participate in the construction of hydrogen bonds, which leads to the better proton conductivity of corresponding MOFs. In 2014, S. K. Ghosh group introduced [Me2NH2]+ cation into a 3D framework, {(Me2NH2)3(SO4)]2[Zn2(ox)3]}n, in which the ion pair of [(Me2NH2)3(SO4)]2+ locates in the void formed by the 2

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Scheme 1. Structural diagrams of carboxylate ligands mentioned in this review.

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5

Scheme 1 (continued)

electrostatic and H-bond interactions [53]. Interestingly, this compound displays superhigh water-mediated r value of 4.2  102 S/cm under 25 °C and 98% RH, and high anhydrous r value of 1.0  104 S/cm under 150 °C. The authors did not offer the Ea value under humidity. But they calculated the Ea value (0.13 eV) under anhydrous conditions, which assumes that a Grotthuss mechanism was operated. By the resolution of DSC and variable-temperature PXRD, they suggested that the high conductivity is from the proton transfer along the H-bonded [Me2NH2]+ Ò and SO2 4 ions. In 2018, this MOF was embedded into the Nafion polymer in accordance with 1 wt% of MOF to NafionÒ, and its high proton conductivity resulted in an 18% increase in the r of the MOF/NafionÒ composite membrane compared to the pure NafionÒ [87]. This convenient method of introducing conductive MOFs into polymer membranes provides useful inspiration for us to find out the practical application of such MOFs in fuel cells.

D. R. Zhu et al. in 2016 also constructed a 3D dimethylammonium-containing MOF [Me2NH2][Eu(ox)2(H2O)]3H2O [88], in which [Me2NH2]+ ions and coordination and crystallization H2O units are distributed with the H-bond network in the pores. This compound indicates a high r value of 2.73  103 S/cm at 55 °C and 95% RH and a Grotthuss proton conduction mechanism (Ea = 0.398 eV). They proposed that the H+ of [Me2NH2]+ can hop along the chains of ox2  [Me2NH2]+  (H2O)n. Moreover, the Eu (III) cation is strongly lewis, which can accelerate the selfdissociation of the coordinated water. As a result, the released H+ can rely on the H-bond networks for rapid transfer. Another [Me2NH2]+-containing example is a mixed metal MOF: (Me2NH2)2[Li2Zr(ox)4] presented by K. Cheetham et al. [89]. This MOF has undergone interesting transformation from insulator-to-proton-conductor upon exposure to humidity. The authors employed the single-crystal sample to determine the

6

Table 1 Structural features and proton conductivities of aliphatic carboxylate-based MOFs. Carboxylates H2ox

H2ox + H2adp

H2ox

H2ox

HFA

L-asp H2fum H2Mes L-mal tart

H3HPA S,S-H4ama a

Structures

Presence of un-coordinated COOH

Synthetic strategy

r (S/cm) a

Ea (eV)

Refs.

0.37 – 0.14 0.63 0.45 0.69 – – 0.23 0.28 0.13 – 0.398 0.64 – – – – – –

[81] [82] [82] [20] [83] [84] [78] [78] [85] [86] [53]

3

[Fe(ox)(H2O)2] [Fe(ox)(H2O)2] [Fe(ox)(H2O)2] (NH4)2(H2adp)[Zn2(ox)3]3H2O K2(H2adp)[Zn2(ox)3]3H2O Rb2(H2adp)[Zn2(ox)3]3H2O (NH4)2(H2adp)[Zn2(ox)3]2H2O (NH4)2(H2adp)[Zn2(ox)3] (NH4)4[MnCr2(ox)6]4H2O (C3N2H5)4[MnCr2(ox)6]5H2O (Me2NH2)3(SO4)]2[Zn2(ox)3]}n

1D 1D 1D 2D 2D 2D 2D 2D 2D 3D 3D

No No No Yes Yes Yes Yes Yes No No No

Microwave synthesis Microwave synthesis Microwave synthesis Hydrothermal synthesis Hydrothermal synthesis Hydrothermal synthesis Hydrothermal synthesis Hydrothermal synthesis Solvent diffusion technique Solvent diffusion technique Solvothermal synthesis

[Me2NH2][Eu(ox)2(H2O)]3H2O ((Me)2NH2)2[Li2Zr(ox)4] {N(n-C4H9)3}[MCr(ox)3] (M = Mn, Fe, Co) {NMe3(CH2COOH)}[FeCr(ox)3]nH2O {NEt3(CH2COOH)}[MnCr(ox)3]nH2O {NBu3(CH2COOH)}[FeCr(ox)3]nH2O {NBu3(CH2COOH)}[MnCr(ox)3]nH2O {NEt3(CH2COOH)}[MnCr(ox)3]2H2O {NEt3(CH2COOH)}[FeIICrIII(ox)3]2H2O {NEt3(CH2COOH)}[FeIIFeIII(ox)3]2H2O LaCr(ox)310H2O LaCo(ox)310H2O LaRu(ox)310H2O LaLa(ox)310H2O LaCo(ox)2.510H2O {[Gd(ma)(ox)(H2O)]n3H2O} {[Dy(ma)(ox)(H2O)]n1.5H2O} (N2H5)[CeEu(ox)4(N2H5)]4H2O (N2H5)[Nd2(ox)4(N2H5)]4H2O [La2(ox)3(H2O)6]4H2O [Er2(ox)3(H2O)6]12H2O [Eu2(CO3)(ox)2(H2O)2]4H2O

3D 3D 2D 2D 2D 2D 2D 2D 2D 2D 1D 1D 2D 2D 1D 3D 3D 3D 3D 2D 3D 3D

No No No No No No No No No No No No No No No No No No No No No No

Facile synthesis in aqueous solution Solvothermal synthesis Solvent volatilization synthesis Reaction in solution at room temperature Reaction in solution at room temperature Reaction in solution at room temperature Reaction in solution at room temperature Reaction in solution at room temperature Reaction in solution at room temperature Reaction in solution at room temperature Solvent diffusion technique Reaction in solution at room temperature Reaction in solution at room temperature Reaction in solution at room temperature Reaction in solution at room temperature Hydrothermal synthesis Hydrothermal synthesis Hydrothermal synthesis Hydrothermal synthesis Stirring at 15 °C Stirring at 15 °C Hydrothermal synthesis

1.3  10 (25 °C, 98% RH)  5.4  109 (SC; 20 °C, 80% RH) 1.5  103 (25 °C, 100% RH) 0.8  102 (25 °C, 98% RH) 1.2  104 (25 °C, 98% RH) 4.3  105 (25 °C, 98% RH) ~7  105 (25 °C, 95% RH) ~1012 (25 °C, 0% RH) 1.7  103 (40 °C, 96% RH) 1.86  103 (22 °C, 88% RH) 1  104 (150 °C) 4.2  102 (25 °C, 98% RH) 2.73  103 (55 °C, 95% RH) 3.9  105 (17 °C, 67% RH) ~1  104 (25 °C, 75% RH) 0.8  104 (25 °C, 65% RH) 1.0  107 (25 °C, 65% RH) 2.0  1011 (25 °C, 60% RH) 0.8  1011 (25 °C, 65% RH) ~1  107 (25 °C, 65% RH) ~1  107 (25 °C, 65% RH) ~1  107 (25 °C, 65% RH) ~1  106 (25 °C, 40–95% RH) ~1  105 (25 °C, 40–95% RH) ~3  108 (25 °C, 40–95% RH) ~3  108 (25 °C, 40–95% RH) ~1  1010 (25 °C, 40–95% RH) 4.7  104 (80 °C, 95% RH) 9.06  105 (80 °C, 95% RH) 3.42  103 (25 °C, 100% RH) 2.70  103 (25 °C, 100% RH) 3.35  107 (95 °C, 100% RH) 1.76  109 (90 °C, 100% RH) 2.08  103 (150 °C)

[Zn2(HCOO)(trz)3]n 2Him@[Zn2(HCOO)(trz)3]n [C2H5NH3][Na0.5Fe0.5(HCOO)3] MIP-202(Zr) MOF-801 [Al(OH)(Mes)]nH2O Zn4(5-mtz)4(L-mal)2(H2O)2 [Cd(L-tart)(4,40 -bipy)(H2O)]n9n(H2O) [Cd(D-tart)(4,40 -bipy)(H2O)]n9n(H2O) [Cd(DL-tart)(4,40 -bipy)(H2O)]n6n(H2O) [Ca(tart)]n4n(H2O) LaHPA-I GdHPA-II [CaCu6(S,S-Hama)3(OH)2(H2O)]32H2O

3D 3D – 3D 3D 3D 2D 3D 3D 3D 3D 3D 3D 3D

No No No No No No No Yes Yes Yes Yes No No No

Solvothermal synthesis Hydrothermal synthesis Solvothermal synthesis Reflux with ambient pressure Solvothermal synthesis Microwave synthesis Hydrothermal synthesis Solvent diffusion technique Solvent diffusion technique Solvent diffusion technique Solvent diffusion technique Gel-assisted preparation Gel-assisted preparation Solvent diffusion technique

7.95  107 (50 °C, 98% RH) 1.51  104 (45 °C, 98% RH) – 0.011 (90 °C, 95% RH) 1.88  103 (25 °C, 98%R H) 1.1  105 (130 °C) 1.33  105 (60 °C, 95% RH) 1.3  106 (85 °C, 95% RH) 1.3  106 (85 °C, 95% RH) 4.5  107 (85 °C, 95% RH) 3.0  105 (25 °C, 100% RH) 5.6  106 (21 °C, 98% RH) 3.2  104 (21 °C, 98% RH) 1.0  105 (24 °C, 95% RH)

If not specified, the r values are obtained by the pellet samples.

0.32 – 0.90 – – 0.88 0.70 0.10 – 0.35 0.47 0.47 (25–90 °C) 0.26 (100–150 °C) 0.40 0.54 0.23 0.22 0.256 0.45 0.81 0.63 0.67 0.77 0.20 0.23 0.20 0.34

[88] [89] [90] [91] [91] [91] [91] [92] [92] [92] [93] [93] [93] [93] [93] [94] [94] [95] [95] [96] [96] [97] [98] [98] [99] [100] [103] [104] [105] [106] [106] [106] [107] [108] [108] [109]

X.-X. Xie et al. / Coordination Chemistry Reviews 404 (2020) 213100

H2ox + H2ma

MOFs

Table 2 Structural features and proton conductivity of aromatic carboxylate-based MOFs. Presence of noncoordinated COOH

Synthetic strategies

r (S/cm)a

Ea (eV)

Refs.

Zr6O4(OH)6(p-BDC)5 (UiO-66-6) Mg(p-BDC)(PyOH) Mg(p-BDC)(PyOH)_Cs

3D 3D 3D

No No No

6.93  103 (65 °C, 95% RH) 8.30  106 (90 °C, 90% RH) 1.61  102 (90 °C, 90% RH)

0.22 0.35 0.19

[112] [47] [47]

m-H2BDC

[In(m-BDC)2{(Me)2NH2}(H2O)2] (In-IA-2D-1) [In(m-BDC)2{(Me)2NH2}(DMF)] In-IA-2D-2

2D 2D

No No

Ligand defects control method Solvothermal reaction Post-synthetic method (loading 10% Cs+ ions into the MOF) Solvothermal reaction Solvothermal reaction

o-H2BDC H3BETC

[[Zn(2-MBIm)(o-BDC)(H2O)]2H2O]n Zr6O5(OH)3(BETC)2(HCOO)5(H2O)2 [MOF-808] Im@MOF-808 HKUST-1 NH4Br@HKUST-1

1D 3D 3D 3D 3D

No No No No No

0.61 0.48 – – 0.19 0.37 0.25 0.69 1.42

[37] [37] [37] [37] [113] [115] [45] [116] [116]

Cu3(BETC)2(H2O)3]4[SiW11MoVO40](C4H12N)530H2O {Co2Cl2(BETC)4/3](Me2NH2)+24/3H2O}n {Mn2Cl2(BETC)4/3](Me2NH2)+24/3H2O}n [Cd2(BETC)2(H2O)2]nn(H2bmib)6n(H2O) MOF-808-OX MOF-808-EDTA UiO-66(Zr)-(CO2H)2 {[Zn(BTC)0.5(DPDS)]5H2O]}n {[Zn(BTC)0.5(DPDS)]2H2O]}n {[Cd(BPTA)(BDA)](DMF)16H2O}n {[Co(BPTA)(BDA)](DMF)13H2O}n {[Cd(BPTA)(OBA)](2DMF)8H2O}n {[Co(BPTA)(OBA)](2DMF)6H2O}n [Cd4(cpip)2(Hcpip)2]nn(H2bmib)n(H2O) {[Mg(BBDC)(H2O)3](H2O)}n Zn3(BBDC)2(pdc)(DMF)6DMF [Zn3K2(3,30 ,4,40 -BPTC)3(DMF)2][Me2NH2]4 [Mg(2,20 ,6,60 -BPTC)0.5(H2O)3]5H2O [Sr2(2,20 ,6,60 -BPTC)(H2O)6]H2O MIP-177-SO4H-LT [In(EBTC)(Me2NH2)]DMF(H2O)5

3D 3D 3D 3D 3D 3D 3D 2D 1D 2D 2D 3D 3D 3D 2D 3D 3D 2D 3D 3D 3D

No No No No Yes Yes Yes No No No No No No No No No No No No No No

Solution volatilization method Solvothermal reaction Encapsulation technique Solvothermal reaction Post-synthetic method (soaking HKUST-1 into saturated EtOH solution of NH4Br) Hydrothermal reaction Solvothermal reaction Solvothermal reaction Hydrothermal reaction Post-synthetic method Post-synthetic method Heating circumfluence Solvent diffusion technique Hydrothermal reaction Reaction in solution Reaction in solution Reaction in solution Reaction in solution Hydrothermal reaction Solution reaction Solvothermal reaction Solvothermal reaction Hydrothermal reaction Hydrothermal reaction Simple synthesis protocol Solvothermal reaction

3.4  103 (27 °C, 98% RH) 4.2  104 (27 °C, 98% RH) 2.6  105 (90 °C, 0% RH) 1.18  105 (90 °C, 0% RH) 1.0  105 (25 °C, 100% RH) 7.58  103 (42 °C, 99% RH) 3.45  102 (65 °C, 99% RH) 1.04  108 8.99  104 (25 °C, 99% RH)

6.37  108 (25 °C, 97% RH) 1.19  103 (50 °C, 65% RH) 2.60  104 (19 °C, 65% RH) 5.4  105 (60 °C, 95% RH) 4.25  104 (80 °C, 98% RH) 1.31  104 (30 °C, 98% RH) 2.3  103 (90 °C, 95% RH) 2.55  107 (80 °C, 95% RH) 4.39  104 (80 °C, 95% RH) 2.2  103 (80 °C, 98% RH) 9.5  104 (80 °C, 98% RH) 1.2  103 (80 °C, 95% RH) 6.6  104 (80 °C, 98% RH) 2.2  105 (60 °C, 98% RH) 1.44  105 (30 °C, under vacuum) 0.95  102 (65 °C, 99% RH) 8.4  103 (27 °C, 98%RH) 2.6  104 (100 °C, 98% RH) 2.7  104 (90 °C, 98% RH) 2.6  102 (25 °C, 95% RH) 3.49  103 (25 °C, 99% RH)

[79] [117] [117] [41] [118] [118] [119] [122] [122] [123] [123] [123] [123] [41] [124] [52] [125] [126] [126] [16] [127]

Fe–MOF Im@Fe–MOF Im–Fe–MOF [Ba2(mH3TPT)(H2O)1.5(CO2)(DMF)1.5] [Ba(H2TPFoCA)(H2O)(DMF)] [Ba2(HTPFiCA)(H2O)4] [Cd5(TCA)2(H2O)2]8DMA16H2O

3D 3D 3D 2D 3D 3D 3D

No No No No No Yes No

Solvothermal reaction Post-synthetic method Solvothermal reaction Solvothermal reaction Solvothermal reaction Solvothermal reaction Solvothermal reaction

RH) RH) RH) RH) RH) RH) RH)

– 0.21 – 0.62 0.14 0.15 0.17 0.96 0.84 – – – – 0.27 0.277 0.45 0.25 0.47 1.18 – 0.105 (55– 65 °C) 0.17 (15– 45 °C) 0.385 0.573 0.436 0.63 0.40 0.32 0.74

Phenyl carboxylates bearing other functional groups H4pmip (Me2NH2)[Eu(pmip)]

2D

No

Solvothermal reaction

H2NIPA H4dobdc

3D 3D

No No

Solvothermal reaction Micarowave reaction

1.25  103 (SC, 150 °C) 3.76  103 (100 °C, 98% RH) 7.17  102 (SC, 75 °C, 98% RH) 1.4  104 (80 °C, 95% RH)

0.38 – 0.13 0.12

[131] [131] [44] [132]

Phenyl carboxylates p-H2BDC

H3PETC + H2ox H3PETC + EDTA H4BTC

H2DBA H2OBA H3cpip H2BBDC H2BBDC + H2pdc 3,30 ,4,40 -H4BPTC 2,20 ,6,60 -H4BPTC H4TBDP H4EBC

H3TPTCA

H3TPT H4TPFoCA H5TPFiCA H3TCA

MOFs

{H[(N(Me)4)2][Gd3(NIPA)6]}3H2O [Ni2(dobdc)(H2O)2]6H2O (Ni-MOF-74)

1.25  104 4.23  103 1.21  102 2.1  105 5.1  105 2.9  103 1.45  106

(60 °C, (60 °C, (60 °C, (25 °C, (25 °C, (25 °C, (80 °C,

98% 98% 98% 99% 99% 99% 85%

X.-X. Xie et al. / Coordination Chemistry Reviews 404 (2020) 213100

Structures

Carboxylates

[128] [128] [128] [129] [129] [129] [130]

(continued on next page) 7

8

Table 2 (continued) Structures

Presence of noncoordinated COOH

Synthetic strategies

r (S/cm)a

Ea (eV)

Refs.

H+@Ni-MOF-74 Co-MOF-74

3D 3D

No No

Post-synthetic method Solvothermal reaction

NaH2CS H3CS

Cu4(CS)2(OH)2(DMF)2 {[In(5-HCS)2(Me2NH2)]DMF(H2O)1.4}n

3D 2D

Yes No

Solvothermal reaction Solvothermal reaction

{[Cu2(sba)2(bpg)2(H2O)3]5H2O}n H2SO4@MIL-101-SO3H

1D 3D

Yes No

Solvent evaporation method Post-synthetic method

H4DBCA H4DTA H4BPDSDC Na2H2DSO

Na2H2DSOD H3BTAA H3BTEA

UiO-66(SH)2 UiO-66(SO3H)2 {[SmK(BPDSDC)(DMF)(H2O)]x(solvent)}n {[Tb4(OH)4(DSO)2(H2O)8](H2O)8}n {[H3O][Cu2(DSO)(OH)(H2O)]9.5H2O}n {[Gd4(OH)4(DSO)2(H2O)8]4.6H2O1.4CH3CN}n {[Dy4(OH)4(DSO)2(H2O)8]4.6H2O1.4CH3CN}n {[Ho4(OH)4(DSO)2(H2O)8]4.6H2O1.4CH3CN}n {[Er4(OH)4(DSO)2(H2O)8]4.6H2O1.4CH3CN}n [Cp3Zr3(l3-O)(l2-OH)3]2(DSOD)34Na4H2O (MOP-1) [CuI3CuII3(BTAA)6(DMF)2(MeOH)(H2O)]3MeOH [CuI6CuII6(BTEA)6(H2O)10(DMF)2]6H2O

3D 3D 3D 3D 3D 3D 3D 3D 3D 3D 3D 0D cluster

No No No No No No No No No No No No

Micarowave reaction Post-synthetic oxidation method Solvothermal reaction Hydrothermal reaction Solvothermal reaction Hydrothermal reaction Hydrothermal reaction Hydrothermal reaction Hydrothermal reaction Solvothermal reaction Solvent evaporation method Solvent evaporation method

0.14 0.12 0.30 1.32 0.31 – – – 0.64 0.21 – 0.23 0.32 0.628 0.45 1.04 0.27 0.25 0.38 0.32 0.225 0.63 0.78 0.35

[132] [133] [133] [134] [135]

4-Hsba H3STA

H3BTPA

[CuI6CuII6(BTPA)6(H2O)10]n

2D

No

Solvent evaporation method

0.86 0.58

[145] [145]

H2SBBA

Ca-SBBA Sr-SBBA Na2[Eu(SBBA)2(FA)]0.375DMF0.4H2O [(Cu1.5H2LLleu)(Ac)H2O]n3H2O [(Cu1.5H2LDleu)H2O]n10H2O Ca2[(H3PPhA)2]2[(H2PPhA)(H2O)2]5H2O (Ca-PiPhtAI) Ca-PiPhtA-NH3 Mg2(H2O)4(H2DBDP)H2O (PCMOF10)

2D 2D 1D 3D 3D 3D 3D

No No No No No Yes Yes

Solvothermal reaction Solvothermal reaction Solvothermal reaction Solvent evaporation method Solvent evaporation method Solvent evaporation method Post-synthetic method

2.2  102 (80 °C, 95% RH) 4.5  103 (SC; 90 °C, 95% RH) 2.5  107 (90 °C, 95% RH) 7.4  104 (95 °C, 95% RH) 1.17  103 (31 °C, 40% RH) 1.25  103 (SC {1 1 0}; 25 °C, 40% RH) 1.20  103 (SC {1 1 0}; 25 °C, 40% RH) 8.66  105 (SC {0 0 1}; 25 °C, 40% RH) 0.94  102 (80 °C, 95% RH) 1.82 (70 °C, 90% RH) 0.92  102 (-40 °C) 2.5  105 (80 °C, 90% RH) 8.4  102 (80 °C, 90% RH) 1.11  103 (80 °C, 98% RH) 1.66  104 (100 °C, 98% RH) 1.9  103 (85 °C, 98% RH) 2.02  106 (80 °C, 95% RH) 2.96  106 (80 °C, 95% RH) 4.56  103 (80 °C, 95% RH) 6.59  103 (80 °C, 95% RH) 1.41  103 (30 °C, 98% RH) 3.78  104 (100 °C, 98% RH) 2.5  105 (100 °C, 98% RH) 9.80  104 (100 °C and aqua-ammonia vapor from 2.0 M NH3H2O solution) 1.16  105 (100 °C, 98% RH) 7.70  104 (100 °C and aqua-ammonia vapor from 2.0 M NH3H2O solution) 8.6  106 (25 °C, 98% RH) 4.4  105 (25 °C, 98% RH) 2.91  102 (90 °C, 90% RH) 1  105 (90 °C, 90% RH) 4.12  106 (90 °C, 90% RH) 5.7  104 (24 °C, 98% RH) 6.6  103 (24 °C, 98% RH)

0.23 0.56 0.10 0.794 0.748 0.32 0.41

[146] [146] [48] [147] [147] [148]

2D

No

Solvothermal reaction

3.55  102 (70 °C, 98% RH)

0.40

[149]

Polycyclic armatic carboxylates H3NTAA [(CuI4CuII4NTAA4)3H2O]n CuI4CuII4NTAA4]n-NH3 H2NDC + p-H2BDC Fe4(p-BDC)2(NDC)(SO4)4(DMA)4 H4SSCC [Cu(H2SSCC)(DMF)4]n [Ca(SSCC)0.5(DMF)2.5]n [Cd(SSCC)0.5(DMF)2]n [Zr6O8(H2O)8(H2SSCC)4] (VNU-23) His8.2  VNU-23 H4PTC {[K8(PTC)2(H2O)1.5]4H2O}n H3CP a-Cu(HCP)(H2O) (a-Cu-1), a-Cu(HCP)(H2O)0.5H2O (a-Cu-2) b-Cu(HCP)(H2O) (b-Cu)

3D 3D 3D 1D 3D 3D 3D 3D 3D 2D 2D 2D

No No No Yes No No No No No Yes Yes Yes

Solvent evaporation method Post-synthetic method Solvothermal reaction Solvothermal reaction Solvothermal reaction Solvothermal reaction Solvothermal reaction Post-synthetic method Solution reaction Solvothermal reaction Solvothermal reaction Solvothermal reaction

4.90  104 (100 °C, 98% RH) 1.13  102 (100 °C, 98% RH) 2.90  102 (60 °C, 95% RH) 3.46  103 (95 °C, 95% RH) 1.27  105 (95 °C, 95% RH) 5  104 (150 °C) 1.1  105 (70 °C, 90% RH) 1.79  102 (95 °C, 85% RH) 1  103 (25 °C, 98% RH) 7.1  108 (25 °C, 95% RH) 2.4  107 (25 °C, 95% RH) 1.5  1011 (60 °C, 95% RH)

0.39 0.37 0.22 0.68 0.17 0.59 0.27 0.23 0.52 0.47 –

[30] [30] [150] [27] [27] [27] [27] [51] [151] [152] [152] [152]

N-heterocyclic carboxylate MH3IDC {Na[Cd(MIDC)]}n PhH3IDC [Sr(l2-PhH2IDC)2(H2O)4]2H2O

3D 3D

No Yes

Hydrothermal reaction Solvothermal reaction

1.04  103 (100 °C, 98% RH) 1.91  106 (90 °C, 98% RH) (100 °C, aqua-ammonia vapor from 1.5 M NH3H2O solution)

0.35 1.07 0.82–1.81

[153] [154] [154]

Carboxylates

PiPhtA

H6DBDP

4.76  102

[136] [137] [40] [40] [139] [140] [141] [142] [142] [142] [142] [143] [144] [145] [145]

X.-X. Xie et al. / Coordination Chemistry Reviews 404 (2020) 213100

H2SBBA + HFA H4Llue

MOFs

3D 1D 3D

Yes Yes Yes

Solvothermal reaction Solvothermal reaction Hydrothermal reaction

1.36  103 (100 °C, 98% RH) 5.74  105 (100 °C, 98% RH) 5.00  105 (100 °C, 98% RH)

0.67 0.48 0.56

[155] [156] [156]

1D 3D

No No

Hydrothermal reaction Solvothermal reaction

0.40 0.67

[24] [157]

[Cu4(HDMPhIDC)4(H2O)4]n [Cd(HDMPhIDC)(H2O)]n {[Co3(DMPhIDC)2(H2O)6]2H2O}n

1D 1D 3D

No No No

Solvothermal reaction Solvothermal reaction Solvothermal reaction

0.95 1.02 0.26

[158] [159] [160]

p-ClPhH3IDC

{[Co3(p-ClPhHIDC)3(H2O)3]6H2O}n

3D

No

Hydrothermal reaction

0.20 1.58

[161] [161]

m-ClPhH3IDC

{[Co3(m-ClPhIDC)2(H2O)6]2H2O}n

3D

No

Solvothermal reaction

1.58 0.25, 0.73

[161] [161]

o-BrPhH2IDC

[Zn(o-BrPhH2IDC)2(H2O)2]EtOH3H2O [Co(o-BrPhH2IDC)2(H2O)2]EtOH3H2O {[Co3(m-BrPhIDC)2(H2O)6]2H2O}n

0D 0D 3D

Yes Yes No

Solvothermal reaction Solvothermal reaction Solvothermal reaction

0.72 0.89 0.56 0.46–0.5

[28] [28] [160] [160]

[Cu(p-IPhHIDC)]n {[Cd(p-TIPhH2IDC)2]H2O}n [Ni8(OH)4(H2O)2(BDP-COOH)6] (Ni-BDP-X) [Nd(mpca)2Nd(H2O)6Mo(CN)8]nH2O [Zn(l-LCl)(Cl)](H2O)2 [Zn(d-LCl)(Cl)](H2O)2 [In(5-TIA)2(Me2NH2)(H2O)] (In-5TIA) [Cd(5-TIA)2(Me2NH2)2(H2O)] (Cd-5TIA) [Cu(bpdc)(H2O)2]n {H[Cu(Hbpdc)(H2O)2]2[PMo12O40]nH2O}n {H[Cu (Hbpdc)(H2O)2]2[PW12O40]nH2O}n [Cu(bpdc)(H2O)2]n {H[Ni(Hbpdc)(H2O)2]2[PW12O40]8H2O}n [Eu(HL)(H2O)3]2H2O [Dy(HL)(H2O)3]2H2O [(Me)2NH2][In(PPTTA)]2.5DMF2H2O [(Me)2NH2][In(PPTTA)]4.5DMF16H2O [(UO2)2(TTTPC)(OH)O(COOH)]1.5DMF7H2O (SCU6) [(UO2)(HTTTPC)(OH)]Br1.5DMF4H2O (SCU-7) SCU-7P [Co(DCDPP)]5H2O (BUT-83)

2D 1D 3D 3D 3D 3D 3D 3D 2D 3D

Yes Yes Yes No No No No No Yes Yes

Solvothermal reaction Hydrothermal reaction Solvothermal reaction Solvent diffusion technique Hydrothermal reaction Hydrothermal reaction Solvothermal reaction Solvothermal reaction Solvent diffusion technique Solvent diffusion technique

6.08  105 (100 °C, 98% RH) 0.92  103 (100 °C, 98% RH)2.58  105 (100 °C, 98% RH) 1.30  104 (100 °C, 98% RH) 4 8.91  10 (100 °C, 98% RH) 4.41  103 (100 °C, aqua-ammonia vapor from 1.5 M NH3H2O solution) 2.47  104 (90 °C, 93% RH) 4.25  102 (100 °C, aqua-ammonia vapor from 7.4 M NH3H2O solution) 7.62  104 (100 °C, 98% RH) 2 2.89  10 (100 °C, aqua-ammonia vapor from 7.4 M NH3H2O solution) 1.14  104 (100 °C, 98% RH) 3.11  104 (100 °C, 98% RH) 7.64  105 (100 °C, 98% RH) 5.07  104 (100 °C, aqua-ammonia vapor from 1.5 M NH3H2O solution) 1.15  103 (100 °C, 98% RH) 1.24  104 (100 °C, 98% RH) 2.22  103 (80 °C, 97% RH) 2.8  103 (80 °C, 98% RH) 4.45  105 (31 °C, 98% RH) 4.42  105 (31 °C, 98% RH) 5.35  105 (28 °C, 98% RH) 3.61  103 (28 °C, 98% RH) 1.55  104 (100 °C, 98% RH) 1.25  104 (100 °C, 98% RH)

0.25 0.32 0.11 0.39 0.34 0.36 0.137 0.163 – 1.02

[17] [157] [162] [164] [165] [165] [22] [22] [166] [166]

3D 3D 3D 3D 2D 2D 2D

Yes Yes Yes Yes No No No

Solvent diffusion technique Solvent diffusion technique Solvothermal reaction Solvothermal reaction Solvothermal reaction Solvothermal reaction Solvothermal reaction

1.56  103 (100 °C, 98% RH) 1.35  103 (100 °C, 98% RH) 1.6  105 (75 °C, 97% RH) 1.33  105 (75 °C, 97% RH) 1.25  103 (100 °C) 1.08  102 (100 °C) 7.66  107 (50 °C, 90% RH)

1.02 1.01 0.91 0.87 0.38 0.29 –

[166] [39] [167] [167] [168] [168] [169]

2D 2D 3D

No No Yes

Solvothermal reaction Solvothermal reaction Solvothermal reaction

1.15  106 (50 °C, 90% RH) 8.77  105 (50 °C, 90% RH) 3.9  102 (80 °C, 97% RH)

– – 0.34

[169] [169] [170]

O-heterocyclic carboxylate H2FDA In3O(FDA)3(H2O)3][NO3] [NH2Me2][In(FDA)2] In2(l2-OH)2(FDA)2(H2O)

3D 2D 3D

No No No

Solution reaction Solution reaction Urothermal synthesis

– – –

[77] [77] [77]

H4FTA

3D

No

Solution reaction

0.57

[171]

1D

No

Solvent evaporation method

–

[172]

1D

Yes

Solvent evaporation method

–

[172]

DMPhH3IDC

m-BrPhH2IDC

p-IPhH3IDC p-TIPhH3IDC H2BDP-COOH H2mpca LCl 5-TIA H2bpdc

H4L H4PPTTA H3TTTPCBr3

H2DCDPP

[Li6(HFTA)2(H2O)3]3H2O

S-heterocyclic carboxylate HSA [Ag(SA)(2-apy)]n {[Ag(3-apy)](SA)(H2O)}n

10  104 (22.5 °C, 99.5% 6.7  106 (22.5 °C, 99.5% 9.5  103 (SC; 22.5 °C, 99.5% -(22.5 °C, 99.5% 1.2  105 (25 °C, 75%

RH) RH) RH) RH) RH)

3.27  102 (25 °C in a KNO3-K4Fe(CN)6-K3Fe (CN)6 electrolyte) 2.25  102 (25 °C in a KNO3-K4Fe(CN)6-K3Fe (CN)6 electrolyte)

X.-X. Xie et al. / Coordination Chemistry Reviews 404 (2020) 213100

Ba(o-CPhH2IDC)(H2O)4]n {[Mn(o-CPhH2IDC)(4,40 -bipy)0.5(H2O)2]3H2O}n {[Zn5(o-CPhH2IDC)2(o-CPhHIDC)2(2,20 -bipy)5] 5H2O}n {[Sr(o-CPhH2IDC)(H2O)2]2H2O}n [Sr(DMPhH2IDC)2]n

o-CPhH4IDC

(continued on next page)

9

[173] –

[173] –

–

Solvent evaporation method If not specified, the r values are obtained by the compacted pellet samples. a

3D [{Ag2(SDA)(hmt)2}3H2O]n

No

2D [Ag2(SDA)(2-apy)2]n

No

Solvent evaporation method

1.99  102 (25 °C in a KNO3-K4Fe(CN)6-K3Fe (CN)6 electrolyte) 4.25  103(25 °C in a KNO3-K4Fe(CN)6-K3Fe (CN)6 electrolyte) 1.98  103 (25 °C in a KNO3-K4Fe(CN)6-K3Fe (CN)6 electrolyte) Solvent evaporation method 1D [Ag(PPh3)2(SDA)]n H2SDA

No

Ea (eV)

r (S/cm)a Synthetic strategies Presence of noncoordinated COOH Structures MOFs Carboxylates

Table 2 (continued)

[173]

X.-X. Xie et al. / Coordination Chemistry Reviews 404 (2020) 213100

Refs.

10

humidity-dependent proton conductivities, and discovered that the conductivity soared from <109 (50% RH; phase I) to 3.9  105 S/cm (67% RH; phase III) at 17 °C. As decreasing the RH to 25%, the MOF is insulating again due to the formation of phase II. By single-crystal diffraction and elemental analysis, the overall composition of I and II was recognized as (Me2NH2)2[Li2Zr (ox)4] and (Me2NH2)2[Li2(H2O)0.5Zr(ox)4], respectively. Although the crystal structure of III is not available, the pair distribution functions were adopted to discuss the local and middle-range order of III. In I, there is no coordinated water molecule, which is considered to be the proton source, so I is not conductive. In II, in spite of the presence of coordination water, it forms a hydrogen bond with the oxalate ions, so it does not conduct protons. In III, another four water molecules can coordinate to the Zr ion as the proton source, thus, the absorbed water molecules as proton carriers can transfer the protons. As reviewed by H. Kitagawa and co-workers in 2016, a number of ox-bridged 2D conductive MOFs [90–92] bearing mixed-metal have been prepared. Three 2D MOFs, {N(n-C4H9)3}[MCr(ox)3] (M = Mn, Fe, Co) have the r values of ca. 104 S/cm at 25 °C and 80% RH, which ascribe to the 2D hydrophilic sheets built by {NH (n-C4H9)3}+ cations [90]. By contrasting the proton conductivities of {NR3(CH2COOH)}[MCr(ox)3]nH2O (R = Me, Et, or Bu; M = Mn or Fe), H. Kitagawa et al. pointed out that the proton conductive behaviours depend on the hydrophilicity of the counteractions [91]. In 2013, several similar conductive MOFs, {NEt3(CH2COOH)} [MnCr(ox)3]2H2O, {NEt3(CH2COOH)}[FeIICrIII(ox)3]2H2O, {NEt3 (CH2COOH)}[FeIIFeIII(ox)3]2H2O were obtained [92]. The same conclusion can be drawn about the effect of the hydrophilicity of cations on the r values. Although the r values of these MOFs mentioned above are not high, the grasp of proton conduction mechanism is deepened through the different effects of counter-cations on proton transport. To overcome the weakness of instability for these MOFs with hydrophilic ions towards humidity mentioned in references [90–92], H. Kitagawa et al. made several analogous MOFs of LaM (ox)310H2O (M = Cr, Co, Ru, La) and researched their proton conduction [93]. The results illustrated that the honeycomb sheet structures, LaLa(ox)310H2O and LaRu(ox)310H2O, have higher proton conductivities than those of ladder structures, LaCr(ox)310H2O and LaCo(ox)310H2O. Also, different proton conduction mechanisms exist in the different structural MOFs. The proton conduction in LaRu(ox)310H2O (Ea = 0.90 eV) and LaCr(ox)310H2O (Ea = 0.32 eV) obeys the vehicle mechanism and Grotthuss mechanism, respectively. As LaCo(ox)310H2O converted to LaCo(ox)2.5 10H2O, the reduction product has an exceedingly low r value of 1  1010 S/cm due to collapse of proton transferring pathway. In 2018, S. Konar group used M(NO3)3 (M = Gd3+ or Dy3+) to hydrothermally react with H2ma to get two 3D magnetic MOFs, {[Gd(ma)(ox)(H2O)]n3H2O} and {[Dy(ma)(ox)(H2O)]n1.5H2O} [94], in which ox2 ions are from the thermal decomposition of H2ma. Both two MOFs are 3D pillar-layered frameworks fabricated by l2-ma, l4-ma and l2-ox linkers and metal ions. At 80 °C and 95% RH, their r values are 4.7  104 and 9.06  105 S/cm, respectively. On the basis of structural analysis, they believed that coordination water molecules could act as proton sources. The calculated Ea values (0.88 and 0.70 eV) indicate that the proton conduction follows a vehicle mechanism. To improve the water stability of oxalate-based MOFs and their room temperature proton conductivity, L. Huang et al. introduced hydrazinium as proton source and the lanthanide ion with high coordination number into the 3D MOFs, (N2H5)[CeEu(ox)4(N2H5)] 4H2O and (N2H5)[Nd2(ox)4(N2H5)]4H2O [95]. As shown in Fig. 3, a 3D pillared-layer framework of (N2H5)[CeEu(ox)4(N2H5)]4H2O can be acquired by the Ln3+ cations and ox2 linkages, in which the complicated H-bonding networks are built by the interaction

X.-X. Xie et al. / Coordination Chemistry Reviews 404 (2020) 213100

11

Fig. 2. The 3D structure of (C3N2H5)4[MnCr2(ox)6]5H2O indicating two types of channels with imidazolium ions. Reproduced rom Ref. [86] with permission from The Royal Society of Chemistry, Copyright 2017.

Fig. 1. Representative H-bond networks of (a) (NH4)2(H2adp)[Zn2(ox)3]3H2O; (b) (NH4)2(H2adp)[Zn2(ox)3]2H2O; and (c) (NH4)2(H2adp)[Zn2(ox)3]nH2O. Reproduced from Ref. [78] with permission, copyright American Chemical Society 2014.

between coordination N2H+5 ion and crystallization water molecules/crystallization N2H+5 ion, between ox2 anion and crystallization water molecules/crystallization N2H+5 ion. The dense H-bond networks provide potential for high proton conduction. Expectedly, compounds (N2H5)[CeEu(ox)4(N2H5)]4H2O and (N2H5)[Nd2(ox)4(N2H5)]4H2O possess high room temperature r values of 3.42  103 and 2.70  103 S/cm at 100% RH, respectively. More interestingly, the proton conductivity and fluorescence emission of the two MOFs alter with the humidity. Through detailed research, the authors found that in MOF (N2H5) [CeEu(ox)4(N2H5)]4H2O, the proton conductivity in the form of log(r) increases linearly with the increase of humidity, while the Eu-based fluorescence emission intensity decreases linearly. These features can make the electric signal of proton conductivity variation versus humidity be translated to optical signal of fluorescence, so it can be recognized by the naked eye. In 2016, S. Kawata group described two single metal ox-based MOFs, 2D [La2(ox)3(H2O)6]4H2O and 3D [Er2(ox)3(H2O)6]12H2O [96], which include hydrophilic 1D channels filled with crystallization H2O units to construct H-bond networks as proton transfer pathway. At 100% RH, the ordinary r values of [La2(ox)3(H2O)6] 4H2O and [Er2(ox)3(H2O)6]12H2O are 3.35  107 S/cm under 95 °C and 1.79  106 S/cm under 90 °C, respectively,

corresponding to the Grotthuss mechanism. Both two compounds indicated a dehydration-rehydration reversible process via crystalline-to-crystalline transformations. There are few reports on proton conduction of oxalate-based MOFs under anhydrous conditions. In addition to the MOF mentioned above [53], there is another interesting example, Z. Zheng and his co-workers reported one Eu(III) MOF, [Eu2(CO3)(ox)2 (H2O)2]4H2O, which was hydrothermally got through the reaction of Eu2(CO3) with H2(ox)2H2O [97]. As denoted in Fig. 4, the Eu(III) carbonate chains are joined by ox2 anions through two types of coordination modes into a 3D framework, in which the hexagonal channels filled with crystallization waters. Consequently, highly ordered H-bond chains are formed by the coordianation waters, ox2 anions and crystallization waters, which forebodes that high proton conductivity is feasible. Firstly, they determined the humidity-dependent proton conductive features at room temperature and 40–90% RHs, and found that RH has slight effection on the conductivity. Subsequently, without additional RH, the AC impedance tests were conducted from 25 to 200 °C. Its r value increases apace with the promoting of temperature, and reaches the highest value (2.08  103 S/cm) under 150 °C. By systematic studies, they discovered that the H-bond chains are responsible for the high r value. The Ea values vary with different temperature ranges: Ea = 0.47 eV (25–90 °C) and Ea = 0.26 eV (100–150 °C). That is to say, as the temperature increases, the aqua ligands in the H-bond network will rotate in favor of proton hopping. Above 160 °C, the aqua ligands will be thermal loss, and thus the H-bond pathway will disintegrate. Thus, a sharp drop in conductivity of this MOF was found. Note that its structure with the H-bonding arrays could be recovered upon rehydration. Also, by the AC determination from a single crystal sample, they realized that the efficient proton transfer occurs along the a-axis direction by the ordered H-bond chains. 2.1.2. Formate-based MOFs There are no proton conductive MOFs prepared by single formic acid in literature due to their unstability. Only several MOFs can be

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X.-X. Xie et al. / Coordination Chemistry Reviews 404 (2020) 213100

Fig. 3. (a) The coordination geometries of Ln3+; (b) 3D pillared-layer framework of (N2H5)[CeEu(ox)4(N2H5)]4H2O. Adapted from Ref. [95].

promote the direct proton transfer through the channel (vehicle mechanism, Ea = 0.54 eV). In addition, the authors also mingled [Zn2(HCOO)(trz)3]n and 2Him@[Zn2(HCOO)(trz)3]n into the PVA film, and found that 2Him@[Zn2(HCOO)(trz)3]n-PVA-10 has best r value of 1.23  104 S/cm at 85 °C and 98% RH. In 2017, S. Pawlus et al. explored the temperature-dependent crystal structural changes and proton conductivity in an anhydrous MOF, [C2H5NH3][Na0.5Fe0.5(HCOO)3] [99] by X-ray diffraction and broadband dielectric spectroscopic (BDS) manners. The phase transition of [C2H5NH3][Na0.5Fe0.5(HCOO)3] from Pn to P21/n could be observed under about 90 °C. For the first time, the authors supply the direct evidence of the anhydrous proton conduction in the MOF by Kramers–Kronig transformation. Moreover, they explained the Grotthuss mechanism with Ea of 0.23 eV by BDS techniques, which is very essential for expounding proton conduction mechanism.

Fig. 4. (a) 3D structure of Eu2(CO3) with H2(ox)2H2O containing 1D channels filled with crystallization waters; (b) H-bond chain array of coordination waters and ox2 units. Reproduced from Ref. [97] with permission from American Chemical Society, Copyright 2014.

found bearing formate ligands and other components, such as Htrz [98] and C2H5NH+3 ion [99]. In 2018, C. X. Zhang group solvothermally synthesized a 3D MOF, [Zn2(HCOO)(trz)3]n, in which formate is from the decomposition of DMF, and coordinates to the central Zn(II) ion [98]. The Zn(II) cations exhibit two kinds of coordination surroundings: [ZnN3O] and [ZnN6]. Afterward, these Zn(II) ions are associated by trz units with l3-g1g1g1 mode to form a 3D framework. After loaded imidazole (Him) molecules into the channels of [Zn2(HCOO)(trz)3]n, a new MOF, 2Him@[Zn2(HCOO)(trz)3]n was obtained. Interestingly, 2Him@[Zn2(HCOO)(trz)3]n has a higher r value of 1.51  104 S/cm (45 °C, 98% RH) than that of [Zn2(HCOO)(trz)3]n (7.95  107 S/cm at 50 °C, 98% RH), which illustrates that the Him units play a pivotal role in elevating the proton conductivity. The authors pointed out that in [Zn2(HCOO)(trz)3]n, the infinite H-bonds between trz units and COO groups facilitate proton hopping (Grotthuss mechanism, Ea = 0.40 eV); in 2Him@[Zn2(HCOO)(trz)3]n, the introduced Him units can generate imidazolium cations, and form Br£nsted acid-base pair with formate anions, which would greatly

2.1.3. Other aliphatic carboxylate-based MOFs In 2018, C. Serre and co-workers provided a novel amino acidbased MOF, MIP-202 (Zr) bearing L-asp with ultrahigh proton conductivity, excellent water and chemical stability [100], which is very easy to prepare in high yield. In MIP-202 (Zr), the 12connected Zr6(l3-O)4(l3-OH)4 node and the L-aspartate spacer are assembled in water forming a 3D structure. It is worth noting that –NH2 linked by amino acids is protonated and coexists with the remaining Cl in the form of –NH+3/Cl pairs. Under high humidity conditions, NH+3 acting as a proton source, interacts with H2O units present in the pores, and Cl in the cavity also interacts with protons to form a complicated 3D H-bond network. As a consequence, it has an ultrahigh r of 0.011 S/cm under 90 °C and 95% RH, which is a promising candidate as practical proton conductor. Another Zr-based porous microcrystalline MOF, MOF-801 was firstly prepared in term of the solvothermal reaction of the unsaturated H2fum with ZrOCl28H2O in mixed DMF/formic acid solution by G. Wißmann [101] in 2012. Later, O. M. Yaghi and his collaborators determined its single crystal structure [102] and water adsorption ability. They discovered that this MOF contains three symmetrically independent cavities with a pore size of 7.4, 5.6, and 4.8 Å and has large water uptake of MOF-801-SC (single crystal; 350 cm3/g) and MOF-801-Pow (powder; 450 cm3/g) at P/P0 being 0.9. This is helpful for the transport of protons in the channels. In 2018, Ren et al. verified that this MOF has the high tolerance for acidic and alkaline solutions and boiling water, and measured its proton conductivity [103]. Its conductivity soars with the increase of temperature and humidity. At 61 °C and 98% RH, the r value attained 4.16  103 S/cm. The Ea being 0.256 eV denotes the proton transfer mechanism is the Grotthuss mechanism. To further improve the applied value of this MOF, a composite film

X.-X. Xie et al. / Coordination Chemistry Reviews 404 (2020) 213100

was prepared by mixing MOF-801 crystallites with poly(vinylidene fluoride)-poly(vinylpyrrolidone) (MOF-801@PP-X, where X stands for the mass percentage of MOF-801 in the membrane). With the increase of MOF-801 content, the proton conductivity of the membrane is significantly improved, which indicates that MOF-801 can directly strengthen the proton transport performance of the membrane. At 52 °C and 98% RH, the r value of MOF-801-PP-60 attained 1.84  103 S/cm, which was selected to fabricate a MEA into H2/O2 fuel cells for practical testing. Its open-circuit voltage (OCV) is 0.95 V under 30 °C and 100% RH, which can maintain after the MEA worked for 26 h. In 2018, Reinsch et al. reported a new 3D Al-MOF, [Al(OH) (Mes)]nH2O using another unsaturated fatty acid (methylfumaric acid) [104]. There are two types of holes in the framework, one is a triangular hole with diameter of about 2 Å, and the other is a hexagonal hole with diameter of about 6 Å. This MOF displays a high surface area of 1040 m2/g, great thermal (350 °C) and chemical stability. The AC impedance test illustrated that as the humidity and temperature increased, the r value of [Al(OH)(Mes)]nH2O could be 1.1  105 S/cm under 130 °C and 100% RH. The Ea value being 0.15 eV (80–100 °C) denotes that the proton conduction is via the Grotthuss mechanism. A 2D MOF, Zn4(5-mtz)4(L-mal)2(H2O)2 [105] was hydrothermally made by using dual ligands, L-mal and 5-mtz, in which two types of chains, [Zn-5-mtz]n and [Zn-L-mal]n chains, were joined by sharing the Zn(II) ions. This MOF stays its thermal stability until 340 °C, and structural stability in H2O at 60 °C. The temperature- and humidity-dependent proton conductive properties can be found. At 60 °C and 95% RH, its r value can achieve 1.33  105 S/cm. The higher Ea value (0.81 eV) implies that a proton conduction vehicle mechanism exists in this MOF. In 2014, S. Konar and co-workers adopted L-, D- and DL-tartaric acid ligands to synthesize three H-bonded 3D MOFs, [Cd(L-tart) (4,40 -bpy)(H2O)]n9n(H2O), [Cd(D-tart)(4,40 -bpy)(H2O)]n9n(H2O) and [Cd(DL-tart)(4,40 -bpy)(H2O)]n6n(H2O) [106]. The single crystal X-ray diffraction (SCXRD) depicts that the former two MOFs are enantiomers and isostructural. The third MOF, [Cd(DL-tart)(4,40 bpy)(H2O)]n6n(H2O) has the similar PXRD pattern and FT-IR spectrum to the former two MOFs, and is racemic. According to the structural analysis, the three MOFs all contain uncoordinated COOH groups, which can interact with hydroxyl groups, coordination and lattice water units involving in the complicated H-bonds. The three MOFs have relatively large uptakes for water vapor (~239, 240 and 184 mg/g, respectively). Nevertheless, these compounds revealed low r values around 106 S/cm at 85 °C and 95% RH. The reason may be that although free carboxyl group and coordinated water molecules are conducive to proton transfer, a large amount of lattice water units in the frameworks hinder the efficient proton transport. In these compounds, the proton conduction obeys the vehicular mechanism (Ea values: 0.63, 0.67 and 0.77 eV, respectively), which further accounts for the above viewpoint. In the same year, D. Saravanabharathi group supplied another example of an interpenetrating 3D MOF, [Ca(tart)]n4n (H2O) [107], which was prepared by a simple solvent diffusion technique. Different from the tart-based MOFs mentioned above, the carboxylate units in [Ca(tart)]n4n(H2O) all took part in the coordination to Ca2+ cations. Additional evidence comes from the absence of any peak around 1700 cm1 in FT-IR spectrum. Nevertheless, the abundant H-bonding network between the tartrate and H2O units also contributes to the proton conduction. The r value of this MOF is 3  105 S/cm under 100% RH and 25 °C. The authors found that when pelletized samples are placed in dry conditions, the proton conductivity decreases rapidly, and when they are put back in high humidity or moist MeOH vapor, the conductivity is restored. By contrast, they could not observe this phenomenon at anhydrous MeOH or at the hydrocarbon vapor. Such

13

comparative researches express the crucial role of H2O units and protic MeOH moieties on proton conductivity. This strategy provides a new way to explore the proton conduction mechanism. The H3HPA ligand including phosphonate and carboxylate units (Scheme 1) was used to synthesize a new family of lanthanide frameworks, Ln3(H0.75HPA)4xH2O (Ln = la, Ce, Pr, Sm, Eu, Gd and Dy; x = 15–16) [108], which manifested reversible dehydration–rehydration processes. Since MOFs, La3(H0.75HPA)4xH2O (LaHPA-I) and Gd3(H0.75HPA)4xH2O (LaHPA-II) have single crystal data, they were selected to explore proton conducting properties. In the two 3D frameworks, there exist 1D channels filled with crystallization waters and H-bond networks entailed in –POH groups and crystallization waters, which will be conducive to the proton transfer. Both MOFs have humidity- and temperature-dependent proton conducting features. The r values of LaHPA-I and LaHPA-II are 6  106 and 3.2  104 S/cm at 21 °C and 98% RH, respectively. In the light of calculated Ea values (0.20 eV of LaHPA-I for 98% RH; 0.23 eV of LaHPA-II for 98% RH), a Grotthuss proton conduction mechanism via H2O units can be deduced. In 2016, T. Geancha and co-workers constructed a versatile chiral 3D bioMOF, [CaCu6(S,S-Hama)3(OH)2(H2O)]32H2O using a biomolecular ligand, S,S-H4ama [109]. As exhibited in Fig. 5, this chiral MOF is an ordered 3D open framework consisting of honeycomb-like hexagonal channels found by oxamidate-linked dinuclear Cu(II) units, Ca2+ cations and carboxylate(aqua/hydrox o)-based linkages. The lattice H2O units occupying the channels are extended into 1D chains by hydrogen bonding along the

Fig. 5. 3D framework of [CaCu6(S,S-Hama)3(OH)2(H2O)]32H2O along the c-axis (upper); the array of H-bonded lattice H2O units and OH ions hosted in the channels (lower). Reproduced from Ref. [109] with permission from American Chemical Society, Copyright 2016.

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c-axis, which will be helpful for the proton transfer. Its proton conductivity increases with increasing RH. The calculated Ea values are 0.42–0.34 eV under the RH range of 60–95% implying that the proton transfer obeys the Grotthuss mechanism via H-bond networks. Furthermore, in term of quantum MD simulations, the proton conduction mechanism of this bioMOF is deeply discussed. Detailed analysis will be described in a individual section later. 2.2. Aromatic carboxylate-based MOFs 2.2.1. Phenyl carboxylate-based MOFs Aromatic carboxylic acids are widely used in the construction of multi-dimensional proton conducting MOFs due to the diversity of coordination modes and strong coordination ability. Also, the p-p stacking force between the aromatic groups would provide additional stabilization energy for the MOFs. Furthermore, the hydrophobic ability of these aromatic groups will make the MOFs have high water stability. Finally, by introduction of more carboxylate groups to aromatic groups, intricate H-bond networks in the frameworks can be constructed. Particularly, the presence of uncoordinated carboxyl groups in complexes will be very beneficial to the study of proton conduction. Table 2 lists the aromatic carboxylate-based MOFs reported recently. As described in the previous section, Zr-based MOFs frequently possess high structural stability and excellent proton conductivity [100–102]. By adopting a phenyl carboxylate ligand, p-H2BDC, one high stable MOF, (Zr6O4(OH)4(p-BDC)6 (UiO-66) was firstly reported by K. P. Lillerud et al. [110] in 2008. Later, P. Behrens et al. recognized that the addition of monocarboxylic acid in the synthesis process would increase the surface area of UiO-66 [111]. In 2015, H. Kitigawa and his collaborators reported the proton conductive properties of UiO-66 [112], and hoped to improve the r values of UiO-66 by the defect control. By adjusting the molar ratio of metals to ligands (6:6; 6:4; 6:3), and adding monocarboxylic acids (acetic acid or stearic acid), six kinds of MOFs, Zr6O4(OH)4.6(p-BDC)5.7 (UiO-66-1), Zr6O4(OH)5.6(p-BDC)5.2 (UiO-66-2), Zr6O4(OH)6.8(p-BDC)4.6 (UiO-66-3), Zr6O4(OH)4(p-BDC)5.3 (O2C–CH3)1.4 (UiO-66-4), Zr6O4(OH)4.8(p-BDC)5.6 (UiO-66-5) and Zr6O4(OH)6(p-BDC)5 (UiO-66-6) of UiO-66 with defects were prepared. When the defects of p-BDC2 ligand increased from 5% to 23%, the conductivity increased next to two orders of magnitude at 65 °C and 95% RH (from 1.30  105 S/cm (UiO-66-1) and 6.61  105 S/cm (UiO-66-2) to 1.01  103 S/cm (UiO-66-3)). Compared with low defect UiO-66-1, conductivity of UiO-66-4 (2.75  105 S/cm) was slightly improved. Surprisingly, high conductivity can be attained by using excessive long-chain fatty acid in the synthesis process. The conductivity of samples UiO-66-5 and UiO-66-6 were 2.63  104 and 6.93  103 S/cm, respectively, although the number of defects was less than those of UiO-66-2 and UiO-66-3. This demonstrates that the number of defects is not the solely factor affecting the conductivity. Furthermore, the reasons for the enhancement of the r values are appraised from the aspects of nitrogen adsorption, water vapor adsorption and activation energy calculation. They believed that the increases of the porosity of the MOFs and lewis acid site are very beneficial to the improvement of proton conductivity in term of defect control. In 2016, R. Vaidhyanathan and co-workers reported a 3D Mg(II) MOF, Mg(p-BDC)(PyOH) bearing mixed ligands, p-BDC and PyOH [47]. Furthermore, after Mg(p-BDC)(PyOH) was soaked into a saturated methanol solution of Cs2CO3, ~10% Cs ions were loaded into the channels of Mg(p-BDC)(PyOH) obtaining a similar MOF, Mg (p-BDC)(PyOH)_Cs. They employed the pelletized samples to explore their water-mediated proton conduction at 30–90 °C and 30%-90% RHs. A 10000-fold increase in the proton conductivity of Mg(p-BDC)(PyOH)_Cs (1.60  102 S/cm) and Mg(p-BDC)(PyOH)

(8.30  106 S/cm) could be discovered at 90 °C and 90% RH. By crystal structural, TG, PXRD, ICP and elemental analyses, they asserted that the partial protons of the PyOH units in Mg(p-BDC) (PyOH) were superseded by oxyphilic Cs+ ions (Fig. 6), which could lead to the increase of H2O units in the framework due to the hydration of Cs+ and the formation of better H-bonding proton transport channels. They also analyzed the reason why Li+ cation could not be introduced in terms of the difference of ion radius, and further pointed out that the introduction of excess amount of Cs+ ions would block the proton transport. Optimal loading of 10% Cs+ ions in Mg(p-BDC)(PyOH) would be appropriate. The halving of activation energy again shows the advantage of this postsynthesis manner (Ea: 0.35 eV for Mg(p-BDC)(PyOH) and 0.19 eV for Mg(p-BDC)(PyOH)_Cs). In 2013, two isomeric 2D In(III) MOFs, [In(m-BDC)2{(Me)2NH2} (H2O)2] (In-IA-2D-1) and [In(m-BDC)2{(Me)2NH2}(DMF)] (In-IA2D-2), were solvothermally prepared by the reaction of m-H2BDC with In(NO3)3 and [(Me)4N+Cl], otherwise m-H2BDC with In (NO3)3 in a mixture of DMF and H2O [37], respectively. Both two MOFs have very similar structures, in which each In(III) ion is bonded by seven O atoms from four m-BDC2 ligands to form a similar secondary building unit (SBU). Moreover, these SBUs are linked by m-BDC2 ligands to establish a sheet structure. Note that both frameworks contain [(Me)2NH2]+ cations that will conduct protons as in other MOFs [53,85,86,88,89]. The r values are 3.4  103 S/cm for In-IA-2D-1 and 4.2  104 S/cm for In-IA-2D2 at 27 °C under 98% RH. More interestingly, In-IA-2D-2 could also transmit protons under anhydrous conditions from 25 to 90 °C. They trusted that the existence of [(Me)2NH2]+ ions and DMF units is as proton carriers and good for proton conduction. When the ambient temperature is above 90 °C, In-IA-2D-2 shows nonconducting behavior under water-free conditions due to the losing of DMF units. A 1D MOF, [[Zn(2-MBIm)(o-BDC)(H2O)]2H2O]n constructed by mixed ligands of o-BDC2 and 2-MBIm was presented by D. Saravanabharathi and co-workers in 2015 [113]. All the water molecules were aligned by H-bonds that obviously facilitate proton conduction. As expected, humidity based conductive features of this compound could be explored. Although its r value is low (1.0  105 S/cm under 25 °C and 100% RH), this study opens a new window for the study of solid state proton conduction of biomimetic complexes. The structural and stable properties of a 3D Zr(III) MOF, Zr6O5(OH)3(BETC)2(HCOO)5(H2O)2 (MOF)-808 bearing BETC3 ligand was previously investigated in 2014 [102,114], which has excellent thermal and water stability. In 2017, X. M. Ren group determined its r under 42 °C and 99% RH (7.58  103 S/cm) by compressed pellets [115]. Also, this MOF has high r value of 2.65  10 3 S/cm at 17 °C and 99% RH. The low Ea value of 0.37 eV advocates that a Grotthuss mechanism could be observed and the protons hopped along the H-bond networks. To investigate the practical application of MOF-808, they doped this compound into organic polymer, poly(vinylidene fluoride, at a ratio of 10–55%, and thereafter studied the r value of the resulting composite membranes. The results reveal that the mechanical properties and durability of the obtained films are very well, showing good r value (1.56  104 S/cm) in deionized H2O under 65 °C. Recently, this group obtained another proton conductive MOF, Im@MOF-808, which was achieved by incorporating the imidazole units into the pores of MOF-808 [45]. The high boiling imidazole units displace some H2O molecules inside the framework, which leads to the improvement of the thermal stability of the H-bond network. Moreover, the introduction of imidazole led to the Ea value to go down to 0.25 eV, so the r value of Im@MOF-808 was swelled by an order of magnitude by comparison with MOF-808 reaching 3.45  102 S/cm under 65 °C and 99% RH.

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Fig. 6. (a) 3D framework of Mg(p-bdc)(PyOH); (b) Schematic diagram of Mg(p-bdc)(PyOH)_Cs after introducing Cs+ ions into the framework by post-synthetic method. Color code: C, teal; N, blue; O, red; Mg, pink; Cs+, bright green. Adapted from Ref. [47].

In 2016, X. M. Ren group deployed the post-synthesis method to soak another 1,3,5-benzenetricarboxylate-based MOF, HKUST-1 into saturated EtOH solution of NH4Br at room temperature and get a MOF, NH4Br@HKUST-1 [116]. The structural determinations denote that the framework of HKUST-1 preserves stable after NH4Br was encapsulated in the pores. They found that the two compounds had temperature- and humidity-dependent proton conducting properties. The r value of NH4Br@HKUST-1 is increased by three/four orders of magnitude in contrast to that of HKUST-1. They confided that the incorporation of NH4Br into the HKUST-1 framework result in the enhancement of proton conduction. Because in NH4Br@HKUST-1 the proton carriers are increased, the dense H-bond network can be formed. However, they did not interpret why the Ea value (1.42 eV) of NH4Br@HKUST-1 is significantly larger than that of HKUST-1 (0.69 eV). We hold the view that it may be due to the higher energy required for the proton dissociation of ammonium. One 3D MOF, Cu3(BETC)2(H2O)3]4[SiW11MoVO40](C4H12N)5 30H2O, is set up by heteropoly blue [SiW11MoVO40]5 and 1,3,5benzenetricarboxylate anions [79]. Its structural feature is very similar to that of the famous MOF, HKUST-1 [116]. The difference is [SiW11MoVO40]5 as guest located in the pores. The presence of a large number of coordination and crystallization H2O units in the framework forebodes that the compound may have good proton conductivity. The experimental results reveal that its r value magnifies with the increase of humidity. Nevertheless, the r value is 6.37  108 S/cm under 25 °C and 97% RH being a very ordinary value. Two high proton conductive isostructural 3D MOFs, {Co2Cl2(BETC)4/3](Me2NH2)+24/3H2O}n and {Mn2Cl2(BETC)4/3] (Me2NH2)+24/3H2O}n were reported by X. H. Bu group in 2016 [117]. The two MOFs includes anion frameworks and (Me2NH2)+ cations and water chains, which will act as proton conducting pathways. Using compacted pellets, their proton conductive behaviors were explored. At 50 °C and 65% RH, the highest r value of {Co2Cl2(BETC)4/3](Me2NH2)+24/3H2O}n is 1.19  103 S/cm. Its Ea value is equal to 0.21 eV. However, due to the low stability of {Mn2Cl2(BETC)4/3](Me2NH2)+24/3H2O}n, it is not easy to get the temperature-dependent r and Ea values. At low temperature of 19 °C, its r value can be determined to be 2.60  104 S/cm under 65% RH. X. Li and co-workers [41] described a 3D anionic porous MOF, [Cd2(BETC)2(H2O)2]nn(H2bmib)6n(H2O) built by carboxylate and protonated bmib ligands, in which H2bmib and crystallization waters were located inside the framework and assembled an extensive H-bond network for proton conduction. Its best r value is 5.4  105 S/cm under 60 °C and 95% RH, and a vehicle proton conduction mechanism could be observed due to the high Ea value of 0.62 eV.

By using the post-synthetic approach, X. Meng et al. successfully adopted EDTA and H2ox to graft into the Zr6 cluster of MOF808 to achieve two corresponding MOFs, MOF-808-EDTA and MOF-808-ox [118]. Although no single crystal data were gained, PXRD, FT-IR and XPS spectra were employed to infer their structures. The r values of MOF-808, MOF-808-EDTA and MOF-808-ox are 8.97  106, 1.31  104 and 4.25  104 S/cm at 80 °C and 98% RH, respectively. The trend of these r changes is relevant to the water affinities of the modified compounds. The adsorbed water molecules undergo the proton transfer with the coordinated EDTA/ox molecules, which easily construct a continuous H-bond network in the channel. The Grotthuss mechanism is associated to the low Ea values [MOF-808 (0.37 eV), MOF-808-EDTA (0.15 eV) and MOF-808-ox (0.14 eV)]. F. Paesani et al. demonstrated a highly stable MOF, UiO-66(Zr)(CO2H)2 from H4BTC in 2016 [119]. The compound incorporates a high concentration of –COOH units and a large number of microcylinder pores, exhibiting a superprotonic conductivity of 2.3  103 S/cm under 90 °C and 95% RH. Ea = 0.17 eV is very low, which shows that proton transport is highly efficient and pertains to the Grotthuss mechanism. Furthermore, they firstly gave a clear explanation on proton conduction mechanism at the molecular level. The kinetics of protons and water molecules can be detected separately by the combination of quasi-elastic neutron scattering (QENS) measurement and MD simulation. At low temperature, H2O units are mainly distributed in tetrahedral cages, which form strong interaction with free –COOH, and preferentially form clusters. Water molecules have a long residence time, and migration of protons between cages and cages is difficult due to imperfect H-bond networks. Under high temperature, waters are evenly distributed in tetrahedral and octahedral cages. The residence time of H2O molecules is lessened by 20 times. This allows protons to be efficiently transferred throughout the UiO-66(Zr)-(CO2H)2 void, ensuring faster diffusion rates. Two Zn-MOFs, 2D {[Zn(BTC)0.5(DPDS)]5H2O]}n and 1D {[Zn (BTC)0.5(DPDS)]2H2O]}n, comprising BTC4 anions and DPDS were firstly prepared and structurally characterized by S. Konar et al. in 2013 [120] and 2014 [121]. Later, their proton conductive properties were surveyed by the same group [122]. Based on the crystal data, the authors pointed out that the water-chains sustained by the hydrogen bonds between the free H2O units and carboxyl units inside the layers of the two MOFs are responsible for proton transport (Fig. 7). The best conductivities for the two compounds are 2.55  107 and 4.39  104 S/cm under 80 °C and 95% RH, respectively. The high Ea values of 0.96 and 0.84 eV for {[Zn(BTC)0.5(DPDS)]5H2O]}n and {[Zn(BTC)0.5(DPDS)]2H2O]}n, respectively, hint that the proton conductivity belongs to a vehicle mechanism. By the control experiment under D2O environment,

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Fig. 7. Water-chain supported by H-bonds in 2D MOF {[Zn(BTC)0.5(DPDS)]5H2O]}n (left) and 1D MOF {[Zn(BTC)0.5(DPDS)]2H2O]}n (right). Reproduced from Ref. [122] with permission from American Chemical Society, Copyright 2015.

they proved the key role of H2O units on the resultant r. Also, the time-dependent measurements further manifest the structural stability of the two compounds. In 2019, utilizing linear bis-pyridyl-tris-amide ligand (BPTA) and angular bicarboxylate ligands (H2DBA and H2OBA), K. Biradha group designed four water-stable MOFs, {[Cd(BPTA)(BDA)](DMF) 16H2O}n, {[Co(BPTA)(BDA)](DMF)13H2O}n, {[Cd(BPTA)(OBA)](2D MF)8H2O}n, and {[Co(BPTA)(OBA)](2DMF)6H2O}n to research their hydrophilicity and proton conductivity [123]. Structural analyses display that BDA2+ ligand has a propensity to connect Cd2+ or Co2+ forming the 1D double chains, which are further linked by BPTA to establish a grid-like network. While, the OBA2+ ligand tend to align metal ions making up 2D-corrugated sheets, which are bridged by BPTA to build up a 3D framework. These amide-based MOFs have highly hydrophilic nature and large amounts of water adsorbed. These four MOFs showed excellent r values around 103 S/cm at 80 °C and 98% RH, which is mainly attributed to the H-bonds formed by the solvent molecules, aimides and carboxylates. The authors did not supply the Ea values. However, this kind of preparation method of attaining high proton conductive MOFs through introducing the mixed hydrophilic ligands and carboxyl ligands is worthy of attention. By using the mixed ligands of H3cpip and bmib, X. Li group synthesized a 3D porous MOF, [Cd4(cpip)2(Hcpip)2]nn(H2bmib)n (H2O) [41]. The voids in this compound were filled with protonated bmib and lattice waters, in which the H-bond network constituted by H2bmib and water units and the anionic framework are helpful for the proton conduction. A moderate r and a Grotthuss mechanism could be discovered in this compound. In 2016, S. Kumar group adopted the BBDC ligand to prepare a 2D Mg(II) MOF, {[Mg(BBDC)(H2O)3](H2O)}n [124]. The unique feature is that the authors investigated the proton conductivity of {[Mg(BBDC)(H2O)3](H2O)}n under vacuum. Subsequently, they discovered that the r value is 1.44  105 S/cm at 30 °C. Indeed, by this method, they clearly deduced that the crystallization water units act as the proton source and the H-bond networks built by coordination water units and carboxylate O atoms serve as proton transferring channel. Another 3D non-interpenetrating MOF bearing two kinds of carboxylate-based ligands, H2BBDC and H2pdc named Zn3(BBDC)2(pdc)(DMF)6DMF was reported by X. Meng et al. [52]. This compound has nanotubular channels and a free void volume of 53.8%, and can adsorb 615 cm3/g of water vapor at room temperature. Electrochemical tests revealed that the proton conductivity is highly dependent on humidity and temperature. This also suggests that the external H2O units play a pivotal role in proton transport. The H-bonds between carboxylic acid groups, coordinated and

uncoordinated DMF units and the high hydrophilicity of the framework are responsible for the high r value (0.95  102 S/cm under 60 °C and 97% RH). There may be a mix of Grotthuss mechanism and partially vehicular mechanism in this MOF (Ea = 0.45 eV). In 2017, Y. Liu et al. discussed the synergistic action of host carboxylate group, 3,30 ,4,40 -H4BPTC, guest [Me2NH2]+ cation and water molecule on the strengthening of proton conduction in a 3D MOF, [Zn3K2(3,30 ,4,40 -BPTC)3(DMF)2][Me2NH2]4 (JLU-Liu44) [125]. In this MOF, each bptc4 ligand joins four [Zn(COO)4]2 SBUs and K+ ions to constitute a 3D anion framework. The H-bonds composed by the carboxylate anions and [Me2NH2]+ cations are located in the pores. The proton conductivity is weak (8.1  107 S/cm at 190 °C) under anhydrous condition. Nonetheless, as RH increases, the conductivity ranges from 8.1  107 (75% RH) to 8.4  103 S/cm (98% RH) at 27 °C meaning that the adsorbed H2O units serve as a vital part in proton conduction. Additionally, they also mentioned that the concentration of [Me2NH2]+ ions is crucial for the r values. Similar prominence of this cation can be found in other similar compounds [37,53,88,89]. Thereafter, proton transport obeys a Grotthuss mechanism (Ea = 0.25 eV). By using 2,20 ,6,60 -H4BPTC ligand, S. Q. Zang and his colleagues thermally synthesized two MOFs, {[Mg(BPTC)0.5(H2O)3]5H2O}n (Mg-BPTC) and {[Sr2(BPTC)(H2O)6]H2O}n (Sr-BPTC) [126]. Mg-BPTC has a layered structure. The interlayer 1D hydrophilic channels are awash with a marvelous amount of coordination and lattice water units to make up a dense H-bond network that can serve as a potential proton transport pathway. The conductivity of Mg-BPTC elevates with the increase of temperature and humidity, and attains 2.6  104 S/cm under 100 °C and 98% RH. The Ea values are 0.47 eV (30–65 °C) and 1.18 eV (65–100 °C), respectively. At 30–65 °C, proton transport relys on the Grottouss and the vehicle mechanism. Above 65 °C, proton transport relies on the vehicle mechanism, and free H2O units in the space and absorbed H2O units can flow between the layers for long-range proton transfer. In 3D Sr-BPTC, similar to Mg-BPTC, the six coordination H2O units and one free H2O unit constitute a heptamer, which is interrelated by carboxylate to construct a H-bond network to conduct protons. Its best r value is 2.7  104 S/cm under 90 °C and 98% RH. Proton transfer is operated by the vehicle mechanism (Ea = 0.77 eV). Both two MOFs retain high structural stability before and after electrochemical testing. Recently, M. Wahiduzzaman and co-workers utilized a green and simple method to prepare a stable TBDP-based Ti(IIII) MOF, MIP-177-SO4H-LT, decorated by HSO 4 unit [16], which was easily acquired by dispersing MIP-177-LT in 6 M of sulfuric acid aqueous solution for one day at 25 °C. The porosity of MIP-177-SO4H-LT maintains after incorporating HSO 4 unit. FT-IR, TEM, UV–Vis and

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TGA-MS determinations demonstrated that half of the formates in MIP-177-LT were substituted by HSO 4 anions. Note that the r value of MIP-177-SO4H-LT (2.6  102 S/cm) is four times higher than that of MIP-177-LT at 25 °C and 95% RH, which illustrates that sulfuric acid treatment is a useful strategy for improving the proton conductivity. By molecular simulation method, they discussed the Grotthuss mechanism, which will be described in detail in the following section. Another In(III)-based MOF, In(EBTC)(Me2NH2)]DMF(H2O)5, that has good hydrolytic and extra thermal stability was produced by solvothermal reaction of H4EBTC with InCl3 [127]. The fully deprotonated EBTC4 anions coordinated and linked the In(III) cations to afford the 3D anionic framework comprising two types of channels, in which Me2NH2 ions, crystallization DMF and water units are accommodated. The proton conductivity of this compound raises slowly with the increasing RH and temperature. When the temperature is higher than 45 °C, there exists a rapid increase. The change of PXRD pattern around 45 °C suggests that the alignment fluctuations of guest units are very beneficial to the efficient proton conduction. Its highest r value is 3.49  103 S/cm at 25 °C and 99% RH. The calculated Ea values within the full temperature range are all less than 0.4 eV implying that the proton transfer is performed through the dense H-bond network obeying the Grotthuss mechanism. As described in Ref. [45], the introduction of guest molecules such as, imidazole, sulfonic acid, and NH+4 cations into the voids of MOFs is an efficient method to upgrade the proton conductivity. In 2017, H. C. Zhou et al. conducted a more detailed investigation on the effect of the different arrangements of imidazole groups loaded in the MOFs system on proton conductivity. Firstly, a blank MOF, Fe-MOF is solvothermally prepared by using H3TPTCA and Fe3O(MeCOO)6 clusters. Secondarily, the imidazole units were successfully introduced into the pores of Fe-MOF to get two kinds of MOFs, imidazole@Fe–MOF (Im@Fe–MOF) and imidazole–Fe–MOF (Im–Fe–MOF) [128]. In Im@Fe–MOF, the imidazole units were encapsulated into the voids. In contrast, in Im–Fe–MOF, the imidazole units coordinate to the metal ion (Fig. 8). Under the same test conditions, the conductivity of Im–Fe–MOF is much higher than that of Im@Fe–MOF and Fe-MOF, which illustrates that the coordinated imidazole is more favorable for proton conduction than the

Fig. 8. Variation of r values with wash times of Fe–MOF, Im@Fe–MOF, and Im–Fe– MOF. Reproduced from Ref. [128] with permission from American Chemical Society, Copyright 2017.

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free imidazole and coordination water units. At 60 °C and 98% RH, the optimized conductivities of Fe-MOF, Im@Fe-MOF and Im–Fe–MOF are 1.25  104, 4.23  103 and 1.21  102 S/cm, respectively. The proton conduction mechanisms of the three compounds were proposed according to the Ea values under 98% RH. To further understand the mechanism in Im–Fe–MOF, DFT calculations were adopted to prove the important role of coordinated imidazole. The results reveal that the imidazole unit not only increased the proton concentration but also assuredly boosted the stability of Im-Fe–MOF. After the samples of the three MOFs were washed three times with water, the r values of Fe-MOF and Im-Fe–MOF maintain steady (Fig. 8). Recently, to illustrate the key role of free carboxyl units in the proton transfer of MOFs, three multifunctional phenyl carboxylate compounds (H3TPA, H4TPFoCa and H5TPFiCa, see Scheme 1) with different number of carboxyl groups were meticulously designed and used to prepare three Ba(II) MOFs, [Ba2(mH3TPT) (H2O)1.5(CO2)(DMF)1.5] (MFM510), [Ba(H2TPFoCA)(H2O)(DMF)] (MFM511) and [Ba2(HTPFiCA)(H2O)4] (MFM512) [129]. The single crystal structural analyses and AC impedance measurements revealed that since all carboxyl groups in 2D MOF, MFM501, participate in the coordination with the metal ions, this compound displays a moderate r value (2.1  105 S/cm under 25 °C and 98% RH). Although there exist two monodentate –COOH units in 3D MOF, MFM511, their mobility is very confined due to the formation of intramolecular H-bonds with the adjacent carboxylate units. As a result, the conductivity of MFM511 is still moderate (5.1  105 S/cm), which is only two times higher than that of MFM510. In contrast, 3D MOF, MFM512 includes uncoordinated free –COOH groups that are located in the unrestricted framework void. Hence, the r value of MFM512 is 100 times higher that of MFM510, and achieves 2.8  103 S/cm at the same conditions. The differences in the conductivity of these MOFs and the conducting mechanism are analyzed from the characteristics of crystal structures, the strength of hydrogen bonding interactions and the theoretical calculation of Ea. More interestingly, the authors utilized QENS technique to further explore the proton dynamics in MFM512 and illustrated that the proton conducting obeys the ‘‘free diffusion inside a sphere” mechanism. Through the determination of elastic incoherent structure factor, they calculated the geometrical information of the free protons in MFM-512, and found that the result is consistent with the ‘‘proton-hopping” mechanism suggested by Ea value. A 3D MOF, [Cd5(TCA)2(H2O)2]8DMA16H2O, with pentanuclear Cd(II) clusters was solvothermally prepared by the reaction of H3TCA with Cd(NO3)24H2O [130]. The presence of a number of water units in the framework and the hydrophilic channels provides favorable conditions for the proton transfer. At 30 °C, the conductivity raised from 7.41  1010 (50% RH) to 6.88  108 S/ cm (80% RH), further evidencing that water units act as proton carriers. The optimized r value is measured to be1.45  106 S/cm at 80 °C and 85% RH. The high Ea value of 0.74 eV suggests that the proton transfer in the framework is carried out according to the Vehicle mechanism. 2.2.2. MOFs based on phenyl carboxylate containing other functional groups Some interesting discoveries have also been made by introducing other substituents (phosphate, hydroxyl, and nitro groups, etc.) to carboxyphenyl compounds. To name only a few, Z. Q. Zang and co-workers reported the unique proton dynamics in a novel 2D acid–base pair proton-conductive MOF, (Me2NH2)[Eu(pmip)] [131] containing pmip4 anions and (Me2NH2)+ cations. In this MOF, the (Me2NH2)+ cations between the [Eu(pmip)] layers can interact with the O atoms of PO2 anions to constitute strong 3 (N–H  O) H-bond chains, which will be very helpful for the proton

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conduction. The r values of single crystal and powder pressed pellet are 1.25  103 (150 °C and N2 atmosphere) and 3.76  103 S/ cm (100 °C and 98% RH), respectively. Moreover, the r value of single crystal was performed along the c-axis, which is consistent with the direction of H-bond chains. Also, the Ea value for the single crystal was calculated to be 0.21 eV meaning that the proton hopping behaviors (Grotthuss mechanism) take place through the Hbond chains. By contrast, the compacted pellet showed extremely low proton conductivity (108–107 S/cm) under 150 °C and anhydrous conditions indicating that the H-bond chains cannot be fully connected in microcrystalline samples. They determined further the water-mediated r value by compacted pellet at different RHs, and found the highest r value being 3.76  103 S/cm under 100 °C and 98% RH. This hints that the water molecules absorbed by this MOF effectively join the tiny-crystals in the pellet to form highly efficient proton transfer. Importantly, the authors demonstrated that proton dynamics (vibrating and transfer) along N– H  O chains can be directly observed by various methods. For example, in situ variable temperature SCXRD indicated that the significant variation of a-axis is higher than those of the b and c axes, which is beneficial to the proton dynamic transfer along the H-bond chain. Also, in situ SCXRD determinations clear showed that as the temperature goes up, H1a slowly leaves its parent N1 atom. When the temperature drops, the atom H1a can return to its original position, which is a reversible process. In situ diffuse reflectance infrared Fourier transform spectrum further confirmed this finding. Moreover, the reversible proton conducting process was verified by the variable-temperature photoluminescence measurements. G. C. Guo group grafted the electron-withdrawing unit, –NO2, into isophthalic acid to enhance the acidity and hydrophilicity of the resulting ligand, H2NIPA (Scheme 1), and further investigated its coordination characteristic. Accordingly, a highly water-stable 3D MOF, {H[(N(Me)4)2][Gd3(NIPA)6]}3H2O was prepared [44]. By adopting a single crystal sample, they found that its best r value can achieve 7.17  102 S/cm at 75 °C and 98% RH. On the basis of structural analysis, H2O vapor adsorption, and calculated Ea value (0.13 eV), they analyzed the conductive mechanism in detail, and mentioned that both the inherent H-bonds within the framework and the formation of a more abundant H-bond network after adsorbing additional H2O molecules have a consequential impact on the proton transport. The elevating temperature enhances the r values by increasing the hydrophilicity of the framework, improving the acidity of free H2O unit and accelerating the proton mobility. By microwave-assisted solvothermal reaction, C. S. Hong group [132] made a 3D porous MOF, [Ni2(dobdc)(H2O)2]6H2O (Ni-MOF74) bearing dobdc4 anion and hexagonal channels, and explored the pH-dependent proton conductive properties. This MOF is intact in boiling water for 7d, and still preserves its structural stability in the H2SO4 aqueous solutions from pH 3.9 to 1.8 for 3d. Further experiments confirmed that the H+ ions were infused into the pores of the framework and that the equilibrium anion was not 2+ SO2 ions were 4 . The MOF was partially dissociated and the Ni charged in equilibrium with SO2 . Consequently, stable H+@Ni4 MOF-74 could be gained. The authors determined their proton conductivities by compressed pellets, and found that H+@Ni-MOF-74 at pH = 1.8 displays a high r value of 2.2  102 S/cm under 80 °C and 95% RH, which is two orders of magnitude larger than that of Ni-MOF-74 under similar conditions. H2O adsorption experiments demonstrated that the acidified MOF is more hydrophilic than the original one. These adsorbed H2O clusters inside H+@NiMOF-74 enhanced the proton conductivity significantly. Both MOFs belong to the Grotthuss proton conduction mechanism with the low Ea values (0.12 eV for Ni-MOF-74 and 0.14 eV for H+@NiMOF-74 under 95% RH).

In 2018, N. C. Jeong et al. also favorably synthesized the same structural MOF, Co-MOF-74, as Ni-MOF-74 [133]. The difference is that they used a single crystal of Co-MOF-74 to measure its proton conductivity along different directions. Subsequently, the high degree of directional migration conductivity of protons in the channel makes the conductivity of c-axis about 1200 times larger than that of a-axis, and 46 000 times larger than that of a pelletized sample. The maximum c-axis conductivity of the compound is 4.5  103 S/cm (pH = 11) accompanying a low Ea value of 0.12 eV. They furthermore treated the single crystal of MOF-74 into aqueous solutions of different pH values (3–11) adjusted by H2SO4 and NH3H2O. By the determinations of 1H NMR, PXRD, and N2 adsorption/desorption isotherms, they verified their structural stability. Then, the proton conduction in these pH-controlled single crystals was examined under 30–90 °C and 95% RH. The r value of the compound increases with the increase of acid and base concentration and temperature. Note that the c-axis r values were promoted 7.4-times after the acid or base treatments. Obviously, the increased proton concentration in the crystals treated by acid improves the proton conductivity, while the increase in the r value of the crystals treated by the base is ascribed to the catalysis of base, which enhances the deprotonation of waters and the proton concentration. Recently, sulfonate–carboxylate ligands have attracted much attention due to the strong coordination ability and various geometrical configurations. In the following part, we will give an overview of proton conducting MOFs constructed by the multidentate aromatic sulfonate–carboxylate ligands. H. J. Zhang group employed a ligand, NaH2CS to solvothermally react with copper nitrate to afford a 3D non-interpenetrating MOF, Cu4(CS)2(OH)2 (DMF)2, carrying [Cu4(OH)2(CO2)4(SO3)2] clusters [134], in which 1D irregular channels (~7.0 Å diameter) could be discovered containing ample H-bonds. These H-bonds are erected by the sulfonate O atoms, carboxylate O atoms, and DMF units, which is fairly good for proton conduction. The compressed Cu4(CS)2(OH)2(DMF)2 pellets are not proton conductive under anhydrous conditions. Its proton conductivity increases with temperature at 95% RH, attaining a maximum value of 7.4  104 S/cm under 95 °C. The calculated Ea value is 1.32 eV revealing that the proton transport is a vehicle mechanism. In 2017, by using H3CS ligand, a 2D anionic layered MOF, {[In(5HCS)2(Me2NH2)]DMF(H2O)1.4}n holding H-bonded Me2NH 2 ions and H2O units was solvothermally prepared [135]. Considering the low stability of this MOF at high humidity, G. K. H. Shimizu and co-workers mainly explored its proton conductivity under low humidity. The highlight of this literature lies in the determination of proton conductivity along three { 1 1 0}, {1 1 0}, and {0 0 1} planes with a single crystal and the comparison with the results obtained from pelletized samples. The authors emphasized that the r value (1.17  103 S/cm) at 25 °C and 40% RH obtained by a pelletized sample is not from dissolution at the grain boundary; the H-bonds along ab crystallographic plane act as the main conducting pathway; contrary to what was previously thought, the pore structure in this MOF does not constitute an efficient proton transport channel but rather acts as a constraint. Recently, A. S. Kumbhar group synthesized a 1D MOF, {[Cu2(sba)2(bpg)2(H2O)3]5H2O}n from a sulfonate–carboxylate ligand (4-Hsba) and a N-containing ligand (bpg) [136]. In this MOF, the crystallization and coordination H2O and hydrophilic units inside the 1D channel elevated the proton transfer. Therefore, its best r value is 0.94  102 S/cm under 80 °C and 95% RH. The conductive mechanism was verified by ab initio electronic structure calculations, and the theoretical Ea value (0.54 eV) is close to the observed value (0.64 eV). A water-stable MOF consisting of STA3 ligand, H2SO4@MIL101-SO3H, was described by Y. Q. Lan and co-workers in 2017

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[137]. Free H2SO4 units are encapsulated into MIL-101-SO3H [formula given as Cr3(H2O)3O(HSTA)2(STA)], which was firstly synthesized in 2014 [138] (Fig. 9). The conductivity of the MOFs, H2SO4@MIL-101-SO3H (3 M), H2SO4@MIL-101 (3 M) and MIL101-SO3H, increases with increasing temperature and RH. At 70 °C and 90% RH, the maximum r values of them are 1.82, 6.09  101 and 6.32  105 S/cm, respectively. Apparently, the r values of the H2SO4-encapsulated MOFs are greatly reinforced. The reason for the improvement lies in that two steady proton sources were observed in theses frameworks. One is the high density Brønsted acidic–SO3H sites. Another is that the sulfuric acid molecules can act as a proton donor, resulting in a significant increase in the conductivity. Their Ea values estimated in accordance with Arrhenius are 0.39, 0.27 and 0.33 eV, respectively. It’s amazing that the r value of H2SO4@MIL-101-SO3H (3 M) at 40 °C is 0.92  102 S/cm and can be enhanced to 3.54  102 S/cm at 10 °C with an activation barrier of 0.17 eV. This shows that high r value is obtained by a valid H-bond network through the Grotthuss mechanism. They believed that the synergy of water molecules, H2SO4 units and -SO3H groups offers multiple protontransfer pathways. The high r value of H2SO4@MIL-101-SO3H (3 M) can be asserted at 40 °C for 20 h. C. S. Hong group deployed a post-synthetic oxidation method to prepare a 3D MOF, UiO-66(SO3H)2 having a super-high r value (8.4  102 S/cm under 80 °C and 90% RH) [40]. The starting material UiO-66(SH)2 was got by the reaction of H4DBCA with Zr4+ in the mixed solvent of DMF and CH3COOH under microwave irradiation. Then, UiO-66(SH)2 was treated by H2O2 for one hour to get the oxidized product, UiO-66(SO3H)2, which has high water stability, can keep its structure over 30 days in boiling water. They determined the r values of UiO-66, UiO-66(SH)2 and UiO-66(SO3H)2 under different RHs and temperatures. The r values of these MOFs emerge temperature- and humidity-dependence. Under 80 °C and 90% RH, the r values of UiO-66(SO3H)2, UiO-66(SH)2 and UiO-66 are 8.4  102, 2.5  105, and 4.3  106 S/cm, respectively. The r value of UiO-66(SO3H)2 was increased by more than 3–4 orders of magnitude compared to that of UiO-66(SH)2 or UiO-66. The Ea values of UiO-66(SO3H)2 and UiO-66(SH)2 are 0.23 and 0.32 eV, respectively, indicating that a Grotthuss proton conduction mechanism in the two MOFs could be discovered. The results once again confirm the pivotal role of –SO3H units in the MOFs. By utilizing a well-designed aromatic sulfonate-carboxylate ligand, H4BPDSDC, Q. Y. Liu et al. prepared three 3D isomorphous

Fig. 9. (a) Host framework of MIL-101-SO3H [–SO3H units as yellow balls; the environment of Cr(III) ions as green polyhedron]; (b) Schematic diagram for the preparation of H2SO4@MIL-101-SO3H. Reproduced rom Ref. [137] with permission from American Chemical Society, Copyright 2017.

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Ln(III) MOFs, {[LnK(BPDSDC)(DMF)(H2O)]x(solvent)}n (Ln = Sm3+, Eu3+, and Pr3+) [139] including 1D rod-shaped heterometallic Ln– K SBUs, hexagonal and trigonal channels. Note that the sulfonate oxygen atoms lined in the surfaces of the channels, which will furnish proton transfer pathways. They chose {[SmK(BPDSDC)(DMF) (H2O)]x(solvent)}n to research its proton conductivity. This compound has humidity- and temperature-dependent proton conductive properties with a high r of 1.11  103 S/cm under 80 °C and 98% RH. In the light of Ea value of 0.628 eV, they deduced that the hydrophilic sites (sulfonate) inside the framework will attract more water molecules into the pores. Then, theses solvated water molecules move along the channels by a vehicle mechanism. S. Q. Zang group used another sulfonate-carboxylate ligand, Na2H2DSO (Scheme 1) to assembly two 3D porous MOFs, {[Tb4(OH)4(DSO)2(H2O)8](H2O)8}n (Tb-DSO) [140] and {[H3O][Cu2(DSO) (OH)(H2O)]9.5H2O}n (Cu-DSO) [141]. Both MOFs incorporate similar tetranuclear [Tb4(l3-OH)4]8+ and [Cu4(l3-OH)2]6+ clusters. These clusters are joined by the carboxylate units of DSO4 ions to construct 3D frameworks. Note that in Tb-DSO, one O atom of sulfonate unit is coordinated to Tb(III) ion, the other uncoordinated O atom of sulfonate unit acting as proton hopping site locates in the channels. In Cu-DSO, two types of channels lined with uncoordinated O atoms of sulfonate unit and filled with a large amount of (H3O)+ and water units. Obviously, the difference of the structure, especially the difference of the filled component in the pores must lead to the difference of the proton conductivity. For Tb-DSO and Cu-DSO, the highest r values are 1.66  104 S/cm at 100 °C and 98% RH and 1.9  103 S/cm under 85 °C and 98% RH, respectively. At high humidity, the calculated Ea values are 0.45 for Tb-DSO and 1.04 eV for Cu-DSO, which implies that the proton conduction mechanism in these two compounds is quite different. In 2018, another four isostructural 3D MOFs, {[Ln4(OH)4(L)2 (H2O)8]4.6H2O1.4CH3CN}n (Ln = Gd, Dy, Ho and Er) constituted by Na2H2DSO ligand were described by S. Konar group [142]. By determining the r values of these MOFs, they found a direct relationship between proton conductivity and metal ion radius. Because the acidity of coordination H2O increases with the increase of metal charge density, the ability of coordination water to release protons enhances with the decrease of metal radius (Gd3+ > Dy3+ > Ho3+ > Er3+). Thus, under 80 °C and 95% RH, the r values of the four MOFs are 2.02  106, 2.96  106, 4.56  103 and 6.59  103 S/cm, respectively. This fact indicates the effect of lanthanide shrinkage on the conductance of MOFs. The Grotthuss mechanism was discussed by the Ea values (<0.4 eV) and indirectly proved by the channel analysis adopting ToposPro program. At the same year, S. Q. Zang group cleverly designed a novel sulfonate–carboxylate ligand, Na2H2DSOS, with a flexible linkage, and consequently synthesized a highly stable 3D Zr-based MOF, [Cp3Zr3(l3-O)(l2-OH)3]2(DSOD)34Na4H2O (MOP-1) bearing 2D H-bond network [143]. In this compound, the Cp3Zr3(l3-O) (l2-OH)3 clusters were connected by DSOD4 anions to construct a 3D framework featuring candy-like cages, in which the –SO 3 groups, coordination and free water units are all involved in the 2D H-bond networks. This makes this MOF show a high r value (1.41  103 S/cm) under 30 °C and 98% RH corresponding to a Grotthuss mechanism (Ea = 0.225 eV). Recently, our group conducted a series of studies of proton conducting MOFs erected by aromatic acyl thiourea-based carboxylate ligands, H3BTAA [144], H3BTEA [145], H3BTPA [145] and H3NTAA [30] (Scheme 1). By the reaction of these similar ligands with Cu (OAc)2, all the prepared MOFs hold the similar unique mixedvalence CuIICuI subunits. For example, in MOFs, {[CuI3CuII3(BTAA)3 (DMF)2(MeOH)(H2O)]3MeOH}n [144], [CuI6CuII6(BTEA)6(H2O)10 I II (DMF)2]6H2O and [Cu6Cu6 (BTPA)6(H2O)10]n [145], the mixedvalence [CuI6CuII6(Latc)6] (Latc = BTAA, BTEA or BTPA) subunits could be constructed. Due to the steric hindrance of naphthalene ring, a

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different subunit of [CuI4CuII4(NTAA)4] was discovered in another MOF, {[CuI4CuII4(NTAA)4]3H2O}n [30]. The stability test depicted that these compounds have good thermal stability below 100 °C and excellent water stability. The more crucial structural features of these MOFs are that the complicated H-bond networks can be constituted inside the frameworks due to the organic ligands containing –C=O, –C=S, –COOH, and two –NH– units, which would be very helpful to the exploration of proton conductivity. In-depth studies illustrated that these MOFs demonstrate altitudinal temperature- and humidity-dependent proton conducting characteristic. The optimized r value of 3D MOF, {[CuI3CuII3(BTAA)3(DMF)2 (MeOH)(H2O)]3MeOH}n is 3.78  104 S/cm under 100 °C and 98% RH, which is one order of magnitude larger than that of a 3D Hbonded organic framework (HOF), HOF-H3BTAA. Under 98% RH, the Ea values of them are similar (0.68 eV for HOF-H3BTAA and 0.63 eV for {[CuI3CuII3(BTAA)3(DMF)2(MeOH)(H2O)]3MeOH}n) suggesting a vehicle proton conduction mechanism. This comparative study provides a new idea for designing new proton conductive materials. Our group mainly performed a comprehensive comparative study on the proton conduction of 0D cluster [CuI6CuII6(BTEA)6(H2O)10(DMF)2]6H2O and 2D MOF, [CuI6CuII6(BTPA)6(H2O)10]n under the H2O and NH3H2O vapors. The experimental results illustrate that compared with water vapor, aqua-ammonia vapor can significantly enhance the r value of the MOF. Under 100 °C and NH3H2O vapors from 2.0 M of aqua-ammonia solution, the r values of [CuI6CuII6(BTEA)6(H2O)10(DMF)2]6H2O and [CuI6CuII6(BTPA)6(H2O)10]n are 9.80  104 and 7.70  104 S/cm, respectively. These values are 40–77 times higher than those in water vapor at 100 °C and 98% RH. When the water vapor is changed to aqua-ammonia vapor, the activation energy of the two MOFs also decreases. The Ea values of [CuI6CuII6(BTEA)6(H2O)10(DMF)2]6H2O and [CuI6CuII6(BTPA)6(H2O)10]n are 0.78 and 0.86 eV, respectively, at 98% RH. Under NH3H2O vapor from 2.0 M of aqua-ammonia solution, Ea values are 0.35 for [CuI6CuII6 (BTEA)6(H2O)10(DMF)2]6H2O and 0.58 eV for [CuI6CuII6(BTPA)6 (H2O)10]n. We deduced that in [CuI6CuII6(BTEA)6(H2O)10(DMF)2] 6H2O, the absorbed NH3 and H2O can easily interact with the crystallization H2O and DMF units and substituent units of the organic ligands to form an efficient H-bond network for proton hopping. Different from the wonderful structural stability of the two compounds, [CuI6CuII6(BTEA)6(H2O)10(DMF)2]6H2O and [CuI6CuII6(BTPA)6(H2O)10]n, under aqua-ammonia vapor [145], the structure of 3D {[CuI4CuII4(NTAA)4]3H2O}n [30] changed after placed into the vapor of concentrated aqua-ammonia solution (28 wt%) for 2.5 h. A new compound [CuI4CuII4(NTAA)4]n-NH3 was obtained carrying two ammonia units and four water units. Both of them have high water stability. It’s worth noting that the r value of [CuI4CuII4 (NTAA)4]n-NH3 (1.13  102 S/cm under 100 °C and 98% RH) is two orders of magnitude higher than that of {[CuI4CuII4(NTAA)4]3H2O}n.

They possess a Grotthuss mechanism illustrating the key role of Hbond network inside the frameworks. Additionally, higher dense H-bonding networks can be formed between NH3 molecules and crystallization waters and functional groups on the organic ligands. By adopting H2SBBA to react with alkali metals (Ca2+, Sr2+ and Ba2+), three different structural 2D MOFs, were prepared by T. Kundu et al. [146]. Therefore, the proton conductive properties and mechanisms of them are different. At 25 °C and 98% RH, r values of Ca-SBBA and Sr-SBBA are 8.6  106 and 4.4  105 S/cm, respectively. The Ea values are 0.23 eV for Ca-SBBA and 0.56 eV Sr-SBBA depicting that there are different conductive mechanisms, which are obviously caused by structural differences. Ba-SBBA did not show any r value due to its instability at the humidity conditions. Wang and his colleagues reported a 2D layered MOF, Na2[Eu (SBBA)2(COO)]0.375DMF0.4H2O in 2018 holding the mixed ligands, H2SBBA and HFA [48]. Its maximum proton conductivity was 8.78  103 S/cm under 90 °C and 90% RH. Because the Ea value is 0.11 eV, protons are transported by the Grotthuss mechanism in theory. However, the O  O distance between two adjacent groups is 5.7141(5) Å, which does not construct a complete H-bond network. ICP-MS and AC impedance tests revealed that when 33.48% of Na+ was replaced by Li+, the r value decreased (2.52  103 S/cm) and Ea increased to 0.15 eV. They proposed a new proton conduction mechanism in which the absorbed waters are distributed in the form of bridging groups and cover the Na+ cation chains associated with the terminal C=O and S=O groups (Fig. 10). M. Dubey et al. synthesized two homochiral Cu-MOFs by using Cu(II) to induce D/L-leucine-derived ligand conformational changes for the first time, and studied their proton conduction properties [147]. At 90 °C and 90% RH, the optimal r values of [Cu1.5(H2LL-leu)(Ac)(H2O)]n3H2O and Cu1.5(H2LD-leu)(H2O)]n10H2O are 1  105 and 4.12  106 S/cm, respectively. The difference in conductivity is closely relevant to the structures. The crystallization water units in [Cu1.5(H2LL-leu)(Ac)(H2O)]n3H2O are highly ordered and linearly arranged, and interacted with the coordinated water units and –COOH groups to build up an effective H-bond network. The number of crystallization H2O units in Cu1.5(H2LD-leu)(H2O)]n10H2O is much higher than that in [Cu1.5(H2LL-leu)(Ac)(H2O)]n3H2O, but a large number of disordered H2O units hinder the efficient transfer of protons, and the circular voids formed by the eight Cu(II) units are hydrophobic due to the existence of isopropyl units. There are few reports about proton conducting MOFs based on carboxylate-phosphate ligands in literature [71,148,149]. In 2014, A. Cabeza and co-workers utilized a rigid multifunctional organic ligand, H4PPhA, to react with CaCl2H2O in water solution. A pillared layered framework, Ca2[(H3PPhA)2]2[(H2PPhA)(H2O)2]5H2O

Fig. 10. Suggested conducting mechanism of Na2[Eu(SBBA)2(COO)]0.375DMF0.4H2O. Reproduced from Ref. [48] with permission of The Royal Society of Chemistry, copyright 2018.

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(Ca-PiPhtA-I), could be produced, in which all uncoordinated POH units, COH units and –COOH groups are pointing toward the interlayer space forming a hydrophilic sheet. Free water molecules interact with each other to form an infinitely extended H-bond network, providing structural guarantee for proton conduction. When Ca-PiPhtA-I was exposed to the vapor from aqueous ammonia (28 wt%) for two hours, they got a new derivative, Ca-PiPhtANH3 carrying 7 ammonia and 16 water units accompanied by a slight structural change. Expectedly, the proton conductivity of Ca-PiPhtA-NH3 (6.6  103 S/cm) was an order of magnitude higher than that of Ca-PiPhtA-I (5.7  104 S/cm) under 24 °C and 98% RH, which indicates that polar guest small molecules such as ammonia and water molecules can generate more efficient proton transfer channels, leading to a significant increase in the r. Using a dicarboxylate-diphosphonate ligand, H6DBDP, a robust layered MOF, Mg2(H2O)4(H2DBDP)H2O was solvothermally prepared [149]. This MOF features a grid-like architecture, in which zigzag ladders composed by Mg2+ cations and H2DBDP4+ anions are further connected by H2DBDP4+ ligands. The extensive Hbond networks can be constructed among the phosphonate and carboxylate, and coordination and free water units in the interlayer acting as an effective proton transferring pathway. This MOF demonstrated a high humidity dependence of r values. For instance, under 70 °C, its r value (3.55  102 S/cm) at 95% RH can sharply drop to 0.72  107 S/cm at 40% RH. The Grotthuss mechanism (Ea = 0.4 eV) implies that the ‘‘proton-hopping” is along the H-bond network. 2.2.3. Polycyclic aromatic carboxylate-based MOFs In 2015, a 3D anionic MOF, Fe4(p-BDC)2(NDC)(SO4)4(DMA)4 (VNU-15) showing a rare fob topology was constructed by T. N. Tu et al. [150]. The DMA counterions and sulphate units form a H-bond network for proton conduction. The r value is enhanced from 2.38  104 S/cm at 30% RH to 2.90  102 S/cm under 60% RH and 95 °C. Ea is 0.22 eV indicating that high proton conductivity is attained by a valid H-bond network through the Grotthuss mechanism. Also, VNU-15 has good heating-cooling stability. This reveals that the use of mixed aromatic carboxylate compounds is beneficial to the construction of structurally stable MOFs with high proton conductivity. S. N. Zhao and his colleagues got three MOFs, 1D [Cu(H2SSCC) (DMF)4]n, 3D [Ca(SSCC)0.5(DMF)2.5]n and [Cd(SSCC)0.5(DMF)2]n by utilizing H4SSCC [27]. Like other sulfonic–carboxylic ligands [139–143], H4SSCC also displays strong coordination ability and complicated coordination modes. All the three compounds show high structural stability after exposed to 95% RH for 48 h, and high thermal stability until 200 °C. However, solely [Cu(H2SSCC) (DMF)4]n indicated a high r value of 3.46  103 S/cm under 95 °C and 95% RH. The other two MOFs showed relatively low r values around 104–105 S/cm. This may be because that all the carboxylate and sulfonate groups in the latter two frameworks are involved in the coordination to metals. In contrast, the two carboxylate groups are protonated and H-bonded with the neighboring sulfonic units and DMF units. Although the calculated Ea value of [Cu(H2SSCC)(DMF)4]n (0.68 eV) is much larger than that of [Ca (SSCC)0.5(DMF)2.5]n (0.17 eV), the high carrier concentration from the free –COOH group in Cu-based MOF makes its proton conductivity much higher than that of Ca-bade MOF. By means of solvothermal reaction of H4SSCC with ZrOCl28H2O, M. V. Nguyen group prepared a novel 3D MOF, {[Zr6O8(H2O)8 (H2SSCC)4](DMA)8}22.93H2O (VNU-23) with the bcu topology (Fig. 11a) [51]. Furthermore, they adopted an anchoring strategy to introduce histamine into the voids of VNU-23 to gain a new MOF, His8.2  VNU-23 (Fig. 11b). Afterward, they compared the water-assisted r values of these two MOFs and found that the proton conductivity (1.79  102 S/cm at 95 °C and 85% RH) of

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His8.2  VNU-23 was significantly improved. The Ea value of 0.27 eV demonstrates that the proton conduction in His8.2  VNU-23 obeys a Grotthuss mechanism. This experimental result illustrated that the crucial role of the re-arrangement of protonated histamine inside the MOF His8.2  VNU-23. In 2016, by using a multifunctional H4PTC ligand, N. Sikda et al. synthesized a 3D pillared-layer MOF, {[K8(PTC)2(H2O)1.5]4H2O}n [151]. The carboxylate oxygen atoms and coordination water units are H-bonded along the c-axis, while guest water and the coordination water units constitute a H-bond network along the a-axis. However, without the electrochemical data from a single crystal sample, the contribution of these hydrogen bonds in different directions to the proton conduction is uncertain. Obviously, the dense H-bond network inside the framework is responsible for the proton transfer, which can be confirmed by the low Ea value of 0.23 eV corresponding to the Grotthuss mechanism. The activation energy decreases with the increase of humidity, illustrating that proton transport becomes easier with the increase of humidity. To be noticed that the r value of this MOF monotonically increases with elevating temperature and humidity. A high value of 1.0  103 S/cm under mild temperature (25 °C) and 98% RH could be attained. Upon exposing the powder sample of {[K8(PTC)2(H2O)1.5]4H2O}n to 98% RH for three days, the authors showed that the extra absorbed water units can interact with the framework to form an extended H-bond network contributing to the proton conduction. Recently, three 2D MOFs, a-Cu(HCP)(H2O) (a-Cu-1), a-Cu(HCP) (H2O)0.5H2O (a-Cu-2) and b-Cu(HCP)(H2O) (b-Cu) based on a carboxylate-phosphonate ligand, H3CP were solvothermally prepared [152]. Slight differences in preparation conditions lead to subtle structural differences in these 2Dstructures. In these MOFs, only phosphonate group in HCP ligand coordinated with the Cu(II) ions serving as a tridentate ligand. The layer topologies of a-Cu-1 and a-Cu-2 are identical, but the H-bond networks between the layers are different due to the different free water units. In b-Cu, double chains composed by {CuO5} and {PO3C} units are bridged by {CuO5} to form a sheet structure. Unexpectedly, although these layered structures contain a large number of uncoordinated –COOH groups, their proton conductivity values are extremely common. For example, under 25 °C and 95% RH, the r values are 7.1  108 and 2.7  107 S/cm for a-Cu-1 and a-Cu-2, respectively. For b-Cu, it exhibits a negligible r of 1.5  1011 S/cm under 60 °C and 95% RH. 2.2.4. N-heterocyclic carboxylate-based MOFs As denoted in Scheme 1 and Table 2, our group mainly deployed ten kinks of substituted imidazole dicarboxylate ligands, MH3IDC, PhH3IDC, o-CPhH4IDC, DMPhH3IDC, p-ClPhH3IDC, m-ClPhH3IDC, o-BrPhH3IDC, m-BrPhH3IDC, p-IPhH3IDC and p-TIPhH3IDC, to construct various proton conducting MOFs [24,28,153–161]. As we all known, 1H-imidazole-4,5-dicarboxylic acid (H3IDC) is a traditional organic multifunctional ligand due to its diverse coordination ability and patterns. In order to enrich the research contents of coordination chemistry and material chemistry, fatty and aromatic substituents were introduced in the 2-position of imidazole group and a variety of imidazole dicarboxylate derivatives were gained. As a result, a large number of structurally variable MOFs bearing above ligands were synthesized to study their applications, such as magnetic, fluorescent, sensing and other properties. Nevertheless, the exploration on proton conduction is extremely limited. When we turned our attention to this field, we were attracted by the unique structural characteristics of these MOFs containing imidazole groups and multiple carboxyl groups, which will be helpful for the construct of proton transfer pathways. Considering the need of practical application in the future, we attach great importance to the structural stability of these MOFs.

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Fig. 11. (a) The structure of VNU-23; (b) Loading of histamine within VNU-23. Reproduced from Ref. [51] with permission of The Royal Society of Chemistry, copyright 2018.

The previous researches showed that all the MOFs adopted have good thermal stability, excellent water stability and wide pH tolerances [24,28,153–161]. This also reflects the advantages of these imidazole dicarboxylic acid derivatives. Firstly, water-mediated proton conduction in these MOFs was investigated. All of these complexes exhibit proton conductivities associated with temperature and humidity. That is to say, their r values enhance with elevating temperature and humidity. The optimal r values of these MOFs are in the range of 103-106 S/cm at 100 °C and 98% RH (Table 2). Note that three MOFs, [Cu(p-IPhHIDC)]n [17], {Na[Cd(MIDC)]}n [153], and Ba(o-CPhH2IDC) (H2O)4]n [155], indicate superhigh r values above 103 S/cm. As shown in Fig. 12, in 2D MOF [Cu(p-IPhHIDC)]n, the imidazole unit and one carboxylate unit bridged Cu2+ cations to form a corrugated 2D layer bearing a great number of uncoordinated –COOH groups. These layers packed each other by intermolecular interactions. To be noticed that each layer is hydrophilic due to the existence of a large amount of free carboxylate units, which will facilitate the formation of H-bond networks with the adsorbed waters, and will be conducive to high proton conductivity. In anionic 3D MOF, {Na[Cd(MIDC)]}n, the carboxylate O atoms and Na+ cations are located in the 1D hydrophilic channels, and interacted with the externally absorbed water molecules can build up a dense and efficient H-bond network

Fig. 12. Three-dimensional packing of [Cu(p-IPhHIDC)]n indicating the hydrophilic sheets. Adapted from Ref. [17].

(1.04  103 S/cm at 100 °C and 98% RH) [153] for the protons hopping through the Grotthuss mechanism (Ea being 0.35 eV). Secondly, we chose five 3D imidazole dicarboxylate-based MOFs, [Sr(l2-PhH2IDC)2(H2O)4]2H2O [154], {[Co3(DMPhIDC)2 (H2O)6]2H2O}n [160], {[Co3(m-BrPhIDC)2(H2O)6]2H2O}n [160], {[Co3(p-ClPhHIDC)3(H2O)3]6H2O}n [161] and {[Co3(mClPhIDC)2(H2O)6]2H2O}n [161], to further investigate their r values under NH3H2O vapors. The results indicated that the r values of these compounds enhanced with the increase of NH3H2O vapor concentration and the increase of temperature. In addition, r value is improved by order of magnitude from 1 to 4 compared with that under pure water vapor. For example, the r value of [Sr(l2-PhH2IDC)2(H2O)4]2H2O is 4.76  102 S/cm under 100 °C and NH3H2O vapor from 1.5 M NH3H2O solution, which is greatly larger than that of [Sr(l2-PhH2IDC)2(H2O)4]2H2O at 100 °C and 98% RH (1.90  106 S/cm). We believe that this is due to the synergistic effect of ammonia and water molecules in the environment and H-bond network within the framework, which leads to the significant improvement of proton conduction of the compounds under aqua-ammonia vapor. Although we have preliminarily discussed the proton conduction mechanism from the crystal structure, water vapor adsorption and ammonia vapor adsorption data, especially the calculation of activation energy, due to the complexity of proton conducting mechanism, we still require gaining more relevant data and evidence to precisely analyze the proton conduction mechanism. Thirdly, encouraged by the fact that the r values of the above five MOFs can be significantly improved under aqua-ammonia vapor, we explored the electrochemical identification of ammonia and amines with three good proton conducting MOFs, [Cu(pIPhHIDC)]n [17], {Na[Cd(MIDC)]}n [153], Ba(o-CPhH2IDC)(H2O)4]n [155] at a certain humidity. The purpose of identification research is mainly to detect the change of impedance value of the MOFs after encountering target gas at room temperature. The homemade device was used to perform the sensing exploration as described in references [17,153,155]. [Cu(p-IPhHIDC)]n displayed prominent sensing features under 68–98% RHs. For example, at 68% RH, it can detect 2 ppm of NH3 and has high response of 8620% toward 130 ppm of NH3, and can select recognize NH3 form seven kinds of interference gases (N2, H2, O2, CO, CO2, benzene, MeOH). Additionally, this compound exhibited an excellent reproducibility. To investigate the sensing properties of {Na[Cd (MIDC)]}n, the impedance of sample was tested toward different

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concentrations of ammonia and volatile amine compounds [methylamine (MA), dimethylamine (DMAE), trimethylamine (TMA) and ethylamine (EA)]. As shown in Fig. 13, the response value increases linearly with increasing gas concentration, and the trend of maximum response at 30 ppm is ammonia (1379%) > MA (934%) > DMAE (548%) > TMA (381%) > EA (231%), where the ammonia sensor’s testing limit can reach 0.5 ppm. In addition, the detection of N2, H2, O2, CO, CO2, benzene, MeOH, n-hexane, and toluene by the same method revealed that MOF {Na[Cd(MIDC)]}n had no sensing ability towords these gases. The MOF Ba(oCPhH2IDC)(H2O)4]n also demonstrated outstanding sensing properties towards NH3 gas under high humidity. However, different from [Cu(p-IPhHIDC)]n, Ba(o-CPhH2IDC)(H2O)4]n exhibits the best sensing ability under 98% RH, which can sense at least 1 ppm of NH3 gas at this humidity. Also, this compound has excellent selectivity and reproducibility. Apparently, similar to the effect of NH3H2O vapor on proton conduction of such MOFs, the synergistic effect of NH3 and amine molecules, adsorbed water molecules and hydrogen bond system within the frameworks during the identification process leads to the sensitive reduction of the impedance of the MOFs. Three novel water- and base-stable MOFs with various functional units [Ni8(OH)4(H2O)2(BDP-X)6] (Ni–BDP-X; X = CHO, CN, COOH) was reported by T. He et al. [162]. Unfortunately, no accurate single crystal data could be gained. However, PXRD tests found that the three compounds have high crystallinity, and the PXRD peaks are highly consistent with the simulated ones of Ni-BDP [163], indicating that MOF Ni-BDP-X (X = CHO, CN, COOH) and Ni-BDP have the very similar structures (Fig. 14). The optimal proton conductivity of Ni-BDP-COOH is 1.21  103 S/cm under 25 °C and 97% RH, which is much higher than that of Ni-BDP-CN (4.87  105 S/cm) and Ni-BDP-CHO (4.27  106 S/cm). It is noteworthy that Ni-BDP-COOH with non-coordinated –COOH groups exhibits better proton conductive features than those of the MOFs with –CHO and –CN groups. The r values of Ni-BDP-COOH improve with increasing temperature, and the highest r of Ni-BDP-COOH is 2.22  103 S/cm at 80 °C and 97% RH. Ea is 0.11 eV, indicating that protons are delivered by the Grotthuss mechanism. By applying mpca ligand, Gao et al. prepared a mixed metal MOF, [Nd(mpca)2Nd(H2O)6Mo(CN)8]nH2O (NdMo-MOF), and investigated the effects of different types of H2O unis inside the framework (coordination and crystallization H2O molecules) on proton conduction [164]. The compound has a 3D structure con-

Fig. 13. Variation of the gas responses as a function of ammonia and amine gas under 98% RH. Reproduced from Ref. [153] with permission of American Chemical Society, copyright 2019.

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Fig. 14. The framework of Ni–BDP and ligands used therein. Adapted from Ref. [162].

taining 1D hydrophilic channels (Fig. 15). TG analysis showed the lattice H2O molecules lost at 80–130 °C and coordination H2O molecules lost at 130–150 °C. The proton conductivity is negligible at 80 °C and 0% RH and at 130 °C, indicating that the coordination H2O molecule is a poor proton conductor. As the humidity increases, the adsorbed H2O units are more easily combined in the MOF channel. The conductivity of NdMo-MOF is enhanced by six orders of magnitude from 4.2  109 (0% RH) to 2.8  103 S/ cm (98% RH). The results show that adsorbed waters are essential for obtaining the high r values. The Ea of NdMo-MOF at 44% RH is 0.39 eV revealing that the proton transfer follows a Grotthuss mechanism. Note that this compound has high thermal stable up to 150 °C making it a suitable candidate for practical application. On the basis of the hydrothermal reaction of a derivative of L-/ D-valine with Zn(OAc)22H2O, four homochiral MOFs, [Zn(l-LCl)) (Cl)](H2O)2, [Zn(l-LBr)(Br)](H2O)2, [Zn(d-LCl)(Cl)](H2O)2, and [Zn(dLBr)(Br)](H2O)2 were made by S. C. Sahoo et al. [165]. The four MOFs exhibit a rare zeolitic unh-topology incorporating a threeperiodic lattice with a parallel helical channel. Interestingly, the

Fig. 15. Crystal structure of the NdMo-MOF viewed along the b-axis, showing the hydrophilic channels (the red spheres denote the oxygen atoms of the lattice water molecules). Reproduced from Ref. [164] with permission from The Royal Society of Chemistry, copyright 2015.

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two crystallization water units form a helical water chain in the helix, which can supply a pathway for proton transport. The r values of [Zn(l-LCl))(Cl)](H2O)2 and [Zn(d-LCl)(Cl)](H2O)2 at 31 °C and 98% RH are 4.45  105 and 4.42  105 S/cm, respectively. Surprisingly, two compounds bearing Br show almost zero proton conductivity under the same conditions. They suggested that different metal-bound halogen atoms make this significant difference. Higher hydrophilicity and electronegativity of chlorine than bromine can make clear the higher r value of [Zn(l-LCl))(Cl)] (H2O)2 than [Zn(l-LBr)(Br)](H2O)2. Apparently, more experiments are needed to explain this strange phenomenon. In 2012, T. Panda et al. used 5-TIA to react with In(III) or Cd(II) synthesizing two isomorphic MOFs, [In(5-TIA)2(Me2NH2)(H2O)] (In-5TIA) and [Cd(5-TIA)2(Me2NH2)2(H2O)] (Cd-5TIA) bearing 1D functionalized nanotubes [22] with internal dimensions of 7.85 Å and 8.23 Å, and the triazoles are all located outside the nanotubes. The 3D framework is built by the weak H-bonds (C–H  O) between the nanotubes (Fig. 16). The proton conductivities of In5TIA and Cd-5TIA are humidity-dependent decreasing upon decreasing RH. Additionally, r value of Cd-5TIA is much more humidity sensitive than that of In-5TIA. The r value increases with the increasing temperature in the range of 4–39 °C for In-5TIA and of 4–28 °C for Cd-5TIA, and then decreases up to 95 °C. As discussed previously, (Me)2NH+2 cations made a significant contribution to proton conduction. For instance, the number of dimethylamine cations in In-5TIA and Cd-5TIA is different. Thus, at 28 °C and 98% RH, proton conductivities for In-5TIA and Cd5TIA are 5.35  105 and 3.61  103 S/cm, respectively. Obviously, the abundance of the H-bond network in the framework could be constructed by more (Me)2NH+2 cations, which leads to the high proton conductivity in Cd-5TIA. Both In-5TIA and Cd5TIA exhibited low Ea values of 0.137 and 0.163 eV, respectively meaning that proton conduction is carried out by a Grotthuss mechanism, and the synergy of (Me)2NH+2 cations, water molecules, and triazoles promotes the movement of protons (Fig. 16). M. Wei and his co-workers employed a N-heterocyclic carboxylic acid ligand, H2bpdc solely, or with Keggin-anions to prepare one 2D MOF [Cu(bpdc)(H2O)2]n [166], and three 3D polyPOM-MOFs, {H[Cu(Hbpdc)(H2O)2]2[PMo12O40]nH2O}n [166], {H

Fig. 16. Schematic representation of the protons hopping along 1D nanochannels for In-5TIA and Cd-5TIA. Reproduced rom Ref. [22] with permission from The Royal Society of Chemistry, Copyright 2017.

[Cu(Hbpdc)(H2O)2]2[PW12O40]nH2O}n [166] and {H[Ni(Hbpdc)(H2O)2]2[PW12O40]8H2O}n [39]. As listed in Table 2, at 100 °C and 98% RH, the maximum r values are of 1.55  104, 1.25  103, 1.56  103 and 1.35  103 S/cm, respectively, which illustrates that the introduction of Keggin-anions into the frameworks gives an improtant contribution on the proton conduction. Evidently, the H-bonds between the Keggin polyanions and coordinated H2O units and between Hbpdc and lattice H2O units are very helpful for the efficient transferring for protons. The high Ea values of the three poly-POM-MOFs are 1.02, 1.02, and 1.01 eV, respectively, implying that a vehicle mechanism impels the proton conduction. M. Zhu et al. reported two isostructural 3D Ln-MOFs, [Eu(HL) (H2O)3]2H2O and [Dy(HL)(H2O)3]2H2O built by double-chain motifs with a N-heterocyclic tetra-carboxylate ligand, H4L [167]. They displayed low r of 1.6  105 and 1.33  105 S/cm, respectively, under 75 °C and 97% RH. Their Ea values are 0.91 and 0.87 eV, respectively, indicating that protons are carried out by the vehicle mechanism. In 2018, Liu et al. using multicarboxylate H4PPTTA ligand and In3+ synthesized two different 3D interpenetrated MOFs, [(Me)2NH2][In(PPTTA)]2.5DMF2H2O (FJU-16) and [(Me)2NH2][In(PPTT A)]4.5DMF16H2O (FJU-17) [168]. FJU-16 and FJU-17 have 4-fold interpenetrated and 2-fold interpenetrated structures, respectively. Both compounds have two forms of square channels. FJU16 has channel sizes of 8.352 Å  11.091 Å (a-axis) and 6.794 Å  7.831 Å (b-axis). The channel sizes of FJU-17 are 8.062 Å  14.759 Å and 20.022 Å  5.928 Å. The large number of lattice waters and (Me)2NH+2 cations in the channels make FJU-16 and FJU-17 have excellent r value without increasing external humidity. The r of FJU-16 and FJU-17 at 40 °C is 2.90  106 and 9.13  105 S/cm, respectively. With the increase of temperature, the highest intrinsic r of FJU-16 and FJU-17 is 1.25  103 (80 °C) and 1.08  102 S/cm (100 °C), respectively. The r value of FJU-17 is larger than that of FJU-16, which is related to the porosity of the structures (73.8% for FJU-17 and 36.4% for FJU-16) and the number of guest molecules. The Ea are 0.38 eV for FJU-16 and 0.29 eV for FJU-17 suggesting that the proton transfer mainly depends on the Grotthuss mechanism. Such MOFs, which have intrinsic proton conductivity without increasing ambient humidity, are very rare. S. Wang and his colleagues successfully prepared two 2D layered uronyl-MOFs, [(UO2)2(TTTPC)(OH)O(COOH)]1.5DMF7H2O (SCU-6) and [(UO2)(HTTTPC)(OH)]Br1.5DMF4H2O (SCU-7) by controlling the degree of hydrolysis of UO2+ 2 ions and using a multicarboxylate ligand, H3TTTPCBr3 [169]. The two compounds can form pseudo-3D structures by H-bonded. In addition, in SCU-7, highly disordered Br exists in interlaminar space, which offers a prerequisite for ion exchange. At 50 and 90% RH, the r values of SCU-6 and SCU-7 are 7.66  107 and 1.15  106 S/cm, respectively. After anion (Br and H2PO 4 ) exchange, the conductivity increased to 8.77  105 S/cm (SCU-7P). This research supplies a novel way to enlarge the r values of cationic MOFs. In 2017, J. R. Li and co-authors designed and obtained one 3D porphyrinic-based MOF, [Co(DCDPP)]5H2O, bearing high-density free –COOH groups and revealing a high r value of 3.9  102 Scm1 at 80 °C under 97% RH [170]. The free –COOH group is located in the 1D hydrophilic channels, and the distance between the adjacent carboxyl group and the hydroxyl O atom is 2.984 and 4.739 Å, respectively, which can construct H-bonds with water molecules. Ea is 0.34 eV, showing that it is a Grotthuss proton conduction mechanism. This means that the protons jump forward along the H-bond network constituted by the –COOH units and adsorbed H2O units. In order to demonstrate the contribution of free carboxyl groups to proton conductivity, they prepared another isostructural MOF Co(DpyDtolP), in which in organic ligand, CH3 was instead of COOH. The results showed that conductivity of Co

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(DpyDtolP) is four orders of magnitude lower than that of [Co (DCDPP)]5H2O under the same conditions. Also, [Co(DCDPP)] 5H2O has wonderful stability and conductivity durability implying that the adopting a bifunctional ligand to prepare stable MOFs with high proton conductivity is an efficient method.

2.2.5. O-heterocyclic and S-heterocyclic carboxylate-based MOFs Although there are limited reports on O-heterocyclic and Sheterocyclic carboxylate-based MOFs in literature, their structural characteristics and proton conductivity also need our attention [77,171–173]. Herein, we should mention three proton conducting MOFs, 3D In3O(FDA)3(H2O)3][NO3], 2D [NH2Me2][In(FDA)2] and 3D In2(l2-OH)2(FDA)2(H2O), built by O-heterocyclic carboxylic acid ligand, H2FDA [77]. They were prepared by the similar reaction of In(NO3)3 with H2FDA according to different reaction ratios and different reaction solvents, which also denotes the complicated affecting factors in the preparation of MOFs. Single crystal structure test shows that In3O(FDA)3(H2O)3][NO3], [NH2Me2][In (FDA)2] and In2(l2-OH)2(FDA)2(H2O) are positive, negative, and neutral frameworks, respectively. Proton conductivity determinations by pellet samples indicate that positive and negative frameworks displayed good r values around 1.0  104 S/cm at mild temperature and 99.5% RH. If a single crystal sample of [NH2Me2][In(FDA)2] was used, its r value can attain 9.5  103 S/cm at 22.5 °C and 99.5% RH. However, r values of the neutral framework, In2(l2-OH)2(FDA)2(H2O), are very low, which can be ignored. The authors only discussed the difference of proton conduction from the different structures, but did not give the proton conduction mechanism. Nevertheless, it is obvious that there exists a more complex H-bond network and electrostatic force within the positive and negative frameworks, which will be very conducive to proton transmission. V. Zima group used a ligand bearing four carboxylate groups (H4FTA) to prepare a 3D MOF, [Li6(HFTA)2(H2O)3]3H2O, comprising two types of tetranuclear clusters (LiO4) [171]. The lattice water units are located in the 1D channels, and accompanied by the coordination water units acting as hydrogen bond donors and acceptors. Thus, a 3D extended H-bonding network in the framework can be constructed, which is good for proton conduction. This MOF displays a moderate r value (1.2  105 S/cm) under 25 °C, 75% RH. The highlight of this paper is that the random-walk approach was adopted for the first time to analyze the impedance data of a polycrystalline MOF. As the authors stated that the random-walk approach assumes that the mobile ion moves randomly. One great virtue of this method is that there is no need to choose any equivalent electrical circuit. Note that the r values obtained by the random-walk approach are consistent with the values obtained by the equivalent electric circuit approach. Two thiophene-based carboxylate ligands, HAS and H2SDA, were utilized to synthesize a series of proton conducting MOFs, 1D [Ag(SA)(2-apy)] [172], 1D 1D {[Ag(3-apy)](SA)(H2O)}n [172], 1D [Ag(PPh3)2(SDA)]n [172], 2D[Ag2(SDA)(2-apy)2]n [173] and 3D [{Ag2(SDA)(hmt)2}3H2O]n [173] by X. F. Zheng group. Note that the AC impedance spectra were acquired by putting a pellet sample in KNO3-K4Fe(CN)6-K3Fe(CN)6 electrolyte, which is different from the method used for most conductive MOFs at a fixed RH in air. Using the experimental method described by the authors, these MOFs show relatively high proton conductivity. For example, the r values of them are 3.27  102, 2.25  102, 1.99  102, 4.25  103 and 1.98  103 S/cm, respectively, at 25 °C. When comparing these conductivity values of other MOFs, we should be aware of the differences in determining methods. The effect of structural dimension on proton conductivity is also discussed by the authors. They pointed out that under high humidity case, H2O units are widespread. Thus, The H-bonded networks of 1D

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MOFs are more consecutive than those in 2D or 3D MOFs leading to the higher r values of 1D MOFs than those of 2D and 3D MOFs. 2.3. Proton conduction mechanism: Insights from molecular dynamics simulations The exploration of conductive mechanism is a crucial issue in the field of proton conducting materials. When people break through the shortcoming of fuzzy structures of traditional organic polymeric materials (Such as Nafion or Nafion-like membrane) and use crystalline solid materials to reveal clearly the channels of proton conduction, the solid-state materials with high crystallinity have been paid more and more attention. With the development of research, MD has become an important aspect of proton conduction mechanism. Recently, QENS and solid-state 1H NMR techniques as powerful tools were adopted to investigate the dynamic motion for protons and further elaborated the conducting mechanism [119,129,174,175]. H. Kitagawa group [74] and X. H. Bu group [66] have given a comprehensive overview of the above new experimental methods and detailed introduction of some MOFs in two latest reviews. Very recently, with the rapid development of theoretical chemistry and computational techniques, people hope to predict or interpret the mechanism of proton conduction at the molecular level. Therefore, MD simulations have attracted immense attention. Taking into account that a few simulation applications on MOFs have been described in several recent reviews [66,74,176], we will mainly review the latest and important findings of molecular dynamics simulations on carboxylate-based MOFs. For instance, in {[Gd(ma)(ox)(H2O)]n3H2O}, the precise location of the adsorbed H2O units after humidification and existence of dense H-bonds involving in these adsorbed H2O units along the a-axis was confirmed by the method of computational simulation adopting grand canonical Monte Carlo calculation [94]. Accordingly, the authors expressed that the different arrangement of adsorbed H2O units by H-bonding in the framework along different channel directions gives different contribution to the proton conduction. In channel A (along the a-axis), a dense arrangement of adsorbed H2O units could be found, compared to those of channel B (along the b-axis). This simulation method provides an effective way for the comprehending the proton-transport dynamics. In 2019, ab initio MD simulations were used to show a Grotthuss-like mechanism proceed by a H+ transport from SO4H moieties of MIP-177-SO4H-LT to adjacent guest water units [16]. Afterward, these protons shuttle along the H-bonding array constructed by the guest H2O units could also be evidenced by above caculations. Such proton transfer behavior is also consistent with the Grotthuss mechanism. By means of quantum MD simulations through the SIESTA program, D. Armentano and co-authors gave a visual picture (Fig. 17) of the proton hopping along the H-bond network for a bioMOF, [CaCu6(S,S-Hama)3(OH)2(H2O)]32H2O [109]. In general, the optimized structure is close to the suggested (Grotthuss-type) mechanism, in which the proton-hopping, cleavage and formation of O– H, and relocation of the H2O units are clearly observed. This provides a useful tool to understand the mechanism at the atomic scale. The proton transferring mechanism in UiO-66(Zr)-(CO2H)2 was theoretically verified by MD simulations, employing the anharmonic multistate empirical valence bond (aMS-EVB3) model in the NVE ensemble [119]. The self-diffusion coefficient Ds of water and Ea value (0.2 eV) are consistent with the observed values from QENS determinations. Also, the simulation results confirmed that the proton diffusivity is restricted at mild temperature and changes fast at higher temperature. In summary, the combination of the two approaches of QENS and MD simulations makes up the deficiency of the missing of microscopic picture about proton conduction in conducting MOFs.

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Fig. 17. Proton-hopping along the H-bonded H2O/OH moieties and the successive relocation: (a) starting real location (from crystal data), (b, c) intermediate suggested processes (the proton hopping and the H2O units relocation occuring), and (d) recovery of the starting location. Reproduced rom Ref. [109] with permission from The Royal Society of Chemistry, Copyright 2016.

To shed light on the proton conduction mechanism in MOF, {[Cu2(sba)2(bpg)2(H2O)3]5H2O}n, A. S. Kumbhar group adopted two kinds of ab initio calculations on the basis of DFT to study the atoms affected by temperature and to calculate the Ea value [136]. First, by computing the root mean square deviation (RMSD), they found that H atoms of H2O units are significantly moved throughout the simulation process indicating the presence of dynamic H-bonded network. This is accordance with understanding of proton-hopping in this MOF. Second, to reduce the computational work, the authors isolated the molecular structural unit and assumed that only one proton hopped along the H-bonds, and then calculated the Ea by Climbing Image-Nudged Elastic Band (Cl-NEB) analysis. The calculated Ea value is 0.54 eV, which is slightly smaller than the observed Ea of 0.64 eV. Nonetheless, this simulation method offers enough information for our understanding of proton conduction mechanism. Due to the intricate structural features of most MOFs, it is not easy to simplify the structures. In addition, limited theoretical models and huge computational workload are not conducive to simulating the proton conduction mechanism at the molecular level. So far, there are very few relevant reports in literature. Apparently, to acquire a deeper comprehending of the proton conduction pathways in the furture, much more computational work is required. 2.4. Research progress of other proton conducting coordination systems 2.4.1. Membrane electrode assembly The ultimate goal of the research is to apply crystalline solid proton conducting materials to practical applications in fuel cells. As the basic research of real-world application, MEA incorporated with proton conductive MOFs is a primary approach. As J. Escori-

huela and co-workers reviewed the progress on composite polyelectrolyte membranes with proton conducting MOFs in 2018 [177], the reports about these membranes as MEAs in practical fuel cells are restricted, especially the MEAs constructed by carboxylate-based MOFs. As described early, only composite membrane, MOF-801-PP-60 [103] and pure MOF, (Me2NH2)[Eu(pmip)] [131] were fabricated into the MEAs in H2/O2 fuel cells. The latter fuel cell exhibits the best OCV of 0.87 V under 80 °C and 98% RH. Therefore, we must look for other coordination systems applied to MEAs to enrich the latest progress on this aspect. In 2016, S. Kitagawa group reported the electrolyte made by MOFs in a fule cell [178]. They embedded proton carriers (H3PO4, H2PO 4 , and H2O) in the defect sites of a 2D MOF, [Zn(H2PO4)2 (Htrz)2]n (ZnP) by adjusting amounts of the H3PO4 to obtain a series of related MOFs, ZnP-2 (4 mmol), ZnP-3 (4.4 mmol), ZnP-4 (4.8 mmol), and ZnP-5 (5.2 mmol). By PXRD determinations, they confirmed that the crystal structures of derivative MOFs are similar to ZnP. Furthermore, the defects and mobile proton carriers were studied by 1H NMR, XAFS, and ICP-AES/EA measurements. AC impedance test indicates that the r values of these derivatives are enhanced from 30 to 150 °C under anhydrous conditions with elevating ratio of H3PO4. The r value of ZnP-5 increases from 0.02 at 30 °C to 4.6 mS/cm at 150 °C. The optimized r value of ZnP-5 is about 10 000 times higher than that of ZnP-2. Consequently, ZnP-5 was chose to apply as a electrolyte in a H2/O2 fuel cell. The best OCV is 0.88 V at 120 °C and keeps for one hour, which can be comparable to that of previous polymeric electrolyte fuel cell. Actually, this group has used the MOF, ZnP to prepare a MEA in a H2/air cell [179]. The OCV is 0.65 and 0.50 V at 25 and 130 °C, respectively. In 2018, N. Anahidzade and co-workers [180] firstly used CrMIL-101 [181] as a precursor to prepare a modified MOF, Cr-MIL101-NH2 by post-synthetic approach. Then, Cr-MIL-101-NH2 was

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anchored in a sulfonated poly(ether sulfone) membrane, which has a high r value of 0.041 S/cm under 160 °C and anhydrous conditions. Its power density and OCV of the single cell were tested under the same conditions. The OCV is 0.92 V suggesting the membrane with no obvious gas permeability. The results denoted that the chemical and mechanical nature of the membrane can be enhanced due to the existence of conductive MOFs. At the same year, H. Mahdavi et al. designed the following preparation process for a polymeric electrolyte membrane [182]: firstly, imidazole was encapsuled in MOF NH2MIL-53(Al) [183] to get a derivative mMOF. Afterwards, mMOF and Si-SO3H nanoparticles react with polysulfone and sulfonated polysulfone. Finally, a nanocomposite membrane was obtained, which was characterized

Fig. 18. Distribution of hydrogen bond networks in CoCa4H2O. Reproduced rom Ref. [184] with permission from The American Chemical Society, Copyright 2015.

by IR, TG, SEM, TEM and PXRD data. The mechanical and thermal features of the obtained membrane were improved after MOF/Si nanoparticles were embedded. Its higher r value is 17 mS/cm under 70 °C as percentage of mMOF/Si-SO3H nanoparticles being 5%. When the nanocomposite membrane was used in a fuel cell, the power density is 40.80 mW/cm2, and the OCV is 0.9 V under 70 °C. Through the proton conductivity and fuel cell performance test, the authors discovered that the imidazole group did not play a pivotal role in the membrane as expected under anhydrous and high temperature conditions. Because the imidazole units may be entrapped by the pores in MOF nanoparticle. In general, the application of proton conductive MOFs or polymeric membranes carrying MOFs as MEAs in fuel cell fields needs to be greatly expanded and deepened. 2.4.2. Single crystal proton conduction studies As discussed above, using a single crystal sample to investigate the proton conduction in the MOFs is a powerful tool to grasp the proton transport pathway and mechanism. The exploration of single crystal proton transfer for carboxylate-based MOFs has been described in the previous section [44,77,82,131,133,135], and we will review the related studies of other coordination systems herein. In 2015, L. M. Zheng group reported a Co(II)-Ca(II) phosphonatebased layered MOF [CoCa(H2ntp)(H2O)2]ClO4nH2O (CoCanH2O) (H6ntp = 1,4,7-triazacyclononane-1,4,7-triyl-tris-(methylenepho sphonic acid)) [184], which can reversibly transform from single crystal to single crystal between two lattice water phase (CoCa2H2O) and four lattice water phase (CoCa4H2O) under 40% and 95% RHs, respectively. The relationship between the change of lattice water molecules and the variable of proton conductivity can be clearly described by combining the structural characterization of single crystal to single crystal and impedance spectra determination. With the increase of lattice H2O units, the H-bond network in the MOF is continuously completed. The proton conductivity of CoCa4H2O powder sample is five orders of magnitude higher than that of CoCa2H2O. Furthermore, by measuring the AC impedance spectra of a single crystal sample in three different

Table 3 The single crystal proton conduction investigations of other coordination systems. MOFs [CoCa(H2ntp)(H2O)2]ClO4nH2O (CoCanH2O)

2D

[P2Mo5O23][C7H7N2]6H2O (NNU-6)

0D

[PMo12O40][C7H7N2]32H2O (NNU-7) [PMo11.04V0.96O40][C3H5N2]4H2O (NNU-8) [Cu2(Htzehp)2(4,40 -bipy)]3H2O

0D 0D 2D

[Zn(H2PO4)2(Htrz)2]n

2D

[In(HIDC)(ox)](NH4)(H2O)1.5

3D

[Cr4In4(HIDC)12]H2O [Cr7.28In0.72(HIDC)12]H2O

a

[Co2Na(3,30 ,4,40 -BPTC)2][Emim]3

3D

[CoLa(notpH)(H2O)6]ClO45H2O

2D

If not specified, the r values are obtained by the compacted pellet samples.

r (S/cm) a

Ea (eV)

Refs.

1.55  10 (25 °C, 95% RH) 1.0  103 (SC, [0 1 0]; 25 °C, 95% RH) 8 4.35  10 (SC, [20–1]; 25 °C, 95% RH) 4.49  106 (SC, [2 0 2]; 25 °C, 95% RH) 1.21  103 (50 °C, 98% RH) 1.92  102 (SC, a-axis; 50 °C, 98% RH) 2.42  104 (SC; b-axis; 50 °C, 98% RH) 8.90  105 (SC, c-axis; 50 °C, 98% RH) 6.87  106 (50 °C, 98% RH) 4.45  104 (50 °C, 98% RH) 1.13  105 (30 °C, 98% RH) 1.39  104 (SC, [100]; 30 °C, 95% RH) 1.52  106 (SC, [0 1 0]; 30 °C, 95% RH) 1.2  104 (150 °C, dry N2) 1.1  104 (SC, [0 0 2]; 150 °C, dry N2) 0.82  103 (23.5 °C, 98.6% RH) 1.11  102 (SC, 23.5 °C, 98.6% RH) 2.3  103 (22.5 °C, 98% RH) 5.8  102 (SC; 22.5 °C, 98% RH) 2.1  103 (22.5 °C, 98% RH) 4.8  102 (SC; 22.5 °C, 98% RH) 4.78  107 (SC, c-axis; 25 °C) 2.63  105 (SC, [110]; 25 °C) 3.50  106 (25 °C, 95% RH) 3.05  104 (SC, a-axis; 25 °C, 95% RH)

0.98 0.90 0.58 0.86 0.24 0.86 0.75 0.53 0.69 0.40 – 0.48 0.56 – – – – 0.66 – – – 0.49

[184]

Structures 5

0.34 0.42

[185]

[186]

[179] [187] [188]

[189] [190]

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directions, it was found that the proton transfer in the materials showed obvious anisotropy: 1  103 S/cm along the direction of [0 1 0], 4  106 S/cm along the direction of [2 0 2] and 4  108 S/cm along the direction of [20–1] at 25 °C and 95% RH (Fig. 18). Compared with the H-bond networks in the structure, the Hbond chain composed of lattice water molecules and ClO 4 anions is the optimal proton transfer direction, while the relatively rigid H-bond network composed of MOF skeleton is not easy to transfer protons. In 2018, a millimeter scale single crystal of a 3D supramolecular complex, [P2Mo5O23][C7H7N2]6H2O (NNU-6) containing [P2Mo5O23]6 anions and protonated benzimidazole was selected to investigate proton conductivity [185]. The best r value along the a-axis is 1.91  102 S/cm, and is much larger than that along the b(2.42  104 S/cm) and c-axis (8.9  105 S/cm) under 50 °C and 98% RH. On the basis of the analyses of crystal structure and the calculations of potential energy surfaces of proton transport between benzimidazole units, the authors suggested that the Hbond networks and p-p stacking between benzimidazole units along the a-axis are more favorable for proton conduction than those along b- and c-axes. For the first time, they depicted that aromatic stacking forces also aid proton transport. G. C. Guo group determined the single-crystal conductivity of a 2D MOF, [Cu2(Htzehp)2(4,40 -bipy)]3H2O (H3tzehp = N-[2-(1Htetra zol-5-yl)ethyl]-L-hydroxyproline) [186], and realized that the highest r value (1.43  103 S/cm under 80 °C and 95% RH) along [100] direction is two orders of magnitude higher than that along [0 1 0] direction. This is good agreement with the direction of the H-bond chain array, which is composed by lattice H2O units and OH group of Htzehp2 ligands. Due to the limitations of this review, some other examples on single crystal proton conduction investigations will be summarized in Table 3.

3. Conclusion and perspectives Herein, we generalize the recent progress on proton conductive carboxylate MOFs in recent years. Indeed, the carboxylate group not only indicates strong coordination ability and various coordination modes to bind to metal ions but also can interact with the components of the frameworks (such as, coordination or lattice solvent molecules, other functional groups) to build up plentiful H-bond networks, providing effective proton transfer pathways. Through the molecular modification and clipping of carboxylic acid ligands, a miraculous number of proton conducting MOFs with pores or channels have been constructed by dexterous strategies. Some of these MOFs exhibit superhigh proton conductivity up to 101-102 S/cm, which can be comparable to that of Nafion. However, looking ahead, we believe that the following aspects require paying more attention to: (1) Synthetic strategy: Elaborate design and preparation of organic building blocks are still the first priority. The carboxylate ligands containing more other functional groups will be helpful for the increase the density of free carboxyl groups constructing effective hydrophilic channels inside the frameworks. Additionally, we believe that it is also a good choice to embed other organometallic groups, such as sandwich structural ferrocene group with special physical and chemical features, into the above carboxylate ligands. By this approach, the obtained ferrocene-based MOFs may have unique spatial structure and special structural stability, and desired proton conductivity. In order to meet the needs of greatly improving the proton conductivity, the

post-synthesis method of proton conducting MOFs also requires to be broadened and precisely regulated. (2) Proton conductive mechanism investigations: In addition to using the traditional calculated Ea values to explore the proton conduction mechanism, as well as the solid state 1H NMR and QENS methods, the computational simulation of MD have recently attracted intense attention. Nonetheless, the current limited theoretical models and the computational ability of the computer limit its further application, and the next step is to develop more available theoretical models for the study of mechanism. With improved testing techniques, it would be an exciting discovery if protons hopping or diffusion could be captured in real time. Finally, as discussed above, proton conduction mechanism of single crystal samples for MOFs requires being expanded urgently. Meanwhile, it should be noted that the accurate proton conduction mechanism in the powder pelleted sample should not be ignored, because the powder samples are easily obtained and mostly used in the future. (3) Real-world applications: The combination of MOFs and membrane to prepare mixed membranes can significantly improve the proton transport properties of membrane materials. Nevertheless, the types and functions of membrane materials need to be enlarged. More MEAs need to be fabricated and applied in more fuel cell systems. In addition to paying attention to the applications of proton conductive MOFs in fuel cells, it is needful to extend the new applications of these conducting MOFs. As an example, the impedance recognition study of ammonia and small amine molecules carried out by our research group. Especially, this offers new development opportunities for these MOFs with poor proton conductivity, which may have not particularly application values in the fields of fuel cells and so on. In general, proton conductive carboxylate-based MOFs have broad development prospects in fuel cells, composite membranes and impedance sensors, and can compensate for the defects of Nafion membrane under high temperature and high RH conditions, and will be widely used in the near future. In this respect, the full combination of chemistry and materials will certainly promote the practical application of such MOFs in proton conduction. In summary, carboxylate-based MOFs are unique functional materials and worthy of further and broader research. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors acknowledge the National Science Foundation of China (Grants 21571156 and J1210060). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ccr.2019.213100. References [1] D. Banham, J. Choi, T. Kishimoto, S.Y. Ye, Adv. Mater. 31 (2019) 1804846. [2] X.X. Wang, V. Prabhakaran, Y.H. He, Y.Y. Shao, G. Wu, Adv. Mater. 31 (2019) 1805126.

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