Design strategies and applications of charged metal organic frameworks

Coordination Chemistry Reviews 398 (2019) 113007

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

Review

Design strategies and applications of charged metal organic frameworks Shu-Na Zhao a,c,1, Yang Zhang b,1, Shu-Yan Song a,⇑, Hong-Jie Zhang a a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China b Center of Advanced Analysis & Computational Science, Zhengzhou University, Zhengzhou 450001, PR China c College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, PR China

a r t i c l e

i n f o

Article history: Received 13 March 2019 Accepted 4 July 2019

Keywords: Anionic MOFs Cationic MOFs Ions exchange Post-synthesis

a b s t r a c t As one unique subclass of metal organic frameworks (MOFs), charged MOFs (anionic and cationic MOFs) have attracted extensive attention owing to the strong electrostatic interactions of their charged frameworks. They can serve as interesting cation/anion exchange materials through the exchange of positive/ negative counterions in the framework with functional guest species like luminescent or catalytic guest molecules to endow with enhanced performance or unique functions. The inherent charged properties of charged MOFs afford them excellent selectivity in adsorption and separation. This review focuses on the state-of-the-art development of charged MOFs. Firstly, a comprehensive summary of the design strategies and synthetic methods of charged MOFs will be presented. Then, we will discuss how guest ion exchange can enhance the performance of charged MOFs and canvass the potential applications in the fields of gas adsorption and separation, luminescence, sensing, pollutants removal, heterogeneous catalysis and proton conduction. Finally, the critical challenges and future perspectives in this promising field are also highlighted. Ó 2019 Elsevier B.V. All rights reserved.

Abbreviations: ad, adeninate; AF, acriflavine; AO
, acid orange A; 440 -bipy, 4,40 -bipyridine; bmib, 1,4-bis(2-methylimidazol-10 -yl)butane; BPDC, biphenyldicarboxylate; BPPO, bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide); bpe, 1,2-bis(4-pyridyl)ethane; bpy, 2,20 -bipyridine; BRB+, butyl rhodamine B; 4-BTAPA, 1,3,5-benzene tricarboxylic acid tris[N-(4-pyridyl)amide]; BTB, 1,3,5-benzene(tris)benzoate; CCT, correlated color temperature; CH3I, iodomethane; COD, 1,5-cyclooctadiene; CPM, crystalline porous materials; CRI, color rendering index; 2,4-D, 2,4-dichlorophenoxyacetic acid; 3D, three-dimensional; DEF, N,N-diethylformamide; DES, deep eutectic solvent; DMA, N,N-dimethylacetamide; DMF, N,N-dimethylformamide; DOBDC4
, 1,4-dioxido-2,5-benzenedicarboxylate; DPA, 2,6-pyridinedicarboxylic acid; DPASB, 1butyl-4-(4-(diphenylamino)styryl)pyridinium; DPASD, (4-(4-(diphenylamino)styryl)-1-dodecylpyridinium; DPASM, 4-(4-(diphenylamino)styryl)-1-methylpyridinium; DPASN, 4-(4-(diphenylamino)styryl)-1-nonylpyridinium; dppe, 1,2-bis(diphenylphosphino)ethane; DSM, 4-(p-dimethylaminostyryl)-1-methylpyridinium; 2-FBA, 2fluorobenzoic acid; FBPDC, 3-fluorobiphenyl-4,40 -dicarboxylate; FDA, 2,5-furandicarboxylic acid; FUM, fumaric acid; GCMC, grand canonical Monte Carlo; H4BPTC, 3,30 ,4,40 -biphenyltetracarboxylic acid; H3BTC, 1,3,5-Benzenetricarboxlic acid; H2BTCA, benzotriazole-5-carboxylic acid; H3BTT, 1,3,5-tris(2H-tetrazol-5-yl)benzene; H2CBPTZ, 3-(4-carboxylbenzene)-5-(2-pyrazinyl)-1H-1,2,4-triazole; HCPBPF6, 1-(4-carboxyphenyl)-4,40 -bipyridinium hexafluorophosphate; H3CPIP, 5-(4-carboxyphenoxy)isophthalic acid; H2EDDA, (E)-4,40 -(ethene-1,2-diyl)dibenzoic acid; H2FTZB, 2-fluoro-4-(1H-tetrazol-5-yl)benzoic acid; H2ImidCl, 1,3-bis(4-carboxy-2,6-dimethylphenyl)-1H-imidazo lium chloride; HIP, 1H-Imidazo[4,5-f][1,10]phenanthroline; H4La, tetrakis[4-(carboxyphenyl)oxamethyl] methane acid; H4Lb, 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid; H3Lc, [1,10 -biphenyl]-3,40 ,5-tricarboxylicacid; H2Ld, 1,3-bis(4-carboxyphenyl)imidazolium; H6Le, 50 ,50000 -((5-((4-(3,6-dicarboxy-9H-carbazol-9-yl)phenyl)ethynyl)-1,3phenylene)bis(ethyne-2,1-diyl))bis((1,10 :30 ,100 -terphenyl]-4,400 -dicarboxylic acid); H12Lh, 5,50 ,50 0 ,50 0 0 ,50 0 0 0 ,50 0 0 0 0 -[1,2,3,4,5,6-phenylhexamethoxyl] hexaisophthalic acid; Hppy, 2phenylpyridine; H3TATB, 4,40 ,40 0 -s-triazine-2,4,6-triyltribenzoic acid; H6TATPT, 2,4,6-tris(2,5-dicarboxylphenylamino)-1,3,5-triazine; [H2TMPyP] [p-tosyl]4, 5,10,15,20-tetra kis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate); H2TPA, terephthalic acid; H2TZB, 4-(1H-tetrazol-5-yl)benzoic acid; ICP-OES, inductively coupled plasma optical emission spectroscopy; IMTA, N,N0 -bis(2,6-dimethyl-3,5-carboxylphenyl)imidazolium chloride; Iso, isoprene; KDP, KH2PO4; LED, light-emitting diodes; Lf4
, 1,10 ,40 ,10 0 ,40 0 ,10 0 0 -quaterphenyl-3,5,30 0 0 ,50 0 0 -tetracarboxylate; Li, 1,2,4,5-tetrakisphosphonomethylbenzene; Lj, 4,40 -(ethane-1,2-diyl)bis(N-(pyridin-2-ylmethylene)aniline; Lk, 1-benzimidazolyl-3,5-bis(4-pyridyl)benzene; Ll, 4,40 -(9,9-dibutyl-9H-fluorene-2,7-diyl)dipyridine; Lm, tris(4-(1H-imidazol-1-yl)phenyl)amine; MB+, methylene blue; MBBs, molecular building blocks; MO
, methyl orange; MOFs, Metal organic frameworks; MVK, methyl vinyl ketone; Na2H2DSOA, disodium-2,20 -disulfonate-4,40 0 -oxydibenzoic acid; Na6Lg, 5,50 ,50 0 -(1,3,5-triazine-2,4,6- triyltriimino)tris-isophthalate hexasodium; N(CN)2
, dicyanamide; 1,4-NDC, 1,4-naphthalenedicarboxylate; NENU, Northeast Normal University; 2-NH2-H2BDC, 2-amino-1,4-benzenedicarboxylic acid; NIR, near-infrared; NLO, nonlinear optics; pc-MBPPO, porous BPPO membrane; PCy3, tricyclohexylphosphine; ppz, piperazine; PSM, post-synthetic modification; py, pyridine; pytpy, 2,4,6-tris(4-pyridyl)pyridine; Qst, isosteric heats of adsorption; RE, rare earth; R6G+, rhodamine 6G; Rh, rhodamine; SBUs, secondary building units; SCSC, single-crystal to single-crystal; SD0, Sudan I; SHG, second-harmonic generation; SO0, solvent orange 7; TBHP, tert-butyl hydroperoxide; TBA, tetrabutylammonium; TCPT, 2,4,6-tris-(4-carboxyphenoxy)-1,3,5-triazine; TEA, tetraethylammonium; TEMA Ms, tris (2-hydroxyethyl)methylammonium methylsulfate; TEOA, triethanolamine; TM, Transition metal; TMA, tetramethylammonium; TNP, 2,4,6-trinitrophenol; TON, turn over number; TPA, tetrapropylammonium; TTCA, triphenylene-2,6,10-tricarboxylate; ZMOF, zeolite-like MOF. ⇑ Corresponding author. E-mail address: [email protected] (S.-Y. Song). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ccr.2019.07.004 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.

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S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

Contents 1. 2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Design strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1. Design strategies of anionic MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2. Design strategies of cationic MOFs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Applications of anionic MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Gas adsorption and separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Luminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2.1. White-light emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2.2. Nonlinear optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2.3. Luminescent sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.3. Organic pollutants removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.4. Proton conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.5. Heterogeneous catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Applications of cationic MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.1. Gas adsorption and separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.2. Colorimetric sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.3. Pollutants removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Summary and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Appendix A. Supplementary data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

1. Introduction Over the past decades, metal organic frameworks (MOFs), a fascinating class of hybrid porous materials, have received extensive attention. The highly tunable structure, composition, size, as well as chemical properties of their pores empower them with great promise for various applications including gas separation and sorption [1,2] magnetism [3,4], luminescence [5–7], heterogeneous catalysis [8–10], chemical sensing [11–13], proton conduction [14], energy storage and conversion [15–17], and biomedicine [18,19]. So far, numerous MOFs with well-defined structures have been reported because of the unlimited combinations of metal nodes and organic ligands. The recent research interest of MOFs has extended to thin MOF films [20] and MOF-polymers hybrid materials [21], which can meet requirements for practical applications. In addition, MOF-derived nanomaterials like carbons, metals/metal oxides and their composite materials with regular morphologies have been springing up, exhibiting remarkable potential in energy storage and conversion [22]. However, most of the hybrid materials are based on neutral MOFs. Charged MOFs (anionic and cationic MOFs), a fascinating subclass of MOFs, show superior performance in various applications through postsynthetic ion exchange with functional ionic guests which cannot be happened in neutral MOFs [23] Table 1. The charged MOFs possess a charged framework with guest counterions either positive or negative for the overall electrical neutrality. They are mainly classified into two types including i) anionic MOFs in which the skeleton shows negative charges thereby requiring extra guest cations to neutralize the overall charge and ii) cationic MOFs where the positive charges of the framework results in the existence of an extra counter anion to maintain the neutrality. The studies of charged MOFs have shown that the counterions residing in the frameworks can be easily exchanged by functional guest cations like luminescent or catalytic guest molecules to endow with enhanced performance or unique functions, making them very promising in various applications including gas adsorption and separation, luminescence, sensing, pollutants removal, heterogeneous catalysis, and proton conduction [24]. This review focuses on the state-of-the-art development of charged MOFs. Firstly, we will present a comprehensive summary

of the design strategies and synthetic methods of charged MOFs. Then, how guest ion exchange can enhance the performance of charged MOFs will be discussed including their potential applications in the fields of gas adsorption and separation, luminescence, sensing, pollutants removal, heterogeneous catalysis, and proton conduction. Finally, the critical challenges and future perspectives in this promising field are also highlighted. 2. Design strategies The approaches to build up a charged MOF are of significant challenge. Therefore, the choice of metal ions or clusters, organic ligands, and solvents is of great importance for designing ionicity in a MOF. In this section, several popular used methods for synthesizing charged MOFs will be summarized (Scheme 1,2). 2.1. Design strategies of anionic MOFs To our best knowledge, some solvents like N,Ndimethylformamide (DMF), N,N-dimethylacetamide (DMA), and N,N-diethylformamide (DEF) can generate [NH2Me2]+ or [NH2Et2]+ cations through hydrolysis under solvothermal conditions, resulting in the formation of anionic MOFs. In their pioneering work, Rosi and coworkers reported the very famous anionic MOF bio-MOF-1, formulated as Zn8(ad)4(BPDC)6O2Me2NH28DMF11H2O, (ad = adeninate, BPDC = biphenyldicarboxylate) with [NH2Me2]+ cations residing in the channels [25]. Hereafter, numerous anionic MOFs have been synthesized in which the countercations are generated from the decomposition of DMF, DMA, or DEF molecules [26–28]. Bu, Feng and coworkers reported an anionic C3N4-type framework [In3(BTC)4]n3n– (H3BTC = 1,3,5Benzenetricarboxlic acid) with different extraframework organic cations (Fig. 1) in the pores under dramatically different synthetic conditions including i) solvothermal assembly in DMF, DEF, or triethanolamine (TEOA); ii) ionic liquid condition in tris(2-hydroxye thyl)methylammonium methylsulfate (TEMA Ms); iii) deep eutectic solvent (DES) condition made from choline chloride and ethyleneurea [29]. The protonated TEOA is easily formed in the synthetic condition and then resides in the anionic framework as the extraframework countercations. The ionic liquid and DES have very rich ionic environments which benefits for the formation of

3

S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007 Table 1 A summary of charged MOFs, ion exchange process and their resulting applications. MOF

Counterions +

Exchanged guest ions

Applications

Ref.

procainamide HCl acridine orange Reichardt’s dye Cs+

Cation-triggered drug release Dye capture

[25] [26]

Metal ion capture Cation-controlled gas sorption properties

[28] [29]

Hydrogen adsorption CO2 adsorption

[30] [34]

Gas-adsorption properties (such as gate sorption, hysteresis, and selectivity) Hydrogen adsorption Proton conductivity Proton conductivity / Selective adsorption of metal ions Proton conductivity Proton conductivity

[40]

bio-MOF-1 [(CH3CH2)2NH2]3[In3(BTB)4]10DEF14H2O

[NH2Me2] [NH2Et2]+

[(CH3)2NH2]4[(UO2)4(TBAPy)3]22DMF37H2O (choline)3[In3(BTC)4]2DMF (Pr4N)3[In3(BTC)4] (Bu4N)3[In3(BTC)4] (Et4N)3[In3(BTC)4]DEF (HTEOA)3[In3(BTC)4] (TEMA)3[In3(BTC)4] (choline)3[In3(BTC)4]eurea {[H2ppz][In2(Lf)2]3.5DMF5H2O} [(CH3)2NH2]2[Tb6(l3 -OH)8(FTZB)6(H2O)6](H2O)22 [(CH3)2NH2]2[Y6(l3 -OH)8(FTZB)6(H2O)6](H2O)52 [(CH3)2NH2]2[Tb6(l3 -OH)8(TZB)6(H2O)6]x(solvent) [(CH3)2NH2]2[Tb6(l3 -OH)8(FTZBP)6(H2O)6]x(solvent) [(CH3)2NH2]2[Y6(l3 -OH)8(FTZBP)6(H2O)6]x(solvent) [(CH3)2NH2]2[Tb6(l3 -OH)8(FBPDC)6(H2O)6]x(solvent) [(CH3)2NH2]2[Tb6(l3 -OH)8(DFBPDC)6(H2O)6]x(solvent) (InL)(Me2NH2)3DMA2H2O (In2X)(Me2NH2)25DMA2H2O |(HPP2+)24|[In48(HImDC)96] [In3O(FDA)3(H2O)3][NO3] [NH2(CH3)2][In(FDA)2] In2(l2-OH)2(FDA)2(H2O) [Zn(trz)(H2betc)0.5]DMF

[NH2Me2]+ Choline Pr4N Bu4N Et4N HTEOA TEMA Choline H2ppz2+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ HPP2+ NO
3 [NH2Me2]+ / -COOH

UiO-66-SO3H UiO-66-2COOH Mg2(DOBDC)LiiOPr UiO-66LiOtBu [In3O(PBA)3(NDC)1.5](NO3) [In3O(PBA)3(BPDC)1.5](NO3) [Th3(L3)3O(H2O)3.78]Cl(C5H14N3Cl)8H2O [Eu(TTP)2](ClO4)3H2OCH3OH [{(Zn0.25)8(O)}Zn6(Ld)12(H2O)29(DMF)69(NO3)2]n {[In(OH)Ld]5(NO3)533H2O14DMF} [Cu2(IMTA)(DMSO)2]2H2O [Ag2(tipm)]2NO31.5H2O [Cd(tipo)(HCOO)(H2O)]NO3DMF [{Zn(L)(H2O)2}(NO3)22H2O]n

–SO3H –COOH Li+ Li+ NO
3 NO
3 Cl
ClO
4 NO
3
NO3 Cl
NO
3 NO
3 NO3


MIL-101/[(Etim-H2BDC)+(Br
)] MIL-100-Cr-F-AlCl3 MIL-100-Fe-F- AlCl3 MIL-101(Cr) - AlCl3 [Me2NH2]3[In3(BTB)4]2DMF2DMA28H2O

Br
/ / / [NH2Me2]+

[(CH3)2NH2][In3O(BTC)2(H2O)3]2[In3(BTC)4]7DMF23H2O [(CH3)2NH2][In3O(BTC)2(H2O)3]2[In(BTC)4/3][In(BTC)4/3(H2O)]2

[NH2Me2]+

/

[CH3NH3][In3O(BTC)2(H2O)3]2[In3(BTC)4] (Et4N)3[In3(TATB)4] [CuL]2(CH3)2NH2DMF [Cu6(L)3(H2O)4(HCOO)]∙Me2NH+2∙20DMF

[NH2Me2]+ [CH3NH3]+ Et4N+ [NH2Me2]+ [NH2Me2]+

/ / / / /

[NC2H8]4Cu5(BTT)3

[NH2Me2]+

/

+

bio-MOF-1 [(CH3)NH2]3[(Cu4Cl)3(BTC)8]9DMA (Et2NH2)3[(Cu4Cl)3(TTCA)8]26DEF [Zn3(TCPT)2(HCOO)][NH2(CH3)2] MIL-101(Cr)-SO3H [(CH3)2NH2]15[(Cd2Cl)3(TATPT)4]12DMF18H2O [(CH3)2NH2]2[Zn8(BTCA)6(2-NH2-BDC)3]8DMF (Me2NH2)3[In3(BTB)4]12DMF22H2O bio-MOF-1 [(H2NMe2)2Cd3(C2O4)4]MeOH2H2O ZJU-28

[NH2Me2] [NH2Me2]+ [NH2Et2]+ [NH2Me2]+ -SO3H [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+

bio-MOF-1 bio-MOF-1

[NH2Me2]+ [NH2Me2]+

/ / / / / / Li+ / / / / / / / TMA+, TEA+, TPA+, TBA+ TMA+, TEA+, TPA+, TBA+ DMA+, Li+, Mg2+ / / / Cu2+ / / / / OG2
OG2
Cr2O72
, MB2
, ReO4
CF3SO
, SCN
/ 3

AO
7 , OG2 , NC /
ReO
4 , TcO4 Cr2O2
7 SCN
, N3
, N(CN)2
, ClO
4 /
Cl Cl
Cl
/ MB+

TMA, TEA, TBA TMA, TEA, TPA Li+ Li+, Mg2+, Ca2+, Co2+, Ni2+ Ag+ [Ir(ppy)2(bpy)]+ [Ir(ppy)2(bpy)]+ DSM, AF Rh = Rh 110/123/6G/B) NH+4, Na+, K+ DPASM, DPASB, DPASN, DPASD DM-n, DP-n DMASM

[41] [42]

[43] [47]

Proton conductivity Proton conductivity Dye capture Dye capture Pollutants removal Luminescent properties Proton conductivity Dye capture Selective adsorption of CO2 from CH4 Pollutants removal Pollutants removal Luminescent properties

[49] [50] [51]

CO2 adsorption and fixation into cyclic carbonate Proton conductivity Proton conductivity Pollutants removal CH4 and CO2 adsorption Pollutants removal

[77] [78]

CH4 and CO2 adsorption

[94]

Hydrocarbons adsorption and separation Selective adsorption of C2H2 and CO2 over CH4 Selective Adsorptive C2H2/CH4 and CO2/CH4 Separation Selective Adsorptive CO2/N2 and CO2/H2 separation Tunable CO2 adsorption H2 adsorption and CO2/N2 separation H2 adsorption Selective adsorption of CO2 over N2 Selective olefin–paraffin separation White-LED White-LED White-LED White-LED Nonlinear optics Nonlinear optics

[95] [97] [98]

Nonlinear optics Sensing

[130] [134]

[53] [54] [68] [69] [71] [74] [75] [76]

[80] [93]

[101] [104] [105] [109] [110] [114] [118] [121] [122] [125] [128] [127]

(continued on next page)

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S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

Table 1 (continued) MOF

Counterions

Exchanged guest ions

Applications

Ref.

{(NH2Me2)[Zn3(l3-OH)(tpt)(TZB)3](DMF)12}n [Mg3(NDC)2.5(HCO2)2(H2O)][NH2Me2]2H2ODMF (H3O+)Eu0.5[EuNa0.5Lh(DMF)(H2O)] (H3O+)2[Eu3NaLh2(DMF)5(H2O)2] bio-MOF-1 bio-MOF-1 [Zn2(btb)2(bbis)](Me2NH2)26DMF

[NH2Me2]+ [NH2Me2]+ H3O+, Eu3+ H3O+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+

Sensing Sensing Sensing

[135] [136] [137]

Sensing Sensing Sensing Pollutants removal

[139] [140] [141]

[HDMA]2[Zn2(BDC)3(DMA)]6DMF

[NH2Me2]+

bio-MOF-1 [(CH3)2NH2]2[(Zn2O)L]5DMF [(CH3)2NH2]+[Zn4((l4-O)(NTB)2(NO2-bdc)0.5]3DMA {[Me2NH2]0.125[In0.125(H2Lb)0.25]xDMF}n

[NH2Me2]+ [NH2Me2]+ [NH2Me2]+ [NH2Me2]+

Sensing Sensing

[142] [143] [144] [145] [146]

[(CH3)2NH2]6[Sr13(O)3(BTTC)8(OH)2(H2O)16] [CH3NH3][Zn(NTB)(NMF)]4.5NMF [(C2H5)2NH2]2[Mn6(Li)(OH)2(H2O)6]4DEF [(CH3)2NH2]4[(Zn4dttz6)Zn3]15DMF4.5H2O

[NH2Me2]+ [CH3NH3]+ [NH2Et2]+ [NH2Me2]+

{[(CH3)2NH2][Co2NaL2(CH3COO)2]xS}n [(CH3)2NH2]4[(UO2)4(TBAPy)3] [(CH3)2NH2]2[Cu3O(SO4)3Cu2L2(DMF)(H2O)]9DMF {[H3O][Cu2(DSOA)(OH)(H2O)]9.5H2O}n [Cd2(BTC)2(H2O)2]nn(H2bmib)6n(H2O) [Cd4(CPIP)2(HCPIP)2]nn(H2bmib)n(H2O) {[(Me2NH2)3(SO4)]2[Zn2(ox)3]}n [La(H5Li)(H2O)4] [Zn(5-sipH)(bpy)]DMF2H2O [Zn(H2O)(5-sipH)(bpe)0.5]DMF [Zn3(5-sip)2(5-sipH)(bpy)](DMF)2[(CH3)2NH2]2 His8.2[Zr6O8(H2O)8(H2SNDC)4] {NR3(CH2COOH)}[MCr(ox)3]nH2O R = Me (methyl), Et (ethyl), or Bu (n-butyl) M = Mn or Fe ZJU-28 MIL-101-SO3 [(CH3)2NH2]2[Y2(Cu-OCPP)(H2O)4]8DMF22H2O rho-ZMOF ZJU-28

[NH2Me2]+ H3O+ H2bmib /

Rh6G / / / Yb3+ Eu3+ Eu3+, Tb3+, Dy3+, Sm3+ RhB, MB Eu3+, Tb3+, Dy3+, Sm3+ RhB, MB, BR2 Eu3+, Tb3+, Rh6G Rho Eu3+, Tb3+, Dy3+, Sm3+ MG, MB, CV, RhB RhB, BR2, CV, MB BR9, BV14 MB Eu3+, Tb3+, Dy3+, Sm3+ MB, BR, CV MLB, MV, RhB / / / / /

-PO3H -SO3H

Sensing Eu3+ probe Sensing Pollutants removal Pollutants removal Pollutants removal Tb3+, Dy3+ probe

[156] [157] [159] [160]

Pollutants removal Pollutants removal Pollutants removal Proton conduction Proton conduction

[162] [163] [164] [171] [172]

Proton conduction Proton conduction Proton conduction

[173] [174] [175]

Proton conduction Proton conduction

[176] [177]

Selective hydrogenation

[180]

aerobic oxidation cyclohexane oxidation

[181] [184] [186]

/ / –SO3H /

/

[NH2Me2]+ –SO3H [NH2Me2]+ [NH2Me2]+ [NH2Me2]+

+

PCN-99 [Et4N]3[In3(BTC)4] MIL-101(Cr)-SO3Na {[Ni(bpe)2(N(CN)2)](N(CN)2)(5H2O)}n

[NH2Me2] [Et4N]+ -SO3Na N(CN)
2

[Zn2(TPA)2(CPB)]2DMFH2O Zn8O(EDDA)4(ad)4(HEDDA)26DMF27H2O [Mn2(HCBPTZ)2(Cl)(H2O)]ClDMF0.5CH3CN [{Zn(Lj)(OH2)2}(NO3)2xG]n Cu2(Lk)2I2MeCNMeOH1.5H2O ([CuLl2(H2O)0.5](NO3)23.25(CH2Cl2)(CH3OH)0.5H2O) {[(Cu4Cl)(CPT)4(H2O)4]3NO35NMP3.5H2O}n ZJU-101 [Ag(bipy)]NO3 [{Ni2(Lm)3(SO4)(H2O)3}(SO4)x(G)]n bio-MOF-1 {[Mg3(ndc)2.5(HCO2)2(H2O)][NH2Me2]2H2ODMF}

PF
6

(dppe)Rh(COD)BF4 (MeCN)2Rh(COD)BF4 Cu2+ [H2TMPyP] [p-tosyl]4 [Pd(CH3CN)4][BF4]2 [FeCp(CO)2(thf)]BF4 [Ir(COD)(PCy3)(py)]PF6 [Rh(dppe)(COD)]BF4 [Ru(Cp*)(CH3CN)3]OTf [Ru(bpy)3]2+ [CpFe(CO)2(L)]+ [Ir(cod)(PCy3)(py)][PF6] N
3

Cl NO
3

/ / /

ClO
4 , PF6 , BF4

NO
3 NO
3 NO
3 NO
3 2
SO4 [NH2Me2]+ [NH2Me2]+

Cl
, Br
, I
, SCN
,N
3 MO 2
Cr2O7
ReO
4 , TcO4

Cr2O2
7 , MnO4 , TcO4 K+ /




the anionic [In3(BTC)4]n3n– framework. Like TEOA, protonated piperazine (ppz) and H2O are also reported as extraframework countercations when synthesizing anionic MOFs [30–32]. The above results demonstrate that the synthetic environments, especially for the solvents, are of great importance for the formation of anionic MOFs. It is well known that metal ions have different coordination modes and then can form different charged secondary building units (SBUs), which results in the construction of charged MOFs. Cao and coworkers reported a series of MOFs with anionic, cationic,

Hydrogenation Photocatalytic DA reaction DA reaction Selective hydrogenation Selective adsorption of CO2, MeOH, EtOH, H2O and acetone MeOH adsorption Selective adsorption of C2 hydrocarbons over CH4 CO2 adsorption tunable luminescence properties visual colorimetric properties visual colorimetric properties Pollutants removal Pollutants removal Pollutants removal Pollutants removal electrochemical properties electrocatalytic ORR properties

[189] [190] [191] [192] [193] [194] [197] [203] [205] [206] [209] [211] [212] [215] [216] [217]

and neutral frameworks based on a flexible tetrapodal ligand tetrakis[4-(carboxyphenyl)oxamethyl] methane acid (H4La) and different metals including transition metal ions, lanthanide ions, and main group metal ions [33]. The divalent metal ions (Co, Mn, Cd) are six coordinated with an octahedral geometry and form a trimetallic cluster which is bound together by the carboxyl groups from L4
a ligands. To complete this trimetallic cluster SBUs, surplus negative charges from the carboxyl groups are introduced into the framework, thus leading to the formation of anionic frameworks. As a result, [NH2Me2]+ countrcations are resided in the pores to

S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

5

Scheme 1. The representative design strategies of anionic MOFs.

Scheme 2. The representative design strategies of cationic MOFs.

maintain the overall electrical neutrality. Though the In centers are also six coordinated with octahedral coordination spheres, the coordination geometry is completed by carboxyl groups and OH– anions. Thus, the cationic framework is formed with OH– as the charge-balancing anions. Similar to the In centers, coordination geometry of seven-coordinated lanthanide ions (Y and Dy) is completed with the help of NO–3 anions, resulting in the formation of cationic frameworks. Meanwhile, a neutral Pb MOF is also achieved in which the carboxyl groups not only complete the distorted {: PbO6} pentagonal-bipyramidal geometry but also satisfy the charge balance. Interestingly, Mohamed Eddaoudi’ group discovered a highly stable hexanuclear rare earth (RE) cluster

[RE6(l3-OH)8(O2C–)12–x(N4C–)x, x = 0, 8] by pioneering using modulators or structural directing agents like 2-fluorobenzoic acid (2FBA), serving as 12-connected molecular building blocks (MBBs) (Fig. 2) [34]. Based on the hexanuclear RE MBBs, a number of highly connected porous anionic RE-MOFs have been successfully synthesized with [NH2Me2]+ as the extraframework countercations [35–37]. Zhang and coworkers reported a series of isostructural anionic MOFs [Me2NH2][M2(BPTC)(l3-OH)(H2O)2] (H4BPTC = 3,30 ,4,40 -biphenyltetracarboxylic acid, M = Co, Co0.83Ni0.17, Co0.55Ni0.45, Co0.13Ni0.87, and Ni) based on a butterfly-like [M4(OH)2(RCO2)8] cluster, featuring a (4,8)-connected scu topology [38]. Additionally, In-MOFs are normally assembled with either

6

S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

Fig. 1. Six Different Organic Cations. Adapted from Ref. [29]. Copyright 2009 American Chemical Society.

cationic SBU [In3O(COO)6]+ or anionic SBUs [In(COO)4]
and [In (COO)2N4]
, which can easily provide charged MOFs [39–41]. Feng and coworkers found that it is able to access different charged InSBUs (cationic, anionic, and neutral) by varying the ratio of In3+ and 2,5-furandicarboxylic acid (FDA) [42]. The different charges of these three In-SBUs are directly translated into different charges on the final frameworks, thus leading to positive, negative, and neutral MOFs. The rational design and use of metal SBUs is a prime strategy to synthesize anionic MOFs. Except for the organic solvents and metal SBUs, the organic ligands can incorporate the uncoordinated carboxyl or sulfonic groups into MOFs, thus leading to the negative frameworks with H+ or Na+ as the countercations [43–47]. Our group reported a ZnMOF with uncoordinated carboxyl group, exhibiting effective and selective separation of Cu2+ ions [43]. A CuMOF with uncoordinated carboxyl group was successfully synthesized based on 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid (H4Lb), showing high proton conductivity [44]. Using 2-sulfonylterephthalic acid monosodium salt as the starting organic ligands, a new functionalized UiO-66-SO3Na was obtained [45–47]. Such a method for constructing of anionic MOFs is a challenge because it is difficult to control the coordinated sites of organic ligands. Post-synthetic modification (PSM) approaches have been proven to be a fascinating way for the functionalization of MOFs [48]. Long and coworkers found that an anionic MOF can be converted from a neutral MOF through PSM method [49]. In their work, the anionic framework was formed by post-synthetically grafting LiiOPr into Mg2(DOBDC) (DOBDC4
= 1,4-dioxido-2,5-ben zenedicarboxylate), which contained coordinatively unsaturated Mg2+ sites in the one-dimensional hexagonal channels. The alkoxide anions coordinated to the unsaturated Mg2+ ions leading to the formation of the negative framework, while leaving Li+ ions as the charge-balancing cations. Using a similar strategy, they reported another work based on the framework of UiO-66(Zr), in which

the coordinatively unsaturated Zr4+ sites were formed through dehydration of the framework [50]. Through post-synthetically grafting LiOtBu into the dehydrated UiO-66(Zr), the OtBu
anions bound to the unsaturated Zr4+ sites, leaving Li+ ions as extra countercations (Fig. 3). Through PSM approach, we can fabricate anionic MOFs based on the well-refined neutral MOFs. However, it usually requires metal SBUs or organic ligands with active sites that allows feasible chemical transformations. Although the formation of anionic MOFs is often challenging, several successful strategies are applied to construct anionic MOFs through rational choice of organic solvents, metal ions or clusters, and organic ligands. The solvents like DMF, DMA, and DEF can generate [NH2Me2]+, [NH2Et2]+ cations through hydrolysis under solvothermal conditions, and ionic liquid can provide very rich ionic environments, thus leading to the formation of anionic MOFs. Using this kind of organic solvents combined with positive charged metal SBUs, stable anionic MOFs with intriguing structure and

Fig. 3. Representation of the grafting process, involving insertion of a lithium alkoxide in dehydrated UiO-66. On the left, the position of the Zr6O4(OH)4(O2CR)12 clusters in the crystal lattice and the structure of the cluster core is shown. On the right, the two-step modification process is depicted, which consists of dehydration of the cluster core and subsequent grafting of lithium tert-butoxide. Zr gray, O red, C dark green, H blue. The aliphatic part of the grafted alkoxide is represented by a light green triangle. Reproduced from Ref. [50]. Copyright 2013 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.

Fig. 2. A combination of RE metal ions and 2-fluorobenzoic acid enables the assembly of a hexanuclear RE cluster. The carbon atoms of the carboxylate ligands (that is, the points of extension) coincide with the 12 vertices of a cuo, which enables the hexanuclear cluster to act as a 12-connected molecular building block for MOF formation. Reproduced from Ref. [36]. Copyright 2014 Macmillan Publishers Limited.

S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

large pores are obtained, presenting potential applications in gas adsorption and separation and pollutants removal. Therefore, the rational design and use of novel negative charged metal SBUs is of great importance for the construction of anionic MOFs with high stability and interesting structures. Furthermore, anionic MOFs can be also achieved by incorporating the uncoordinated carboxyl or sulfonic groups into MOFs with H+ or Na+ as the countercations. However, it is a great challenge for controlled synthesis of MOFs with active sites on organic ligands and metal SBUs, which also is the prerequisite for the formation of anionic MOFs through PSM approach. Therefore, more efforts should be devoted to explore the controlled synthesis of MOFs with active sites on organic ligands and metal SBUs. 2.2. Design strategies of cationic MOFs As mentioned above, some cationic metal SBUs can directly lead to the formation of cationic frameworks. In 2013, Feng and coworkers utilized the [In3O(COO)6]+ SBU (Fig. 4a) to synthesize cationic frameworks [51]. Rosi and coworkers built up a series of isoreticular cationic RE-MOFs with pcu topology based on a fascinating positive SBU [RE4(l3-OH)4(COO)6]2+ (Fig. 4b) [52]. Using an attractive positive SBU [Th3(COO)9O(H2O)3.78]+ (Fig. 4c), Wang and coworkers prepared the first mesoporous cationic framework [Th3(Lc)3O(H2O)3.78]Cl(C5H14N3Cl)8H2O (SCU-8, H3Lc = [1,10 -biphe nyl]-3,40 ,5-tricarboxylicacid) [53]. The highly disordered chloride anions are served as the extraframework charge-balancing anions. More efforts should be devoted to the design and synthesis of new positive SBUs for the construction of cationic MOFs with controllable topologies and porous structures. The most commonly used method to synthesize cationic MOFs is to bind metal cations to neutral nitrogen donor ligands [54–56]. It will afford [ML]+ positive frameworks with weakly coordinated or unbond anions as charge-balancing anions. Undoubtedly, 4,40 -bipyridine (4,40 -bipy, L1) (Scheme 3) is the most widely used nitrogen donor ligand for the construction of cationic frameworks [57,58]. Oliver and coworkers reported a cationic MOFs consisting Cu+ cations and 4,40 -bipy ligands with formula Cu2(4,40 -bipy)2(O3SCH2CH2SO3)3H2O [59]. The ɑ,x-alkanedisulfonate not only served as counteranions but also bound to the adjacent cationic MOF layers through weak electrostatic interaction. The pyridyl ligands like 1,3-bis(4-pyridyl)propane (L2), N,N’-bis(3-pyridylme thyl)-1,5-diaminopentane dihydronitrate (L3) and bis(pyridinecar boxamido)alkanes (L4, L5) also can be used to synthesize cationic MOFs [60–62]. The pyridyl groups of the ligands can coordinate to metal cations, while the self-complementary amide groups can extend the networks into higher dimension through hydrogenbonded interactions. Biradha and coworkers prepared a series of cationic MOFs with two bis(pyridinecarboxamido)alkanes ligands and different Cu2+ salts [62]. They found that the exchange of counteranions from the cationic MOFs could trigger the

Fig. 4. Structure of the [In3O(COO)6]+ (a), [RE4(l3-OH)4(COO)6]2+ (b), and [Th3(COO)9O(H2O)3.78]+, (c) SBU. (In3+, blue spheres; RE, green spheres; Th, dark yellow spheres; O, red spheres; C, grey spheres; H atoms omitted for clarity).

7

crystal-to-crystal transformations. Kitagawa and coworkers successfully prepared a new base-type cationic framework by using a three-connector ligand 1,3,5-benzene tricarboxylic acid tris[N(4-pyridyl)amide] (4-BTAPA, L6) [63]. Owing to the uniformly ordered amide groups of the ligands on the channel surfaces, the base-type cationic MOFs could selectively accommodate and activate guests. Lee and coworkers employed tripyridyl triamides ligands with C3-symmetry (L7-L9) to synthesize 3D hydrogen-bonded cationic MOFs [64]. A new cationic MOF with ClO
4 as counteranions was achieved via a single-crystal to single-crystal (SCSC) anion exchange process. However, they found that the Cl
anions in the cationic MOFs could not be exchanged with BPh
4 anions. They explained that the anion exchange method depended on the relation between the size of the anions and the size of the cavities in the cationic MOFs. Using two novel tripodal ligands 1,3,5-tris(1-imidazolyl)benzene (L10) and 1,3-bis(1-imidazolyl)-5-(imidazol-1-ylmethyl)benzene (L11), Tang and coworkers reported six noninterpenetrating cationic MOFs [65]. In 2013, Wu and coworkers came up with 1,3,5-tris (1-imidazolyl)benzene (L10) ligand to build up two positive frameworks with different cage-to-cage connections [66]. The charge-balancing anions (NO–3 or ClO
4 ) played a very important role on the structural assembly. The organic ligands containing cationic imidazolium group are another class of widely used organic linkers for the construction of cationic MOFs [67]. In 2012, Bharadwaj and coworkers utilized 1,3-bis(4-carboxyphenyl)imidazolium ligand to build up a novel cationic MOF [{(Zn0.25)8(O)}Zn6(Ld)12(H2O)29(DMF)69(NO3)2]n (H2Ld = 1,3-bis(4-carboxyphenyl)imidazolium, L17) with an unprecedented {Zn8O} cluster [68]. Using the same cationic imidazolium-based dicarboxylate ligand and a neutral chain-like [In(OH)(CO2)2]n SBU, Su and coworkers reported a unique cationic framework {[In(OH)Ld]5(NO3)533H2O14DMF} featuring a rare snub square tessellation pattern [69]. Kaskel and coworkers prepared two cationic MOFs [Cu(Imid)(H2O)](Cl)0.5(NO3)0.5(H2O)0.5(EtOH)0.5 and [Zn4(Imid)5](NO3)3(H2O)12 (H2ImidCl = 1,3-bis(4-car boxy-2,6-dimethylphenyl)-1H-imidazolium chloride, L18) with different topologies using an imidazolium salt linker H2ImidCl with Zn2+ and Cu2+, respectively [70]. The structure of [Zn4(Imid)5] (NO3)3(H2O)12 was made up of binuclear zinc paddle-wheel units (Zn2(COO)3) and extended to a three-dimensional (3D) structure interconnected through dicarboxylates of the organic linkers. Whereas, [Cu(Imid)(H2O)](Cl)0.5(NO3)0.5(H2O)0.5(EtOH)0.5 showed a two-dimensional cationic layer structure with binuclear copper paddle-wheel clusters (Cu2(COO)4). Combination of N,N0 -bis(2,6-d imethyl-3,5-carboxylphenyl)imidazolium chloride (IMTA, L19) with Cu(NO3)2.5H2O afforded a cationic MOFs [Cu2(IMTA) (DMSO)2]2H2O [71]. In addition, a semi-rigid carboxylate imidazolium ligand (L20) was designed by Prabusankar and coworkers [72]. Owing to the flexibility of the two carboxylate wings, a series of transition metal(II) cationic MOFs with different topological structures were successfully achieved. Wu and coworkers used another semi-rigid carboxylate imidazolium ligand 1,10 -methylene bis-(3-(4-carboxy-2-methylphenyl)-1H-imidazol-3-ium, L21) to synthesize a 3D chiral framework with 1D helical cylindered channels [73]. As a positive framework, it required Cl
ions as the extraframework counteranions. The structure of organic ligands can directly affect the structures and properties of cationic MOFs. Therefore, design and synthesis of organic ligands with novel structures (such as L12-L16, L22) (Scheme 4) [65,74–77] can enrich the family of cationic MOFs and afford cationic MOFs with fascinating properties for practical applications (Scheme 5). PSM approach can be also used to convert neutral MOFs into cationic MOFs. Feng and coworkers created a cationic framework from a neutral MOF based on the differential affinity between distinct metal ions and framework anionic species [78]. They

8

S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

Scheme 3. The representative neutral nitrogen donor ligands used to construct cationic MOFs.

demonstrated that Al3+ ions which exhibited stronger affinity to F
ions could strip the Cr3+/Fe3+-bonded F
ions in MIL-100-Cr/Fe away from Cr3+/Fe3+ sites, thus resulting in the formation of positive frameworks with mobile Cl
ions as the charge-balancing anions. Using the same strategy, Xu and coworkers obtained a cationic MOF (MIL-101-Fe-NH2)+Cl
[79]. Subsequently, they integrated the (MIL-101-Fe-NH2)+Cl
with the bromomethylated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO) to form a porous BPPO membrane (pc-MBPPO). In 2017, Ye and coworkers prepared a cationic frameworks (MIL-101-Cr)+ with mobile Cl
anions by anion stripping [80]. The N-alkylation of amine or pyridyl groups is another method to change the framework charge from neutral to positive [81]. Kim and coworkers prepared a rigid homochiral MOF [Zn3(l3-O)(1H)6]2H3O12H2O (1-H = (4R,5R)-(and (4S,5S)-)2,2-Dimethyl-5-[(4 -pyridinyl amino)carbonyl]-1,3-dioxolane-4-carboxylic acid, D-POST-1) with free pyridyl groups aligning in the channel of the framework [82]. After immersing POST-1 in DMF with excess iodomethane (CH3I) at room temperature for 2 h, the N-alkylation product was successfully achieved, leading to the formation of a

cationic framework with free iodide ions as the charge-balancing anions. Cationic MOFs are rarer than anionic MOFs. However, the synthetic strategies of cationic MOFs are quite similar to that of anionic MOFs. Like anionic MOFs synthesized by negative charged metal SBUs, the positive charged metal SBUs can directly lead to the formation of cationic frameworks. Therefore, investigating characteristics of metal ions and design of novel charged metal SBUs is very important for the construction of charged MOFs with high stability and interesting structures. PSM approach can be also used to convert neutral MOFs into cationic MOFs. To realize the charge transformation, the active groups on metal SBUs or organic ligands are necessary. Using special organic ligands like nitrogen donor ligands and imidazolium-based carboxylate ligands is the most commonly used method to synthesize cationic MOFs. The structure of organic ligands can directly affect the structures and properties of cationic MOFs. Therefore, design and synthesis of organic ligands with novel structures can enrich the family of cationic MOFs and afford cationic MOFs with fascinating properties for practical applications.

S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

9

Scheme 4. The representative cationic imidazolium ligands used to construct cationic MOFs.

Scheme 5. The representative applications of charged MOFs.

3. Applications of anionic MOFs 3.1. Gas adsorption and separation Owing to the large surface areas and highly tunable pore structures, MOFs are recognized as particularly promising porous materials for gas adsorption and separation. Numbers of research work on MOFs for gas separation and purification have been reported in recent decades [83,84]. Among them, MOF-5 [85], MIL-101 [86], UMCM-2 [87], NU-100 [88], NU-110E [89] exhibited high surface areas and fascinating adsorption properties. The surface areas of MOFs reached 3800 m2/g in 2005 [86], 5200 m2/g in 2009 [87], and a remarkable 7000 m2/g in 2012 [89]. The strong host-guest interactions between MOFs and gases not only enhance the capability of gas uptake but also realize the selective adsorption. Modifying organic ligands with functional sites such as pyridyl, amino, hydroxyl groups, and sulfonic groups is widely used to enhance the interactions of MOFs and gases [90–92]. Furthermore, the chargeinduced forces in charged MOFs also benefit for gas adsorption

[93]. In this section, we will firstly discuss the adsorption properties based on the structures of anionic MOFs. Then, the effect of charge-balancing cations exchange on gas uptake will be explained in detail. In 2010, Bu and coworkers synthesized three anionic MOFs [(CH3)2NH2][In3O(BTC)2(H2O)3]2[In3(BTC)4]7DMF23H2O (CPM-5, CPM = crystalline porous materials), [(CH3)2NH2][In3O(BTC)2(H2O)3]2[In(BTC)4/3][In(BTC)4/3(H2O)]2solvent (CPM-50 ), and [CH3NH3][In3O(BTC)2(H2O)3]2[In3(BTC)4]solvent (CPM-6), featuring interesting cage-within-cage structure [94]. A large truncated octahedral cage (named as In24 cage) that formed by monomeric four-connected [In(O2CR)4]
sites encapsulated a small truncated tetrahedral cage (named as In12 cage) which was built from trimeric [In3(O)(O2CR)6(H2O)3]+ clusters (Fig. 5a). Although the pore space of In24 cage was too large for small gas molecules storage, the interconnection of the In24 cage and the In12 cage partitioned the In24 cage into multiple domains with the pore radius in the range of 4.00–1.68 Å. Furthermore, open In3+ sites could be generated after removing the dangling water ligands. Therefore, CPM-5

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could be used as a promising candidate for gas uptake due to the synergistic effect of charge separation, pore space partitioning, and open In3+ sites (Fig. 5b,c). The active CPM-5 possessed a BET surface area of 580 m2/g, and took up 1.24 wt% of H2 at 77 K and 1 atm, 81.3 cm3/g and 54.5 cm3/g of CO2 under 1 atm at 273 K and 299 K, respectively. The CPM-6 with smaller chargebalancing cations exhibited significantly enhanced gas uptake capacity, which had a BET surface area of 596 m2/g and adsorbed 1.88 of H2 at 77 K and 1 atm, 106.7 cm3/g and 65 cm3/g of CO2 under 1 atm at 273 K and 299 K, respectively. However, both CPM-5 and CPM-6 exhibited lower CH4 adsorption with the CO2/CH4 uptake ratios of ca. 7.5–3.2 between 0.01 and 1 bar. By utilizing 4,40 ,400 -s-triazine-2,4,6-triyltribenzoic acid (H3TATB), Cao and coworkers synthesized an anionic MOF (Et4N)3[In3(TATB)4] (FJI-C1) with Et4N+ as the charge-balancing cations in the pores [95]. With large unit cell volume and high BET surface area (1726.3 m2/g), it adsorbed 1.49 wt% of H2 at 77 K and 1 atm, rising to 3.31 wt% at 77 K and 32 bar. Meanwhile, it also exhibited a high CO2 adsorption uptake (41.2 cm3/g at 298 K and 1 bar) and a moderate CH4 adsorption uptake (9.7 cm3/g at 298 K and 1 bar). The high CO2 adsorption uptake could be ascribed to the interaction between Et4N+ cations and CO2 [96]. Furthermore, it not only showed high C2 and C3 hydrocarbons adsorption capacities, but also exhibited high C3H8/CH4, C2H2/CH4, C2H6/CH4, and C2H4/CH4 separation selectivity of 14.6, 9.7, 9.0, and 6.6, respectively (Fig. 5d), which can be attributed to the strong interactions between C2 and C3 hydrocarbons and the ethyl groups of the Et4N+ countercations and the selectively blocking effect of the Et4N+ cations. In addition, owing to the internal hydrophilic porous surface, FJI-C1 could selectively adsorb the polar vapors over nonpolar molecules. Although benzene is a nonpolar molecule, FJI-C1 exhibited remarkable benzene adsorption compared to some polar molecules like ethanol and could selectively adsorb benzene over cyclohexane. This surprising adsorption capacity of benzene could be ascribed to the p-p interactions between benzene and the striazine rings of the H3TATB linkers in FJI-C1. This result indicated that the charge-balancing cations in anionic MOFs not only can maintain the overall charge neutrality, but also can improve the gas adsorption capacity of anionic MOFs through its size effect and the interactions with gases. The environment of the pores in anionic MOFs also can influence the separation selectivity of anionic MOFs. The integration of accessible N-donor groups including pyridine, imidazole, triazole as well as tetrazole into the porous surfaces of MOFs can significantly affect the capacity and selectivity of gas adsorption [97–100]. Thus, the incorporation of a N-rich 1,2,3,4-tetrazole ring into MOFs can enhance the gas uptake performance, especially for CO2 because of the dipole-quadrupole interactions between CO2 molecules and nitrogen sites. Using a N-rich ligand 1,3,5-tris(2H-tetrazol-5-yl)benzene (H3BTT), Teng and coworkers successfully assembled an anionic MOF [NC2H8]4Cu5(BTT)3xG (G = guest of DMA and H2O), which featured a giant multi-prismatic nanoscale cage and high CO2/N2 and CO2/H2 adsorption selectivity of 69.7 and 577.2 at 273 K, respectively [101]. Eddaoudi and coworkers demonstrated a general strategy to synthesize highly connected anionic RE-MOFs by using RE clusters as the SBUs and 2-FBA as the modulator. The high thermal and chemical stability, open RE sites, large surface areas, and charge separation of these RE-MOFs render them as promising candidates for gas capture and separation. A series of 12-connected RE-fcu-MOFs are successfully prepared by using nonfunctional/functional 2-connected bridging ligands including 1,4-naphthalenedicarboxylate (1,4NDC) [35], fumaric acid (FUM) [102], 2-fluoro-4-(1H-tetrazol-5-yl) benzoic acid (H2FTZB), 4-(1H-tetrazol-5-yl)benzoic acid (H2TZB), and 3-fluorobiphenyl-4,40 -dicarboxylate (FBPDC) [34].

The RE-fcu-MOFs exhibited excellent C4H10/CH4 [35], H2S/CO2, H2S/CH4 [102], and CO2/N2 [34] selectivity, owing to the structural features of RE-fcu-MOFs and the interactions between functional ligands and associated gases. By simply changing the 2-connected linkers to triangular ligands like 1,3,5-benzene(tris)benzoate (BTB) and 50 ,50000 -((5-((4-(3,6-dicarboxy-9H-carbazol-9-yl)phenyl)ethy nyl)-1,3-phenylene)bis(ethyne-2,1-diyl))bis((1,10 :30 ,100 -terphenyl]4,400 -dicarboxylic acid) (H6Le), two 18-connected RE-gea-MOFs were obtained, which exhibited CH4 storage, C3H8/CH4 and n-C4H10/CH4 separations [36]. When using 4-connected organic linkers, a series of 12-connected RE-ftw-MOFs were successfully synthesized, featuring the selective removal of n-C4H10 and C3H8 from natural gas and excellent selectivity of n-C4H10 over C3H8 or iso-C4H10 [103]. Post-synthetic ion exchange of charge-balancing species in anionic MOFs can result in unusual pore partition effects. Rosi and coworkers introduced a series of organic cations with different sizes including tetramethylammonium (TMA), tetraethylammonium (TEA), and tetrabutylammonium (TBA) through postsynthetic cation exchange into the pores of a very famous anionic MOF bio-MOF-1[104]. The N2 adsorptions of the parent MOF and ion-exchanged MOFs at 77 K were carried out to check the effect of the tetraalkylammonium cations on the pore space and BET surface area. As expected, with increasing the size of the organic cations, the pore volume and BET surface area decreased from 0.75 cm3/g and 1680 m2/g for bio-MOF-1 to 0.37 cm3/g and 830 m2/g for TBA@bio-MOF-1 (Fig. 6a). The CO2 adsorption isotherms of the parent MOF and ion-exchanged MOFs at 273 K were also studied to determine the impact of cation exchange on CO2 uptake capacity. Surprisingly, TMA@bio-MOF-1 exhibited the highest CO2 adsorption capacity of 4.5 mmol/g at 1 bar, followed by TEA@bio-MOF-1 (4.2 mmol/g at 1 bar) (Fig. 6b). The parent material bio-MOF-1 just adsorbed 3.41 mmol/g CO2 at the same condition. The authors concluded that the smaller pore volumes benefited for the CO2 capture, which further confirmed by the results of isosteric heats of adsorption (Qst). To investigate the synergistic effect of open metal sites and extraframework chargebalancing cations on the gas uptake capacities, Zhang and coworkers synthesized an anionic MOF [(CH3)NH2]3[(Cu4Cl)3(BTC)8] 9DMA (CuBTC) with open Cu2+ sites and obtained TMA@CuBTC, TEA@CuBTC, and TPA@CuBTC (TPA = tetrapropylammonium) via cation exchange [105]. The parent material CuBTC, which had the largest BET surface area but lowest Qst, adsorbed 5.53 mmol/g CO2 at 273 K and 1 bar, while TPA@CuBTC, which had the smallest BET surface area, just adsorbed 2.67 mmol/g CO2 at 273 K and 1 bar. These results demonstrated that the suitable pore is of great importance for the gas storage. Furthermore, these anionic MOFs showed excellent adsorption selectivity of CO2 over N2 at STP and high H2 uptake capacity owing to the synergistic effect of open metal sites and suitable pore volume tuned by extraframework charge-balancing cations. The results illustrated that systematically tuning the pore dimensions of anionic MOFs through postsynthetic ion exchange is a very promising route to optimize their gas adsorption capacities. Loading of metal ions into MOFs such as Li+, Na+, Mg2+, Co2+, and Ni2+ can improve the overall adsorption performance, owing to the strong interactions between gases and free metal sites [106]. The anionic MOFs are ideal platforms to study the effect of loading metal ions on gas adsorption properties such as H2, CH4, and CO2 [107,108]. In 2009, Schröder and coworkers reported a doubly interpenetrated anionic MOF {[H2ppz][In2(Lf)2]3.5DMF5H2O}1, (NOTT-200 L4
= 1,10 ,40 ,100 ,400 ,100 0 -quaterphenyl-3,5,300 0 ,5000 -tetracar f boxylate), and its Li+-exchange sample {Li1.5[H3O]0.5[In2(Lf)2] 11H2O}1 (NOTT-201) to evaluate the H2 adsorption properties (Fig. 6c) [30]. NOTT-200 exhibited a hysteretic adsorption behavior for the adsorption and release of N2 and H2, whereas NOTT-201 showed no hysteresis but an enhanced uptake capacity (Fig. 6d).

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Fig. 5. (a) View of the 3D structure of CPM-5. Some BTC ligands interconnecting In12 and In24 cages are omitted for clarity. (b) N2 and H2 adsorption isotherms of CPM-5 and CPM-6. (c) CO2 and CH4 adsorption isotherms of CPM-5 and CPM-6. Reproduced from Ref. [94]. Copyright 2010 American Chemical Society (d) Adsorption selectivities on FJIC1 at 298 K calculated using IAST for equimolar mixtures of C3H8/CH4, C2H2/CH4, C2H6/CH4, and C2H4/CH4. Reproduced from Ref. [95]. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The H2 adsorption capacities for NOTT-200 and NOTT-201 under 1.0 bar were 0.96 wt% and 1.02 wt%, respectively. The significantly different adsorption behavior of NOTT-200 and NOTT-201 could be ascribed to the extraframework charge-balancing cations. The bulky H2ppz2+ dication could create a kinetic trap for H2 and acted as a reversible gate to control the access and release of associated gases, thus leading to the hysteretic adsorption behavior. The open Li+ sites in MOTT-201 through cation exchange not only resulted in the absence of hysteretic adsorption properties, but also increased the BET surface area and the Qst for H2. Lu and coworkers explored the H2 adsorption properties of a sodalite-type anionic MOF (Et2NH2)3[(Cu4Cl)3(TTCA)8]26DEF (CuTTCA, TTCA = triphenylene2,6,10-tricarboxylate) and its Li+-exchange sample CuTTCA-Li+ [109]. The H2 uptakes at 1 bar for CuTTCA and CuTTCA-Li+ were 0.91 and 1.14 wt%, respectively. The H2 adsorption behavior of CuTTCA and CuTTCA-Li+ at low pressures was due to the stronger H2 binding interaction with exposed Li+ sites than that of Et2NH+2 cations, which further confirmed by the results of Qst (4.74 kJ/mol for CuTTCA-Li+, 3.38 kJ/mol for CuTTCA). In addition, H2 adsorption capacities for CuTTCA and CuTTCA-Li+ under 50 bar were 3.29 and 4.75 wt%, respectively. This result could be attributed to the enhanced pore volume and BET surface area of CuTTCA-Li+ by replacing Et2NH+2 cations with smaller Li+ ions. Recently, Suh and coworkers demonstrated that metal cationsexchange of charge-balancing cations within an anionic framework [Zn3(TCPT)2(HCOO)][NH2(CH3)2] (SNU-1000 , TCPT = 2,4,6-tris-(4-c arboxyphenoxy)-1,3,5-triazine) could enhance the uptake capacity,

selectivity, and isosteric heat of the CO2 adsorption in SNU-1000 [110]. In this work, the [NH2(CH3)2]+ cations in SNU-1000 were replaced by Li+, Mg2+, Ca2+, Co2+, and Ni2+ ions to generate SNU-1000 -Li, SNU-1000 -Mg, SNU-1000 -Ca, SNU-1000 -Co, and SNU-1000 -Ni, respectively. As for the CO2 uptake, SNU-1000 -Co took up 16.8 wt% of CO2 at 298 K and 1 atm (Fig. 7a), which was higher than most of the MOF-based materials [111,112]. The Qst of CO2 adsorption for SNU-1000 -Metal improved to 37.4–34.5 kJ/mol in comparison to that (29.3 kJ/mol) of SNU-1000 (Fig. 7b), which was presumably ascribed to the strong interaction between CO2 and the coordinated water molecules of the metal ions, as well as the electrostatic field of the charge-balancing metal ions. Furthermore, the CO2/N2 selectivity at room temperature was improved to 40.4–31.0 kJ/mol in comparison to 29.3 and 25.5 kJ/mol of SNU-1000 . The p-complexes of Ag+ ions can be formed with olefin molecules through carbon–carbon double bonds. Based on this investigation, MOFs with exposed Ag+ sites can be used to separate olefin from olefin–paraffin mixtures [113]. Chen and coworkers successfully immobilized Ag+ ions into MIL-101(Cr)-SO3H through cation exchange with AgBF4 to generate MIL-101(Cr)-SO3Ag (Fig. 7c) [114]. MIL-101 (Cr)-SO3H possessed a BET surface area of 1856 m2/g and a total pore volume of 1.35 cm3/g, while MIL-101(Cr)-SO3Ag had a lower BET surface area of 1253 m2/g and a lower pore volume of 1.00 cm3/g. Although the BET surface area and pore volume of MIL-101(Cr)SO3Ag decreased due to Ag+-exchange, MIL-101(Cr)-SO3Ag exhibited impressive adsorption capacity and selectivity of olefins. MIL-101

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Fig. 6. Gas adsorption experiments. (a) N2 adsorption isotherms of bio-MOF-1 and ion-exchanged samples (77 K) and (b) CO2 adsorption isotherms of bio-MOF-1 and ionexchanged samples (273 K) (bio-MOF-1, navy; TMA@bio-MOF-1, green; TEA@bio-MOF-1, purple; TBA@bio-MOF-1, orange). Reproduced from Ref. [104]. Copyright 2010 American Chemical Society (c) Views of framework structures of NOTT-200 and NOTT-201. Space-filling framework structure of NOTT-200 (left) and NOTT-201 (right) viewed along the crystallographic b axis. The H2ppz2+ dications in the channel B of NOTT-200 can be completely exchanged with Li+ ions in the channel C of NOTT-201. (d) N2 sorption isotherms for desolvated NOTT-200 and desolvated NOTT-201 at 78 K. Reproduced from Ref. [30]. Copyright 2009 Macmillan Publishers Limited.

(Cr)-SO3Ag took up about 1.1 mmol/g of C2H4 and 1.5 mmol/g of C3H6 at 303 K and 5 KPa, which was 5-fold and 2-fold enhancement respectively than those of MIL-101(Cr)-SO3H (0.21 mmol/g for C2H4 and 0.75 mmol/g for C3H6). The Qst of C2H4 and C3H6 for MIL-101(Cr)SO3Ag at close to zero loading was about 120 and 101 kJ/mol, respectively, which was much higher than those for MIL-101(Cr)-SO3H (35 kJ/mol for C2H4 and 41 kJ/mol for C3H6), while MIL-101(Cr)SO3Ag showed no special interactions with alkanes, thus resulting in the excellent C2H4/C2H6 and C3H6/C3H8 selectivity of MIL-101 (Cr)-SO3Ag at room temperature (Fig. 7d). MOF-based materials have been proven to be promising candidates for gas adsorption and separation due to their high porous structures and large surface areas. Researchers persevere to increase the surface areas of MOFs for the superior adsorption capacity. In addition, selective adsorption of targeted gas molecules is more important in practical applications. MOFs with specific functional groups such as pyridyl, amino, hydroxyl groups, and sulfonic groups are achieved to improve their selectivity of targeted gas molecules. Interestingly, anionic MOFs show excellent adsorption selectivity due to the strongly interactions between gases and guest cations as well as the tunable pore structures by cations exchange with different guest species. However, anionic MOFs could undergo structural change or collapse after exchanging with other cationic guests. Therefore, other alternative method (like one pot way) should be explored to realize cation exchange in anionic MOFs. Undoubtedly, design and synthesis of anionic MOFs with high stability like RE-fcu-MOFs, RE-gea-MOFs, and REftw-MOFs which reported by Eddaoudi and coworkers is of great challenge and more efforts are needed. Furthermore, novel cationic guest species that has specific interactions with targeted gas molecules should be explored.

3.2. Luminescence Luminescent MOFs have drawn extensive attention because of their easily introducing light-emitting building blocks and the potential applications in light-emitting devices, chemical sensing, and photocatalysis [115,116]. Their luminescence performances can be efficiently adjusted due to the unlimited choices of metal nodes and organic linkers. In addition, the guest chromophores with appropriated sizes in the frameworks can offer another dimension of tunability in luminescence properties. Through post-synthetic ion exchange, anionic MOFs provide an interesting platform for incorporating ionic chromophores like metal complexes, organic dyes, and RE ions into their cavities. Herein, the luminescence properties of anionic MOFs after encapsulating cationic guest chromophores and their applications in white-light emission, nonlinear optics (NLO) as well as luminescent sensing are systematically investigated.

3.2.1. White-light emission White-LED (light-emitting diodes) materials are gradually replacing incandescent and fluorescent lamps, due to their lower energy consumptions, higher efficiencies, and longer lifetimes. Currently, almost all White-LEDs start from a blue-emitting LED combined with one or two phosphor materials. Most of these materials show the disadvantage of a relatively low color rendering index (CRI) and high correlated color temperature (CCT) due to the deficiency in red emission [117]. Thus, anionic MOFs emerge as promising candidates for developing high efficient White-LED materials due to their tunable metals nodes and emission wavelengths. In addition, their luminescent efficiency can be improved

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Fig. 7. (a) CO2 sorption isotherms of SNU-1000 -Metal at 298 K. Filled shapes: adsorption. Open shapes: desorption. (b) Qst of CO2 adsorption. Reproduced from Ref. [110]. Copyright The Royal Society of Chemistry 2012 (c) Immobilization of Ag(I) into a metal–organic framework with–SO3H sites. (d) Single-component sorption isotherms for various hydrocarbons in MIL-101(Cr)-SO3Ag at 303 K. Reproduced from Ref. [114]. Copyright The Royal Society of Chemistry 2015.

by encapsulating luminescent guest molecules with high quantum yield into the pores of MOFs. In 2013, Li and co-workers reported a new strategy to develop highly efficient white light emission by combining the independent emissions from two components, namely blue emission from the anionic MOF and yellow emission from the encapsulated IrIII complex [118]. This mesoporous blue-emitting (kem = 425 nm) anionic MOF [(CH3)2NH2]15[(Cd2Cl)3(TATPT)4]12DMF18H2O (CdTATPT, H6TATPT = 2,4,6-tris(2,5-dicarboxylphenylamino)-1,3,5triazine) contained two types of cages with the dimensions of approximate 2 and 3 nm, which can be used to load the IrIII complex [Ir(ppy)2(bpy)]+ (Hppy = 2-phenylpyridine, bpy = 2,20 -bipyridine, kem = 570 nm) via ion exchange process (Fig. 8a). The resultant composite exhibited two emission peaks at 425 and 530 nm, and the intensity of the emission at 530 nm increased monotonically with the increasing of the IrIII amount (Fig. 8b). At an optimal concentration of 3.5 wt% IrIII, a high-quality white light was obtained with CIE coordinates of (0.31, 0.33), a high CRI value of 80, and moderate CCT value of 5900 K. The quantum yield was measured to be 20.4%, among the highest of white-light-emitting MOFs reported so far [119,120]. Furthermore, White-LED applications using this material were fabricated, which shows superior performance. Using a similar strategy, Su and co-workers successfully introduced the yellow emission IrIII into a blue emission anionic MOF [(CH3)2NH2]2[Zn8(BTCA)6(2-NH2-BDC)3]8DMF (NENU-524, NENU = Northeast Normal University, H2BTCA = benzotriazole-5carboxylic acid, 2-NH2-H2BDC = 2-amino-1,4-benzenedicarboxylic acid) to yield the [Ir(ppy)2(bpy)]+@NENU-524 composites [121].

By careful adjustment the concentration of IrIII, [Ir(ppy)2(bpy)]+ @NENU-524 (3.86 wt% IrIII) showed a pure white light emission with CIE coordinates of (0.300, 0.336), high quantum yield up to 15.2% and high stability. In addition, organic dyes can be also encapsulated into the pores of a luminescent MOF to achieve White-LEDs phosphor. Through ion exchange process, ZJU-28  DSM/AF composites were obtained by introduction both the red emission dye 4-(p-dimethy laminostyryl)-1-methylpyridinium (DSM) and the green emission dye acriflavine (AF) into a blue-emitting anionic MOF (Me2NH2)3[In3(BTB)4]12DMF22H2O (ZJU-28) [122]. The emission color of the obtained ZJU-28  DSM/AF composites can be easily modulated by adjusting the relative concentration of DSM and AF. The optimal white light emission was realized when the concentration of DSM and AF was adjusted to 0.02 wt% and 0.06 wt%, respectively. As expected, a broadband white light was obtained with CIE coordinates of (0.34, 0.32), a high CRI value of 91, CCT value of 5327 K, and significantly enhanced fluorescence quantum efficiency 17.4%. White-LEDs device was fabricated by simply coating a thin layer of the white light ZJU-28  DSM/AF composites on the surface of the commercially available ultraviolet LED (365 nm), indicating that the dye-encapsulated MOFs are promising for practical lighting applications. Similarly, a series of Rh@bio-MOF-1 (Rh = rhodamine) with an internal QE as high as 79% was fabricated by Xie and coworkers via a solvothermal reaction followed by cation exchanges, showing greatly color tunability from green to yellow to red by controlling the species and concentration of encapsulated dyes [123]. The White-LEDs, fabricated by combining a 450 nm blue

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Fig. 8. (a) Scheme of the encapsulation of [Ir(ppy)2(bpy)]+ in CdTATPT. (b) Room temperature emission spectra of CdTATPT and [Ir(ppy)2(bpy)]+@ CdTATPT with different concentration of [Ir(ppy)2(bpy)]+: black, emission spectrum of CdTATPT; red, 0.52 wt% [Ir(ppy)2(bpy)]+ at CdTATPT; green, 1.04 wt%; blue, 3.5 wt%; cyan, 3.7 wt%; pink, 4.5 wt %; yellow, 7.5 wt%; orange, 8.8 wt%. All measurements were performed on solid samples at an excitation wavelength (kex) of 370 nm. Reproduced from Ref. [118]. Copyright 2013 Macmillan Publishers Limited.

chip, Y3Al5O12:Ce3+ phosphor and RhB@bio-MOF-1, exhibited a high CRI of 80, a luminous efficiency of 156 lm/W, and a medium CCT 5225 K under a drive current of 10 mA. 3.2.2. Nonlinear optics Nonlinear optics (NLO), which describes the nonlinear relationship between dielectric polarization P and electric field E in optical media, has aroused widespread applications in the laser industry and optoelectronic technologies [124,125]. The second-harmonic generation (SHG) is one of the most common NLO behaviors, in which two photons with the same frequency interact with a nonlinear material and generate a new photon with twice the energy of the initial photons [126]. In general, an SHG-active material must lack a center of symmetry. Considering the various available organic ligands, the well-defined geometry of metal centers and the highly directional coordination bonds of metal-ligand, MOFs have been a key candidate in the development of NLO materials [127]. In addition, the NLO properties can be tuned by introducing cationic guest chromophores within the anionic frameworks. The first example of tuning of NLO properties of an anionic MOF by cationic guests was demonstrated by Cui et al. in an anionic octupolar 3D complex [(H2NMe2)2Cd3(C2O4)4]MeOH2H2O, crys 3d [128]. This tallizing in the noncentrosymmetric space group I4 anionic MOF which contains [H2NMe2]+ cation exhibited a SHG intensity of 150 versus a-quartz, which is about 15 times higher than that standard KH2PO4 (KDP). With both polyhedral 3553 and 3454 cages in the framework, the structure showed high ion exchange capacities for cations NH+4, Na+, and K+. The SHG intensity changed to 155, 90, and 110, respectively, versus a-quartz, after exchanging the cations in with NH+4, K+, and Na+. In addition, the re-exchanged product with [H2NMe2]+ cations was also found to give a powder SHG intensity of 147 versus a-quartz, which is consistent with the value of parent MOF. The above result proved that the decrease in the crystallinity had very little effect on the NLO properties of this compound and the change in the SHG intensity must be a cation-dependent phenomenon. In principle, the encapsulation of ordered dipolar chromophores in the porous structure MOFs should also be possible to fabricate nonlinear optical materials; fortunately, this has been realized by Cui and coworkers [129]. Through ion exchange process, four different pyridinium hemicyanine cationic chromophores with a systematically tuned alkyl chain length (DPASM (4-(4-(diphenyla

mino)styryl)-1-methylpyridinium), DPASB (1-butyl-4-(4-(dipheny lamino)styryl)pyridinium), DPASN (4-(4-(diphenylamino)styryl)-1 -nonylpyridinium), and DPASD (4-(4-(diphenylamino)styryl)-1-do decylpyridinium)) can be easily incorporated into the framework of ZJU-28, as displayed in Fig. 9. As a result, the transformation of ZJU-28 material from NLO inactive to NLO active material was realized by the encapsulation of these dipolar chromophores, with powder SHG intensities of 0.25 (ZJU-28  DPASM), 0.28 (ZJU28  DPASB), 3.5 (ZJU-28  DPASN), and 18.3 (ZJU-28  DPASD), respectively, versus a-quartz. The SHG intensities of these materials are heavily dependent on the incorporated guest dipolar chromophores; the longer the terminal alkyl chain, the stronger the SHG intensity of the resulting material. Thus, the highest SHG intensity of 18.3  a-quartz was obtained in ZJU-28  DPASD because of its longest alkyl chain. A similar strategy was also adopted by the same group to incorporate cationic dyes, DM-n and DP-n (where n = number of carbon chains on the pyridinium), into the 1D channel of MOF [130]. SHG measurements of the resultant bio-MOF-1  DM-n and bio-MOF1  DP-n materials prove that the NLO properties were influenced by the length of alkyl chain and the concentration of dyes. 3.2.3. Luminescent sensing Owing to their ordered crystalline structures, tunable pore sizes, and various functional sites like Lewis acidic/basic sites, hydrogen bond sites, as well as open metal sites, MOF-based materials bloomed out as luminescent sensors for metal ions, small organic molecules, explosive chemicals, vapors, etc. in recent years [131–133]. The charged frameworks or introducing charged guests species into the pores of charged MOFs can serve as the specific recognition sites, while the incorporation of charged fluorescent chromophores into charged MOFs can form new luminescent emission which can be used as the sensing signals [134,135]. Maji and coworkers reported an anionic MOFs {[Mg3(NDC)2.5(HCO2)2(H2O)][NH2Me2]2H2ODMF}, (MgNDC) which not only exhibited a turn-off sensing of Cu2+ ion but also could specific sensitize Eu3+ ion through cation exchange with [NH2Me2]+ countercations [136]. By simply immersing desolvated MgNDC in different metal chloride solutions (0.01 M), Cu2+@MgNDC was easily achieved with 88% replacement of the [NH2Me2]+ countercations, while Mn2+, Co2+, Ni2+ and Zn2+ showed very low replacement according to the results of Inductively Coupled Plasma Optical

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Fig. 9. Schematic illustration of pyridinium hemicyanine chromophores incorporated into ZJU-28. Reproduced from Ref. [129]. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Emission Spectroscopy (ICP-OES) analysis. The selective Cu2+ capture could be ascribed to the flexible geometry of Cu2+ ion. In addition, the emission at 410 nm of MgNDC was significantly quenched after exchanging with Cu2+ ion, due to the ligand field splitting of d-orbitals of Cu2+. Thus, MgNDC can be used as a Cu2+ sensor with the quenching constant of 1.986  10
3 M
1. Furthermore, RE3+@MgNDC (RE = Eu, Tb, Dy, Sm) was also achieved by soaking MgNDC in 0.01 M solutions of nitrate salts of RE3+. The results of ICP-OES analysis indicated that a similar amount of RE3+ (62% with Sm3+, 68% with Eu3+, 75% with Tb3+ and 77% with Dy3+) have been exchanged in MgNDC. Upon excited at 317 nm, Eu3+@MgNDC exhibited characteristic emission of Eu3+ accompanying by the decrease of the emission at 410 nm, while other RE3+ was failed to be sensitized evidenced by the absence of the characteristic emission peaks of Dy3+, Sm3+, or Tb3+ in the emission spectra. Therefore, MgNDC can also serve as a Eu3+ ion probe. Our group synthesized an anionic EuMOF, which exhibited a fascinating reversible temperature-stimulated SCSC transformation [137]. At room temperature, (H3O+)Eu0.5[EuNa0.5Lg(DMF)(H2O)](solvent)x, (Na6Lg = 5,50 ,500 -(1,3,5-triazine-2,4,6-triyltriimino)tris-iso phthalate hexasodium) has a free carboxylate group as further hook and free Eu3+ ions filled in the square channels of the anionic frameworks. When decreasing the temperature, the intrinsic free Eu3+ ions can be fast captured by the carboxylate hooks to generate a new anionic MOFs (H3O+)2[Eu3NaLg2(DMF)5(H2O)2](solvent)y. Both two anionic MOFs can detect highly explosive 2,4,6trinitrophenol (TNP) through luminescent turn-off response at room and even low temperatures. The unique luminescent performances of RE ions including large Stokes shifts, characteristic sharp emissions, long lifetime, as well as high color purity with high quantum yields make them excellent sensing signals in MOF-base sensors [138]. Particularly,

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Eu3+ and Tb3+ exhibit strong and characteristic emissions, and Yb3+ and Er3+ emit in the near-infrared (NIR) region which can be used in biological applications. As early as 2011, Rosi and coworkers obtained a Yb3+@bio-MOF-1 with NIR emission at 980 nm through cation exchange [139]. The O2 detection experiment was performed in solid-state condition using Yb3+@bio-MOF-1 to evaluate its potentiality as luminescent sensor. As shown in Fig. 10, the NIR emission of Yb3+@bio-MOF-1 was decreased about 40% upon introducing O2 within 5 min and kept constant for the duration of exposure. Importantly, the Yb3+ signal could restore to its original intensity after exposure to N2. The reversibly detecting O2 based on NIR emission made Yb3+@bio-MOF-1 excellent candidate as luminescent sensor in biological environments. After that, a lot of RE3+@anionic MOFs were successfully prepared through cation exchange for sensing organic amine vapors [140], nitrobenzene [141], and organic solvent molecules [142]. Impressively, Zheng and coworkers designed a fast and facile ratiometric luminescent sensor Tb3+/Eu3+@bio-MOF-1 for an anthrax biomarker DPA (DPA = 2,6-pyridinedicarboxylic acid) [143]. In this ratiometric sensor, bio-MOF-1 served as the supporter and protector of RE ions and provided biocompatibility for the practical applications, where as Tb3+ ion acted as response unit to recognize DPA, Eu3+ ion was as a internal reference. Because of the highly effective energy transfer from Tb3+ to Eu3+, Tb3+/Eu3+@bio-MOF-1 (Tb:Eu = 5.89) still exhibited strong orange-red emission. After adding DPA, the fluorescence color of Tb3+/Eu3+@bio-MOF-1 (Tb:Eu = 5.89) changed from orange-red to green, which could be easily observed by naked eyes. The results of luminescence decay confirmed that the ligand-tometal energy transfer between DPA and Tb3+ could interrupt the energy transfer from Tb3+ to Eu3+, resulting in the enhancement of Tb3+ emission and the decrease of Eu3+ emission. Furthermore, this interesting ratiometric DPA sensor showed high selectivity and a low detection limit of 34 nM, indicating its potential in practical applications. Owing to the excellent electronic and optical properties and the high photoluminescent quantum yield, fluorescent dyes also can be served as fascinating chromophore guests [144,145]. Our group developed an interesting strategy for dual-emitting detection of organic solvent molecules using RhB@InMOF (InMOF = {[Me2NH2]0.125[In0.125(H2Lb)0.25]xDMF}n) [146]. RhB@InMOF exhibited a blue emission at 430 nm of organic ligands and a red emission at 590 nm of RhB molecules. The organic solvent molecules had different effects on the energy transfer between organic ligands and the dye, therefore the peak height ratios of ligand-to-dye moieties could act as detectable signals of analytes. When meeting DMF, RhB@InMOF showed larger emission peak-height ratio (7.51) of ligand-to-dye moieties than the solid RhB@InMOF and other organic solvents including acetone, ethyl acetate, 1,4-dioxane, chloroform, dichloromethane, tetrahydrofuran, cyclohexane, and acetonitrile (Fig. 11a). In addition, this RhB@InMOF could distinguish the benzene homologues and methanol homologues (Fig. 11b-d). The result indicates that the dual-emitting dye@anionic MOFs are very promising ratiometric luminescent sensors for detecting different analytes, particularly the homologues with similar structures. Through post-synthetic ion exchange with ionic chromophores such as metal complexes, organic dyes, and RE ions, anionic MOFs show interesting luminescent properties and have potential in white-LED, nonlinear optics, and chemical sensing. The quantum yield of anionic MOF-based white-LED materials can be improved by encapsulating IrIII complexes. Then, how to use these materials in engineered forms should be explored. Furthermore, NIR emission-based sensors were realized by encapsulating RE ions like Yb into the cavities of anionic MOFs, which can be in biological environments.

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Fig. 10. (a) Yb3+ emission profile under N2 (black) and O2 (blue). (D) Decrease in Yb3+ signal over time after exposure to O2 (blue) and revival of Yb3+ signal over time upon exposure to N2 (black). Reproduced from Ref. [139]. Copyright 2011 American Chemical Society.

Fig. 11. (a) Emission peak-height ratios between ligand and dye moieties in RhB@InMOF after adsorption of DMF, acetone, ethyl acetate, 1,4-dioxane, chloroform, dichloromethane, tetrahydrofuran, cyclohexane, and acetonitrile excited at 345 nm at room temperature. (b) The solvent-dependent emission peak-height ratios of ligandtodye moieties in the luminescence spectra of RhB@InMOF after adsorption of benzene, toluene, ethylbenzene, o-xylene, m-xylene, p-xylene, Cl-benzene, Br-benzene excited at 345 nm at room temperature. (c) Emission peak heights of ligand (black) and dye (hatched) moieties. (d) The emission peak-height ratios between ligand and dye moieties in RhB@InMOF after adsorption of methanol, ethanol, n-propanol, isopropanol, n-butanol, n-pentanol, n-hexanol, and cyclohexanol excited at 345 nm at room temperature. Reproduced from Ref. [146]. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.3. Organic pollutants removal Nowadays, more and more wastewater has been generated from the paper, textiles, and printing industries with the development of industrialization [147]. It contains a variety of highly toxic and long lasting organic compounds, thus resulting in deterioration of the environment and diverse adverse health effects. Therefore, various approaches including adsorption, electrochemical oxidation, photocatalysis, and biodegradation, have been developed for removing the hazardous pollutants [148–152]. Among

them, adsorption is considered to be one of the most competitive methods for organic pollutants removal, due to high efficiency, low energy-cost, flexible design as well as easy operation [153]. MOFs exhibit excellent capacity in the removal of organic pollutants owing to their high porosity and large surface area [154,155]. Notably, anionic MOFs are very promising candidates for selective capture and separation of targeted organic pollutants through strong electrostatic interactions [156–158]. In 2015, Ma and coworkers synthesized a microporous anionic MOFs [(C2H5)2NH2]2[Mn6(Lh)(OH)2(H2O)6]4DEF (H12Lh = 5,5-

S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007 0

,500 ,5000 ,50000 ,500000 -[1,2,3,4,5,6-phenylhexamethoxyl] hexaisophthalic acid) and explored its potential in the selective capture and separation of organic dyes in detail [159]. The fresh crystalline anionic MOF samples were immersed in DEF solutions of different types of organic dyes (methylene blue (MB+), Sudan I (SD0), solvent orange 7 (SO0), acid orange A (AO
) and methyl orange (MO
)) as well as their mixtures (MB+/SD0, MB+/SO0, MB+/AO
and MB+/MO
). The adsorption performance of organic dyes was determined by UV– Vis spectroscopy. As expected, this anionic MOF only absorbed MB+ with the crystals’ color varying from pale yellow to blue (Fig. 12a-d). The authors inferred that the ion exchange process between MB+ and [(C2H5)2NH2]+ cations in the channels was occurred, thus resulting in the selectively capture of MB+ from the mixed organic dyes. Furthermore, MB+, rhodamine 6G (R6G+), butyl rhodamine B (BRB+) which have the same positive charge but different molecular sizes were chosen to evaluate the size effect of organic dyes on adsorption properties. When soaking the as-prepared anionic MOF samples in DEF solution of MB+, R6G+, BRB+, as well as their mixtures MB+/R6G+ and MB+/BRB+, almost no R6G+ and BRB+ were captured (Fig. 12e,f). This is because that R6G+ and BRB+ with large molecular size were excluded from the pores of anionic frameworks and thus could not be exchanged with [(C2H5)2NH2]+ cations. Various published works demonstrate the potential of anionic MOFs for selective adsorption and separation of organic dyes [156–168]. 3.4. Proton conduction Recently, MOFs have emerged as suitable candidates for proton conduction owing to their facile fabrication, high chemical stability, and the feasibility of hybridization with other materials. In addition, the high crystalline nature of MOFs can give insight into the conduction mechanism and help us to optimize the proton conduction pathway [169]. Generally, proton conduction of MOFs arises from the incorporating functional groups (such as carboxylic, sulfonic, phosphonic, or hydroxyl groups) and guest proton carrier species (e.g. water, acid, imidazole, H3O+, NH+4, Emim+, etc.) in the pores of MOFs [170]. Therefore, anionic MOFs with extraframework charging-balancing cations are the best choice for proton conduction in MOF family. Mak and coworkers reported a porous anionic {[H3O][Cu2(DSOA)(OH)(H2O)]9.5H2O}n (Cu-DSOA) with a sulfonatecarboxylate ligand disodium-2,20 -disulfonate-4,40 0 -oxydibenzoic acid (Na2H2DSOA), featuring two large hydrophilic channels decorated with the uncoordinated O atoms from sulfonate groups and occupied by charge-balancing hydronium cations [171]. It showed proton conductivity of 1.9  10
3 Scm
1 at 85 °C and 98% relative humidity. Another two anionic frameworks [Cd2(BTC)2(H2O)2]nn (H2bmib)6n(H2O) and [Cd4(CPIP)2(HCPIP)2]nn(H2bmib)n(H2O) (bmib = 1,4-bis(2-methylimidazol-10 -yl)butane, H3CPIP = 5-(4carboxyphenoxy)isophthalic acid) were reported by Xu and coworkers with protonated H2bmib as the charge-balancing cations in their void spaces [172]. These two anionic MOFs exhibited proton conduction due to the protonation of bmib and the extensive hydrogen bonds, and their proton conductivity were 5.4  10
5 Scm
1 and 2.2  10
5 Scm
1 at 333 K and 95% relative humidity, respectively. An oxalate-based anionic MOF {[(Me2NH2)3(SO4)]2[Zn2(ox)3]}n was synthesized and showed excellent proton conduction performance in both anhydrous and humidified regimes [173]. The oxalate-based MOF comprised an anionic framework [Zn2(ox)3]2
n and a cationic supramolecular net [(Me2NH2)3(SO4)]+n, which was formed by hydrogen-bonding and electrostatic interactions between [Me2NH2]+ cations and sulfate anions (Fig. 13). This compound not only showed proton conductivity of 1.0  10
4 Scm
1 under anhydrous conditions at 150 °C, but also exhibited a high water-assisted proton conductiv-

17

ity of 4.2  10
2 Scm
1 25 °C and 98% relative humidity. The activation energy values calculated from Arrhenius plots were 0.129 eV, which indicated that the proton transportation was operated by Grotthuss mechanism. These results suggest that the charge-balancing guest species in the channel of the anionic MOFs play a very important role in the performance of proton conductivity. Tuning the hydrophilicity and acidity of the channel of MOFs by introducing specific functional groups is a notable strategy for proton conduction. Shimizu and coworkers presented a water-stable MOF [La(H5Li)(H2O)4] (PCMOF-5, Li = 1,2,4,5-tetrakisphosphonome thylbenzene), featuring non-coordinating phosphonic acid groups [174]. It exhibited proton conductivity over 10
3 Scm
1 at 60 °C and 98% relative humidity and required low activation energy for proton conduction, which indicated a highly effective Grotthuss mechanism. The remarkable performance of proton conductivity of PCMOF-5 can be attributed to the non-coordinating phosphonic acid groups and water molecules in the pores as well as the presence of the hydrogen bond pathway. Kitagawa and coworkers demonstrated that the free sulfonic acid groups lined within the pore of MOFs afforded the high proton conductivity at ambient temperature and low humidity (60% relative humidity) [175]. In another interesting work, Tu and coworkers reported a Zr-based MOF [Zr6O8(H2O)8(H2Lb)4] (VNU-23) with free sulfonic acid groups decorated on the channels, and anchored histamine molecules as the proton transfer agent in the channel to form His8.2  VNU-23 (Fig. 14a) [176]. The histamine molecules not only are stable in its protonated form, preventing itself from being released, but also can help to form the proton conduction pathway by attracting water molecules from the surrounding environment, leading to the enhancement of proton conductivity. As a result, His8.2  VNU-23 showed a maximum proton conductivity of 1.79  10
2 Scm
1 at 95 °C and 85% relative humidity without obvious loss of proton conduction for at least 120 h. An oxalate-bridged layered MOF {NR3(CH2COOH)}[MCr(ox)3]nH2O (R = Me (methyl), Et (ethyl), or Bu (n-butyl), and M = Mn or Fe) (Fig. 14b), which featured a cationic component with carboxylic acid groups as the proton carriers, was employed by Kitagawa and coworkers to investigate how to promote the proton conduction [177]. The proton conductivity depended on the hydrophilicity of the cationic components. As a result, Me-FeCr, the most hydrophilic sample, exhibited a high proton conductivity of 0.8  10
4 Scm
1 at a low humidity of 65% relative humidity and 25 °C, and Et-MnCr exhibited the proton conductivity of 10
4 Scm
1 at 25 °C but 80% relative humidity, whereas Bu-FeCr and Bu-MnCr just showed the proton conductivity at 25 °C and 60% relative humidity of 2  10
11 Scm
1 and 0.8  10
11 Scm
1, respectively. The results of the water vapor adsorption (Fig. 14c) clearly showed that the capacity of water molecules adsorption influenced by the interlayer hydrophilicity was a key role in contributing to the high proton conductivity. Therefore, it is of great importance to enhance the hydrophilicity and acidity of the channel of MOFs for proton conduction. The H-bond, hydrophilicity and acidity are essential factors for proton conduction. Anionic MOFs are potential candidates for proton conduction, due to the easy modulation of the H-bond, hydrophilicity and acidity through ion-exchange. 3.5. Heterogeneous catalysis MOFs have emerged as promising candidates in heterogeneous catalysis owing to their high surface areas as well as tunable pore sizes and chemical microenvironments [178,179]. However, the incorporation of catalytic species into MOFs commonly requires covalent sites on the metal nodes or organic linkers of MOFs, resulting in the reduction of the valence electrons activity of the

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Fig. 12. Temporal evolution of UV–Vis absorption spectra of equimolar MB+/SD0 (a), MB+/SO0 (b), MB+/AO
(c), MB+/MO
(d), MB+/R6G+ (e) and MB+/BRB+(f). The photographs show the colors of the mixed solutions and of the crystalline samples, before and after 96 h of organic dye absorption. Reproduced from Ref. [159]. Copyright The Royal Society of Chemistry 2015.

Fig. 13. (a) The tris-chelated D3-symmetric [Zn2(ox)3]2
subunit. (b) Crystal structure of {[(Me2NH2)3(SO4)]2[Zn2(ox)3]}n. (c) Hydrogenbonding interactions between dimethyl ammonium cations and sulfate anions in {[(Me2NH2)3(SO4)]2[Zn2(ox)3]}n. d) 3D supramolecular [[(Me2NH2)3(SO4)]+n net formed by hydrogen bonding between dimethyl ammonium cations and sulfate anions. C gray, N blue, O red, S yellow, Zn orange. In Fig. 13b and d, H atoms are omitted for clarity. Reproduced from Ref. [173]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

encapsulated catalytic species [180,181]. Such obstacle can be avoided through the one-step ion exchange strategy for immobilization of cationic catalytic species into anionic MOFs. Metalloporphyrins are widely used in catalytic oxidation owing to their unique electronic structures, physical and chemical

properties [182]. A suitable host matrix should isolate one metalloporphyrin molecule per cavity, and protect methalloporphyrins against aggregation, leaching, or degradation [183]. Eddaoudi and coworkers used a robust aniconic MOF rho-ZMOF (zeolite-like MOF) as a platform to immobilize a cationic porphyrin 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) ([H2TMPyP] [p-tosyl]4) (Fig. 15 a-c) [184]. The pore sizes of the truncated octahedral cage (ɑ-cage) in rho-ZMOF matched very well with porphyrin molecules, and the window sizes of the ɑ-cage were small enough to prevent the porphyrin molecules from leaching. In addition, charge-induced interactions between the anionic framework of rho-ZMOF and the cationic [H2TMPyP][p-tosyl]4 molecules played an crucial role in the encapsulation processes. After post-synthetically metalizing with various transition metal ions, Mn-RTMPyP@ZMOF was used as an example to access the catalytic performance of cyclohexane oxidation. It showed a total yield (cyclohexane to cyclohexanol/ cyclohexanone) of 91.5% and a noticeably high turn over number (TON) of 23.5 after 24 h under neat conditions (65 °C, tert-butyl hydroperoxide (TBHP) as the oxidant) (Fig. 15d). In addition, it exhibited selectivity of cyclohexanol and cyclohexanone without further oxidation and retained catalytic activity and selectivity after 11th cycle under the reaction conditions. These results suggest that charged MOFs with stability and suitable porous structures are promising host for incorporating catalytic active species to enhance the catalytic performance. Transition metal (TM) complexes are the most commonly used homogeneous catalysts. Considerable efforts have been devoted to anchor TM catalysts onto solid-state hosts for recycling because some of the low cost in comparison with the precious metal catalysts [185]. MOFs are very promising supports for the immobilization of catalytic active species owing to their uniform pores and appropriate grafting sites on the metal nodes or organic linkers. Interestingly anionic MOFs can form a catalyst+@MOF
material through one-step cation exchange due to the strong charge-induced interactions. Sanford and coworkers employed this

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19

Fig. 14. (a) Plausible mechanism for anchoring of histamine. Reproduced from Ref. [176]. Copyright The Royal Society of Chemistry 2018. (b) The crystal structure of BuMnCr: stacking view along the layers. (c) Water vapor adsorption isotherms of the MOFs at 298 K. The blue, red, green, and purple symbols correspond to Me-FeCr, Et-MnCr, Bu-FeCr, and NBu4, respectively. Reproduced from Ref. [177]. Copyright 2012 American Chemical Society.

one-step cation exchange method to partially replace [NH2Me2]+ in the anionic framework of ZJU-28 by different TM catalysts including [Pd(CH3CN)4][BF4]2, [FeCp(CO)2(thf)]BF4 (Cp = ƞ5-C5H5), [Ir (COD)(PCy3)(py)]PF6 (COD = 1,5-cyclooctadiene, PCy3 = tricyclohexylphosphine, py = pyridine), [Rh(dppe)(COD)]BF4 (dppe = 1,2-b is(diphenylphosphino)ethane), and [Ru(Cp*)(CH3CN)3]OTf (Fig. 15e) [186]. [Rh(dppe)(COD)]@ZJU-28 exhibited even higher catalytic activity of 1-octene hydrogenation than the homogeneous counterpart and can be reused till 4 times without obvious loss of the reactivity at ambient conditions (Fig. 15f,g). Che and coworkers synthesized a series of cationic cyclometalated of gold(III) and platinum(II) complexes to prepare AuIII@MOF [187] and PtII@MOF [188] through one-step ion exchange. The AuIII@MOFs can be used as size-selective and reusable heterogeneous photocatalysts for the aerobic oxidation of secondary amines to imines and five other reactions, including Mannich-type reaction and aza-Henry reaction under aerobic conditions. Whereas the PtII@MOFs can catalyze photoinduced formation of C–C bond, dehydrogenation as well as dehydrogenative cyclization, the catalytic performance of which is higher than the corresponding homogeneous Pt(II) complexes. The widely used photoactive cation [Ru(bpy)3]2+ was successfully encapsulated into a robust anionic framework PCN-99 by Zhou and coworkers in 2016 [189]. [Ru(bpy)3]2+@PCN-99 showed relatively high catalytic efficiency for aerobic oxidative hydroxylation of arylboronic acids. However, a reduced catalytic performance of [Ru(bpy)3]2+@PCN-99 in comparison to homogeneous [Ru(bpy)3] Cl2 catalyst was observed because the framework of PCN-99 hindered the access of substrates to the catalytic [Ru(bpy)3]2+ sites. Rosseinsky and coworkers selected the anionic [Et4N]3[In3(BTC)4] MOF as a suitable host due to its structural and chemical stability under the organometallic catalytic reaction conditions [190]. An organometallic cationic Lewis acidic catalyst [CpFe(CO)2(L)]+ ([Fp–L]+, L = weakly bound solvent) partially replaced [Et4N]+ in the anionic framework and was held in the pores via strong charge-induced interactions as well as weaker H-bonding interactions. The resultant [(Fp–L)0.6(Et4N)2.4]3[In3(BTC)4] material can act

as a recyclable heterogeneous catalyst in a simple DA reaction between isoprene (Iso) and methyl vinyl ketone (MVK) to form the 1,4-(para) and 1,5-(meta) isomers of methylacetylcyclohexene. Very recently, the same group used the anionic MOF MIL-101(Cr)SO3Na as the robust host for immobilization of a cationic Crabtree’s catalyst [Ir(cod)(PCy3)(py)][PF6] through one-step cation exchange process [191]. The [Ir(cod)(PCy3)(py)]@MIL-101(Cr)-SO3Na was an efficient heterogeneous catalyst for non-functionalized alkenes hydrogenation both in solution and the gas phase. The catalytic performance of [Ir(cod)(PCy3)(py)]@MIL-101(Cr)-SO3Na for hydrogenation of non-functionalized alkenes was evaluated in CH2Cl2 under mild conditions. MIL-101(Cr)-SO3Na showed no catalytic activity for the oct-1-ene hydrogenation, whereas [Ir(cod)(PCy3) (py)]@MIL-101(Cr)-SO3Na could completely catalyze the hydrogenation of oct-1-ene to n-octane at low loadings, which was comparable to the results of homogeneous catalyst [Ir(cod)(PCy3)(py)] [PF6]. The conversion of [Ir(cod)(PCy3)(py)]@MIL-101(Cr)-SO3Na for hindered aliphatic 2-methylhex-1-ene and cyclohexene was lower than that of the homogeneous counterpart, suggesting that the hydrogenation took place in the pores of [Ir(cod)(PCy3)(py)] @MIL-101(Cr)-SO3Na but not on its surface. Interestingly, the catalytic activity of [Ir(cod)(PCy3)(py)]@MIL-101(Cr)-SO3Na for oct1-ene hydrogenation exhibited 6-fold enhancement than that of the nonporous homogeneous counterpart in the gas/solid reaction, which mainly achieved because dispersion of [Ir(cod)(PCy3)(py)]+ in the porous anionic host can increase the accessible catalytic sites. In addition, the hydrogenation selectivity of [Ir(cod)(PCy3) (py)]@MIL-101(Cr)-SO3Na was obviously enhanced through suppressing the competing isomerization reaction, which can be attributed to the extended coordination sphere interactions between [Ir(cod)(PCy3)(py)]+ and the chemical microenvironments of the pores of MIL-101(Cr)-SO3Na. Thus, the rational design and synthesis of charged MOFs with well-defined chemical environments can significantly enhance the catalytic activity and selectivity of the encapsulated catalysts, making MOF-based materials promising hosts for anchoring catalytic species.

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Fig. 15. (a) Eight-coordinate MBB that could be represented as a TBU, (b) [H2TMPyP]4+ porphyrin, (c) crystal structure of rho-ZMOF (left), hydrogen atoms omitted for clarity, and schematic presentation of [H2TMPyP]4+ porphyrin ring enclosed in rho-ZMOF R-cage (right, drawn to scale). (d) Cyclohexane catalytic oxidation using MnRTMPyP@ZMOF as a catalyst at 65 °C. Yield % based on TBHP, 1 equiv consumed per alcohol produced and 2 equiv consumed per ketone produced. Reproduced from Ref. [184]. Copyright 2008 American Chemical Society (e) Proposed heterogenization of single-site transition-metal catalysts in ZJU-28 via cation exchange. (f) Hydrogenation of 1-octene to n-octane in neat 1-octene catalyzed by [Rh(dppe)(COD)]@ZJU-28 (blue) and homogeneous [Rh(dppe)(COD)]BF4 (red). Conditions: 0.0013 mmol [Rh]. The maximum possible TON is 5000 (g) Recycling of [Rh(dppe)(COD)]@ZJU-28 in the neat reduction of 1-octene to octane. Reproduced from Ref. [186]. Copyright 2013 American Chemical Society.

The construction of stable anionic MOFs with large pore, increasing the catalytic active cation exchange efficiency, and controlling the spatial location of catalytic active cations in anionic MOFs should be considered for the ongoing work of this strategy.

4. Applications of cationic MOFs 4.1. Gas adsorption and separation Like anionic MOFs, exchange of charge-balancing anions is an efficient approach to turn the pore size, shape, and surface properties of cationic MOFs and thus improve their performance of gas adsorption and separation. In 2007, Kitagawa and coworkers designed a twofold interpenetrated cationic MOF {[Ni(bpe)2(N (CN)2)](N(CN)2)(5H2O)}n (bpe = 1,2-bis(4-pyridyl)ethane, N(CN)-2
= dicyanamide), which featured a rectangular channel for charge-balancing N(CN)
2 anions and a hexagonal channel occupied by guest water molecules [192]. Although the pore size of the dehydrated phase was large enough, N2, O2 and Xe could not pass into the pores due to the strong interactions between these gas molecules and the pore windows at 77 K, whereas CO2 can diffuse into the pores owing to the dipole-induced-dipole interactions of CO2 with the p-electron clouds of bpe ligands and Ni(II) polar groups (Fig. 16a). Impressively, it also showed MeOH (Fig. 16b),

EtOH, H2O and acetone adsorption at 298 K with hysteretic sorption behavior, which was because of the H-bonding interactions between the incoming molecules and the cationic framework. In addition, the charge-balancing N(CN)
2 anions of parent framework can be selectively replaced by N–3 anions (Fig. 16c). Interestingly, the smaller size of N–3 anions resulted in an increase in its pore size and thereby an improved performance of CO2 capture (Fig. 16d). In another work of Kitagawa and coworkers, they prepared a cationic framework [Zn2(TPA)2(CPB)]2DMFH2O by using the mixture ligands of terephthalic acid (H2TPA) and 1-(4-carboxyphenyl)-4,40 -bipyridinium hexafluorophosphate (HCPBPF6) [193]. Its degassed phase adsorbed ac. 150 mL/g methanol at 298 K, corresponding to five molecules of methanol per formula unit. The methanol adsorption profile showed distinct hysteretic sorption behavior, suggesting a dynamic structural transformation induced by the inclusion of guest methanol molecules. The higher Qst of methanol (50–95 kJ mol
1) indicated the strong interactions between the cationic pyridinium surface and methanol molecules. Therefore, the charged frameworks of cationic MOFs play a crucial role of gas capture and separation through charge-induced interactions. Owing to the different polarizability properties between C2 hydrocarbons and C1 methane, charged MOFs might be promising candidates for selective adsorption and separation of these small hydrocarbons through charge-induced forces. A 3D cationic framework ZJU-48 with 1D pores (9.1  9.1 Å2) along the c axis was

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Fig. 16. (a) Robust a-polonium-type three-dimensional interpenetrating porous framework of {[Ni(bpe)2(N(CN)2)](N(CN)2)(5H2O)}n with multiple functionality. (b) Methanol adsorption isotherm for vapour adsorption in{[Ni(bpe)2(N(CN)2)](N(CN)2)}n (c) Schematic diagram of the pore size controlled by the anion exchange. (d) Comparison of carbon dioxide adsorption isotherm in as-synthesized (yellow) {[Ni(bpe)2(N(CN)2)](N(CN)2)}n and N–3 exchanged (green) {[Ni(bpe)2(N(CN)2)](N3)}n frameworks. Reproduced from Ref. [192]. Copyright 2007 Nature Publishing Group.

fabricated by a solvothermal reaction of (E)-4,40 -(ethene-1,2-diyl) dibenzoic acid (H2EDDA), ad, and Zn(OAc)22H2O [194]. ZJU-48 showed high stability and a BET surface area of 1450 m2/g, which promoted the authors to investigate the potential applications for selective adsorption and separation of small hydrocarbons. The activated phase ZJU-48a took up 77.83 cm3/g of C2H2, 65.65 cm3/g of C2H4, 80.66 cm3/g of C2H6 but 11.96 cm3/g of CH4 at 1 atm and 273 K; 57.07 cm3/g of C2H2, 39.43 cm3/g of C2H4, 43.78 cm3/g of C2H6 but 7.82 cm3/g of CH4 at 1 atm and 296 K. The Qst was determined to explain the selective separation of ZJU-48a for small hydrocarbons. The Qst of C2H2, C2H4, and C2H6 on ZJU-48a were 15.6, 21.7 and 28.0 kJ mol
1, respectively, whereas the Qst of CH4 was 14.0 kJ mol
1. The higher Qst of C2 hydrocarbons on ZJU-48a was mainly due to the larger van der Waals interactions as well as charge-induced interactions between the cationic framework and C2 hydrocarbons. The ZJU-48a showed C2H6/CH4 adsorption selectivity of more than 6 in a range of pressure to 100 KPa, which demonstrated its potential for the C2/C1 separation in industrial applications. This work will facilitate more research endeavor to separate C2/C1 hydrocarbons with high selectivity by using charged MOFs. CO2 molecules are high polarizability and have the quadrupole moment [195]. Therefore, the polar MOFs with various recognition sites have been gained extensive interest for CO2 capture. Zhang and coworkers fabricated a cationic azolate framework [Zn7(IP)12] (OH)2 (MAF-34, HIP = 1H-Imidazo[4,5-f][1,10]phenanthroline) [196]. It showed high adsorption selectivity of CO2 over N2 owing

to the uncoordinated imidazolate nitrogen sites on the pore surface of desolvated MAF-34. In another novel work, Wang and coworkers built up a stable cationic MOF [Mn2(HCBPTZ)2(Cl)(H2O)] ClDMF0.5CH3CN by using a N-rich pyrazinyl triazolyl carboxyl ligand 3-(4-carboxylbenzene)-5-(2-pyrazinyl)-1H-1,2,4-triazole (H2CBPTZ), which featured an unusual (3,4)-connected 3,4T1 network and 1D channels [197]. It took up 2.2 cm3(STP)g
1 of N2, 15.3 cm3(STP)g
1 of CH4, whereas it showed an exceptionally high CO2 adsorption of 70.1 cm3(STP)g
1 (13.8 wt%) at 100 kPa and 298 K, which was much higher than that of the most MOF materials [198–200]. The high CO2 loading and the high selectivity was achieved due to the synergistic effect of uncoordinated nitrogen sites, exposed metal centers, positive charged framework, as well as Cl
basic sites, confirmed by grand canonical Monte Carlo (GCMC) simulations. 4.2. Colorimetric sensing Luminescent cationic MOFs commonly fabricated by incorporation of d10 metal ions and neutral N-donor ligands [201]. The charge-balancing anions in the pores of cationic MOFs are weakly bonded to the metal centers and thus can be exchanged by other anions with different size, shape and coordinating nature [202]. In this process, the cationic MOFs show tunable luminescent behavior and interestingly can be used as colorimetric sensors. In a pioneering work reported by Ghosh and coworkers, a luminescent cationic MOF [{Zn(Lj)(MeOH)2}(NO3)2xG]n (Lj = 4,40 -(ethane-

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S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

1,2-diyl)bis(N-(pyridin-2-ylmethylene)aniline) was devised and exhibited an interesting guest- and anion-driven dynamic structural behavior [76]. Notably, anion-exchanged compounds showed different luminescent behaviors (Fig. 17a,b). For weakly coordinat
ing anions (ClO
4 and N(CN)2 ), luminescence enhancement of 84.14% and 70.72% respectively was observed, whereas for strongly coordinating anions (SCN
and N–3), a decrease of luminescence was observed (80.36% and 29.53%, respectively) (Fig. 17c). Another work from the same group reported a homochiral cationic MOF [{Zn(Lj)(OH2)2}(NO3)2xG]n, which showed luminescence enhance

ment after exchanging with ClO
4 , PF6 , and BF4 (Fig. 17d) [203]. The anion-dependent tunable luminescence behavior of the cationic MOFs indicates their potential in chemical sensing. In addition, cationic MOFs show interesting guest-dependent visual colorimetric properties after encapsulating special guest molecules or ions [204–207]. Dong and coworkers developed a naked-eye colorimetric sensor Cu2(Lk)2I2MeCNMeOH1.5H2O (Lk = 1-benzimidazolyl-3,5-bis(4-pyridyl)benzene) for humidity and formaldehyde via a SCSC fashion [205]. The color of Cu(I)-MOF changed from bright yellow to red dark due to the replacement of methanol and acetonitrile by atmospheric water molecules, and the emission of Cu(I)-MOF was strongly quenched by increasing the humidity levels and exposition time. Impressively, when exposed to formaldehyde vapors, the encapsulated water molecules can be exchanged by formaldehyde molecules, accompanying with the restoration of emission and the crystal color change. This can be attributed to the hydrogen bonding interactions between formaldehyde molecules and the framework of Cu (I)-MOF. The low detection limit (0.016 ppm) and the wide temperature range (from rt to 75 °C) of Cu(I)-MOF made it an excellent candidate for detecting formaldehyde pollutants in the real environment. The same group also prepared a cationic Cu(II)

MOF ([CuLl2(H2O)0.5](NO3)23.25(CH2Cl2)(CH3OH)0.5H2O) (Ll = 4,40 -(9,9-dibutyl-9H-fluorene-2,7-diyl)dipyridine) with chargebalancing NO–3 anions in the 1D channels [206]. As shown in Fig. 18a, the Cu(II) MOF showed rapid naked-eye color changes simply by immersing the as-prepared samples into an aqueous solution of NaCl, KBr, KI, KSCN and NaN3, respectively. This indicated that Cu(II) MOF is an excellent colorimetric sensor with characteristic colors. In addition, this MOF can selectively capture absorb Cl
from a Cl
/Br
mixture, and I
from a Br
/I
mixture, and SCN
from a SCN
/N–3 mixture (Fig. 18b) owing to their selective substitutions. Another visual colorimetric sensor was achieved in {[Cu(pytpy)]NO3CH3OH}1 (pytpy = 2,4,6-tris(4-pyridyl) pyridine) by an anion exchange process [207]. Cationic MOFs showed interesting visual colorimetric properties through anion-exchange, which might have potential in optical switches, light-emitting devices, and recording materials. 4.3. Pollutants removal Similar to anionic MOFs, cationic MOFs can selectively capture anionic pollutants through strong electrostatic interactions [75,208]{Chen, 2015 #247}. For example, Feng and coworkers built up a platform based on 9-connected [In3O(COO)6]+ SBUs and a series of mixed pyridine/carboxylate linkers to investigate the capacity of cationic MOFs for selective adsorption and separation of organic anionic dyes [51]. Various azodyes with different sizes and charges were selected to replace the nitrate counterions of the cationic InMOFs. As expected, the cationic InMOFs only adsorbed the anionic azodyes, whereas the neutral and cationic azodyes were excluded. Furthermore, the anion exchange dynamics depended on the magnitude of charge, and the molecular weight, shape and size of the dyes. Moreover, the controlled

Fig. 17. (a) Effects of anion exchange on the solid-state luminescence properties of [{Zn(Lj)(MeOH)2}(NO3)2xG]n. (b) Luminescence intensity of different anion-exchanged samples. (c) Bar diagram showing luminescence enhancement and quenching of anion-exchanged samples with respect to [{Zn(Lj)(MeOH)2}(NO3)2xG]n. Reproduced from Ref. [76]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (d) Solid-state luminescence spectra of [{Zn(Lj)(OH2)2}(NO3)2xG]n and other anion-exchanged compounds at 298 K. Reproduced from Ref. [203]. Copyright 2014 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.

S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

23

Fig. 18. (a) Color change of [CuLl2(H2O)0.5](NO3)2 resulted from anion exchange. (b) Anion selectivity based on [CuLl2(H2O)0.5](NO3)2. Reproduced from Ref. [206]. Copyright The Royal Society of Chemistry 2012.

desorption behavior of cationic InMOFs made them excellent candidates for drug molecules release. A cationic cage-based body-centered MOF was reported by Cheng and coworkers, which featured selective adsorption and desorption of MO
[209]. The heavy-metal contaminants in waste water have been become a serious global issue respect to both environment and human health [210]. Surprisingly, cationic MOFs show high capac
ity for removing heavy metals like Cr2O2
7 , MnO4 , as well as the radioactive contaminant TcO
[74,211,212,215]. For instance, Qian 4 and coworkers synthesized a cationic ZrMOF ZJU-101 from MOF867 through a post-synthetic modification approach [211]. It contains immobilized N+–CH3 groups on the surfaces of the channel and counteranions NO–3 in the channel. ZJU-101 exhibited superior adsorption capacity for Cr2O2
7 (245 mg/g), which was much higher than its parent MOF-867 owing to the presence of the cationic N+–CH3 groups (Fig. 19a-d). Furthermore, ZJU-101 showed high


– 2

selectivity toward Cr2O2
7 over Cl , Br , NO3, SO4 , I and F due 2
to the strong coulombic attraction between Cr2O7 and the cationic

framework and suitable size of Cr2O2
for the pores of ZJU-101. 7 Wang and coworkers used a low-dimensional cationic MOF [Ag(bipy)]NO3 (denoted as SBN) equipped with abundant exposed

Ag+ sites for ReO
4 /TcO4 removal in 2017 [212]. ReO4 anion, a sur
99 rogate for radioactive TcO4 , was used as an example to investigate the removal capacity of SBN. Impressively, SBN not only showed excellent ReO
4 removal capacity up to 786 mg/g, which was much higher than the most of reported materials (Fig. 19e) [74,213,214], but also had a high selectivity of ReO
4 due to the formation of strong Ag–O–Re bonds and much denser hydrogen bond network with bipy ligands. It was found that a recrystallizationbased complete and irreversible phase transformation was occurred after the replacement of NO–3 by small excess ReO
4 (Fig. 19f), resulting in the superior adsorption and selectivity of
ReO
4 . Therefore, SBN can be used as promising sorbent for TcO4 and further evaluated for long-term storage of Tc. By using a neutral ligand, tris(4-(1H-imidazol-1-yl)phenyl)amine (Lm), as the backbone, a water-stable cationic framework [{Ni2(Lm)3(SO4)

Fig. 19. (a) UV spectra of aqueous dichromate solution during adsorption test with MOF-867. (b) photographs of MOF-867 for the removal of dichromate in aqueous solution with time. (c) UV spectra of aqueous dichromate solution during the adsorption test with ZJU-101. (d) photographs of ZJU-101 for the removal of dichromate in aqueous solution with time. Reproduced from Ref. [211]. Copyright The Royal Society of Chemistry 2015 (e) Sorption isotherms of ReO
4 by SBN compared with LDH, NDTB-1, PuroliteA532E, and Yb3O(OH)6Cl materials. (f) SCSC transformation from SBN to SBR. Reproduced from Ref. [212]. Copyright 2017 American Chemical Society.

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S.-N. Zhao et al. / Coordination Chemistry Reviews 398 (2019) 113007

(H2O)3}(SO4)x(G)]n (G = DMF, H2O) with charge-balancing SO2
4 anions aligned in the channel was synthesized by Ghosh and coworkers [215]. Interestingly, this cationic MOF showed selective
and colorimetric aqueous-phase adsorption of Cr2O2
7 and MnO4 , and further can act as model for the removal of TcO
. These results 4 offer a strategically approach for the heavy metal ion removal by using cationic MOFs as sorbents and will benefit to the development of MOF-based sorbents. Cationic MOFs showed desirable features for selective adsorption of anionic heavy metals. However, the poor water stability of cationic MOFs limited their practical applications. Thus, improving the water stability is the major challenge of cationic MOFs. Furthermore, employing cationic MOFs in engineered forms like adsorbents should be explored.

5. Summary and perspectives In this review, we provide an in-depth view of charged MOFs (both anionic and cationic MOFs), including the general design strategies and synthetic methods of charged MOFs, and how guest ion exchange enhances their performance in various applications. We have summarized the commonly used strategies to construct charged MOFs through rational choice of solvents, metal ions or clusters, and organic ligands. The [NH2Me2]+ and [NH2Et2]+ cations that generates from the hydrolysis of DMF, DMA, and DEF are commonly as the countercations in anionic MOFs. Thus, solvents like DMF, DMA, DEF, as well as the ones can provide very rich ionic environments like ionic liquid lead to the formation of charged MOFs. Meanwhile, the charged metal SBUs can directly lead to the formation of charged MOFs. Therefore, investigating characteristics of metal ions and rational design of novel charged metal SBUs is of great importance for the construction of charged MOFs with high stability and interesting structures. In addition, charged MOFs can be also achieved by using special organic ligands. Some organic ligands can incorporate the uncoordinated carboxyl or sulfonic groups into MOFs, leading to the negative frameworks with H+ or Na+ as the countercations, whereas the nitrogen donor ligands and imidazolium-based carboxylate ligands is commonly used to synthesize cationic MOFs. The structure of organic ligands can directly affect the structures and properties of charged MOFs. Therefore, design and synthesis of organic ligands with novel structures can enrich the family of charged MOFs and afford charged MOFs with fascinating properties for practical applications. Though great successful achievements have been obtained recently, it is still a great challenge to control the coordinated sites of organic ligands under the reaction conditions. PSM approach has been proved a fascinating way to convert neutral MOFs into anionic MOFs or cationic MOFs with well-refined structures. However, it usually requires metal SBUs or organic ligands with active sites that allows feasible chemical transformations. Thus, more efforts should be devoted to explore the controlled synthesis of MOFs with active sites on organic ligands and/or metal SBUs. The inherent extraframework ions in charged MOFs provide another dimension of advantage through ion exchange with functional ionic species which cannot be obtained in neutral MOFs. MOF-based materials have been proven to be promising candidates for gas adsorption and separation because of their high porous structures and large surface areas. Charged MOFs realize the adsorption selectivity due to the strongly charge-induced interactions between the charged frameworks and targeted gas molecules. Furthermore, through ion exchange with smaller ionic species or metal ions, the adsorption performance can be significantly improved. Luminescent MOFs can be widely used in lightemitting devices and chemical sensing because of their easily

introducing light-emitting building blocks into the frameworks. For charged luminescent MOFs, the luminescence quantum yield can be remarkably improved by encapsulating ionic chromophores with high quantum yield like Ir complexes, and the dual-emission detection as well as the visual colorimetric sensing are successfully achieved by ion exchange. Thus, the charged luminescent MOFs are more close to the practical applications in light-emitting devices and chemical sensing. MOFs exhibit excellent capacity in the removal of organic pollutants owing to their high porosity and large surface area. Charged MOFs can achieve the selective removal of targeted pollutants through strong electrostatic interactions. That is, anionic MOFs can selectively capture cationic pollutants while cationic MOFs can removal anionic pollutants through ion exchange. In addition, charged MOFs also show potential in proton conduction, heterogeneous catalysis, and drug delivery. Very recently, charged MOFs was used to prepare nitrogen-doped porous carbons for electrochemical applications [216,217], suggesting that charged MOFs are promising candidates for energy conversion. Furthermore, the charged frameworks make this subclass of MOFs potential as the ion-exchange resins. To realize the practical applications that mentioned above, charged MOFs should overcome the thermochemical stability both in the synthetic conditions and the post-synthetic ion-exchange process. The counterions in the charged frameworks and the functional ionic species that can be encapsulated into the charged frameworks are limited. Researchers should devote more efforts to explore more accessible counterions, particularly more functional ionic species with fascinating properties, thus to improve the performance of charged MOFs or afford new functions of charged MOFs. Although various methods have been used for synthesizing charged MOFs, further investigations are still necessary for controlled synthesizing charged MOFs with novel structures and active sites. The most common way to load the functional guest species into the pores of charged MOFs is through postsynthetic ion-exchange process. However, this method is difficult to control the loading place and loading amount of the functional guest species. More controllable methods for loading functional guest species into the pores of charged MOFs should be explored. Furthermore, the leaching of the functional guest species after using is the most important challenge of charged MOF materials. Once these major hurdles are overcome, the advantages of charged frameworks would enable charged MOFs to be the next generation MOFs and make them very promising materials in the practical applications of gas adsorption and separation, luminescence, sensing, pollutants removal, heterogeneous catalysis, proton conduction, drug delivery, as well as energy conversion. Acknowledgements The authors are grateful for the financial aid from the National Natural Science Foundation of China (Grant Nos. 21590794, 21771173, and 21521092), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), the Science & Technology Department of Jilin Province (Grant No. 20180101179JC), and the CAS-CSIRO project (Grant No. GJHZ1730). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ccr.2019.07.004. References [1] M. Woellner, S. Hausdorf, N. Klein, P. Mueller, M.W. Smith, S. Kaskel, Adv. Mater. (2018) e1704679.

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