Introduction

During the last two decades, metal–organic frameworks (MOFs)/porous coordination polymers (PCPs) composed of metal ions and organic bridging ligands as main building units and, in some cases, inorganic ligands, have been investigated extensively because of their versatile structural diversity, high structural controllability (pore size, shape, dimension, flexibility and surface environment), high crystallinity, and their potential porous properties/functionalities in a variety of research fields such as storage and separation, catalysis, drug delivery and sensing ability.1, 2, 3 The porous properties of MOFs/PCPs can be finely tailored by not only chemical modification of organic ligands and judicious choice of components but also a variety of techniques such as solid solution formation, defect engineering, core-shell structure, crystal morphology and size control, and so on. For example, the appropriate combination of organic bridging ligands provided the Zn(II) MOF, [Zn4O(bte)4/3(bpdc)] (bte=4,4′,4′′-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tribenzoate, bpdc=biphenyl-4,4′-dicarboxylate), with ultra-high Brunauer–Emmett–Teller and Langmuir specific surface areas close to 6000 and 10 000 m2 g−1, respectively.4 Unlike porous inorganic zeolites and porous carbon materials, MOFs often give flexible structures. As coordination bonds, hydrogen bonds and van der Waals interactions that are used to assemble each component are weaker (bonding energies are 10~200 kJ mol−1) than covalent and ionic bonds (200~1000 kJ mol−1), reversible rotation, bending and breaking of these weaker bonds easily occurs, resulting in structural changes. One representative of flexible MOFs is [Cr(OH)(1,4-bdc)] (MIL-53(Cr), 1,4-bdc=1,4-benzenedicarboxylate), in which local reorientation of the coordination bond triggered by guest adsorption/desorption induced the structural transformation between narrow pore and large pore forms.5 In addition, many MOFs show high crystallinity as with porous inorganic zeolites, which is advantageous for understanding deeply the structure–property relationship using single-crystal and powder X-ray diffraction techniques. For example, the unprecedented highly selective CO sorption from a CO/N2 mixture, which is difficult to separate, was achieved using the flexible Cu(II) MOF, [Cu(aip)] (aip=5-azidoisophthalate).6 Successful determination of the crystal structures of the dried and CO-adsorbed forms using powder X-ray diffraction demonstrated that the synergetic effect of the interaction between CO and the Cu(II) axial sites and a structural transformation contributed to such high selectivity.

At present, it is all but impossible to comprehend completely the reports on MOFs because of the huge, ever-growing number of reports that have been published. In this review article, therefore, we focused on fluorine-functionalized MOFs/PCPs due to their fluorine-specific, interesting porous properties. A fluorine atom has discriminating characteristics, the highest electronegativity and small electric polarizability, which causes the development of a variety of unique characteristics, such as low boiling point, fluorous phase, selective gas absorbability, hydrophobicity and high chemical stability, in fluorine-containing molecules/materials. One of the best-known fluorine-containing materials is polytetrafluoroethylene, an organic polymer containing only carbon and fluorine atoms invented by the DuPont company. Polytetrafluoroethylene exhibits high thermal and chemical stability, low dielectric constant, high insulation property, high water and oil repellency, and non-adhesive property, all of which are caused by fluorine atoms, and is used extensively in our society.

In 2014, Pachfule and Banerjee7 published an excellent review article on fluorine-functionalized MOFs/PCPs, in which MOFs/PCPs with a variety of fluorine-containing building blocks and their structure-property (gas adsorption and separation) relationships were introduced. Herein, we provides a comprehensive review of fluorine-functionalized MOFs/PCPs including not only latest results on the basis of the kinds of fluorinated building blocks but also a much wider range of fluorine-related properties (hydrophobicity, adsorption/separation, perfluoroarene–arene interaction, structural flexibility, ionic conductivity and low-energy C–F bond). The fluorinated components, crystal structures and porous properties of fluorine-functionalized MOFs/PCPs are discussed with the relations between porous properties and fluorine characters. The papers that did not discuss on fluorine-porous property relationships in fluorine-functionalized MOFs/PCPs are not covered here. Table 1 summarizes fluorine-functionalized MOFs/PCPs introduced in this review.

Table 1 Summary of fluorine-functionalized MOFs/PCPsa
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MOFs containing fluorine- and trifluoromethyl-substituted organic ligands

It is well known that a perfluoroarene unit forms an intermolecular face-to-face perfluoroarene–arene interaction (the origins of which are van der Waals and quadrupole–quadrupole interactions) with an arene unit.46 Fujita and colleagues8, 47 reported the crystal structures of a large number of Cd(II) MOFs with fluorine-functionalized organic ligands, as shown in Figure 1. The observed MOFs formed one-, two- and three-dimensional porous structures with non-fluorinated aromatic guest molecules, some of which formed intermolecular arene–perfluoroarene interactions between fluorinated ligands and non-fluorinated aromatic guests. For example, the two-dimensional MOF, {[Cd(NO3)2(2,6-bpfn)2]•2(p-dimethoxybenzene)} (2,6-bpfn=2,6-bis(4-pyridylmethyl)hexafluoronaphthalene, Figure 1c), exhibited face-to-face interactions between the perfluoroarene rings and the guest arene rings with the shortest C•••C distance of 3.411 Å inside the grids (Figure 2a).8 In addition, the guest afforded the weak C–H•••F hydrogen bond (H•••F distance=2.653 Å and C–H•••F angle=126.1°) with the neighboring two-dimensional sheet (H27A•••F2 in Figure 2b).48

Figure 1
figure 1

Fluorine-substituted organic ligands with the pyridine coordination sites reported by Fujita and colleagues.8, 47

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Figure 2
figure 2

Crystal structure of {[Cd(NO3)2(2,6-bpfn)2]•2(p-dimethoxybenzene)}. (a) Top view of the two-dimensional sheet. The guest p-dimethoxybenzene molecules are coloured red. (b) View of the intermolecular hydrogen bonds. Reproduced from Kasai et al.8

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The polar C–F bond may be useful for a preferable gas adsorption site. Cheetham and colleagues9 found an enhanced enthalpy of H2 adsorption in the three-dimensional fluorinated MOF, {[Zn5(triazole)6(tetrafluoroterephthalate)2(H2O)2]•4H2O}, using commercially available fluorinated tetrafluoroterephthalic acid (Figure 3a). This MOF had a small pore and fluorine atoms exposed on the pore surface (Figure 4a), which was expected to cause high enthalpy of H2 adsorption (–8 kJ mol−1 at low coverage, Figure 4b), comparable to MOFs with coordinatively unsaturated metal centers that considerably increase this value. After this report, the H2 adsorption sites were closely investigated using inelastic neutron scattering spectroscopy and molecular simulation by Space and colleagues.10 Theoretical calculation is a powerful tool to support and predict intermolecular interactions between coordination frameworks and guest molecules. The inelastic neutron scattering spectroscopy and simulation results concluded that the most favorable adsorption site is the vicinity of the Zn-coordinated H2O, the fluorine and the carboxylate oxygen atoms of tetrafluoroterephthalate ligands in small pores as shown in Figure 4c. To elucidate the effect of fluorination on the gas-sorption properties of MOFs, a systematic investigation was performed by Bu and colleagues11 using the three-dimensional MOFs, {[Ni0.5(tpt)0.5(R-opa)0.5(H2O)0.5]•x(guest)} (tpt=2,4,6-tri(4-pyridyl)-1,3,5-triazine and H2-R-opa=phthalic acid with different functional groups), in which each phthalate and tpt ligand bridges two and three Ni(II) ions, respectively, to form the porous framework. The increase in the number of fluorine atoms from 3-fluorophthalic acid, 3,6-difluorophthalic acid to 3,4,5,6-tetrafluorophthalic acid (Figure 3b–d) resulted in the positive effect of fluorine on H2 and CO2 adsorption capacity. Snurr and colleagues12 succeeded in the enhancement of CO2/N2 and CO2/CH4 selectivities in the three-dimensional MOF, [Zn2(L1)] (H4L1=4,4′,4′′,4′′′-benzene-1,2,4,5-tetrayltetrabenzoic acid), using the fluorinated 4-(trifluoromethyl)pyridine terminal ligand (Figure 3e). The as-synthesized sample had paddlewheel Zn2 dimers and the N,N-dimethylformamide (DMF) molecules were coordinated to their axial sites. Using the post-synthetic modification approach, the coordinated DMF could be successfully exchanged with the bigger 4-(trifluoromethyl)pyridine. Although this exchange led to a lower specific surface area (800 vs 390 m2 g−1), the CO2/N2 and CO2/CH4 selectivities increased, especially remarkably higher CO2/N2 selectivity at low pressure (22 vs 42 (ideal adsorbed solution theory (IAST) selectivity for equimolar binary mixture)), at the same time. The authors proposed that this enhancement in selectivity can be attributed to (1) the attractive interaction between the polar CF3 group and CO2 and (2) the formation of smaller pores by the introduction of more bulky 4-(trifluoromethyl)pyridine ligands.

Figure 3
figure 3

Molecular structures of organic ligands with fluorine or trifluoromethyl substituents, (a) tetrafluoroterephthalic acid, (b) 3-fluorophthalic acid, (c) 3,6-difluorophthalic acid, (d) 3,4,5,6-tetrafluorophthalic acid, (e) 4-(trifluoromethyl)pyridine, (f) 4,4′-(hexafluoroisopropylidene)bis(benzoic acid), (g) 4,4′-{[3,5-bis(trifluoromethyl)phenyl]azanediyl}dibenzoic acid, (h) 3,5-bis(trifluoromethyl)-1,2,4-triazole, (i) trifluoroacetic acid, (j) 4-(trifluoromethyl)benzoic acid and (k) pentafluorobenzoic acid.

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Figure 4
figure 4

(a) Crystal structure of {[Zn5(triazole)6(tetrafluoroterephthalate)2(H2O)2]•4H2O} viewed down the b axis. Guest H2O molecules are omitted to show one-dimensional pores down the b axis. (b) H2 adsorption isotherms at 77 K (black) and 87 K (red), and Qst plot. Reproduced from Hulvey et al.9 (c) Depiction of an adsorbed H2 molecule (orange) on the most favorable adsorption site as determined from simulation. Carbon atoms are depicted in cyan, hydrogen in white, nitrogen in blue, oxygen in red, fluorine in pink and zinc in silver. Reproduced from Forrest et al.10

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The hydrophobic character of the fluorinated pore surface provides a preferable adsorption and a high capacity of hydrophobic guests. Monge et al.13 reported the three-dimensional MOF [Zn(L2)] (H2L2 is shown in Figure 3f), in which the helical Zn(II)-carboxylate chains are connected by the L2 ligands to form two kinds of very different parallel channels. One of the channels had walls formed by CF3 substituents of ligands. The as-synthesized sample included guest H2O molecules in the other hydrophilic channels but the desolvated form selectively took in hydrophobic heptane guests within its CF3-decorated channels. Ghosh and colleagues14 found that the desolvated form of the two-dimensional MOF, {[Cu4(L3)4(DMF)4]•3DMF} (H2L3 is shown in Figure 3g), exhibited excellent water-repellent and oil/water separation properties derived from CF3 substituents on the L3 ligand. The fluorinated MOF, [Ag6(tz)6] (tz=3,5-bis(trifluoromethyl)-1,2,4-triazolate, Figure 3h), reported by Omary and colleagues,49 showed the three-dimensional porous framework consisting of tetranuclear [Ag4(tz)6] clusters connected by three-coordinate Ag(I) centers and had both large semi-rectangular channels (~12.2 and 7.3 Å) and small diamond-shaped cavities (~6.6 and 4.9 Å) coated with CF3 groups of the fluorinated tz ligands (Figure 5a). Because of its CF3-coated channels and cavities, this MOF adsorbed a negligible amount of water even under almost 100% relative humidity condition and retained its original porous structure after soaking in water for several days.15 In contrast, a high adsorption amount of C6–C8 hydrocarbons such as benzene, toluene, p-xylene, cyclohexane and n-hexane, the most common oil components, was observed (Figure 5b). Such hydrophobic character is effective in the field of oil-spill clean-up. A hydrophobic space derived from fluorine atoms was also suitable for investigating properties of water clusters themselves due to the negligible interaction of water molecules with the pore walls. Omary and colleagues16 reported water cluster confinement in the hydrophobic cavities using the same fluorinated MOF, [Ag6(tz)6]. From Raman and IR spectroscopy and theoretical calculations, it was suggested that a small number of pentamer water clusters were formed in the large pores and the binding energy between the water clusters and the CF3-decorated walls was weak.

Figure 5
figure 5

(a) Crystal structure of [Ag6(tz)6]. In the right figure, the small cavities are denoted by black circles that surround the large channels. (b) Adsorption/desorption isotherms for benzene, toluene, p-xylene, cyclohexane and n-hexane. Closed and open symbols indicate adsorption and desorption, respectively. Reproduced from Yang et al.15, 49

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A hydrophobic nature of fluorine-containing materials contributes to a high stability to water. Senkovska and colleagues17 succeeded in the incorporation of the hydrophobic fluorinated monocarboxylate ligands such as trifluoroacetate, 4-(trifluoromethyl)benzoate and pentafluorobenzoate (Figure 3i–k, respectively) to the three-dimensional Zr MOF of [Zr6O6(OH)2(tdc)4(HCOO)2] (DUT-67-Fa, H2tdc=2,5-thiophenedicarboxylic acid) using the solvent-assisted ligand incorporation technique.45 The exchange of HCOO− for these fluorinated monocarboxylates was performed by immersing the parent DUT-67-Fa in a DMF solution of the respective carboxylic acid. The tolerance of the obtained carboxylate-exchanged materials toward the removal of adsorbed water could be significantly enhanced compared with the parent DUT-67-Fa MOF.

MOFs functionalized with inorganic fluorinated anions

Inorganic fluorinated anions as illustrated in Figure 6 are also good building blocks for the construction of fluorine-functionalized MOFs/PCPs. AF62− anions (A=Si, Ge, Sn and Ti) can be used as bridging ligands for MOFs due to their high density of negative charge on fluorine atoms.18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 29, 50 There are many AF62−-bridged MOFs, [M(AF6)(L)2], in which M is Fe(II), Co(II), Ni(II), Cu(II) or Zn(II) cations, and L is pyrazine (pyz) or bipyridine-type neutral ligands, and their prototypes are [M(SiF6)(4,4′-bpy)2] (M=Zn(II) or Cu(II), 4,4′-bpy=4,4′-bipyridine), which form the three-dimensional porous primitive cubic structure constructed from two-dimensional [M(4,4′-bpy)2]n sheets and inorganic SiF62− pillars.18, 19 The Cu(II) compound, [Cu(SiF6)(4,4′-bpy)2] (SIFSIX-1-Cu), was shown to have permanent pores and exhibit a high uptake of CH4 (6.5 mmol g−1 at 298 K and 36 atm) and selective CO2 uptake over CH4 (10.5 (IAST CO2/CH4 (50:50) selectivity) at 298 K and 1 atm), and selective C2H2 uptake over C2H4 (8.37 (IAST C2H2/C2H4 (50:50) selectivity) at 298 K and 1 atm).19, 20, 21 In general, this type of MOF with Co(II), Ni(II) and Zn(II) is obtained in non-aqueous condition because H2O is preferentially coordinated to these metal centers instead of the weak Lewis base SiF62− (pKa=1.92).21 In contrast, the Cu types can be prepared even in the presence of H2O. Cu(II) complexes have longer axial coordination bonds than equatorial bonds because of a Jahn–Teller effect and, therefore, there is a relatively large contribution of electrostatic interaction to the axial coordination bonds, which enables the preferential coordination of a SiF62− dianion to the Cu(II) axial site compared with a neutral H2O molecule. Use of the Cu(II) axial sites permitted even a stable bridge of metal centers by an inorganic fluorinated PF6− monoanion. We succeeded in the synthesis of the three-dimensional MOF, {[Cu2(PF6)(NO3)(4,4′-bpy)4]•2PF6}, in which the PF6− and NO3− anions alternately bridged the Cu(II) axial sites of adjacent two-dimensional layers with Cu–F and Cu–O distances of 2.676(4) and 2.320(5) Å, respectively (Figure 7a).25 In fact, this MOF was stable after the removal of guest molecules and the desolvated form showed a type I adsorption isotherm for N2 at 77 K with a Brunauer–Emmett–Teller specific surface area of 559 m2 g−1, suggesting the presence of stable micropores (Figure 7b). The SiF62− pillars had an important role in the formation of favorable MOF-sorbate interactions via non-coordinated F atoms, which was confirmed using modeling studies, and X-ray and neutron diffraction analysis.21, 23, 26 The exceptional CO2 separation ability of the three-dimensional MOF, [Zn(SiF6)(pyz)2] (SIFSIX-3-Zn, pyz=pyrazine), was reported by Nugent et al.,23 in which the modeling studies revealed close interactions between electropositive carbon atoms of CO2 molecules and four negatively charged fluorine atoms of SiF62− pillars. Xing and colleagues21 crystallographically characterized the C2D2-adsorbed structure of SIFSIX-1-Cu using the powder neutron diffraction technique. C–D•••F hydrogen bonds (2.063 Å) were observed between C2D2 and SiF62– dianions, and the other D interacted with a neighboring C2D2 molecule with C–D•••C distances of 3.063 and 3.128 Å, which synergistically contributed to high selective C2H2 separation from a C2H2/C2H4 mixture (Figure 8). Space and colleagues28 recently found that the SiF62− anion connected other type of cationic two-dimensional sheets, [Cu3(4-(pyridin-4-yl)acrylate)], to form the neutral three-dimensional [Cu3(4-(pyridin-4-yl)acrylate)(SiF6)] (fsc-2-SIFSIX) with both coordinatively unsaturated Cu(II) and SiF6 fluorine sites for a CO2 gas trapping. Computational studies revealed that a primary CO2 adsorption site was the two Cu(II) cations of adjacent paddle-wheel [Cu2(CO2R)4] moieties and a secondary adsorption site was the two equatorial SiF62− fluorine atoms.

Figure 6
figure 6

Inorganic fluorinated building units.

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Figure 7
figure 7

(a) Porous structure with bridged PF6– anions (the non-coordinated PF6– anions are omitted for clarity in the central figure) and (b) N2 adsorption isotherms at 77 K in {[Cu2(PF6)(NO3)(4,4′-bpy)4]•2PF6}. Reproduced from Noro et al.25

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Figure 8
figure 8

(a) Crystal structure and (b) experimental column breakthrough curves for C2H2/C2H4 (50:50) separation at 298 K and 1 atm in [Cu(SiF6)(4,4′-bpy)2] (SIFSIX-1-Cu). SIFSIX-2-Cu-i is [Cu(SiF6)(4,4′-dipyridylacetylene)2] (i=interpenetrated). Reproduced from Cui et al.21

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In contrast to the organic fluorinated ligands, it is very difficult to modify inorganic fluorinated ligands at will. However, Eddaoudi and colleagues29, 30 succeeded in the preparation of a fine-tuned fluorinated MOF by modification of the inorganic fluorinated anions. They used a NbOF52− dianion instead of SiF62− to afford the three-dimensional MOF, [Ni(NbOF5)(pyz)2]. Because of its longer Nb–F bond length (1.899(1) Å) than the Si–F bond (1.681(1) Å) and greater Lewis basicity, this MOF provided an appropriate pore space for capturing trace CO2, separating propylene form propane and enhanced water stability.

Inorganic fluorinated monoanions such as PF6−, BF4−, CF3SO3− and (CF3SO2)2N− have been used as counter anions of ionic liquids.51 As the negative charge of these anions is delocalized by fluorine atoms, electrostatic interaction between these anions and cations is weaker than that between non-fluorinated anions and cations, which causes low melting points. The same situation, that is, weaker intermolecular interactions, should be observed between fluorinated anions and neutral molecules. In addition, the poor metal-bridging ability of these monoanions contributes to the lowering of a framework dimensionality from three dimensions to two and one dimension. Therefore, the introduction of inorganic fluorinated monoanions to MOFs/PCPs may afford flexible materials showing gate-sorption/breathing behaviors. The two-dimensional MOF [Cu(BF4)2(4,4′-bpy)2] (ELM-11; ELM, elastic-layer-structured MOF) was the first flexible material with inorganic fluorinated anions.31 This Cu(II) MOF, which was prepared by dehydration of the precursor one-dimensional coordination polymer {[Cu(BF4)2(4,4′-bpy)(H2O)2]•4,4′-bpy},52 had a two-dimensional square-grid framework with weakly coordinated BF4– anions at the Cu(II) axial sites (Figure 9a) and showed gate-sorption behaviors for CO2 with interlayer expansion/shrinkage (Figure 9b).31, 53 After this finding, the derivatives [M(A)2(4,4′-bpy)2] (M=Cu, A=CF3SO3; M=Cu, A=PF6; M=Co and A=CF3SO3) were separately reported.32, 33, 34 These MOFs formed similar two-dimensional square-grid frameworks with weakly coordinated CF3SO3− and PF6− (Cu), and coordinated CF3SO3− (Co) monoanions but showed different sorption behaviors from the parent ELM-11; there were two sorption events: the first uptake was a micropore filling and the second uptake was caused by a gate-sorption process with an expansion of the interlayer distance and sliding between the layers. Using these anions, it is also possible to fabricate one-dimensional flexible MOFs exhibiting gate sorption.35, 36, 37, 38, 39, 40, 54 We reported the one-dimensional flexible MOF, [Cu(PF6)2(bpetha)2] (bpetha=1,2-bis(4-pyridyl)ethane).35, 36, 54 This MOF exhibited doubly linked one-dimensional chain structures consisting of Cu(II) ions and bent bpetha ligands with weakly coordinated PF6− monoanions at its axial sites. CO2 and C2H2 gases were selectively adsorbed to this MOF with structural changes.35 The coordination state of the PF6− monoanions was retained during the change in structures and the adsorbed CO2 gas may interact with the fluorine atoms of weakly coordinated PF6− monoanions. This MOF also showed selective uptake of a larger 2-butanone guest from 2-butanone/EtOH and 2-butanone/MeOH mixtures.36 The 2-butanone guest with an sp2 coordinated oxygen atom could coordinate to the Cu(II) axial sites instead of the PF6− monoanion, whereas the EtOH and MeOH guests with a sterically crowded sp3 coordinated oxygen atom were hard to coordinate to the sterically crowded Cu(II) axial sites that are formed by the coordination of four bpetha pyridine moieties from the equatorial direction, which was the origin of the high selectivity for the larger 2-butanone guest.

Figure 9
figure 9

(a) Two-dimensional structure and (b) change in the sample volume on increasing CO2 pressure (from the left, 0, 6.66, 13.3, 26.7, 34.7, 45.3 and 101 kPa) at 273 K in [Cu(BF4)2(4,4′-bpy)2]. Reproduced from Kondo et al.31

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Ionic liquids with inorganic fluorinated monoanions have been found to adsorb CO2 gas selectively over other gases through F•••CO2 interactions. In some MOFs with inorganic fluorinated monoanions, selective CO2 adsorption and separation were observed.33, 37, 39 We reported selective CO2 adsorption over CH4 in the one-dimensional MOF, [Cu(PF6)2(bpp)2] (bpp=1,3-bis(4-pyridyl)propane).39 The bent bpp ligands and the Cu(II) ions formed a doubly linked chain with weakly coordinated PF6− anions at the axial sites (Figure 10a). At 298 K, this MOF adsorbed CO2 gas but CH4 and H2O were hardly adsorbed (Figure 10b), suggesting an availability for separation of the CO2/CH4 mixture under dry and even humid conditions. In fact, it was experimentally proven that this MOF showed high equilibrium and kinetic separation for CO2 over CH4 under realistic conditions, using a mixed gas at room temperature and in a humid environment (Figure 10b and c). The moderate heat of CO2 adsorption (Qst=–18 to −31 kJ mol−1), which was experimentally determined, and the calculated interaction energy using a model structure (Figure 10d)32 suggested that the fluorine atoms of the weakly coordinated PF6− anions contribute to the interaction sites with adsorbed CO2 molecules. To elucidate the interactions between inorganic fluorinated monoanions and guest molecules in detail, it is important to determine the crystal structures with adsorbed guest molecules. Recently, the CO2-adsorbed structures of ELM-11 and one-dimensional MOF [Cu(BF4)2(bpp)2]38 showing CO2 gate sorption were successfully determined from synchrotron powder X-ray diffraction data.55, 56 In the CO2-adsorbed ELM-11 (ELM-11⊃2CO2), one CO2 interacted with two neighboring BF4− anions with F•••C distances of 2.918 and 2.932 Å,55 whereas the CO2-adsorbed one-dimensional MOF, {[Cu(BF4)2(bpp)2]•0.7CO2}, formed F•••CO2 interactions (F•••C=2.61(6) Å, see Figure 11a).56 If single crystals of MOFs with inorganic fluorinated monoanions have permanent micropores and retain their single crystallinity after removal of guest molecules, they could be available for determining guest-adsorbed structures and discussing interactions between inorganic fluorinated monoanions and guest molecules by single-crystal X-ray diffraction measurements. However, such permanent single crystals are difficult to synthesize, because the inorganic fluorinated monoanions often give flexible MOFs. We succeeded in the preparation of a stable single crystal of the two-dimensional MOF, {[Cu(CF3SO3)(bpp)2]•PF6}, using an anion-mixing method.41 Two coexistent types of inorganic fluorinated anions, CF3SO3− and PF6−, with different Lewis basicities, enabled the formation of a higher dimensional, in this case two-dimensional, framework compared with the corresponding one-dimensional MOFs with only one type of anion, [Cu(A)2(bpp)2] (A=CF3SO3 and PF6). In the acetone-including crystal {[Cu(CF3SO3)(bpp)2]•PF6•acetone}, the guest acetone molecules located in micropores formed weak hydrogen-bonding interactions with the bridged CF3SO3− (C•••O=3.34(2) Å) and non-coordinated PF6– anions (C•••F=3.11(2) Å), as shown in Figure 11b. After the removal of acetone guests, the single crystallinity was unchanged and we could determine the crystal structure of the completely desolvated form {[Cu(CF3SO3)(bpp)2]•PF6}. As this MOF adsorbed CO2, a direct visualization of the interaction between fluorinated monoanions and adsorbed CO2 may be possible using a single-crystal X-ray diffraction technique.

Figure 10
figure 10

Structure and gas separation properties of [Cu(PF6)2(bpp)2]. (a) Schematic view of the structure. (b) Single-gas and mixed-gas equilibrium adsorption properties at 298 K under dry and humid conditions. The single-gas adsorption isotherms for CO2 and CH4 are shown in filled red circles and blue squares, respectively. The open black circles, squares and triangles indicate the CO2, CH4 and total adsorption amounts, respectively, for the mixed gas of CO2:CH4=40:60 (mol). The filled black symbols correspond to their adsorption amounts for the mixed gas of CO2:CH4:H2O=39.98:59.96:0.06 (mol), in which each adsorption amount is calculated according to the hypothesis that water is hardly adsorbed at all. The mixed-gas adsorption measurements were repeated five times. (c) Breakthrough curves of CO2/CH4 mixture (measurement condition: CO2:CH4=40:60 (mol), the measurement temperature=298 K, the total pressure=800 kPa and the space velocity=3 min–1). The red circles represent CO2 and the blue squares represent CH4. (d) Optimized structure of [Cu(PF6)2(pyridine)4] and CO2. Reproduced from Noro et al.33, 39

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Figure 11
figure 11

(a) CO2-adsorbed structure in {[Cu(BF4)2(bpp)2]•2CO2} (ELM-11⊃2CO2). Reproduced from Tanaka et al.55 (b) Crystal structure and view of the interaction between the acetone guest and inorganic fluorinated CF3SO3– and PF6– anions in {[Cu(CF3SO3)(bpp)2]•PF6•acetone}.

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Ionic liquids including inorganic fluorinated monoanions are good candidates as safe electrolytes in electrochemical devices because of their flame resistance, extremely low volatility, high thermal and electrochemical stability, and high ionic conductivity. However, their ionic conductivity dramatically decreases below their freezing points. Kitagawa and colleagues42 reported a low-temperature ionic conductor obtained by the incorporation of the ionic liquid, EMI-TSFA (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide) within the three-dimensional MOF, [Zn(MeIM)2] (ZIF-8, HMeIM=2-methylimidazole). The ionic conductivity of EMI-TSFA@ZIF-8 was higher than that of bulk EMI-TFSA below 250 K because there is no freezing transition of the nanosized ionic liquid formed in the restricted pore space.

Perfluoroalkane-functionalized MOFs

As is the case with previously introduced fluorine-containing MOF building units, perfluoroalkanes can provide polar adsorption sites. Furthermore, the assembly of these substituents is useful for the construction of a specific fluorophilic field and preferential adsorption field for O2 and CO2. There are two methods to adopt perfluoroalkyl substituents into MOFs; one is to use bridging ligands with their substituents and the other is the use of terminal ligands bearing them. The two-dimensional MOF, {[Co(NCS)2(L4)2]•x(guest)} (guest is ethylene glycol or C6F14 and L4 is 1,3-bis(4-pyridylethynyl)-2-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxy)benzene shown in Figure 12a), with the pendant perfluoroalkyl C6F13 chains were synthesized by Fujita and colleagues.43 The stacking of the distorted two-dimensional grid frameworks created pores occupied by the C6F13 chains and the guest molecules. When ethylene glycol guests (boiling point: 471 K) were included, they started to be removed from the pores around 313 K, similar to neat ethylene glycol. On the other hand, the clathrated perfluorohexane (boiling point: 329 K) began to evaporate at around 268 K, which was much higher than neat perfluorohexane (~223 K), indicating high fluorophilicity and organophobicity. Matsuda and colleagues reported the densely perfluorobutyl-functionalized two-dimensional MOF, {[Cu(bpbtp)(L5)(DMF)]•DMF} (H2bpbtp=2,5-bis(perfluorobutyl)terephthalic acid, L5=2,5-bis(perfluorobutyl)-1,4-bis(4-pyridyl)benzene, Figure 12b and c), in which its pore surface was covered with perfluorobutyl substituents. The installation of perfluorobutyl groups to both neutral and anionic bridging ligands enabled the formation of densely fluorinated pores (Figure 13a). The desolvated form of this MOF was found to show preferential adsorption for CO2 and O2 over N2, Ar and CO with hysteresis, which may be caused by the high dissolving ability of perfluoroalkanes for CO2 and O2. Using the solvent-assisted ligand incorporation technique, Snurr and colleagues45 succeeded in the incorporation of terminal perfluoroalkane carboxylate (Figure 12d) within the three-dimensional Zr-based MOF, [Zr6(μ3-OH)8(OH)8(TBAPy)2] (NU-1000, H4TBAPy=1,3,6,8-tetrakis(p-benzoic acid)pyrene), as shown in Figure 13b. The octahedral Zr6 cluster has eight terminal –OH ligands, which can be exchanged with a variety of monocarboxylate ligands, including perfluoroalkane carboxylate, without destruction of the framework. Detailed sorption measurements and theoretical modeling confirmed that (1) the NU-1000 functionalized with a perfluoroalkane carboxylate exhibited a systematically higher value for the heat of adsorption than the parent NU-1000 with increasing the length of the perfluoroalkane chain and (2) the Zr6 cluster and perfluoroalkane synergistically acted as the primary CO2 adsorption sites. The C–F dipole in the perfluoroalkane carboxylate contributed to the favorable C–F•••CO2 interaction.

Figure 12
figure 12

Structures of organic bridging and terminal ligands with perfluoroalkane substituents, (a) 1,3-bis(4-pyridylethynyl)-2-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxy)benzene, (b) 2,5-bis(perfluorobutyl)terephthalic acid, (c) 2,5-bis(perfluorobutyl)-1,4-bis(4-pyridyl)benzene and (d) perfluoroalkanecarboxylic acid.

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Figure 13
figure 13

(a) Crystal structure of {[Cu(bpbtp)(L5)(DMF)]•DMF}. Perfluorobutyl groups are coloured blue. Reproduced from Jeon et al.44 (b) Crystal structure and solvent-assisted ligand incorporation in [Zr6(μ3-OH)8(OH)8(TBAPy)2] (NU-1000). Reproduced from Deria et al.45

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Postsynthetic trifluoromethyl modification of MOF using a plasma-enhanced chemical vapor deposition

Decoste et al.57 reported a unique technique to modify an internal pore surface of MOF with trifluoromethyl groups. The postsynthetic treatment of the famous three-dimensional hydrophilic MOF, [Cu3(btc)2] (HKUST-1, H3btc=1,3,5-benzenetricarboxylic acid),58 with a plasma-enhanced chemical vapor deposition of perfluorohexane yielded a hydrophobic form of HKUST-1 (herein referred to as ‘HKUST-1 plasma’). The HKUST-1 plasma got CF3 groups on the surface of the pores with maintenance of the overall crystal structure of HKUST-1 and the presence of the CF3 groups had an important role in perfluorohexane loading. Thus, the HKUST-1 plasma with pefluorohexane guests showed an enhanced stability to ammonia and water and an enhanced ammonia adsorption capacity compared with untreated HKUST-1.

Conclusion and outlook

In this review article, we organized the structures and porous properties of MOFs/PCPs decorated with fluorinated building blocks such as fluorine- or trifluoromethyl-functionalized organic bridging ligands, inorganic fluorinated anions and perfluoroalkane-functionalized organic ligands. The manipulation of porous structures using fluorine-containing building blocks leads to fascinating porous properties such as high hydrophobicity, preferable adsorption for targeted gases, flexible pores and specific perfluoroarene–arene interaction. In addition, we expect that fluorine functionalization has great potential to give other novel and beneficial properties to MOFs/PCPs.

The existence of high-energy C–H and/or O–H oscillators in luminescent coordination compounds causes a decrease in emission intensity. Utilization of fluorinated organic ligands with low-energy C–F bonds is effective for reducing/eliminating the non-emissive process associated with vibrational relaxation. Chen et al.59 compared the luminescent properties of two three-dimensional MOFs, {[Er2(1,4-bdc)3(DMF)2(H2O)]•H2O} and {[Er2(tetrafluoroterephthalate)3(DMF)(H2O)]•DMF}. The partially desolvated fluorinated MOF {[Er2(tetrafluoroterephthalate)3(DMF)]•DMF} showed higher Er(III)-based emission intensity than the desolvated non-fluorinated MOF [Er2(1,4-bdc)3]. The hfa (hexafluoroacetylacetonate) chelating ligand is also a good building unit for the construction of MOFs exhibiting strong emissions. Hasegawa and colleagues60 reported the one-dimensional Eu(III) coordination polymers, [Eu(hfa)3(L)] (L=bidentate phosphane oxide ligands). In particular, a high-emission quantum yield (ΦLn=83% in the solid state) was observed in [Eu(hfa)3(dppcz)] (dppcz=3,6-bis(diphenylphosphoryl)-9-phenylcarbazole). Although the luminescent properties given in this paragraph are not related to porous properties, it is expected that this strategy to use fluorinated ligands is available for the construction of luminescent MOFs showing a significant guest-responsive change in emission properties in future sensing devices.

Fine control of framework flexibility is one of the challenging issues in MOF/PCP chemistry. A solid-solution approach based on organic bridging ligands such as substituted dicarboxylates has been applied to tune the flexibility.61, 62 Inorganic fluorinated monoanions have the potential to endow MOFs/PCPs with flexibility and the degree of flexibility is dependent on the kind of these monoanions. For example, we found inorganic monoanion-dependent acetone adsorption properties of porous assemblies of coordination complexes, [Cu(A)2(pyridine)4] (A=PF6–, BF4–, CF3SO3– and CH3SO3–).63 The desolvated forms had no pores and showed no N2 and CO2 adsorption at all. However, the adsorption isotherms for acetone at 283 K clearly indicated that the porous assemblies of coordination complexes with PF6 or BF4 monoanions took in acetone guests with structural changes, whereas the porous assemblies of coordination complexes with CF3SO3 or CH3SO3 monoanions exhibited no response to acetone. Furthermore, these monoanions are often coordinated to the metal centers in a monodentate manner, which is effective in an unalterable framework topology during complete or partial anion displacement. We believe that these characteristics in inorganic fluorinated monoanions contribute to the controllable flexibility in MOFs/PCPs by mixing more than two kinds of monoanions.

Charge- and electron-transfer MOFs/PCPs composed of electron donor and acceptor units may show fascinating electronic properties (magnetic, conductive and ferroelectric properties) coupled with porous properties.64 Modification of organic ligands by fluorine atom with the highest electronegativity and small van der Waals radius enables to drastically change the degree of charge/electron transfer with less effect in steric hindrance, resulting in a fine tuning of electronic structures and properties. A series of electronically active carboxylate-bridged paddle-wheel Ru dimers can be utilized as an electron-donor building unit in MOFs/PCPs frameworks with electron-acceptor building units.

In addition, it is important to provide new synthetic methods to obtain fluorine-functionalized MOFs/PCPs. Partial incorporation of fluorine-containing molecules into pores of MOFs/PCPs may be a new approach to obtain fluorine-dominated porous properties. Uemura and colleagues65 hint at the possibility of this synthetic approach. Oligo(vinylidene fluoride) was confined in 1 × 1 nm2 pores of the three-dimensional MOF, [Tb(1,3,5-benzenetrisbenzoate)],66 without any fluorine substituents to elucidate the dynamics of oligo(vinylidene fluoride) in restricted space. Although the parent MOF showed a typical type I N2 isotherm with the Brunauer–Emmett–Teller specific surface area of 730–930 m2 g–1, the obtained composite adsorbed negligible amount of N2 gas, indicating no accessible pores after the incorporation of oligo(vinylidene fluoride). However, we expect that if pores can be filled with an appropriate amount of fluorine-containing molecules, the composite may retain sufficient accessibility for other guests with a fluorine-modified pore surface. Partial incorporation of ionic liquids with inorganic fluorinated anions into MOFs/PCPs is also effective for creating a fluorine-modified pore surface.42

Finally, we anticipate that this review will attract much attention among not only MOF/PCP scientists but also many researchers involved in other scientific fields and provide opportunities to investigate new and/or reported fluorine-functionalized MOFs/PCPs towards future applications.