Stepwise Expansion of Layered Metal–Organic Frameworks for

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Stepwise Expansion of Layered Metal−Organic Frameworks for Nonstochastic Exfoliation into Porous Nanosheets Vonika Ka-Man Au,†,§,∥ Kazuki Nakayashiki,†,∥ Hubiao Huang,† Shun Suginome,† Hiroshi Sato,*,† and Takuzo Aida*,†,‡ †

Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

J. Am. Chem. Soc. 2019.141:53-57. Downloaded from pubs.acs.org by DUKE UNIV on 01/10/19. For personal use only.

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ABSTRACT: A layered metal−organic framework (MOF) with a porous kagomé lattice, kgmSMe, was synthesized by complexation of 5-methylthioisophthalate (SMe-ip) with Cu2+ in MeOH. As observed by powder XRD, kgmSMe (state I), when immersed in aprotic polar solvents such as THF, underwent stepwise interlayer expansion into a monolayer-expanded state (state III) through a bilayer-expanded state (state II). We successfully obtained the single-crystal structures of states I−III. Of interest, when further immersed in appropriate solvents, state II and III crystals preferentially exfoliated into bilayer and monolayer MOF nanosheets, respectively. The stepwise expansion followed by exfoliation, thus developed, may enable a nonstochastic approach to the selective synthesis of ultrathin porous nanosheets from layered MOF crystals.

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wo-dimensional (2D) materials1−4 with periodically arranged in-plane nanopores with uniform diameters have the potential for highly selective separation membranes. Postsynthetic approaches by generating nanopores in 2D materials is one of the promising synthetic methods.5 However, ordered pores are difficult to generate. Meanwhile, layered porous MOFs,6−14 if properly exfoliated, can provide 2D nanosheets with periodically arranged in-plane nanopores. As a pioneering example, the group of Zamora employed ultrasonication for exfoliating layered porous MOFs to obtain inorganic nanosheets.6 Related examples have later been reported, but the exfoliation process is stochastic and does not ensure the layer-number-selective synthesis of nanosheets.6−14 MOF nanosheets have also been prepared by bottom-up methods including interfacial synthesis with and without surfactants or layer-by-layer synthesis on certain substrates.15,16 However, these bottom-up methods again do not guarantee the layer-number-selective synthesis of MOF nanosheets.16 Here we report the first nonstochastic approach toward the selective exfoliation of layered porous MOFs into 2D nanosheets. In the present work, we focused attention on kgmSMe, a layered Cu(II)-MOF with a kagomé lattice (Figure 1),17 which showed an interesting phenomenon that its single crystal, when immersed in N,N-dimethylformamide (DMF) (Figure 1f, Movies S1 and S2) or tetrahydrofuran (THF) © 2018 American Chemical Society

Figure 1. (a) Synthesis of kgmSMe and a microscopic image of its green hexagonal crystals. Single-crystal structures of (b) a Cu(II)paddlewheel unit, (c) a kagomé lattice, (d) a porous monolayer, and (e) a layered MOF of kgmSMe. Hydrogen atoms are omitted for clarity. (f) Snapshots of kgmSMe upon expansion in DMF.

(Movie S3), expands rapidly just like an accordion. A detailed study showed that kgmSMe unprecedentedly undergoes a stepwise and reversible interlayer expansion (Figures 2, 3, and S9). The expansion of kgmSMe (state I) occurs up to a monolayer-expanded state (state III) through a bilayerexpanded state (state II). Fortunately, we succeeded in obtaining the crystal structures of states I−III (Figure 2). Received: September 14, 2018 Published: December 23, 2018 53

DOI: 10.1021/jacs.8b09987 J. Am. Chem. Soc. 2019, 141, 53−57

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Figure 2. Single-crystal X-ray structures of as-synthesized (state I), bilayer-expanded (state II), and monolayer-expanded (state III) kgmSMe crystals in the stepwise and reversible expansion. Hydrogen atoms are omitted for clarity. The crystal structures were optimized using the SQUEEZE command. Depending on the expansion profiles of kgmSMe, tested solvents are categorized into groups 1−3. Stepwise expansion from state I to state III via state II was confirmed by powder X-ray diffraction (XRD) (Figures 3 and S9) when asterisked solvents in group 3 were utilized.

We also found that expanded kgmSMe crystals in states II and III, when gently stirred for 24 h in appropriate solvents, preferentially exfoliate into bilayer and monolayer nanosheets, respectively (Figure 4). In contrast with kgmSMe, kgmOMe, an analogous kagomé MOF having −OMe groups in place of the −SMe groups, expanded only in DMF and N,N-diethylformamide (DEF) (Figures S10−S12) and was successively exfoliated in DEF into monolayer nanosheets, preferentially (Figure S17). Layered Cu(II)-MOFs, kgmSMe and kgmOMe, were synthesized by the reaction of 5-methylthio- (SMe-H2ip) and 5methoxyisophthalic acid (OMe-H2ip) with Cu(NO3)2·3H2O, respectively, in MeOH at 20 °C (Figure 1a).18 The singlecrystal structure of kgmSMe showed that Cu(II)-paddlewheel units (Figure 1b) are mutually connected by isophthalate units, affording an infinite 2D porous kagomé lattice (Figure 1c and d). This 2D lattice stacks normally on top of each other, affording a layered MOF having triangle and hexagonal nanopores (Figure 1e). The crystal structure of kgmOMe is essentially identical to that of kgmSMe, where the interlayer distances are 6.8 and 6.7 Å for kgmSMe and kgmOMe, respectively (Figures 2 and S1). The layer structure is supported by the O···H hydrogen-bonding interactions (3.23 and 3.18 Å for kgmSMe and kgmOMe, respectively) between the O atom of a water molecule coordinated to the axial position of Cu2+ in a paddlewheel unit and the H atom of an isophthalate unit located in an adjacent layer. Although assynthesized kgmSMe and kgmOMe, as observed by thermogravimetry upon heating, lost guest solvent molecules (MeOH and water) at around 80 °C, they were stable thereafter up to ca. 310 °C (Figure S2). The crystalline sample of kgmSMe exhibited a green color, while that of kgmOMe showed a blue color. In the solid state at

Figure 3. Powder XRD patterns of kgmSMe upon exposure to THF vapor and simulated patterns of as-synthesized (state I), bilayerexpanded (state II), and monolayer-expanded (state III) kgmSMe crystals. The black-, red-, and blue-colored broken lines indicate the positions of representative diffractions indexed to state I, state II, and state III, respectively.

20 °C, kgmSMe displayed two absorption bands at 344 and 714 nm, while kgmOMe showed its absorption bands at 309 and 711 nm (Figure S3). In each case, the higher-energy band is assignable to the intraligand π−π* absorption, while the lowerenergy absorption band is due to a metal-centered electronic transition characteristic of Cu(II) complexes.19 The wavenumber difference of 3290 cm−1 between the higher-energy bands of kgmSMe and kgmOMe can be accounted for by the fact that −SMe is more electron-donating than −OMe (Figure S4). When exposed to THF vapor, kgmSMe gradually changed its color from green to blue. When a single crystal of assynthesized kgmSMe was immersed in THF at 20 °C, kgmSMe (state I) quickly expanded to afford a bilayer-expanded state (state II) in 5 min. Upon continuous immersion, the state II crystal further expanded to furnish a monolayer-expanded state (state III) in 1 day (Figure 2). Of interest, for state II, a bilayer with an interlayer distance of 7.0 Å is a principal constituent, which stacks on top of each other with a separation of 10.0 Å. The interlayer O···H bond that supports the layered structure is 3.31 Å long (Figure S13b), which is 0.08 Å longer than that before the expansion (state I; 3.23 Å). Further to note, the 54

DOI: 10.1021/jacs.8b09987 J. Am. Chem. Soc. 2019, 141, 53−57

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observed for kgmSMe occur reversibly. We also confirmed a similar stepwise and reversible interlayer expansion of kgmSMe in dioxane by powder XRD and single-crystal X-ray analysis (Figure S9 and Table S4). We investigated the effects of solvents for the interlayer expansion of kgmSMe by powder XRD and found that they can be categorized into three different groups depending on the expansion profiles (Figure 2); the solvents in group 1 are inert for the expansion (Figure S6), those in group 2 preferentially afford a bilayer-expanded state (state II) (Figure S7), while those in group 3 induce full expansion to afford a monolayerexpanded state (state III) (Figure S8). Although there are a variety of different parameters that potentially affect the layer expansion of kgmSMe, we examined four parameters of individual solvents, surface tension, dipole moment, relative permittivity, and Hansen solubility parameters, where the observed interlayer expansion profiles of kgmSMe were better correlated with surface tension than the other three parameters (Table S7). The best solvents for obtaining the monolayerexpanded state (state III) include THF and dioxane (Figure S8), while those for the bilayer-expanded state include 2methyltetrahydrofuran (MeTHF), a mixture of THF and MeOH (3:1 v/v), and cyclopentanone (Figure S7). We noticed that the stepwise interlayer expansion observed for layered crystalline kgmSMe leads to its preferential exfoliation into bilayer and monolayer nanosheets in MeTHF and dioxane, respectively (Figure 2). As a typical example, a MeTHF (1 mg/mL) suspension of bilayer-expanded kgmSMe (state II) was gently stirred at 20 °C for 24 h. When a supernatant of the resultant suspension was subjected to atomic force microscopy (AFM), we observed a large number of platelets (Figures 4a and S16). Their height distribution histogram20 (Figure 4c) indicated that the average thickness of 80% of the observed platelets is 2.0 ± 0.5 nm, which is close to the bilayer thickness of 2.2 nm estimated from state II (Figure 4e). Next, we likewise stirred a dioxane (1 mg/mL) suspension of monolayer-expanded kgmSMe (state III) at 20 °C for 24 h. AFM imaging of a supernatant taken from the resulting suspension again showed a large number of platelets (Figure 4b). The height distribution histogram (Figure 4d) indicated that 85% of the observed platelets are most likely monolayer nanosheets considering that their average thickness of 1.3 ± 0.3 nm is close to the monolayer thickness of 1.5 nm estimated from state III (Figure 4f). The solvent-dependent exfoliation profiles of kgmSMe described above were well supported by transmission electron microscopy (TEM; Figure S18). Furthermore, scanning electron microscopy (SEM)-energydispersive X-ray (EDX) analysis demonstrated that the exfoliated nanosheets carry O, S, and Cu as expected (Figure S19). We also investigated the behaviors of kgmOMe in a variety of solvents listed in Figure 2 under conditions analogous to those for kgmSMe. As determined by powder XRD (Figures S10− S12), kgm OMe remained structurally unchanged when immersed in all the solvents except DMF and DEF. In DEF, kgmOMe expanded to a monolayer-expanded state (state III) at 20 °C in 24 h and subsequently exfoliated into monolayer nanosheets, preferentially (Figure S17). Meanwhile, in DMF, kgmOMe expanded to give a mixture of the bilayer and monolayer-expanded states at 20 °C in 24 h. Upon further immersion under the same conditions, the formation of an illdefined exfoliated mixture resulted. Compared with kgmSMe, kgmOMe appears to be less active for the solvent-induced

Figure 4. (a, b) AFM images of the exfoliated mixtures obtained by gentle stirring of (a) MeTHF and (b) dioxane suspensions of kgmSMe at 20 °C for 24 h and (c, d) their height distribution histograms using ∼80 platelets. Insets show pictures of their supernatants representing a Tyndall effect. (e, f) Schematic representations of the side views of (e) bilayer and (f) monolayer nanosheets with their thicknesses estimated from state II and state III (Figure 2), respectively.

transition from state II to state III in THF was accompanied by the elongation of the interlayer distance from 7.0 Å to 10.8 Å. These expansion phenomena were followed by powder XRD (Figure 3): Upon exposure of kgmSMe to THF vapor at 20 °C, the diffraction peaks having c-axis-related Miller indices shifted toward a smaller 2θ region, as a result of the interlayer expansion of kgmSMe (Figure 2). The (001) peak of assynthesized kgmSMe, for example, shifted from 12.9° to 5.2° in 2θ, thereby confirming the large increment of the d-spacing from 6.8 Å (state I) to 17.0 Å (state II). This transition started within 4 h and then subsided in 12 h (Figure 3). Furthermore, when a droplet of THF was placed onto bilayer-expanded kgmSMe (state II), further expansion took place within 1 h to afford state III with a (001) diffraction at 8.2° in 2θ (interlayer distance = 10.8 Å). The crystalline transition from state II to state III, thus observed for the powder sample, was accompanied by an instantaneous color change from green to blue as a consequence of the decrease in the absorbance at 714 nm (Figure S5). Notably, when fully expanded kgmSMe (state III) was placed in air at 20 °C, its powder XRD recovered the initial pattern observed for as-synthesized kgmSMe. This interlayer contraction was supported, though preliminarily, by single-crystal structure analysis (Figure S15). Namely, the interlayer expansion and contraction thus 55

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Takeshi Kitao, and Mr. Yujiro Nagasaka from the University of Tokyo for their help in SEM-EDX measurements. We also thank Prof. Ryotaro Matsuda and Dr. Akihiro Hori from Nagoya University for their assistance in Raman spectroscopy, Ms. Liming Liu from the University of Tokyo for her help in TEM measurements, and Dr. Xiang Wang from Riken for his help in AFM height profile analysis.

expansion. In relation to this point, the interlayer O···H-bond in kgmOMe (state I; 3.18 Å; Figure S14) is a little shorter than that in kgmSMe (state I; 3.23 Å; Figure S13a). In conclusion, by using kgmSMe with a porous kagomé lattice, we successfully demonstrated the first example of stepwise and reversible interlayer expansion of a layered MOF in appropriate solvents and preferential exfoliation of the resulting expanded states into bilayer and monolayer nanosheets in a nonstochastic manner (Figure 2). Exfoliation of layered MOFs has the potential of obtaining nanosheets with periodically arranged in-plane nanopores. Intercalation behaviors of the bilayer- and monolayer-expanded states of kgmSMe are an interesting subject worthy of further investigation for the precision synthesis of hybrid 2D materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b09987. Experimental details and the synthetic procedures, crystallographic data, powder XRD patterns, TGA profiles, diffuse reflectance spectra of kgmSMe and kgmOMe , TEM and SEM micrographs of kgm SMe suspended in MeOH, THF, and dioxane, Raman spectra of kgmSMe, and diffuse reflectance spectra of SMe- and OMe-H2ip (PDF) Crystallographic data in cif format (ZIP) Movie showing expansion phenomena of kgm SMe immersed in DMF (MPG) Movie showing expansion phenomena of kgm SMe immersed in DMF (MPG) Movie showing expansion phenomena of kgm SMe exposed to saturated vapor of THF (MPG)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Vonika Ka-Man Au: 0000-0001-9202-9422 Hiroshi Sato: 0000-0002-5899-1340 Takuzo Aida: 0000-0002-0002-8017 Present Address §

Department of Science and Environmental Studies, The Education University of Hong Kong, 10 Lo Ping Road, Tai Po, New Territories, Hong Kong, People’s Republic of China. Author Contributions ∥

V. K.-M. Au and K. Nakayashiki contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science (JSPS) via a Grant-in-Aid for Scientific Research (S) (18H05260) on “Innovative Functional Materials based on Multi-Scale Interfacial Molecular Science” for T.A., and 17H05357 (Coordination Asymmetry) and 18H04501 (Soft Crystal) for H.S. V.K.-M.A. acknowledges the receipt of a JSPS Postdoctoral Fellowship for Research in Japan administered by the JSPS. We thank Prof. Takashi Uemura, Dr. 56

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DOI: 10.1021/jacs.8b09987 J. Am. Chem. Soc. 2019, 141, 53−57