Hydrogen Uptake by an Inclined Polycatenated Dynamic Metal

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Hydrogen Uptake by an Inclined Polycatenated Dynamic Metal− Organic Framework Based Material Dilip Kumar Maity, Arijit Halder, Goutam Pahari, Fazle Haque, and Debajyoti Ghoshal* Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India S Supporting Information *

framework (1′) shows appreciable H2 uptake capacity of 1.94 wt % at 77 K and 1 bar of pressure. To the best of our knowledge, this is the first example of an inclined polycatenated MOF-based material that shows appreciably high H2 uptake capacity. Dark-blue single crystals of 1 were obtained by mixing an aqueous solution of Cu(NO3)2·3H2O with a mixed-ligand aqueous methanolic solution of 1,2-bis-(4-pyridyl)ethane (bpe) and disodium 2-nitroterephthalate (Na2-2-ntp) at room temperature using a slow diffusion method. The compound crystallizes in the monoclinic Ia space group (Table S1), and the structure analysis reveals that 1 is a polycatenated 2D + 2D → 3D framework12 through the parallel/diagonal inclined polycatenation of two identical sets of parallel 44-squarelike (sql) layers.12 The hexacoordinated copper(II) center in 1 exhibits an axially elongated octahedral geometry due to Jahn− Teller distortion, confirmed by electron spin resonance spectroscopy (Figure S1). Here each copper(II) center is connected with two oxygen atoms (O1 and O3a) of two different bridging 2-ntp2− and two nitrogen atoms (N1 and N2) of two different bpe linkers in the basal plane along with two axial H2O molecules (O1W and O2W; Figures 1a and S2a and Table S2). Here, each 2-ntp2− ligand is connected in a bismonodentate fashion between two adjacent copper(II) centers that are parallelly pillared by the bpe linkers to form a single 44sql layer (Figure 1b). Finally, two identical sets of parallel 44-sql layers are polycatenated in an inclined way, followed by the interpenetration of two adjacent single layers, resulting in the formation of a 2D + 2D → 3D inclined polycatenated framework12 (Figure 1c,d). Unlike most of the nonporous polycatenated systems, the structure of 1 is found to be microporous13 with a total solvent-accessible estimated void14 of 658.0 Å3, which is 27.54% of the total crystal volume (2389.1 Å3). The framework is further stabilized by intermolecular hydrogen-bonding, π···π, and C−H···π interactions (Figure S2b and Tables S3 and S4). Structure analysis with TOPOS15 suggests that the overall framework exhibits a 4c uninodal net with the point symbol {44.62}. It also indicates that the entangled type of 1 belongs to the 2D + 2D → 3D inclined polycatenation class (orientations of group 1, [1, 1, 0], and group 2, [1, −1, 0]; the angle of groups 1 and 2 is 82.5°, inclined), and the nonequivalent six-membered shortest rings are catenated and connected in eight different ways (Table S5). Thermogravimetric analysis revealed the formation of a dehydrated sample at around 100 °C. The dehydrated sample

ABSTRACT: A 2D + 2D → 3D inclined polycatenated dynamic metal−organic framework of {[Cu(4-bpe)(2ntp)(H2O)2]·2H2O}n [1, where 2-ntp2− = 2-nitroterephthalate and 4-bpe = 1,2-bis-(4-pyridyl)ethane] has been synthesized and characterized. The variable-temperature powder X-ray diffraction study indicates the dynamic nature of the inclined polycatenated framework, and the dehydrated framework with exposed metal centers exhibits excellent type I H2 adsorption of 1.94 wt % at 77 K and 1 bar of pressure. he impetus development in the field of functional metal− organic frameworks (MOFs)1 has placed aside the frameworks of interpenetrated or polycatenated structures.2 For a long time, such frameworks have been believed to be nonporous and hence considered unsuitable for exhibiting the celebrated functionalities3 of the MOFs. A few interpenetrated structures4 showing voids in their transformed structures have recently been reported. Such interpenetrated MOFs showed functionalities similar to those of their noninterpenetrated analogues. This observation has opened up a new area of flexible MOFs5 that can undergo physical/chemical transformation in the presence of external stimuli like pressure, heat/light energy, etc. Moreover, their structural flexibility allows them to alter their basic structures by the aforesaid external stimuli, leading to the unusual adsorption of different gases including hydrogen.6 The proper design of hydrogen storage materials is an important issue because storage is one of the major challenges in the effective use of hydrogen energy.7,8 Although several MOFs have been studied for hydrogen storage, the use of flexible5 MOFs for this application is very limited, in spite of them having high specific surface area, tunable porosity, open metal sites,4,9 etc. It has also been observed that catenated frameworks2 have several advantages over noncatenated analogues in terms of their use in hydrogen storage.10 Thus, a careful blend of flexibility in polycatenated structures11 would be an interesting idea for the exploration of suitable hydrogen storage devices. Among the entangled/ catenated networks,12 the inclined polycatenated networks are very uncommon, and to date, none of them have been studied for hydrogen storage. Herein, we report the synthesis of an interesting 2D + 2D → 3D inclined polycatenated12 framework of {[Cu(4-bpe)(2ntp)(H2O)2]·2H2O}n (1), which exhibits temperature-dependent structural dynamicity to form an expanded framework (1′) upon the release of H2O molecules. The transformed

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© XXXX American Chemical Society

Received: November 9, 2016

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DOI: 10.1021/acs.inorgchem.6b02719 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

form the activated 1 (1′), exhibiting new peaks below 2θ = 10° and between 15° and 20°. Because of the loss of single crystallinity of 1 above 100 °C, the single-crystal structure analysis of 1′ could not be performed, but to clarify the structural transformation (at 120 °C), we have indexed the PXRD pattern of 1′ (Figure S6) using the TREOR program.16 The determined unit cell parameters are a = 9.9465 Å, b = 9.3007 Å, c = 32.5896 Å, α = 90.00°, β = 113.66°, γ = 90.00°, and V = 2761.40 Å3 for 1′, representing a monoclinic system with the space group P121. The volume expansion of 1′ is possibly due to elongation of the unit cell along the crystallographic c axis, and this huge expansion has also been supported by the appreciable uptake of different gas molecules by the expanded framework of 1′. Moreover, the IR spectra (Figure S7) of as-synthesized forms exhibit a broad band at 3468 cm−1 corresponding to the H2O molecules, which is missing in the IR spectra of its dehydrated form (Figure S7). For the dehydrated framework, the corresponding carboxylate CO (1617 cm−1) and C−O (1353 cm−1) bands broadened because of a change in the coordination environment around the metal center (Figure S7). Upon exposure of H2O vapor for 7 days, the original parent network 1 was regenerated, which is also confirmed by the IR study (Figure S7). The UV−vis diffuse-reflectance spectra of the as-synthesized, dehydrated, and rehydrated forms of 1 were recorded in the solid state at room temperature (Figure S8). It shows a reversible visual color change of blue−cyan−blue with absorbance maxima of 620, 589, and 623 nm probably due to the d−d transition for 1, 1′, and rehydrated 1′ (1), respectively. The expanded framework of 1′ exhibits a typical type I N2 adsorption up to 250 cc/g at P/P0 ≈ 0.99 (Figure S9). The nature of the N2 adsorption isotherm signifies the permanent microporous nature17 of 1′. Notably, 1′ also exhibits appreciably high, type I H2 adsorption with a maximum value of 216 cc/g (i.e., 1.94 wt %) at 77 K and 1 bar of pressure (Figure 3). The high uptake for H2 may be attributed to the

Figure 1. (a) View of the coordination environment around copper(II) with a atom labeling scheme in 1. Color code: Cu, green; O, red; N, blue; C, black. (b) Single 2D sheet constructed of both 2-ntp2− and 4-bpe ligands. (c) Interpenetration of two adjacent single 44-sql layers. (d) Simplified topological representation of 2D + 2D → 3D inclined polycatenation of two identical sets of parallel 44sql layers in 1.

was found to be stable up to 225 °C (Figure S3). Compound 1 exhibits reversible structural dynamicity (Scheme 1) as well as Scheme 1. Schematic Representation Showing Possible Flexibility in the Polycatenated Framework

bulk phase purity, as proven by the solid-state powder X-ray diffraction (PXRD) in different states (Figures S4 and S5). From the variable-temperature PXRD experiment in the range 30−240 °C (Figure 2), it is clear that the structural transformation begins at 30 °C and completes at ∼100 °C to

Figure 3. H2 adsorption−desorption isotherm up to (a) 1 bar and (b) 60 bar of pressure.

opening of the polycatenated nets due to the structural transformation of 1 as discussed above (Figure 2). To check the reliability and reproducibility of the study, the measurement was repeated five times, taking fresh samples in each case, which give very similar results. We also repeated the measurement three times on the same sample, which resulted in 216 cc/g (1.94 wt %), 209 cc/g (1.88 wt %), and 180 cc/g (1.62 wt %) consecutively at the same temperature and pressure ranges (Figure S10). The gradual decrease in H2 uptake is probably due to the partial rupture of the framework during the degassing process. Hence, it clearly reveals that the optimum opening of the polycatenated framework occurs when it was degassed the first time at 120 °C. Interestingly, the high-

Figure 2. PXRD patterns of 1 in the temperature range of 30−240 °C. B

DOI: 10.1021/acs.inorgchem.6b02719 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

possibilities and prospects in the development of hydrogen storage materials.

pressure H2 adsorption study shows comparatively less uptake18 of 130 cc/g (1.17 wt %) at 62 bar (Figure 3), which is probably due to more partial rupture of the framework at high pressure. This is further supported by the PXRD pattern of 1′ after high-pressure measurement, showing fewer peaks having lesser intensity compared to the PXRD pattern taken after normal H2 adsorption measurement (Figure S5). This high amount of H2 adsorption at 77 K and 1 bar of pressure for 1′ may be due to an effective pore size with appreciable void spaces and the presence of coordinatively unsaturated metal centers.9 During dehydration of 1, the axially coordinated H2O molecules were released and formed vacant sites on each copper(II) to form metal hydride or hydrogen complexes.9 However, unsaturation in coordination may inspire stronger physisorption and is concomitant with the lowering of the density of the framework, which may facilitate the loss of the terminal ligands.4 The reported grand canonical Monte Carlo simulation18a,19 reveals that the small pores created by catenation play an active role in confining the H2 molecules more closely, indicating that the catenated MOFs have higher H2 uptake than the noncatenated MOFs at low pressure.18a Both the ambient- and high-pressure H2 adsorption measurements of 1′ show a small hysteresis in the adsorption− desorption profile, possibly due to the flexible nature of polycatenated structure, and also for the moderate chemical interaction between the open metal sites in 1′ and a hydrogen molecule.20 To the best of our knowledge, it is the first example showing H2 adsorption for an inclined polycatenated MOFbased material. It is also worth mentioning that the amount of H2 uptake is higher than or comparable to (Table S6) the earlier reported value of interpenetrated MOFs.18b,21 The activated framework of 1 (1′) also exhibited 9.5 cc/g H2 uptake at 298 K and 0.99 bar (Figure S11). Sorption with some other gases (e.g., CH4 at 298 K and CO2 at 195, 273, and 298 K) and vapors (e.g., H2O and EtOH at 298 K) was also investigated for 1′. The framework of 1′ exhibited a significant amount of CO2 uptake up to 153, 67, and cc/g measured at 195, 273, and 298 K, respectively, at 1 bar of pressure (Figures S9 and S12). Selective adsorption of CO2 over CH4 (26 cc/g) was also found at 298 K and 1 bar of pressure (Figure S9), which has also been observed in interpenetrated/catenated isoreticular MOFs.10 This elevated CO2 selectivity has been accounted for as a result of the small pore size and interpenetrated adsorption sites.10 CO2, N2, and CH4 adsorption capacities of 1′ are also higher than or comparable to those of known MOFs (Table S7). In the case of solvent vapor (Figure S13), it shows type V H2O adsorption up to 156 cc/g at 298 K and P/P0 ≈ 0.9. At a pressure P/P0 region of 0.09−0.17, a sharp uptake of H2O adsorption (74 cc/g) occurred, which is equivalent to 1.8 mol of H2O per formula unit, signifying the loss of two coordinated H2O molecules at the low-pressure region. The H2O uptake capacity is more compared to the uptake of less polar EtOH vapor (up to 143 cc/g) at the same pressure and temperature. The H2O adsorption profile also exhibits a larger hysteresis in the adsorption−desorption curve compared to that of the EtOH profile, possibly because of the strong adsorbate− adsorbent interaction in the case of H2O.22 To summarize, we report here the first example of a 2D + 2D → 3D inclined polycatenated dynamic MOF-based material showing significant H2 uptake up to 1.94 wt %. Although the amount is moderately high, the important finding of this work is the utilization of a new class of material for effective hydrogen storage. This will certainly open up some new

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02719. Synthetic and experimental details, other physicochemical analyses, and some sorption isotherms (PDF) X-ray crystallographic data in CIF format (CCDC 1502229) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Debajyoti Ghoshal: 0000-0001-8820-8209 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial assistance by the Science and Engineering Research Board, India (Grant SB/S1/IC-06/ 2014). D.K.M. and F.H. acknowledge the University Grants Commission for their research fellowship. Dr. T. K. Maji of JNCASR, Bangalore, India, is gratefully acknowledged for the high-pressure H2 adsorption study.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b02719 Inorg. Chem. XXXX, XXX, XXX−XXX