Chromium(II) Metal–Organic Polyhedra as Highly Porous Materials

Jul 25, 2017 - (16, 17) Through careful design of favorable interactions, MOPs can be used to template bulk porous materials. ...... J. A.; Bell , J.;...
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Chromium(II) Metal−Organic Polyhedra as Highly Porous Materials Jinhee Park,*,† Zachary Perry,‡ Ying-Pin Chen,‡,§ Jaeyeon Bae,† and Hong-Cai Zhou*,‡,§ †

Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology, Daegu 42988, Republic of Korea ‡ Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United States § Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77840, United States S Supporting Information *

ABSTRACT: Herein we report for the first time the synthesis of Cr(II)-based metal−organic polyhedra (MOPs) and the characterization of their porosities. Unlike the isostructural Cu(II)- or Mo(II)-based MOPs, Cr(II)-based MOPs show unusually high gas uptakes and surface areas. The combination of comparatively robust dichromium paddlewheel units (Cr2 units), cage symmetries, and packing motifs enable these materials to achieve Brunauer−Emmett−Teller surface areas of up to 1000 m2/g. Reducing the aggregation of the Cr(II)-based MOPs upon activation makes their pores more accessible than their Cu(II) or Mo(II) counterparts. Further comparisons of surface areas on a molar (m2/mol cage) rather than gravimetric (m2/g) basis is proposed as a rational method of comparing members of a family of related molecular materials. KEYWORDS: metal−organic polyhedra (MOPs), chromium(II) paddlewheel, chromium(II) porous materials, air-stable MOPs, oxygen adsorption

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INTRODUCTION Porous metal−organic materials (MOMs) are largely divided into metal−organic frameworks (MOFs) and metal−organic polyhedra (MOPs), based on their dimensionalities.1−4 MOFs, which have 3-D structures, generally maintain their porous structures even after the removal of guest molecules because of their structural rigidity.5 In contrast, MOPs are individual discrete cages formed by ligands that bridge metal centers.3,6−10 MOP structures have been shown to maintain their inherent porosities in the solid state.11−15 Although their structures are intrinsically porous, MOPs generally undergo random aggregation, or collapse upon solvent removal due to the lack of strong bonding between cages. This aggregation results in the blocking of pore windows by adjacent MOPs, limiting diffusion through the material. Consequently, the gas adsorption of the resulting material is relatively low compared to the predicted surface area for an intact structure, removing the advantage of using an inherently porous building block.16,17 Through careful design of favorable interactions, MOPs can be used to template bulk porous materials. Recently, two strategies for the prevention of the unfavorable aggregation of MOPs have been reported. In the first case, MOPs were dispersed into the pores of porous materials such as mesoporous silicas and metal−organic frameworks, which inhibited aggregation by spatially separating individual MOPs. In this manner, pore windows and adsorption sites were accessible even after solvent removal.18−20 Second, interdigitated MOPs, having welldefined chain structures, showed significantly enhanced gas uptakes compared to discrete MOPs.15 In this case, the solventmediated hydrophobic interactions of bulky functional groups, © 2017 American Chemical Society

such as triisopropylsilylethynyl, was necessary to direct the selfassociation of dicopper paddlewheel units (Cu2 units). Dimetallic paddlewheel units (M2 units), consisting of dimetallic cores and four bridging carboxylates, have served as fundamental building blocks for metal−organic materials because of their symmetric geometries and stabilities.21,22 Among M2 units, dichromium paddlewheels (Cr2 units) are well-known to form infinite-chain structures through selfassociation.23 This self-association originates from the nature of the quadruple Cr−Cr bond, the high Cr−O bond dissociation energy, and preferential Cr2-unit coordination geometry. We hypothesized that MOPs consisting of Cr2 units might undergo cage-to-cage packing through reorganization and self-association of the Cr2 units after axial ligand removal. Regardless of the functional groups on the ligands, the thermally activated process for the Cr(II) system reported herein affords highly porous MOP materials with Brunauer−Emmett−Teller (BET) surface areas in excess of 1000 m2/g, which are among the highest surface areas achieved for any known metal−organic cage-based system. These are also the first Cr(II)-based MOPs reported in the literature. Three different isophthalate ligands, isophthalate (isoph), 5tert-butylisophthalate (TBI), and 5-triisopropylsilylethynylisophthalate (TEI) were used in this study. The reaction between Cr2(OAc)4 and the acidic forms of the ligands afforded purple block crystals. Because of the air sensitivity of Cr2+ ions, Received: June 28, 2017 Accepted: July 25, 2017 Published: July 25, 2017 28064

DOI: 10.1021/acsami.7b09339 ACS Appl. Mater. Interfaces 2017, 9, 28064−28068

Research Article

ACS Applied Materials & Interfaces

where. For example, the N2 uptakes at 77 K and 1 bar by TBI24Mo24, TBI24Cu24, and TEI24Cu24 were 125,29 15,26 and 4.5 cm3/g,28 respectively. The calculated BET and Langmuir surface areas of the Cr-MOPs are listed in Table S2. In particular, Cr-2 showed BET and Langmuir surface areas of 1044 and 1406 m2/g, respectively. These surface areas are comparable to those of typical MOFs that are well-known for framework rigidity. This indicates that the pore windows are still open and the gas adsorption sites are available, even after the rearrangement of the MOP cages upon activation. We attribute this unusually high gas uptake to well-controlled interactions among MOPs over short ranges that prevent random MOP aggregation. The only comparable porous materials containing Cr2+ ions are Cr3(BTC)2 (BTC3−, benzenetricarboxylate) and Cr-BTT (BTT3−, 1,3,5-benzenetristetrazolate) reported by the Long group.30−32 Both Cr3(BTC)2 and Cu3(BTC)2 maintain their porous structures after solvent removal. This is because the cages in these MOFs are linked entirely through a mixture of covalent and dative bonds provided by the additional carboxylate groups of the BTC ligands. Thus, the surface areas and pore sizes of the Crand Cu-based MOFs were comparable to each other after complete activation. It is worth mentioning that the Cr-MOPs are the only Cr(II) materials with inherent porosity which can grow large single crystals. To more accurately portray the gas sorption abilities of these cage-based materials, molar BET surface areas (m2/mol) were also used (Table S2). Since the inner-cage volume is the same for all three cages, one would expect that the sizes of the functional groups would largely determine the differences in surface area per cage, but not per gram. This comparison is useful as it allows for a comparison of uptake relative to structural metrics. As expected, the unadorned cage Cr-1 shows the lowest molar surface area. This is demonstrated structurally, as the cages rest against one another in the crystal structure, in turn limiting the surface to little more than the interior of the cage. The expected trend of functional group length to surface area does not hold when the 5-position of the isophthalate is expanded by replacement of the hydrogen with a tert-butyl or triisopropylsilylethynyl group. This is probably the result of unavoidable random aggregation of the Cr-MOPs. However, Cr-2 and Cr-3 subsequently showed increased molar surface areas, by over 50% compared to that of Cr-1. There may be several possible factors that enable the cage-tocage packing of the activated Cr-MOPs; the first involves hydrophobic interactions among adjacent MOPs, and the second involves the potential self-association of Cr2 units. As shown in Figure 2, however, hydrophobic interactions are not critical in the presented Cr-based systems, as Cr-3, consisting of bulky TEI units shows the lowest gas uptake (cm3/g). The potential self-association between an outer Cr in a Cr2 unit, and the O atom of a nearby carboxylate in an adjacent MOP is largely governed by the distance between them (d in Figure S1) and the directionality of the Cr in relation to O. The distance (d) in the crystal structures of Cr-1, Cr-2, and Cr-3tetra are 8.5, 8.1, and 13.2 Å, respectively (Figure 2e−g). There is an approximately inverse correlation between N2 uptake and the distance between the Cr and O, as shown in Figure S18. This indicates that the Cr2 units in the tightly packed MOPs, Cr-1 and Cr-2, might easily undergo cage-to-cage packing upon removal of the coordinating solvents, resulting in retaining porosity, while the less densely crystallized Cr-3 prefers to undergo random aggregation. Gas uptake by Cr-3tetra and Cr-

all syntheses and treatments of the Cr-MOPs were performed in an Ar-filled glovebox. The Cr-MOPs obtained in this manner are isostructural to their Cu/Mo/Ru(II)-based counterparts that were reported by Yaghi’s group in 2001, as well as other groups.24−26 In the crystal structures of Cr-MOPs, one Cr2 unit is assembled with four carboxylates from four different isophthalate ligands, and in the overall structure the 24 ligands, as linear edges, coordinate to 12 Cr2 units, as vertices, leading to the formation of cuboctahedral cages (green cages in Figure 1). Because of the axial coordination of solvents, the Cr−Cr

Figure 1. Crystal structures of Cr-1 (a), Cr-2 (b), and Cr-3 (c). H atoms of the ligands and coordinating solvent molecules are omitted for clarity. Color schemes: Cr, green; O, pink; Si, orange; C, light blue (Cr-1), blue (Cr-2), and dark blue (Cr-3). The green polyhedra represent the polyhedral molecular geometries of Cr-1, Cr-2, and Cr-3 when the metal clusters are considered as vertices, and the ligands as edges.

bond distance in the crystal structure is 2.36−2.39 Å, consistent with that reported (2.36 Å), and longer than the bond length in unsolvated dichromium paddlewheels (1.97 Å).27 Cr24isoph24 (Cr-1) and Cr24TBI24 (Cr-2) exhibit the I4/m space group. Cr24TEI24 (Cr-3) initially crystallized in I4/m. When the concentration of the ligands and Cr2(OAc)4 solution was halved; however, Cr-3 crystallized with the R3̅c space group. For simplicity, Cr-3 in I4/m and R3c̅ are referred to as Cr-3tetra and Cr-3rho, respectively. We demonstrated our hypothesis by showing enhanced gas uptake capacities in Cr-1, Cr-2, and Cr-3. For N2 adsorption by these MOPs at 77 K, Cr-1 and Cr-2 were activated by toluene exchange followed by evacuation under reduced pressure at 200 °C for 10 h. Because of its solubility in toluene, Cr-3 was activated after hexane exchange. The Cu24TEI24 MOP also exhibited solubility in a variety of nonpolar solvents including toluene, diethyl ether, and benzene, among others.15,28 Because polar solvents can cause a loss of crystallinity by inducing random disorder, or solvent-assisted cross-linking of the CrMOP cages similar to the previously published behavior of the Cu24TEI24 MOP,15 they were not used for solvent exchange. This loss in crystallinity makes it difficult to correlate Cr-MOP packing in their crystal structures and the gas uptakes of these Cr-MOPs. Upon activation, these MOPs exhibited significant color changes from purple to yellow due to removal of the coordinating solvents, indicating that the samples were ready for gas adsorption experiments (Figure S3). The materials are readily oxidized by air as evidenced by the appearance of a green color (Figure S3). Hence, the isotherms reported herein are from samples that were subjected to rigorous air free handling and were determined not to be oxidized by visual inspection. The N2 uptakes by Cr-1, Cr-2, Cr-3tetra, and Cr-3rho at 0.95 P/P0 were 292, 387, 230, and 131 cm3/g, respectively, which were exceptionally high compared with isostructural Cu- and Mo-based MOPs reported else28065

DOI: 10.1021/acsami.7b09339 ACS Appl. Mater. Interfaces 2017, 9, 28064−28068

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Figure 3. N2 adsorption isotherms for Cu-2 (a) and Cr-2 (b) activated at 80, 120, 160, and 200 °C.

Figure 2. (a) N2 adsorption isotherms for Cr-1, Cr-2, Cr-3tetra, and Cr3rho. Crystal packing of Cr-1 (b), Cr-2 (c), and Cr-3tetra (d). The arrows in (e)−(g) indicate the distances between adjacent Cr units of Cr-1 (e), Cr-2 (f), and Cr-3tetra (g).

contrast, until the activation temperature was raised to 160 °C, Cr-2 showed negligible gas uptake. The N2 uptake of Cr-2, when activated at 200 °C, finally rose to 387 cm3/g (Figure 3b). The sorption isotherms of Cr-1, Cr-2, Cr-3, and Cu-2 reported herein exhibited the inflection of the adsorption isotherms and the hysteresis of the desorption branches.33 This behavior is ascribed to slow diffusion of gas molecules due to the blockage of the open windows which comes from the random arrangement and aggregation of the MOP cages after activation.16,34,35 This blockage makes it difficult for gas molecules to fill the pores and escape from the pores during the adsorption and desorption process, respectively. There are two possible reasons for the different activation behavior observed for Cu-2 and Cr-2. First, Cu-2 is less thermally stable than Cr-2, as shown by their thermogravimetric analysis (TGA) curves (Figures S16 and S17). The prolonged exposure to high temperature (200 °C) might decompose the cuboctahedral structure of Cu-2. Second, the higher activation temperatures required for the Cr-MOPs may result from strong coordination bonds between the coordinating solvent molecules and the chromium centers. As observed for other Cr(II) compounds, Cr-2 also shows O2 chemisorption at 295 K.31 The steep increase in O2 adsorption at low pressure originates from the chemisorption of O2 at relatively high temperature (Figure 4a). The O2 uptake was as high as 34 cm3/g at 295 K and 750 mmHg, indicating approximately one O2 molecule per Cr2 unit. However, N2 uptake under the same conditions was negligible. It is worth mentioning that the O2 uptake at 295 K in porous metal− organic materials is generally attributed to the physisorption of the gas molecules on the surface. In this case, O2 adsorption is very low.36 In this report, O2 adsorption is due to the chemisorption based on the reaction of O2 with the Cr2 units. This relatively high O2 adsorption at 295 K indicates that the potential applications of Cr-MOPs in O2 production and separation applications.32

3rho was compared to demonstrate how the crystal packing of Cr-MOPs affects porosity. The distance d in Cr-3rho is 14.3 Å, longer than that, 13.2 Å, in Cr-3tetra (Figure S2). Cr-3tetra and Cr-3rho exhibited N2 uptakes of 230 and 130 cm3/g, respectively. In addition, the BET surface areas of Cr-3tetra and Cr-3rho are 579.8 and 708.5 m2/g, respectively. The porosity of Cr-3tetra is comparable to that of interdigitated Cu24TEI24.15 The different gas uptakes and surface areas of Cr3tetra and Cr-3rho reveal that MOP cage packing is an important factor in maintaining porosity. Although d in Cr-3tetra and Cr3rho are different by 1.1 Å, the Cr and O atoms in Cr-3tetra are three-dimensionally well-aligned, indicating a relatively high possibility that Cr-3tetra will undergo cage-to-cage packing. However, the cage-to-cage packing of Cr-3rho can only occur in the [1 1 1] plane because the Cr2 units in other directions do not face each other (Figure S2). In the same vein, the Cr2 units in Cr-2 are almost linearly aligned as indicated by arrows in Figure 2f. In other words, Cr−Cr (in one Cr2 unit) and O (in the other Cr2 unit on an adjacent cage) for Cr-2 are linearly aligned; this explains why this MOP exhibits the highest gas uptake among the Cr-MOPs. However, Cr−Cr (in one Cr2 unit) and O (in the other Cr2 unit) for Cr-1 and Cr-3 are not linearly aligned in such a manner. Therefore, we are proposing that they do not have an optimally arranged packing to facilitate self-association induced cage-to-cage packing. To make a comparison, the Cu(II) analog of Cr-2, Cu24TBI24 (for simplicity Cu-2), was prepared according to the literature and activated under the same condition. Cu-2 showed step-bystep increases in N2 uptake, up to 98 cm3/g at 0.95 P/P0, when the activation temperature was elevated from 80 to 260 °C, consistent with that reported.26 However, this uptake suddenly decreases when Cu-2 was activated at 200 °C (Figure 3a). In 28066

DOI: 10.1021/acsami.7b09339 ACS Appl. Mater. Interfaces 2017, 9, 28064−28068

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09339. Detailed experimental procedures for the syntheses of Cr-1, -2, and -3; single-crystal X-ray crystallographic data, TGA curves, FT-IR spectra, and additional gas adsorption isotherms for the Cr-MOPs (PDF) Cr-1 data (CIF) Cr-2 data (CIF) Cr-3tetra data (CIF) Cr-3rho data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jinhee Park: 0000-0001-7755-5889 Hong-Cai Zhou: 0000-0002-9029-3788 Notes

Figure 4. (a) O2 adsorption isotherms for Cr-2. The inset shows the color change exhibited by Cr-2 after O2 adsorption at 295 K. (b) H2 adsorption isotherms of Cr-2, Cr-2′, and Cr-2′ in air for 1 month, and Cu-2.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.P. and J.B. were supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF-2016R1C1B2009987 and NRF-2016M2B2A9912217). ChemMatCARS Sector 15 is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE1346572. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. Y-P.C., Z.P., and H.-C.Z. were supported by the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0001015 for material synthesis and characterization. We also thank Jihye Park for her assistance with some of the gas sorption measurements.

We next confirmed the stability of the oxidized Cr-MOPs. Compared to Cr-2, the oxidized Cr-2 (for simplicity, Cr-2′) showed a decreased capacity for H2 uptake, from 149 to 71 cm3/g at 77 K and 810 mmHg. Interestingly, this uptake was comparable to that of Cu-2 (70 cm3/g at 77 K and 810 mmHg). H2 adsorption isotherms for Cr-2, Cr-2′, and Cr-2′ in air for 1 month, and Cu-2 are shown in Figure 4b. The H2 uptake of Cr-2′ indicates that the oxidized Cr-MOP, in which Cr2+ is transformed into Cr3+, still preserves its porous structure, whereas random aggregation was not able to be prevented. Unlike Cu-MOPs that are unstable in air, Cr-2′, consisting of high valence Cr3+ ions, showed excellent air resistance. Even after it was left in air for 1 month, H2 uptake as high as 63 cm3/g at 77 K and 810 mmHg was maintained. This improved stability relative to the Cu-MOP indicates that the coordination bond of Cr3+−O in the oxidized Cr2 unit is strong.

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REFERENCES

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CONCLUSION We have presented the first example of Cr(II)-based MOPs. The Cr-MOPs presented herein act as template materials that exhibit exceptionally high gas adsorptions. Several features have been identified as possible guidelines for designing new porous Cr-MOP based materials. First, the cages need to have a close packing arrangement with possible electrostatic contacts. Second, once packed the pore windows need to be wellaligned for porosity to be maintained; any misalignment of the cage positions may result in blocked pores that prevent gas diffusion. The Cr-2 MOP, consisting of tert-butyl isophthalate as the bridging ligand, shows exceptionally high gas uptake and surface area. Further characterization of the activated Cr-MOPs will be conducted to provide an understanding of the mechanisms of the short-range ordering of Cr-MOP cages. 28067

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DOI: 10.1021/acsami.7b09339 ACS Appl. Mater. Interfaces 2017, 9, 28064−28068