- Email: [email protected]
The roles of rod-packed organic units for preparing stable porous coordination polymers with carboxylate ligands
Inorganic Chemistry Communications 61 (2015) 53–56
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
The roles of rod-packed organic units for preparing stable porous coordination polymers with carboxylate ligands Jingui Duan ⁎, Wanqin Jin State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China
a r t i c l e
i n f o
Article history: Received 18 July 2015 Received in revised form 14 August 2015 Accepted 23 August 2015 Available online 31 August 2015 Keywords: Porous coordination polymers Rod-packed organic units Stability La Adsorption
a b s t r a c t Despite tremendous efforts, designing and preparing water, chemical, and moisture stable porous coordination polymers (PCPs) with carboxylate ligands remain a challenge. Here, we firstly and systemically investigated the roles of rod-packed organic units for preparing the stable PCPs with different carboxylate ligands. The experimental results demonstrated that the rod packed organic units with short distance of adjacent ligands, bridged by La3+ metal chain, can form water, chemical, and moisture stable PCPs. © 2015 Elsevier B.V. All rights reserved.
In recent years, the fascinating discovery in the area of functional, nanostructured materials is the emergence of large numbers of structurally defined porous coordination polymers (PCPs) [1,2]. Their versatile architectures and customizable chemical properties could enable them to be attractive candidates for gas storage [3–7], separation [8–10] and catalysis [11,12]. Water, chemical, and moisture stabilities are important factors for any sorbent that has potential to be used for practical applications, especially for PCP materials [13–19]. Till now, almost twenty thousand of PCPs have been reported [20]; however, unfortunately, some of the most promising PCPs are limited because of instability with respect to moisture and water [4,21,22]. Therefore, for offering an opportunity to address this problem, it is essential to find and understand the factors that affect the stability of porous frameworks, especially for the framework with carboxylate ligands. PCPs are crystalline materials consisted of metal ions/clusters and organic ligands, which are bridged together to form extended porous frameworks. The stability of PCPs should be dominated by the strength of coordination bonds between ligands and metal centers. Generally, the methods for enhancing the strength or preventing the corrodibility of coordination bonds were believed as rational ideas for accessing stable frameworks [23]. For example, by introducing hydrophobic alkyl-chains around coordinate sites, IRMOF-1 showed an enhanced moisture-resistant [24]. Additionally, the rational-design methods of postsynthetic modification and polymer coating can produce ⁎ Corresponding author. E-mail address: [email protected] (J. Duan).
http://dx.doi.org/10.1016/j.inoche.2015.08.016 1387-7003/© 2015 Elsevier B.V. All rights reserved.
moisture/water-resistant and superhydrophobic PCPs with intact porosity [25,26]. Further, the strategy of using polyazolate-bridging ligands can lead to frameworks with strong metal–nitrogen bonds, providing a greater chemical and thermal stability compared to their carboxylate-based counterparts [13]. However, the generated water and moisture stable PCPs by employing the above methods will suffer the decreasing of their pore size and volume, and further, some approached only feasible to some specific PCPs. Very recently, we reported two La-based PCPs with easily-prepared carboxylate ligands [16,17]. Their good water and chemical resistance encouraged us to speculate that the coordination of ahighly-coordinated metal center (from conventional 4 or 6 to 8 or 9) with rod-packed organic units may be a new method to prepare stable PCP, because the increase of connectivity can possibly improve the strength of connection between metal clusters and ligands, and more importantly, the well-packed organic units could provide a tight space for preventing the attack of M–O bonds from water or moisture. If this idea works well, it will definitely facilitate the design and preparation of stable PCPs with such great number of carboxylate ligands. We are interested in the construction of porous coordination polymers with interesting properties based upon carboxylate ligands [27-29]. Herein, in order to confirm this idea, four La-based PCPs (La–BTC, La–BTB-1, La–BTB-2, La–BTN) with three different carboxylate ligands (1,3,5-Benzenetricarboxylic acid: H 3 BTC; 1,3,5-Tris(4-carboxyphenyl)benzene:H3 BTB; 1,3,5-Tri(6hydroxycarbonylnaphthalen-2-yl) benzene: H3BTN) were synthesized and structurally characterized. Powder X-ray diffraction pattern (PXRD) and gas adsorption experiments of treated samples
54
J. Duan, W. Jin / Inorganic Chemistry Communications 61 (2015) 53–56
demonstrated that the noticeable variation of structural stability should be attributed to different types of ligand arrangements along metal chains. These four PCPs were solvothermally synthesized from a mixture of La(NO3)3·6H2O and H3BTC, H3BTB and H3BTN in different solvent systems, respectively (Fig. 1). Two of them (La–BTN and La–BTB-1) as reported in our previous work [16,17] and the third one (LaBTB-2) were prepared according to Walton's work [30]. The phase purities of the PCPs were confirmed by PXRD. The new structure of La–BTC was determined by single-crystal X-ray diffraction analysis. Thermogravimetric analyses (TGA) show that the four PCPs are thermally stable up to 400 °C under N2 atmosphere. Single crystal X-ray diffraction revealed that the structure of [La(BTC)(H2O)]3·xGuest is chiral, with the space group of P43. The asymmetric unit of it consists of one La3+ ion, one BTC ligand and one coordinated water. The La3 + ion is nine-coordinated and binds to eight oxygen atoms from six different BTC3 −ligands and one water molecule. Each BTC3 −ligand connects to six different La3 + ions. The LaO8(H2O) polyhedra are connected by corner-sharing to form a chiral of –La–O–C–O–La– along the c-axis. These chains are bridged together by the BTC3−ligands to form a 3D framework (Fig. 1a), with the pore size of 7 × 7 Å2. By comparing these four structures, we found that the space groups of them are all chiral. All of the coordination geometries of the La center were completed by eight oxygen atoms from six different carboxylate groups and one coordinated water/DMF molecule. The metal–oxygen chains with closed La/La distances of 4.224 to 4.293 Å were bridged by BTC, BTB and BTN ligands in those series of PCPs (Fig. S8). However, it is worth noting that the arrangements of the ligands in them show significant differences. For La–BTC, the adjacent BTC moieties adopt the end-to-end type, while that in other compounds took the face-to-face type. For La–BTB-1, the distances between the adjacent benzene rings are around 4 Å, which means that there is no enough space for any extra guest molecules (even including H2 molecules) to go inside between every two adjacent benzene rings. Thus, the well-packed benzene rings with a high concentration of protons act like a wall that can provide a hydrophobic environment along the channel. However, the extended distance of adjacent ligands was found in La–BTN and La–BTB-2 (Fig. 2). Therefore, we can theorize that the coordination
geometries of La3+ completed by rod-packed units with different distances will bring the PCPs with varied water and moisture resistance. Although three of the structures have been reported, their water, chemical, and moisture stabilities, especially at high temperature, have not been well explored. Further, the roles of rod-packed organic units in influencingthe stabilities of PCPs have also not been reported before. Thus, systemic study of the stabilities of those four La–PCPs is very necessary. Generally, the combined results show that PXRD and gas adsorption were the most effective methods for indentifying the integrity of porous materials. The frameworks with high stability should keep their original PXRD patterns, as well as their gas uptake capacities. The fresh samples of those four PCPs were soaked in hot (60 °C and 100 °C) aqueous HCl (pH = 2), aqueous NaOH (pH = 12) and pure water for 24 h. After the temperature cools down, part of the samples was used for PXRD pattern collection (Figs. S9–S12) and the rest of them were used for gas sorption experiments (exchanged by methanol for three times and degassed at 120 °C for 24 h, Figs. S14–S17). As shown in Figs. S9–S12, the framework of La–BTC transformed to another unknown phase, whereas, the PXRD patterns of La–BTB-1, La–BTB-2, and La–BTN matched well with their original form, indicating the integrity of their frameworks. Meanwhile, the gas adsorption results demonstrated that treated La–BTC did not show any gas uptakes at low pressure range. The very low amount of gas uptake in it at around 95 kPa should be ascribed to the adsorption of apertures between particles. Thus, La–BTC cannot maintain its porosity under such condition. For La–BTB-2, the gas uptake of treated samples decreased from 300 cm3·g−1 to ~20 cm3·g−1 as well as the disappearance of gate opening at P/P0 = 0.3, even if the PXRD pattern of the samples remained the same after treatment. Interestingly, for La–BTN, the gas uptake of treated samples (pH = 2 and 12) at 100 °C just decreased a little, and moreover, the gas uptake of La–BTN (pH = 7) is nearly the same as that of the fresh one, exhibiting a good integrity of the framework. Moreover, the treated La–BTB-1 maintained the same gas uptakes as its fresh sample, except for the sample that was treated under acid solution (pH = 2) at 100 °C. To further explore the stability of these materials, we treated the activated sample of them at 80 °C and 80% RH for 24 h. The PXRD patterns (Figs. S9–S12) and gas uptakes (Figs. S18–S21) of the samples reveal that La–BTC transforms to that unknown phase without porosity
Fig. 1. The packing view of those four PCPs (inner surfaces: yellow, outer surfaces: gray).
J. Duan, W. Jin / Inorganic Chemistry Communications 61 (2015) 53–56
55
Fig. 2. The view of six adjacent ligands along the La chains in each of them. Probe radius for organic surface is 1.4 Å.
again. La–BTB-2 also changed to a new unknown phase; however, the significant gate-opening behavior that has been found in the fresh sample disappeared after the moisture treatment. This interesting and rare phenomenon indicates that moisture treatment may contribute to the change of framework flexibility. More importantly, the gas uptakes of the samples before and after treatment decreased only a little in La–BTN, but the capacity remainedalmost same in the structure of LaBTB-1. Those results strongly indicate the good integrity of the framework, which could be confirmed by the PXRD again. In order to easily evaluate the stability of these four PCPs, we defined a concept of relative integrity, and which is the ratio of the maximum gas uptake after and before the hydro/chemical treatments. As shown in Fig. 3, we found that a striking feature of those four La-based PCPs is the enhanced value of relative integrity associated with the tightness of
organic rods. Therefore, taking the crystal structures into consideration, we can conclude that the noticeable impact of the structural stability of these series of La-based PCPs should be assigned as different ligand arrangements along the metal chains, as the coordination number of La3+is almost the same in these four PCPs. The shorter distance between the adjacent ligand units will provide less opportunity for water to attack the M–O bonds which is strongly associated with the relative integrity of the porous framework. In summary, a new porous structure (La–BTC), and other three previously reported La-based PCPs, La–BTB-1, La–BTB-2, and La–BTN, were synthesized and characterized. PXRD and gas sorption measurements demonstrated that the rod-packed organic units with short distance of adjacent ligands, bridged by highly-coordinated metals chain, can access the water, chemical, and moisture stable PCPs. Thus, we believe that this discovery can substantially benefit for preparing stable PCPs with carboxylate ligands. Work along this strategy via other metal centers or polydentate carboxylate ligands is currently underway in our laboratory. Acknowledgments The authors gratefully acknowledge supports from the National Natural Science Foundation of China (No. 21301148), Talent Development Program of Nanjing Tech University (39801116) and Foundation of State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201406). Appendix A. Supplementary material
Fig. 3. The relative integrity of these series of PCPs.
CCDC 1047216 contains the supplementary crystallographic data for La–BTC. These data can be obtained free of charge via http://www.ccdc. cam.ac.uk/conts/retrieving.html, or from the CCDC, 12 Union Road, Cambridge CB21EZ, UK; or Email: [email protected]. Additional
56
J. Duan, W. Jin / Inorganic Chemistry Communications 61 (2015) 53–56
experimental details as supplementary information are available at http://dx.doi.org/10.1016/j.inoche.2015.08.016. References [1] S. Kitagawa, R. Kitaura, S. Noro, Functional porous coordination polymers, Angew. Chem. Int. Ed. 43 (2004) 2334–2375. [2] H. Li, M. Eddaoudi, M. O'Keeffe, O.M. Yaghi, Design and synthesis of an exceptionally stable and highly porous metal–organic framework, Nature 402 (1999) 276–279. [3] Y. Yan, S.H. Yang, A.J. Blake, M. Schroder, Studies on metal–organic frameworks of Cu(II) with isophthalate linkers for hydrogen storage, Acc. Chem. Res. 47 (2014) 296–307. [4] H. Furukawa, N. Ko, Y.B. Go, N. Aratani, S.B. Choi, E. Choi, A.O. Yazaydin, R.Q. Snurr, M. O'Keeffe, J. Kim, O.M. Yaghi, Ultrahigh porosity in metal–organic frameworks, Science 329 (2010) 424–428. [5] H.X. Deng, S. Grunder, K.E. Cordova, C. Valente, H. Furukawa, M. Hmadeh, F. Gandara, A.C. Whalley, Z. Liu, S. Asahina, H. Kazumori, M. O'Keeffe, O. Terasaki, J.F. Stoddart, O.M. Yaghi, Large-pore apertures in a series of metal–organic frameworks, Science 336 (2012) 1018–1023. [6] K. Sumida, D.L. Rogow, J.A. Mason, T.M. McDonald, E.D. Bloch, Z.R. Herm, T.H. Bae, J.R. Long, Carbon dioxide capture in metal–organic frameworks, Chem. Rev. 112 (2012) 724–781. [7] L.J. Murray, M. Dinca, J.R. Long, Hydrogen storage in metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1294–1314. [8] E.D. Bloch, W.L. Queen, R. Krishna, J.M. Zadrozny, C.M. Brown, J.R. Long, Hydrocarbon separations in a metal–organic framework with open iron(II) coordination sites, Science 335 (2012) 1606–1610. [9] J.R. Li, J. Sculley, H.C. Zhou, Metal–organic frameworks for separations, Chem. Rev. 112 (2012) 869–932. [10] J.R. Li, R.J. Kuppler, H.C. Zhou, Selective gas adsorption and separation in metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1477–1504. [11] J. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Metal–organic framework materials as catalysts, Chem. Soc. Rev. 38 (2009) 1450–1459. [12] L. Ma, C. Abney, W. Lin, Enantioselective catalysis with homochiral metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1248–1256. [13] V. Colombo, S. Galli, H.J. Choi, G.D. Han, A. Maspero, G. Palmisano, N. Masciocchi, J.R. Long, High thermal and chemical stability in pyrazolate-bridged metal–organic frameworks with exposed metal sites, Chem. Sci. 2 (2011) 1311–1319. [14] N.T.T. Nguyen, H. Furukawa, F. Gandara, H.T. Nguyen, K.E. Cordova, O.M. Yaghi, Selective capture of carbon dioxide under humid conditions by hydrophobic chabazite-type zeolitic imidazolate frameworks, Angew. Chem. Int. Ed. 53 (2014) 10645–10648. [15] T.A. Makal, X. Wang, H.C. Zhou, Tuning the moisture and thermal stability of metal– organic frameworks through incorporation of pendant hydrophobic groups, Cryst. Growth Des. 13 (2013) 4760–4768.
[16] J.G. Duan, M. Higuchi, R. Krishna, T. Kiyonaga, Y. Tsutsumi, Y. Sato, Y. Kubota, M. Takata, S. Kitagawa, High CO2/N2/O2/CO separation in a chemically robust porous coordination polymer with low binding energy, Chem. Sci. 5 (2014) 660–666. [17] J.G. Duan, M. Higuchi, S. Horike, M.L. Foo, K.P. Rao, Y. Inubushi, T. Fukushima, S. Kitagawa, High CO2/CH4 and C2 hydrocarbons/CH4 selectivity in a chemically robust porous coordination polymer, Adv. Funct. Mater. 23 (2013) 3525–3530. [18] H.L. Jiang, D.W. Feng, K.C. Wang, Z.Y. Gu, Z.W. Wei, Y.P. Chen, H.C. Zhou, An exceptionally stable, porphyrinic Zr metal–organic framework exhibiting pH-dependent fluorescence, J. Am. Chem. Soc. 135 (2013) 13934–13938. [19] S.J. Yang, C.R. Park, Preparation of highly moisture-resistant black-colored metal organic frameworks, Adv. Mater. 24 (2012) 4010–4013. [20] H. Furukawa, K.E. Cordova, M. O'Keeffe, O.M. Yaghi, The chemistry and applications of metal–organic frameworks, Science 341 (2013) 974–986. [21] O.K. Farha, A.O. Yazaydin, I. Eryazici, C.D. Malliakas, B.G. Hauser, M.G. Kanatzidis, S.T. Nguyen, R.Q. Snurr, J.T. Hupp, De novo synthesis of a metal–organic framework material featuring ultrahigh surface area and gas storage capacities, Nat. Chem. 2 (2010) 944–948. [22] D. Han, F.L. Jiang, M.Y. Wu, L. Chen, Q.H. Chen, M.C. Hong, A non-interpenetrated porous metal–organic framework with high gas-uptake capacity, Chem. Commun. 47 (2011) 9861–9863. [23] D.Y. Ma, Y.W. Li, Z. Li, Tuning the moisture stability of metal–organic frameworks by incorporating hydrophobic functional groups at different positions of ligands, Chem. Commun. 47 (2011) 7377–7379. [24] J. Yang, A. Grzech, F.M. Mulder, T.J. Dingemans, Methyl modified MOF-5: a water stable hydrogen storage material, Chem. Commun. 47 (2011) 5244–5246. [25] J.G. Nguyen, S.M. Cohen, Moisture-resistant and superhydrophobic metal–organic frameworks obtained via postsynthetic modification, J. Am. Chem. Soc. 132 (2010) 4560–4561. [26] W. Zhang, Y. Hu, J. Ge, H.L. Jiang, S.H. Yu, A facile and general coating approach to moisture/water-resistant metal–organic frameworks with intact porosity, J. Am. Chem. Soc. 136 (2014) 16978–16981. [27] J.G. Duan, J.F. Bai, B.S. Zheng, Y.Z. Li, W.C. Ren, Controlling the shifting degree of interpenetrated metal–organic frameworks by modulator and temperature and their hydrogen adsorption properties, Chem. Commun. 47 (2011) 2556–2558. [28] J.G. Duan, Z. Yang, J.F. Bai, B.S. Zheng, Y.Z. Li, S.H. Li, Highly selective CO2 capture of an agw-type metal–organic framework with inserted amides: experimental and theoretical studies, Chem. Commun. 48 (2012) 3058–3060. [29] J.G. Duan, M. Higuchi, M.L. Foo, S. Horike, K.P. Rao, S. Kitagawa, A family of rare earth porous coordination polymers with different flexibility for CO2/C2H4 and CO2/C2H6 separation, Inorg. Chem. 52 (2013) 8244–8249. [30] B. Mu, F. Li, Y.G. Huang, K.S. Walton, Breathing effects of CO2 adsorption on a flexible 3D lanthanide metal–organic framework, J. Mater. Chem. 22 (2012) 10172–10178.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
The roles of rod-packed organic units for preparing stable porous coordination polymers with carboxylate ligands Jingui Duan ⁎, Wanqin Jin State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China
a r t i c l e
i n f o
Article history: Received 18 July 2015 Received in revised form 14 August 2015 Accepted 23 August 2015 Available online 31 August 2015 Keywords: Porous coordination polymers Rod-packed organic units Stability La Adsorption
a b s t r a c t Despite tremendous efforts, designing and preparing water, chemical, and moisture stable porous coordination polymers (PCPs) with carboxylate ligands remain a challenge. Here, we firstly and systemically investigated the roles of rod-packed organic units for preparing the stable PCPs with different carboxylate ligands. The experimental results demonstrated that the rod packed organic units with short distance of adjacent ligands, bridged by La3+ metal chain, can form water, chemical, and moisture stable PCPs. © 2015 Elsevier B.V. All rights reserved.
In recent years, the fascinating discovery in the area of functional, nanostructured materials is the emergence of large numbers of structurally defined porous coordination polymers (PCPs) [1,2]. Their versatile architectures and customizable chemical properties could enable them to be attractive candidates for gas storage [3–7], separation [8–10] and catalysis [11,12]. Water, chemical, and moisture stabilities are important factors for any sorbent that has potential to be used for practical applications, especially for PCP materials [13–19]. Till now, almost twenty thousand of PCPs have been reported [20]; however, unfortunately, some of the most promising PCPs are limited because of instability with respect to moisture and water [4,21,22]. Therefore, for offering an opportunity to address this problem, it is essential to find and understand the factors that affect the stability of porous frameworks, especially for the framework with carboxylate ligands. PCPs are crystalline materials consisted of metal ions/clusters and organic ligands, which are bridged together to form extended porous frameworks. The stability of PCPs should be dominated by the strength of coordination bonds between ligands and metal centers. Generally, the methods for enhancing the strength or preventing the corrodibility of coordination bonds were believed as rational ideas for accessing stable frameworks [23]. For example, by introducing hydrophobic alkyl-chains around coordinate sites, IRMOF-1 showed an enhanced moisture-resistant [24]. Additionally, the rational-design methods of postsynthetic modification and polymer coating can produce ⁎ Corresponding author. E-mail address: [email protected] (J. Duan).
http://dx.doi.org/10.1016/j.inoche.2015.08.016 1387-7003/© 2015 Elsevier B.V. All rights reserved.
moisture/water-resistant and superhydrophobic PCPs with intact porosity [25,26]. Further, the strategy of using polyazolate-bridging ligands can lead to frameworks with strong metal–nitrogen bonds, providing a greater chemical and thermal stability compared to their carboxylate-based counterparts [13]. However, the generated water and moisture stable PCPs by employing the above methods will suffer the decreasing of their pore size and volume, and further, some approached only feasible to some specific PCPs. Very recently, we reported two La-based PCPs with easily-prepared carboxylate ligands [16,17]. Their good water and chemical resistance encouraged us to speculate that the coordination of ahighly-coordinated metal center (from conventional 4 or 6 to 8 or 9) with rod-packed organic units may be a new method to prepare stable PCP, because the increase of connectivity can possibly improve the strength of connection between metal clusters and ligands, and more importantly, the well-packed organic units could provide a tight space for preventing the attack of M–O bonds from water or moisture. If this idea works well, it will definitely facilitate the design and preparation of stable PCPs with such great number of carboxylate ligands. We are interested in the construction of porous coordination polymers with interesting properties based upon carboxylate ligands [27-29]. Herein, in order to confirm this idea, four La-based PCPs (La–BTC, La–BTB-1, La–BTB-2, La–BTN) with three different carboxylate ligands (1,3,5-Benzenetricarboxylic acid: H 3 BTC; 1,3,5-Tris(4-carboxyphenyl)benzene:H3 BTB; 1,3,5-Tri(6hydroxycarbonylnaphthalen-2-yl) benzene: H3BTN) were synthesized and structurally characterized. Powder X-ray diffraction pattern (PXRD) and gas adsorption experiments of treated samples
54
J. Duan, W. Jin / Inorganic Chemistry Communications 61 (2015) 53–56
demonstrated that the noticeable variation of structural stability should be attributed to different types of ligand arrangements along metal chains. These four PCPs were solvothermally synthesized from a mixture of La(NO3)3·6H2O and H3BTC, H3BTB and H3BTN in different solvent systems, respectively (Fig. 1). Two of them (La–BTN and La–BTB-1) as reported in our previous work [16,17] and the third one (LaBTB-2) were prepared according to Walton's work [30]. The phase purities of the PCPs were confirmed by PXRD. The new structure of La–BTC was determined by single-crystal X-ray diffraction analysis. Thermogravimetric analyses (TGA) show that the four PCPs are thermally stable up to 400 °C under N2 atmosphere. Single crystal X-ray diffraction revealed that the structure of [La(BTC)(H2O)]3·xGuest is chiral, with the space group of P43. The asymmetric unit of it consists of one La3+ ion, one BTC ligand and one coordinated water. The La3 + ion is nine-coordinated and binds to eight oxygen atoms from six different BTC3 −ligands and one water molecule. Each BTC3 −ligand connects to six different La3 + ions. The LaO8(H2O) polyhedra are connected by corner-sharing to form a chiral of –La–O–C–O–La– along the c-axis. These chains are bridged together by the BTC3−ligands to form a 3D framework (Fig. 1a), with the pore size of 7 × 7 Å2. By comparing these four structures, we found that the space groups of them are all chiral. All of the coordination geometries of the La center were completed by eight oxygen atoms from six different carboxylate groups and one coordinated water/DMF molecule. The metal–oxygen chains with closed La/La distances of 4.224 to 4.293 Å were bridged by BTC, BTB and BTN ligands in those series of PCPs (Fig. S8). However, it is worth noting that the arrangements of the ligands in them show significant differences. For La–BTC, the adjacent BTC moieties adopt the end-to-end type, while that in other compounds took the face-to-face type. For La–BTB-1, the distances between the adjacent benzene rings are around 4 Å, which means that there is no enough space for any extra guest molecules (even including H2 molecules) to go inside between every two adjacent benzene rings. Thus, the well-packed benzene rings with a high concentration of protons act like a wall that can provide a hydrophobic environment along the channel. However, the extended distance of adjacent ligands was found in La–BTN and La–BTB-2 (Fig. 2). Therefore, we can theorize that the coordination
geometries of La3+ completed by rod-packed units with different distances will bring the PCPs with varied water and moisture resistance. Although three of the structures have been reported, their water, chemical, and moisture stabilities, especially at high temperature, have not been well explored. Further, the roles of rod-packed organic units in influencingthe stabilities of PCPs have also not been reported before. Thus, systemic study of the stabilities of those four La–PCPs is very necessary. Generally, the combined results show that PXRD and gas adsorption were the most effective methods for indentifying the integrity of porous materials. The frameworks with high stability should keep their original PXRD patterns, as well as their gas uptake capacities. The fresh samples of those four PCPs were soaked in hot (60 °C and 100 °C) aqueous HCl (pH = 2), aqueous NaOH (pH = 12) and pure water for 24 h. After the temperature cools down, part of the samples was used for PXRD pattern collection (Figs. S9–S12) and the rest of them were used for gas sorption experiments (exchanged by methanol for three times and degassed at 120 °C for 24 h, Figs. S14–S17). As shown in Figs. S9–S12, the framework of La–BTC transformed to another unknown phase, whereas, the PXRD patterns of La–BTB-1, La–BTB-2, and La–BTN matched well with their original form, indicating the integrity of their frameworks. Meanwhile, the gas adsorption results demonstrated that treated La–BTC did not show any gas uptakes at low pressure range. The very low amount of gas uptake in it at around 95 kPa should be ascribed to the adsorption of apertures between particles. Thus, La–BTC cannot maintain its porosity under such condition. For La–BTB-2, the gas uptake of treated samples decreased from 300 cm3·g−1 to ~20 cm3·g−1 as well as the disappearance of gate opening at P/P0 = 0.3, even if the PXRD pattern of the samples remained the same after treatment. Interestingly, for La–BTN, the gas uptake of treated samples (pH = 2 and 12) at 100 °C just decreased a little, and moreover, the gas uptake of La–BTN (pH = 7) is nearly the same as that of the fresh one, exhibiting a good integrity of the framework. Moreover, the treated La–BTB-1 maintained the same gas uptakes as its fresh sample, except for the sample that was treated under acid solution (pH = 2) at 100 °C. To further explore the stability of these materials, we treated the activated sample of them at 80 °C and 80% RH for 24 h. The PXRD patterns (Figs. S9–S12) and gas uptakes (Figs. S18–S21) of the samples reveal that La–BTC transforms to that unknown phase without porosity
Fig. 1. The packing view of those four PCPs (inner surfaces: yellow, outer surfaces: gray).
J. Duan, W. Jin / Inorganic Chemistry Communications 61 (2015) 53–56
55
Fig. 2. The view of six adjacent ligands along the La chains in each of them. Probe radius for organic surface is 1.4 Å.
again. La–BTB-2 also changed to a new unknown phase; however, the significant gate-opening behavior that has been found in the fresh sample disappeared after the moisture treatment. This interesting and rare phenomenon indicates that moisture treatment may contribute to the change of framework flexibility. More importantly, the gas uptakes of the samples before and after treatment decreased only a little in La–BTN, but the capacity remainedalmost same in the structure of LaBTB-1. Those results strongly indicate the good integrity of the framework, which could be confirmed by the PXRD again. In order to easily evaluate the stability of these four PCPs, we defined a concept of relative integrity, and which is the ratio of the maximum gas uptake after and before the hydro/chemical treatments. As shown in Fig. 3, we found that a striking feature of those four La-based PCPs is the enhanced value of relative integrity associated with the tightness of
organic rods. Therefore, taking the crystal structures into consideration, we can conclude that the noticeable impact of the structural stability of these series of La-based PCPs should be assigned as different ligand arrangements along the metal chains, as the coordination number of La3+is almost the same in these four PCPs. The shorter distance between the adjacent ligand units will provide less opportunity for water to attack the M–O bonds which is strongly associated with the relative integrity of the porous framework. In summary, a new porous structure (La–BTC), and other three previously reported La-based PCPs, La–BTB-1, La–BTB-2, and La–BTN, were synthesized and characterized. PXRD and gas sorption measurements demonstrated that the rod-packed organic units with short distance of adjacent ligands, bridged by highly-coordinated metals chain, can access the water, chemical, and moisture stable PCPs. Thus, we believe that this discovery can substantially benefit for preparing stable PCPs with carboxylate ligands. Work along this strategy via other metal centers or polydentate carboxylate ligands is currently underway in our laboratory. Acknowledgments The authors gratefully acknowledge supports from the National Natural Science Foundation of China (No. 21301148), Talent Development Program of Nanjing Tech University (39801116) and Foundation of State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201406). Appendix A. Supplementary material
Fig. 3. The relative integrity of these series of PCPs.
CCDC 1047216 contains the supplementary crystallographic data for La–BTC. These data can be obtained free of charge via http://www.ccdc. cam.ac.uk/conts/retrieving.html, or from the CCDC, 12 Union Road, Cambridge CB21EZ, UK; or Email: [email protected]. Additional
56
J. Duan, W. Jin / Inorganic Chemistry Communications 61 (2015) 53–56
experimental details as supplementary information are available at http://dx.doi.org/10.1016/j.inoche.2015.08.016. References [1] S. Kitagawa, R. Kitaura, S. Noro, Functional porous coordination polymers, Angew. Chem. Int. Ed. 43 (2004) 2334–2375. [2] H. Li, M. Eddaoudi, M. O'Keeffe, O.M. Yaghi, Design and synthesis of an exceptionally stable and highly porous metal–organic framework, Nature 402 (1999) 276–279. [3] Y. Yan, S.H. Yang, A.J. Blake, M. Schroder, Studies on metal–organic frameworks of Cu(II) with isophthalate linkers for hydrogen storage, Acc. Chem. Res. 47 (2014) 296–307. [4] H. Furukawa, N. Ko, Y.B. Go, N. Aratani, S.B. Choi, E. Choi, A.O. Yazaydin, R.Q. Snurr, M. O'Keeffe, J. Kim, O.M. Yaghi, Ultrahigh porosity in metal–organic frameworks, Science 329 (2010) 424–428. [5] H.X. Deng, S. Grunder, K.E. Cordova, C. Valente, H. Furukawa, M. Hmadeh, F. Gandara, A.C. Whalley, Z. Liu, S. Asahina, H. Kazumori, M. O'Keeffe, O. Terasaki, J.F. Stoddart, O.M. Yaghi, Large-pore apertures in a series of metal–organic frameworks, Science 336 (2012) 1018–1023. [6] K. Sumida, D.L. Rogow, J.A. Mason, T.M. McDonald, E.D. Bloch, Z.R. Herm, T.H. Bae, J.R. Long, Carbon dioxide capture in metal–organic frameworks, Chem. Rev. 112 (2012) 724–781. [7] L.J. Murray, M. Dinca, J.R. Long, Hydrogen storage in metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1294–1314. [8] E.D. Bloch, W.L. Queen, R. Krishna, J.M. Zadrozny, C.M. Brown, J.R. Long, Hydrocarbon separations in a metal–organic framework with open iron(II) coordination sites, Science 335 (2012) 1606–1610. [9] J.R. Li, J. Sculley, H.C. Zhou, Metal–organic frameworks for separations, Chem. Rev. 112 (2012) 869–932. [10] J.R. Li, R.J. Kuppler, H.C. Zhou, Selective gas adsorption and separation in metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1477–1504. [11] J. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Metal–organic framework materials as catalysts, Chem. Soc. Rev. 38 (2009) 1450–1459. [12] L. Ma, C. Abney, W. Lin, Enantioselective catalysis with homochiral metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1248–1256. [13] V. Colombo, S. Galli, H.J. Choi, G.D. Han, A. Maspero, G. Palmisano, N. Masciocchi, J.R. Long, High thermal and chemical stability in pyrazolate-bridged metal–organic frameworks with exposed metal sites, Chem. Sci. 2 (2011) 1311–1319. [14] N.T.T. Nguyen, H. Furukawa, F. Gandara, H.T. Nguyen, K.E. Cordova, O.M. Yaghi, Selective capture of carbon dioxide under humid conditions by hydrophobic chabazite-type zeolitic imidazolate frameworks, Angew. Chem. Int. Ed. 53 (2014) 10645–10648. [15] T.A. Makal, X. Wang, H.C. Zhou, Tuning the moisture and thermal stability of metal– organic frameworks through incorporation of pendant hydrophobic groups, Cryst. Growth Des. 13 (2013) 4760–4768.
[16] J.G. Duan, M. Higuchi, R. Krishna, T. Kiyonaga, Y. Tsutsumi, Y. Sato, Y. Kubota, M. Takata, S. Kitagawa, High CO2/N2/O2/CO separation in a chemically robust porous coordination polymer with low binding energy, Chem. Sci. 5 (2014) 660–666. [17] J.G. Duan, M. Higuchi, S. Horike, M.L. Foo, K.P. Rao, Y. Inubushi, T. Fukushima, S. Kitagawa, High CO2/CH4 and C2 hydrocarbons/CH4 selectivity in a chemically robust porous coordination polymer, Adv. Funct. Mater. 23 (2013) 3525–3530. [18] H.L. Jiang, D.W. Feng, K.C. Wang, Z.Y. Gu, Z.W. Wei, Y.P. Chen, H.C. Zhou, An exceptionally stable, porphyrinic Zr metal–organic framework exhibiting pH-dependent fluorescence, J. Am. Chem. Soc. 135 (2013) 13934–13938. [19] S.J. Yang, C.R. Park, Preparation of highly moisture-resistant black-colored metal organic frameworks, Adv. Mater. 24 (2012) 4010–4013. [20] H. Furukawa, K.E. Cordova, M. O'Keeffe, O.M. Yaghi, The chemistry and applications of metal–organic frameworks, Science 341 (2013) 974–986. [21] O.K. Farha, A.O. Yazaydin, I. Eryazici, C.D. Malliakas, B.G. Hauser, M.G. Kanatzidis, S.T. Nguyen, R.Q. Snurr, J.T. Hupp, De novo synthesis of a metal–organic framework material featuring ultrahigh surface area and gas storage capacities, Nat. Chem. 2 (2010) 944–948. [22] D. Han, F.L. Jiang, M.Y. Wu, L. Chen, Q.H. Chen, M.C. Hong, A non-interpenetrated porous metal–organic framework with high gas-uptake capacity, Chem. Commun. 47 (2011) 9861–9863. [23] D.Y. Ma, Y.W. Li, Z. Li, Tuning the moisture stability of metal–organic frameworks by incorporating hydrophobic functional groups at different positions of ligands, Chem. Commun. 47 (2011) 7377–7379. [24] J. Yang, A. Grzech, F.M. Mulder, T.J. Dingemans, Methyl modified MOF-5: a water stable hydrogen storage material, Chem. Commun. 47 (2011) 5244–5246. [25] J.G. Nguyen, S.M. Cohen, Moisture-resistant and superhydrophobic metal–organic frameworks obtained via postsynthetic modification, J. Am. Chem. Soc. 132 (2010) 4560–4561. [26] W. Zhang, Y. Hu, J. Ge, H.L. Jiang, S.H. Yu, A facile and general coating approach to moisture/water-resistant metal–organic frameworks with intact porosity, J. Am. Chem. Soc. 136 (2014) 16978–16981. [27] J.G. Duan, J.F. Bai, B.S. Zheng, Y.Z. Li, W.C. Ren, Controlling the shifting degree of interpenetrated metal–organic frameworks by modulator and temperature and their hydrogen adsorption properties, Chem. Commun. 47 (2011) 2556–2558. [28] J.G. Duan, Z. Yang, J.F. Bai, B.S. Zheng, Y.Z. Li, S.H. Li, Highly selective CO2 capture of an agw-type metal–organic framework with inserted amides: experimental and theoretical studies, Chem. Commun. 48 (2012) 3058–3060. [29] J.G. Duan, M. Higuchi, M.L. Foo, S. Horike, K.P. Rao, S. Kitagawa, A family of rare earth porous coordination polymers with different flexibility for CO2/C2H4 and CO2/C2H6 separation, Inorg. Chem. 52 (2013) 8244–8249. [30] B. Mu, F. Li, Y.G. Huang, K.S. Walton, Breathing effects of CO2 adsorption on a flexible 3D lanthanide metal–organic framework, J. Mater. Chem. 22 (2012) 10172–10178.