Toward a robust porous coordination polymer: the inhibition of mutual movement between interpenetrating sub-networks by introduction of multiple C–H⋯π interactions

Abstract

Two isostructural porous coordination polymers based on a tetrahedral ligand and azamacrocyclic Ni(II) complexes were synthesized and structurally characterized. This work demonstrates that a robust porous coordination polymer can be stabilized by intermolecular C–H⋯π contacts between the sub-networks. Furthermore, these compounds exhibit two kinds of gas separation mechanism related to the different pore sizes they possess.

---

Porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) that exhibit regular and definite porous structures have attracted extensive interest in the last two decades. The unique and tunable feature of PCPs may serve as a platform to study various properties, such as gas adsorption and separation, magnetism, catalysis, non-linear optics and so on.^1–10 Despite numerous PCPs have been reported, some of them are not robust enough to be further utilized due to structural collapse after the activated process or water immersion. Several effective strategies have been put forward to regulate the robustness of PCPs, such as the utilization of functional ligand,^11–13 the inducement of interpenetration in the framework,^14–16 and the use of strong coordination bond^17–20 as well as the introduction of weak interactions,^21–23 etc. In this paper, we demonstrate an effective strategy to construct robust porous coordination polymer by introduction of multiple C–H⋯π interactions. Two microporous coordination polymers were obtained, which exhibit the selectivity of CO₂ over N₂.

Compound [Ni(cyclam)]₂(MTB)·8H₂O·4DMF (3) (MTB⁴⁻ = methanetetrabenzoate and cyclam = 1,4,8,11-tetraazacyclotetradecane) reported by Suh et al. has a diamondoid sub-network with a large cavity.²⁴ In spite of the 4-fold interpenetration, there is still considerable channel throughout the structure. However, such a PCP based on rigid MTB⁴⁻ ligand collapsed upon removal of guest molecules. The collapse can be ascribed to the relative slip of adjacent diamondoid sub-networks.^25–27 To inhibit such mutual movement between interpenetrating sub-networks, we chose two cyclam-like macrocyclic metallic tectons as precursors to enhance the C–H⋯π interaction between adjacent sub-nets (Scheme 1). Two new PCPs, namely, [(NiL¹)₂(MTB)]·8H₂O·DMF (1) and [(NiL²)₂(MTB)]·10H₂O (2) (L¹ = 1,3,6,8,11,14-hexza-tricyclooctadecane and L² = 1,3,6,8,12,15-hexza-tricycloeicosane) were obtained. Compound 1 is a less stable PCP, but compound 2 is stable.

Scheme 1 The strategy employed to construct robust porous coordination polymer.

The X-ray crystal structure analyses of compounds 1 and 2 reveal that they are isostructural and crystallize in the tetragonal space group of P4̄b2 with an asymmetric unit that contains 1/2 of (NiL^1,2)²⁺ cation, 1/4 of (MTB)⁴⁻ ion and several guest molecules. As shown in Fig. 1a, all nickel atoms adopt distorted [NiN₄O₂] octahedral geometries, where four nitrogen atoms come from ligand L^1,2 and two oxygen atoms from two individual (MTB)⁴⁻ ligands at the axial positions. Every (MTB)⁴⁻ ligand links to four different (NiL^1,2)²⁺ fragments, resulting a 3D dia sub-network. The diamondoid cage size is 1.7 nm × 4.9 nm (Fig. 1b). The large size of diamondoid cage permits 4-fold equivalent networks to interpenetrate each other (Fig. 1c). In spite of 4-fold interpenetration, compounds 1, 2 still possess a significant solvent-accessible space (Fig. 1d). The extra methylene groups in compound 2 expose on the porous channel, rendering the different pore size of two compounds (Fig. S1†). The aperture of compound 1 and 2 is with an estimated size of ∼5.5 Å and ∼3.6 Å respectively that excludes the van der Waals radius. The void volume in compounds 1, 2 is calculated by PLATON to be 28.8% and 21.7% of the total crystal volume respectively.²⁸

Fig. 1 (a) The molecular structure of compound 1, #: 1.5 − x, 1.5 − y, 2 − z. (b) Diamonded cage of the sub-network. (c) The 4-interpenetration of diamonded sub-nets. (d) Pore channel in compound 1.

The crystal structures of compounds 1, 2 are very similar to a known example [Ni(cyclam)]₂(MTB)·8H₂O·4DMF (3). Careful examination of the crystal structures shows that obvious difference of 1–3 comes from weak interactions between the adjacent interpenetrating sub-networks. Two cyclam-liked macrocyclic metallic tectons possess five-membered ring or six-membered ring above and below the pseudo-plane of azamacrocyclic ligands L¹ and L², respectively (Fig. S2†). Significantly, these five- and six-membered rings point to adjacent benzene ring of (MTB)⁴⁻, forming weak C–H⋯π interactions (Fig. S3†). The distance of C–H⋯π interaction within the scope of high level ab initio calculations.^29–32 Due to –(CH₂)₂– group on L¹ and –(CH₂)₃– group on L², the number of C–H⋯π interaction sites in compound 2 is more than compound 1, and the least for compound 3. The different C–H⋯π interactions lead to different physical properties in compounds 1–3. Compounds 1 and 3 can easily dissolve in water, however, compound 2 is stable in a wide range of pH (Fig. S4†). This phenomenon reveals that the weak C–H⋯π interaction can enhance the chemical stability. The stability of compounds 1–3 after desolvation is different. The XRD patterns of desolvated compound 1 (1d) show a slight shift, along with the appearance of a few peaks compared with as-synthesized sample (Fig. S5a†). This is most probably due to the slip of sub-nets. Significantly, the desolvated compound 2 (2d) exhibits identical XRD patterns, typical of robust porous structure (Fig. S5b†). The results demonstrate that compound 1 is a less stable PCP and compound 2 is stable. In consideration of the unstable of desolvated compound 3, the mutual movement of diamonded sub-nets can be inhibited by introduction of multiple C–H⋯π, thus giving rise to robust PCP.

Thermogravimetric analyses (TGA) show that all the guest solvents readily lost in the range of 30–200 °C for compound 1 (cal. 16.2%, found 15.9%) and in the range of 30–125 °C for compound 2 (cal. 13.3%, found 11.9%) (Fig. S6†). Hence compound was activated at 200 °C for compound 1 and 150 °C for compound 2 under high vacuum to obtain desolvated samples of 1d and 2d. The porosity of 1d and 2d was measured by N₂ at 77 K and CO₂ at 195 K. As shown in Fig. 2a, the sorption for 1d revealed typical type-I isotherm with saturated N₂ uptake of 177.3 cm³ g⁻¹ and CO₂ uptake of 202.8 cm³ g⁻¹, which is characteristic of a microporous material. The corresponding BET surface area, Langmuir surface and pore volume is of 556.7 m² g⁻¹, 733.1 m² g⁻¹ and 0.28 cm³ g⁻¹ respectively. The mean pore size is 4.5 Å revealed by Horvath–Kawazoe mode (Fig. S7†), which is smaller than that calculated from the single crystal structure analysis. The smaller pore size also indicates the sliding of interpenetrated networks.

Fig. 2 The adsorption and desorption isotherm of 1d (a) and 2d (b) for N₂ at 77 K and CO₂ at 195 K.

As for 2d, the sample can only absorb CO₂ at 195 K with saturated uptake of 180.3 cm³ g⁻¹ (Fig. 2b). Compound 2d does not show any appreciable uptake of N₂ at 77 K due to the limitation of pore size. From the single crystal structure, the pore size is smaller than kinetic diameter of N₂ (3.8 Å), but larger than that of CO₂ (3.3 Å).² Hence, the unique pore size can be utilized to separate CO₂ over N₂.

As shown in Fig. 3, the CO₂ uptakes under 800 mmHg pressure for 1d are 75.3 cm³ g⁻¹ at 273 K and 45.3 cm³ g⁻¹ at 293 K, 88.8 cm³ g⁻¹ (273 K) and 49.4 cm³ g⁻¹ (293 K) for 2d. The zero coverage Q_st was calculated to be 49.7 kJ mol⁻¹ and 38.8 kJ mol⁻¹ for 1d and 2d, respectively, indicating strong affinity toward CO₂ (Fig. S8†). It has been proved that functional groups on the porous surface and smaller pore size benefit the CO₂ storage.^33–35 In these compounds, the uncoordinated oxygen atoms of –COO⁻ groups expose on the porous channel and the pore size is in the range of ultramicropore. These merits should provide strong binding ability to CO₂.

Fig. 3 The gas uptake of 1d (a) and 2d (b) for N₂ at 273 K and CO₂ at 273 K and 293 K.

To evaluate the selectivity of CO₂ over N₂, the adsorption isotherms of N₂ at 273 K were also measured (Fig. 3). At 273 K, 1d merely absorbs 1.44 cm³ g⁻¹ of N₂ at 1 atm. The adsorptive ratio of CO₂/N₂ is about 52 at 1 atm, which is comparable to many similar PCPs.^12,36–39 Since the aperture of 2d prohibits nitrogen into pores, 2d also shows absolutely gas selective adsorption ability.

The mechanism for the selectivity of CO₂ over N₂ for 1d and 2d is described in Fig. S11.† Since the pore size of 1d is larger than CO₂/N₂ and polar groups are located on the porous surface, 1d follows mechanism I. Moreover, mechanism I suits for the vast majority of PCPs. Due to the addition of –CH₂– group on porous surface, the pore size of 2d is larger than CO₂, but small than N₂. Hence, the gas adsorption property of 2d follows mechanism II. To date, examples that obey mechanism II are rare.⁴⁰ For the separation of CO₂ over N₂, the development of compounds that follows mechanism II is important. Theoretically, such compounds can completely separate CO₂ over N₂ from the gaseous mixture.

Conclusions

By slight modification of the macrocyclic metallic tectons, two isostructural porous coordination polymers based on methanetetrabenzoate are synthesized and characterized. The systematic comparison of the analogous compounds demonstrates that robust porous coordination polymer can be gained by introduction of multiple C–H⋯π interactions between sub-nets.

Acknowledgements

This work was supported by the 973 program (2013CB933403) and the National Natural Science Foundation of China (No. 91222104, 21771103 and 20121318518). We thank Bo Liu of Northwest University for the measurement of CO₂ adsorption at 195 K.

References

1. G. Ferey, Chem. Soc. Rev., 2008, 37, 191.
2. J.-R. Li, R. J. Kuppler and H.-C. Zhou, Chem. Soc. Rev., 2009, 38, 1477.
3. L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. van Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105.
4. M. P. Suh, H. J. Park, T. K. Prasad and D.-W. Lim, Chem. Rev., 2012, 112, 782.
5. V. Guillerm, D. Kim, J. F. Eubank, R. Luebke, X. Liu, K. Adil, M. S. Lah and M. Eddaoudi, Chem. Soc. Rev., 2014, 43, 6141.
6. A. Schneemann, V. Bon, I. Schwedler, I. Senkovska, S. Kaskel and R. A. Fischer, Chem. Soc. Rev., 2014, 43, 6062.
7. S. Qiu, M. Xue and G. Zhu, Chem. Soc. Rev., 2014, 43, 6116.
8. T. Zhang and W. Lin, Chem. Soc. Rev., 2014, 43, 5982.
9. W.-Y. Gao, M. Chrzanowski and S. Ma, Chem. Soc. Rev., 2014, 43, 5841.
10. B. Seoane, J. Coronas, I. Gascon, M. E. Benavides, O. Karvan, J. Caro, F. Kapteijn and J. Gascon, Chem. Soc. Rev., 2015, 44, 2421.
11. T. A. Makal, X. Wang and H.-C. Zhou, Cryst. Growth Des., 2013, 13, 4760.
12. L. Jiang, X.-R. Meng, H. Xiang, P. Ju, D.-C. Zhong and T.-B. Lu, Inorg. Chem., 2012, 51, 1874.
13. D. Ma, Y. Li and Z. Li, Chem. Commun., 2011, 47, 7377.
14. X. Zhao, J. Dou, D. Sun, P. Cui, D. Sun and Q. Wu, Dalton Trans., 2012, 41, 1928.
15. H.-L. Jiang, T. A. Makal and H.-C. Zhou, Coord. Chem. Rev., 2013, 257, 2232.
16. X. Jiang, Z. Li, Y. Zhai, G. Yan, H. Xia and Z. Li, CrystEngComm, 2014, 16, 805.
17. J. J. Low, A. I. Benin, P. Jakubczak, J. F. Abrahamian, S. A. Faheem and R. R. Willis, J. Am. Chem. Soc., 2009, 131, 15834.
18. J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga and K. P. Lillerud, J. Am. Chem. Soc., 2008, 130, 13850.
19. H. J. Choi, M. Dincă, A. Dailly and J. R. Long, Energy Environ. Sci., 2010, 3, 117.
20. D. Feng, K. Wang, J. Su, T.-F. Liu, J. Park, Z. Wei, M. Bosch, A. Yakovenko, X. Zou and H.-C. Zhou, Angew. Chem., Int. Ed., 2015, 54, 149.
21. D.-S. Zhang, Z. Chang, Y.-F. Li, Z.-Y. Jiang, Z.-H. Xuan, Y.-H. Zhang, J.-R. Li, Q. Chen, T.-L. Hu and X.-H. Bu, Sci. Rep., 2013, 3, 3312.
22. E. Y. Lee and M. P. Suh, Angew. Chem., Int. Ed., 2004, 43, 2798.
23. Z.-Z. Lu, R. Zhang, Y.-Z. Li, Z.-J. Guo and H.-G. Zheng, J. Am. Chem. Soc., 2011, 133, 4172.
24. Y. E. Cheon and M. P. Suh, Chem.–Eur. J., 2008, 14, 3961.
25. C.-T. He, P.-Q. Liao, D.-D. Zhou, B.-Y. Wang, W.-X. Zhang, J.-P. Zhang and X.-M. Chen, Chem. Sci., 2014, 5, 4755.
26. T. K. Prasad and M. P. Suh, Chem.–Eur. J., 2012, 18, 8673.
27. T. K. Maji, R. Matsuda and S. Kitagawa, Nat. Mater., 2007, 6, 142.
28. A. J. Spek, J. Appl. Crystallogr., 2003, 36, 7.
29. S. Tsuzuki, Annu. Rep. Prog. Chem., Sect. C: Phys. Chem., 2012, 108, 69.
30. J. Ran and M. W. Wong, J. Phys. Chem. A, 2006, 110, 9702.
31. M. Nishio, Y. Umezawa, K. Honda, S. Tsuboyama and H. Suezawa, CrystEngComm, 2009, 11, 1757.
32. M. Nishio, Phys. Chem. Chem. Phys., 2011, 13, 13873.
33. D.-M. Chen, N. Xu, X.-H. Qiu and P. Cheng, Cryst. Growth Des., 2015, 15, 961.
34. A. Demessence, D. M. D'Alessandro, M. L. Foo and J. R. Long, J. Am. Chem. Soc., 2009, 131, 8784.
35. K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Chem. Rev., 2011, 112, 724.
36. P. Ju, L. Jiang and T.-B. Lu, Inorg. Chem., 2015, 54, 6291.
37. W.-Y. Gao, Y. Niu, Y. Chen, L. Wojtas, J. Cai, Y.-S. Chen and S. Ma, CrystEngComm, 2012, 14, 6115.
38. B. Li, M. Chrzanowski, Y. Zhang and S. Ma, Coord. Chem. Rev., 2015 DOI:10.1016/j.ccr.2015.05.005.
39. W.-Y. Gao, Y. Chen, Y. Niu, K. Williams, L. Cash, P. J. Perez, L. Wojtas, J. Cai, Y.-S. Chen and S. Ma, Angew. Chem., Int. Ed., 2014, 53, 2615.
40. P. Kanoo and T. K. Maji, Eur. J. Inorg. Chem., 2010, 3762.

---

Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic file in CIF format, XRD patterns of compounds and desolvated compounds, additional figures, tables and TGA figures, etc. CCDC 1420174 and 1420175 for compounds 1 and 2, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra18848b

---