Induction of trimeric [Mg₃(OH)(CO₂)₆] in a porous framework by a desymmetrized tritopic ligand

Abstract

The use of a desymmetrized tritopic ligand with both carboxyl and pyridyl functionalities leads to the first occurrence of the [Mg₃(μ₃-OH)(CO₂)₆] trimer as the 3-D framework building block in a porous crystal that shows relatively high H₂ uptake (1.37% at 77 K and 1 atm).

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Crystalline metal–organic frameworks (MOFs) have attracted a great deal of attention because of their fascinating topologies¹ and potential applications such as H₂storage² and CO₂ capture.³ Most MOF-type materials reported to date are based on 3d- or 4f- elements (e.g.zinc, copper, cobalt and nickel).⁴ In comparison, the synthesis of highly porous materials based on lightweight main group elements such as Li, Be, B, Mg, and Al is considerably more challenging, but highly desirable, because with these lightweight elements, the formation of low-formula-weight materials would be expected, which, together with porosity, could significantly help improve the gravimetric storage capacity.⁵

Among these lightweight elements, magnesium is particularly interesting, because it bears a number of similarities to the transition metal ions commonly used to make MOFs.⁶ For example, it often adopts similar coordination chemistry to 3d elements, in part because of the comparable ionic radii (Mg²⁺, 72 pm; Cu²⁺, 73 pm; Zn²⁺, 74 pm). The similar coordination chemistry is most evident in the structures of metalloporphyrins. In MOF chemistry, such similarity between Mg and 3d elements is highlighted by a series of MOF-74 (also called CPO-27) isostructural solids which have been found to exhibit the highest CO₂ uptake under ambient conditions.⁷

Despite the aforementioned similarities, porous Mg-MOFs are still considered a rarity among the vast collection of MOFs.⁸ While plenty of evidence exists for the similarity between Mg and 3d elements when they are in various monomeric forms, relatively few examples are known of Mg-cluster-based structures. For example, a literature search shows that no Mg–carboxylate frameworks contain [M₃(μ₃-O/μ₃-OH)(CO₂)₆] building blocks,⁹ even though many examples are known for 3d elements. Furthermore, to our knowledge, square paddle-wheel units which are a common occurrence with Co, Ni, Cu, and Zn-based MOFs are also not found in Mg-MOFs.

Since MOF’s structures are often based on metal clusters, whether Mg can adopt cluster structures comparable to those of 3d elements is critical to the eventual development of Mg-MOF chemistry. Thus, it is important to study Mg-MOF chemistry in order to identify synthetic and structural factors that influence the formation of Mg clusters.

The well-known trinuclear clusters of the type [M₃(μ₃-O/μ₃-OH)(CO₂)₆] (M = Cr, Fe, Co, Ni, etc.) represent an interesting building unit for porous crystalline framework construction.^10,11 Such building blocks have been used in the construction of some highly porous materials. Undoubtedly, the discovery of such symmetrical building blocks based on Mg trimers can provide fresh opportunities for constructing novel low-density Mg-MOF materials.

To help stabilize trimeric clusters and induce the formation of elusive Mg trimers, triangular ligands with pyridyl and carboxyl groups would be a good choice because such a ligand has a unique ability to stabilize the trimeric structure by bonding to the edge of the trimer using the carboxyl groups while simultaneously using the pyridyl group to attach to the apical position of the trimer.¹² In this work, by using a triangular ligand pyridine-3,5-bis(phenyl-4-carboxylic acid) (H₂PBPC), we successfully obtained an interesting magnesium-based crystalline porous framework, namely, {[(CH₃)₂NH₂][Mg₃(μ₃-OH)(PBPC)₃]·xSolvent}_n (CPF-3), which is the first Mg-MOF example constructed from the [Mg₃(μ₃-OH)(CO₂)₆] trimer. The gas adsorption properties demonstrate that even though the presence of extra-framework [(CH₃)₂NH₂]⁺ cations leads to a significantly decreased BET surface area, its H₂ sorption properties is comparable to the structurally analogous mixed-valent Ni or Fe-containing neutral frameworks.¹³

Treatment of Mg(NO₃)₃·6H₂O and H₂PBPC in DMA afforded gray cubic crystals of CPF-3.‡Single-crystal X-ray diffraction analysis§ reveals that CPF-3 crystallizes in the rhombohedral space group R3̄c. The oxygen at the center of the trimer, which lies on a site with the crystallographic 32 (D₃) symmetry, links three crystallographically equivalent Mg cations (on the 2-fold axis) to form a planar [Mg₃(OH)] cluster with Mg–O bonds and Mg⋯Mg separations of 1.9814(11) and 3.432(2) Å, respectively. Each pair of Mg centers is bridged by two carboxyl groups from separate PBPC ligands above and below the [Mg₃(OH)] plane. The trimer is encapsulated by six carboxylate groups and three pyridyl groups to form a [Mg₃(OH)(CO₂)₆] SBU (Fig. 1a). To date, there are quite a few similar trimers reported in the literature. Usually, the center bridging group in the trimer is regarded as μ₃-OH or μ₃-O for the +2 and +3 mixed-valent cores, such as [Ni^II₂Ni^III(OH)],¹³ [Co^II₂Co^III(OH)],¹² [Fe^IIFe^III₂(O)],¹³ and μ₃-O for the trivalent cores like [In^III₃(O)].^11,14 The current [Mg₃(OH)(CO₂)₆] trimer in CPF-3 is the first Mg trimer building block used to construct 3-D MOFs.

Fig. 1 (a) The [Mg₃(μ₃-OH)(CO₂)₆] trimer building block; (b) the linkage between triangular PBPC ligands and the Mg-trimers; (c) the distorted cubic cage constituted by eight Mg-trimers; (d) the schematic representation of the 3D framework of CPF-3.

As shown in Fig. 1(b), each [Mg₃(OH)(CO₂)₆] trimer is connected to nine BPC ligands via six carboxylate groups and three Mg–N bonds, and each pyridine ring is connected to three Mg–trimer clusters to give a 3-D (3,9)-connected framework with the short Schläfli symbol of (4².6)₃(4⁶.6²¹.8⁹) (Fig. S1†). If the [Mg₃(OH)(CO₂)₆] clusters are considered as nodes, CPF-3 is a 12-connected uninodal net with a Schläfli symbol of (3¹⁸.4⁴².5⁶) (Fig. S2†). It should be noted that the binary inorganic compound LaCl₃ also has nine-connected nodes of a tricapped trigonal prism and three-connected nodes of a triangle. However, LaCl₃ shows a Schläfli symbol of (4³)₃(4¹².6¹⁵.8⁹). The different topological types of CPF-3 and LaCl₃ are also indicated by different space groups: the former belongs to R3̄c, while the latter belongs to P6₃/m. Alternatively, each trinuclear cluster [Mg₃(OH)(CO₂)₆] also connects with six nearest trimers via double linkers (Fig. S3†). As depicted in Fig. 2(c), distorted cubic cages with a diameter of about 11 Å are generated, which link with adjacent ones to give a well-known 6-connected a-Po net (Fig. 2(d)). Thus, CPF-3 exhibits high porosity. The PLATON calculation indicates that this framework has a total guest-accessible volume of 9252 ų, which occupies approximately 62.2% of the volume of the whole crystal.

Fig. 2 N₂ and H₂ adsorption isotherms at 77 K for CPF-3.

Thermogravimetric analysis of CPF-3 indicates a weight loss (about 30%) in the temperature range 30–240 °C, corresponding to the release of solvent molecules. A plateau is reached above 240 °C before the structure decomposes at about 440 °C (Fig. S4†). Compared with the structurally analogous Ni or Fe-containing neutral frameworks,¹³ the present magnesium material exhibits comparable or perhaps improved thermal stability. The architectural stability and permanent porosity of CPF-3 were also confirmed by measuring the gas adsorption properties (N₂ and H₂) on a Micromeritics ASAP 2020 surface area and pore size analyzer. Prior to the measurements, the sample was degassed at 100 °C under vacuum for 12 h, and the stability of the guest-free material was confirmed by the PXRD patterns (Fig. S5†). As can be seen from Fig. 2 and S6,†CPF-3 exhibits a typical type I adsorption isotherm characteristic of permanent microporosity. The Langmuir and Brunauer–Emmett–Teller (BET) surface areas were calculated to be 685 and 490 m² g⁻¹, respectively. Obviously, the existence of [(CH₃)₂NH₂]⁺ cations significantly decrease the BET surface areas, which is just about one third of the Ni or Fe neutral analogues. However, CPF-3 can still adsorb 153 cm³ g⁻¹ (1.37 wt%, Fig. S7†) of H₂ at 1.0 atm and 77 K, which is comparable to that of the two neutral analogues and some other well-known frameworks such as bio-MOF-1¹⁵ and ZIF-8.¹⁶

In conclusion, we have successfully constructed a highly-connected magnesium-based crystalline porous framework in which the [Mg₃(μ₃-OH)(CO₂)₆] trimer is observed as the integral part of the 3-D porous framework for the first time. The discovery of this Mg trimer suggests the potential of using other highly symmetrical Mg-based building blocks for the construction of MOF frameworks, which should provide new opportunities for the development of novel low-density H₂storage or CO₂ capture materials.

This work was supported by the Department of Energy-Basic Energy Sciences under Contract No. DE-SC0002235 and by the NSF (X. B. DMR-0846958).

Notes and references

1. (a) S. R. Seidel and P. J. Stang, Acc. Chem. Res., 2002, 35, 972; (b) O. R. Evans and W. Lin, Acc. Chem. Res., 2002, 35, 511; (c) N. W. Ockwig, O. Delgado-Friederichs, M. O'Keeffe and O. M. Yaghi, Acc. Chem. Res., 2005, 38, 176; (d) R. J. Hill, D. L. Long, N. R. Champness, P. Hubberstey and M. Schröder, Acc. Chem. Res., 2005, 38, 335; (e) X. M. Chen and M. L. Tong, Acc. Chem. Res., 2007, 40, 162; (f) R. Robson, Dalton Trans., 2008, 5113; (g) J. R. Long and O. M. Yaghi, Chem. Soc. Rev., 2009, 38, 1201 and reference therein; (h) H. He, G. J. Cao, S. T. Zheng and G. Y. Yang, J. Am. Chem. Soc., 2009, 131, 15588; (i) S. T. Zheng, J. Zhang, X. X. Li, W. H. Fang and G. Y. Yang, J. Am. Chem. Soc., 2010, 132, 15102; (j) S. Natarajan and P. Mahata, Chem. Soc. Rev., 2009, 38, 2304.
2. (a) S. S. Kaye and J. R. Long, J. Am. Chem. Soc., 2005, 127, 6506; (b) M. Latroche, S. Surblé, C. Serre, F. Millange and G. Férey, Angew. Chem., Int. Ed., 2006, 45, 8227; (c) A. G. Wong-Foy, A. J. Matzger and O. M. Yaghi, J. Am. Chem. Soc., 2006, 128, 3494; (d) Y. L. Liu, J. F. Eubank, A. J. Cairns, J. Eckert, V. C. Kravtsov, R. Luebke and M. Eddaoudi, Angew. Chem., Int. Ed., 2007, 46, 3278; (e) S. Q. Ma, D. F. Sun, J. M. Simmons, C. D. Collier, D. Q. Yuan and H. C. Zhou, J. Am. Chem. Soc., 2008, 130, 1012; (f) M. Dincă and J. R. Long, Angew. Chem., Int. Ed., 2008, 47, 6766; (g) Y. Yan, X. Lin, S. H. Yang, A. J. Blake, A. Dailly, N. R. Champness, P. Hubberstey and M. Schroder, Chem. Commun., 2009, 1025.
3. (a) S. Bourrelly, P. L. Llewellyn, C. Serre, F. Millange, T. Loiseau and G. Férey, J. Am. Chem. Soc., 2005, 127, 13519; (b) B. D. Chandler, D. T. Cramb and G. K. H. Shimizu, J. Am. Chem. Soc., 2006, 128, 10403; (c) R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O'Keeffe and O. M. Yaghi, Science, 2008, 319, 939; (d) A. O. Yazaydn, R. Q. Snurr, T. H. Park, K. Koh, J. Liu, M. D. LeVan, A. I. Benin, P. Jakubczak, M. Lanuza, D. B. Galloway, J. J. Low and R. R. Willis, J. Am. Chem. Soc., 2009, 131, 18198; (e) J. An, S. J. Geib and N. L. Rosi, J. Am. Chem. Soc., 2010, 132, 38.
4. (a) P. Mahata, A. Sundaresan and S. Natarajan, Chem. Commun., 2007, 4471; (b) P. Mahata, D. Sen and S. Natarajan, Chem. Commun., 2008, 1278; (c) E. Neofotistou, C. D. Malliakas and P. N. Trikalitis, CrystEngComm, 2010, 12, 1034; (d) E. Neofotistou, C. Malliakas and P. N. Trikalitis, Chem.–Eur. J., 2009, 15, 4523; (e) E. Neofotistou, C. D. Malliakas and P. N. Trikalitis, Inorg. Chem., 2007, 46, 8487.
5. (a) X. Luo, D. Luo, H. Zeng, M. Gong,Y. Chen and Z. Lin, Inorg. Chem., 2011, 50, 8697; (b) S. Zheng, Y Li, T. Wu, R. A. Nieto, P. Feng and X. Bu, Chem.–Eur. J., 2010, 16, 13035; (c) J. Zhang, T. Wu, C. Zhou, S. Chen, P. Feng and X. Bu, Angew. Chem., Int. Ed., 2009, 48, 2542; (d) T. Wu, J. Zhang, C. Zhou, L. Wang, X. Bu and P. Feng, J. Am. Chem. Soc., 2009, 131, 6111.
6. (a) M. Dincă and J. R. Long, J. Am. Chem. Soc., 2005, 127, 9376; (b) K. L. Gurunatha, K. Uemura and T. K. Maji, Inorg. Chem., 2008, 47, 6578; (c) J. A. Rood, B. C. Noll and K. W. Henderson, Inorg. Chem., 2006, 45, 5521; (d) I. Senkovska and S. Kaskel, Eur. J. Inorg. Chem., 2006, 4564; (e) K. C. Kam, L. M. Young and A. K. Cheetham, Cryst. Growth Des., 2007, 7, 1522; (f) A. Mesbah, C. Juers, F. Lacouture, S. Mathieu, E. Rocca, M. Francois and J. Steinmetz, Solid State Sci., 2007, 9, 322; (g) Z. Hulvey and A. K. Cheetham, Solid State Sci., 2007, 9, 137; (h) I. Senkovska, J. Fritsch and S. Kaskel, Eur. J. Inorg. Chem., 2007, 5475.
7. S. R. Caskey, A. G. Wong-Foy and A. J. Matzger, J. Am. Chem. Soc., 2008, 130, 10870.
8. (a) S. Ma, J. A. Fillinger, M. W. Ambrogio, J. L. Zuo and H. C. Zhou, Inorg. Chem. Commun., 2007, 10, 220; (b) J. Zhang, S. Chen, H. Valle, M. Wong, C. Austria, M. Cruz and X. Bu, J. Am. Chem. Soc., 2007, 129, 14168; (c) R. P. Davies, R. J. Less, P. D. Lickiss and A. J. P. White, Dalton Trans., 2007, 2528; (d) W. Zhou, H. Wu and T. Yildirim, J. Am. Chem. Soc., 2008, 130, 15268; (e) P. D. C. Dietzel, R. Blom and H. Fjellvag, Eur. J. Inorg. Chem., 2008, 3624; (f) Z. Guo, G. Li, L. Zhou, S. Su, Y. Lei, S. Dang and H. Zhang, Inorg. Chem., 2009, 48, 8069; (g) C. Volkringer, T. Loiseau, J. Marrot and G. Férey, CrystEngComm, 2009, 11, 58; (h) Y. E. Cheon, J. Park and M. P. Suh, Chem. Commun., 2009, 5436; (i) A. Mallick, S. Saha, P. Pachfule, S. Roy and R. Banerjee, J. Mater. Chem., 2010, 20, 9073.
9. A. M. Bohnsack, I. A. Ibarra, P. W. Hatfield, J. W. Yoon, Y. K. Hwang, J. S. Chang and S. M. Humphrey, Chem. Commun., 2011, 47, 4899.
10. (a) S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K. Kim, Nature, 2000, 404, 982; (b) A. C. Sudik, A. W. Millward, N. W. Ockwig, A. P. Côté, J. Kim and O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 7110; (c) G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé and I. Margiolaki, Science, 2005, 309, 2040; (d) A. C. Sudik, A. P. Côté, A. G. Wong-Foy, M. O'Keeffe and O. M. Yaghi, Angew. Chem., Int. Ed., 2006, 45, 2528; (e) Y. B. Zhang, W. X. Zhang, F. Y. Feng, J. P. Zhang and X. M. Chen, Angew. Chem., Int. Ed., 2009, 48, 5287; (f) G. Jiang, T. Wu, S. Zheng, X. Zhao, Q. Lin, X. Bu and P. Feng, Cryst. Growth Des., 2011, 11, 3713.
11. (a) S. Zheng, J. T. Bu, Y. Li, T. Wu, F. Zuo, P. Feng and X. Bu, J. Am. Chem. Soc., 2010, 132, 17062; (b) S. Zheng, J. J. Bu, T. Wu, C. Chou, P. Feng and X. Bu, Angew. Chem., Int. Ed., 2011, 50, 8858.
12. X. M. Zhang, Y. Z. Zheng, C. R. Li, W. X. Zhang and X. M. Chen, Cryst. Growth Des., 2007, 7, 980.
13. J. H. Jia, X. Lin, C. Wilson, A. J. Blake, N. R. Champness, P. Hubberstey, G. Walker, E. J. Cussen and M. Schröder, Chem. Commun., 2007, 840.
14. X. Gu, Z. Lu and Q. Xu, Chem. Commun., 2010, 46, 7400.
15. J. An, S. J. Geib and N. L. Rosi, J. Am. Chem. Soc., 2010, 132, 38.
16. (a) K. S. Park, N. Zheng, A. P. Côté, J. Y. Choi, R. Huang, F. J. Uribe-Romo, H. K. Chae, M. O'Keeffe and O. M. Yaghi, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 10186; (b) X. C. Huang, Y. Y. Lin, J. P. Zhang and X. M. Chen, Angew. Chem., Int. Ed., 2006, 45, 1557.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC reference number 845965. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt12215d

‡ Synthesis of CPF-3: A solution of Mg(NO₃)₃·6H₂O (0.256 g, 1.0 mmol) and H₂PBPC (0.317 g, 1.0 mmol) in 8 ml DMA was heated at 120 °C for 5 days, and the mixture was then cooled to room-temperature. After washing with DMA and ethanol, grey cubic crystals of CPF-3 were obtained (yield: about 60% based on H₂BPC). FT-IR (solid KBr pellet/cm⁻¹): 3415(br), 1607(s), 1546(w), 1406(vs), 1186(w), 1107(w), 1021(w), 862(w), 789(m), 710(w), 484(w).

§ Crystal data for CPF-3: C₅₇H₃₃N₃O₁₃Mg₃, M = 1040.79, rhombohedral, a = 19.4789(3) Å, b = 19.4789(3) Å, c = 45.5070(12) Å, V = 14953.3(5) ų, T = 150(2)K, space group R3̄c, Z = 6, ρ_c = 0.693. 16968 reflections measured, 2948 independent reflections (R_int = 0.0450), R₁ = 0.0772 (I > 2σ(I)). wR₂ = 0.2446 (all data). The routine SQUEEZE was applied to the structures in order to remove diffuse electron density associated with badly disordered [(CH₃)₂NH₂]⁺ cations and DMA molecules.

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