Avishek
Dey
a,
Michael R.
Dworzak
a,
Kaushalya D. P.
Korathotage
b,
Munmun
Ghosh
a,
Jahidul
Hoq
b,
Christine M.
Montone
b,
Glenn P. A.
Yap
b and
Eric D.
Bloch
*ab
aDepartment of Chemistry & Biochemistry, University of Delaware, Newark, Delaware 19716, USA
bDepartment of Chemistry, Indiana University, Bloomington, Indiana 47405, USA. E-mail: edbloch@iu.edu
First published on 5th January 2024
Chemically and thermally stable permanently porous coordination cages are appealing candidates for separations, catalysis, and as the porous component of new porous liquids. However, many of these applications have not turned to microporous cages as a result of their poor solubility and thermal or hydrolytic stability. Here we describe the design and modular synthesis of iron and cobalt cages where the carboxylate groups of the bridging ligands of well-known calixarene capped coordination cages have been replaced with more basic triazole units. The resultingly higher M–L bond strengths afford highly stable cages that are amenable to modular synthetic approaches and potential functionalization or modification. Owing to the robust nature of these cages, they are highly processable and are isolable in various physical states with tunable porosity depending on the solvation methods used. As the structural integrity of the cages is maintained upon high activation temperatures, apparent losses in porosity can be mediated by resolvation and crystallization or precipitation.
The development of supramolecular cages, and permanently porous cages more specifically, essentially mirrors the development of MOFs.23 Where many supramolecular constructs rely on second or third-row metals and neutral donor ligands,24 the vast majority of cages for which surface areas have been reported, are based on carboxylic acid-type ligands.4 Among these, both paddlewheel and tetranuclear building units are quite common where both can be incorporated into diverse structure types where overall cage geometry and size can be tuned via the bridging ligand or transition metal cations that comprise them. Calixarene-capped cages are featured prominently here and offer the advantage in that they are additionally modifiable at their capping units, which are based on either sulfonylcalix[4]arene (Fig. 1) or thiacalix[4]arene. These structures adopt any one of several structure types depending on ligand used in their syntheses and most commonly present as octahedral cages25 with ligands situated at their face (tritopic linker) or edge (linear ditopic linker) or as box-like or square structures (bent ditopic linker).26–28 Both sulfonyl- and thiacalixarene capped cages have been reported for many different transition metal cations29–33 and their synthesis and assembly is well understood and relatively predictable.34
In an effort to increase the coordination stability of these types of cages, which can still be prone to decomposition via hydrolysis, we turned to related families of MOFs that illustrate this concept. Calixarene-capped cages are structurally analogous to a specific pore within PCN-9,35 a carboxylate-based MOF that is isomorphous to the tetrazole, triazole, and pyrazole-based MOFs M-BTT, M-BTTri, and M-BTP, respectively.36–38 Ligand tuning was shown to be a viable route toward increasing the stability of these types of MOFs, where anionic N-donor ligands afforded more stable materials as compared to carboxylic acids with stability increasing with pKa with carboxylate < tetrazolate < triazolate < pyrazolate.36–38 Toward improving the stability of permanently porous coordination cages, we sought to incorporate triazole or pyrazole groups into otherwise carboxylate-based structures as previous work has established that these types of ligands are compatible with this cage synthesis strategy.26–28,39
Specifically, the reaction of FeCl2·4H2O with H3BTTri or H2BDTri in DMF at 120 °C for one day, followed by the addition of an ethanoic solution of sc4a, affords a crystalline product in high yield. Single-crystal X-ray diffraction confirms the structure adopts the expected shape in an octahedral cage based on six 4-metal caps terminated by sc4a and connected with 8 BTTri3− ligands at the faces of the octahedron. As expected, given the structural similarities between the analogous MOFs PCN-9 and Mn-BTT,35,37 the coordination environments of the Fe cations in the structures are similar to our previously reported Fe-calixarene structures based on carboxylic acids and feature pseudooctahedral coordination environments (Table S5†) (Fig. 2). Each iron cation is coordinated to a μ4 species and bridged by two phenolic oxygen atoms, a sulfonyl oxygen, and finally bridged by N atoms from a shared triazole ligand. Although we were unable to refine charge-balancing cations in the structure, the M–L bond distances point to an all Fe(II) cage.13 Given the slightly elongated linker as compared to H3BTC, this octahedral cap displays longer cap-cap distances along an edge (17.4 vs 14.0 Å). In the solid state, the cages adopt face-to-face packing arrangements where cage–cage interactions are largely governed by tBu–tBu interactions between structures. The cobalt analog of this material, which is isolated under similar, albeit one-pot conditions, has a similar composition and structure. Single-crystal X-ray diffraction shows the structure, [(Co4OHsc4a)6(BTTri)8]6−, has a similar unit cell and solid-state packing to the iron structure. This is not entirely unexpected as the metal sites in them are saturated, and the solid-state cage–cage packing is governed by cap–cap interactions.
Although we were unable to obtain diffraction-quality single crystals for the linear ditriazolate ligand, the isophthalic acid analog, 1,3-BDTri, afforded crystalline material upon reaction with CoCl2·6H2O and cap in a DMF:MeOH (3:1) ratio after heating at 120 °C for 12 hours. The structure is analogous to the iron and cobalt cages based on H3BTTri, although as a 4-vertex cage, it presents in a box-like or square geometry (Fig. 2). The extended packing of this cage, which crystallizes in P-1, also mirrors that of the analogous H2BDC cage where packing in 2-D is governed by cap–cap interactions, and packing in the third dimension is based on ligand–ligand interactions. The cobalt coordination environment in this structure, [(Co4OHsc4a)4(BDTri)8]4−, is essentially identical to that in the tristriazoleate-based cage and is isostructural to previously reported tetrazole cages that adopt the same square-like structures.39
As is the case for all permanently porous coordination cages, special care during isolation, solvent exchange, and activation must be given to ensure optimal surface area. Initial PXRD patterns confirm the bulk sample, which matches the predicted pattern based on single-crystal analysis, has significant mobility in the solid state upon solvent exchange. Thermogravimetric analysis indicates that after initial mass loss attributed to the evacuation of pore-bound solvent, all three materials display no further mass loss until at least 400 °C. Although this gives no indication of porosity after heating to elevated temperatures, it confirms the enhanced structural stability of these cages as compared to carboxylate-based analogs. In order to optimize the surface area for these materials, we screened exchange solvents as the cages have moderate to good solubility in a number of organic solvents. Ultimately, room temperature methanol exchanges followed by heating under flowing N2 or dynamic vacuum afforded samples with the highest porosities. Under these conditions, we recorded N2 accessible BET (Langmuir) surface areas of 581 (919), 559 (1067), and 307 (616) m2 g−1 for [(Fe4OHsc4a)6(BTTri)8]6−, [(Co4OHsc4a)6(BTTri)8]6−, and [(Co4OHsc4a)4(BDTri)8]4−, respectively. These values are higher than those reported for most H3BTC or H2BDC cages capped with sc4a, whose BET surface areas are typically 200–300 m2 g−1. This is likely a result of the slightly expanded pore volumes of the triazole cages as compared to the carboxylate structures.
To further investigate the stability that was possibly evidenced by the TGA profiles, we initiated a detailed study targeting solvation and thermal stability. As the cobalt cages had limited solubility, we focused on the iron-based cage [(Fe4OHsc4a)6(BTTri)8]6−. It has appreciable solubility in dichloromethane, chloroform, dimethylacetamide, acetone, and ethyl acetate. To further elucidate thermal stability, TGA data were collected at 5 °C min−1 ramp rates to 100, 200, 300, 400, 500, and 600 °C for six different samples. Although the TGA curves for these samples were essentially identical and only show mass losses as a function of temperature, IR spectra on materials after the heating phases confirm that only minor compositional variations are obvious after heating to 400 °C (Fig. 3).
While the complement of TGA and IR suggested minimal cage degradation up to at least 400 °C activation temperatures, cage packing in the solid state, and thus porosity, may be impacted by heating. A thorough activation screen confirms that although the cage has an optimal activation temperature of 150 °C, it maintains porosity at elevated heating, where the Langmuir surface area of 858 m2 g−1 decreases to 689, 634, 433, and 389 m2 g−1 after heating to 200, 250, 300, or 400 °C, respectively. A prime example of the benefits of cage solution processability is given here as samples with diminished porosity as a result of overheating can simply be recrystallized from volatile solvent to restore the 3-D packing of the cages. A relatively rapid recrystallization of a thermally-degraded sample (∼200 m2 g−1) from chloroform and subsequent heating to 150 °C actually affords a slightly higher surface area than the starting sample with 626 (929) vs. 581 (919) m2 g−1. Interestingly, recrystallization of an as-synthesized sample from chloroform, followed by activation at 150 °C, affords a significantly lower surface area of 216 (436) m2 g−1. Recrystallization from acetone, dichloromethane, or ethyl acetate all gave samples with lower surface areas.
In addition to solid-state stability, the solvent processability and stability of these cages can even aid in characterization as MALDI was used to confirm the presence of the cage, the first time mass spectrometry was used for calixarene-capped cages (Fig. S15†). The cages also display good hydrolytic stability with no dissolution or decomposition after boiling in water for one week, although the surface areas of samples treated in this manner are typically ∼10–15% lower than starting cages.
Footnote |
† Electronic supplementary information (ESI) available: Adsorption isotherms, crystallographic information, spectroscopic data. CCDC 2260661–2260663. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt03365a |
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