UoC-6: a first MOF based on a perfluorinated trimesate ligand

1 Introduction

Metal-organic frameworks (MOFs) are a still emerging class of compounds with almost 108,000 entries in the current MOF subset (version 5.42, Feb. 2021) of the CCDC database [1], [2], [3]. Although not all of these entries obviously fulfill the recommendations of the IUPAC to be classified as MOFs [4], this large number is still astonishing taking into account that MOF research started only in 1999 with the discovery of MOF-5 [5] and HKUST-1 [6]. These two MOFs consist of Zn₄O tetrahedra and Cu₂ paddlewheel units, resp. as nodes, which are connected by aromatic polycarboxylates to form porous 3D structures. In MOF-5 1,4-benzenedicarboxylate (typically abbreviated to BDC²⁻) functions as a linker, for which the MOF subset of the CCDC database gives 2890 entries accounting for approx. 2.68% of all MOFs in this subset. For 1,3,5-benzenetricarboxylate (BTC³⁻), which is used for the construction of HKUST-1, 1737 entries (∼1.61%) are found. Obviously, these two linkers play an important role in the chemistry of MOFs and numerous examples have been published to modify these linkers by substitution or elongation to polyphenyl backbones [7].

Another interesting approach is the incorporation of fluorine atoms as substituents, as this might strongly affect the chemical properties of the resulting MOF – cp. the properties of polyethylene and Teflon – without changing the size of the resulting pores too much due to the similar radii of H and F atoms. With respect to 1,3,5-benzenetricarboxylate, it is surprising that only the synthesis of a monofluorinated variant was reported until recently [8], which was used by Fröba and co-workers to synthesize UHM-31 [9], which is isostructural to HKUST-1. We were able to establish synthesis protocols also for the di-(dF-BTC^3–) and tri-/perfluorinated trimesate linkers (pF-BTC^3–) [10]. With these linkers available, we were able to synthesize coordination polymers with protonated linkers and K⁺, Ba²⁺, Cu²⁺ [10], and Sr²⁺ [11] nodes. Very recently, we synthesized and characterized a first MOF based on Sc³⁺ and a completely deprotonated dF-BTC^3– linker [12]. We named this compound UoC-4 (University of Cologne), although the opening windows to its pores were too small for any guest molecules to pass, so that no type I gas adsorption isotherm could be obtained. These investigations of isostructural compounds led to several important and very general conclusions. Firstly, the thermal and chemical stability of the coordination polymers decrease with increasing fluorination of the linker. Secondly, the affinity to guest molecules (here: solvent molecules) seems to increase with a higher degree of fluorination of the linker [10]. Finally, the fluorination of the linker leads to higher torsion angles between the phenyl and the carboxylate moieties of the BTC linkers [10, 11], i.e., with higher numbers of fluoro substituents the mean torsion angle increases significantly. This seems to be the reason why an isostructural variant of HKUST-1 with dF-BTC^3– or pF-BTC^3– ligands has not been synthesized up to now. In the following, we present a first MOF with the pF-BTC^3– linker. It contains Sc³⁺ nodes like UoC-4 with the dF-BTC^3– linker [12]. Both MOFs are similar, but not isostructural. The reasons for this will be given.

2 Experimental section

2.3 Synthesis of [ Sc ( p F − BTC ) ] ∞ 3

Approx. 14 mg (0.0314 mmol) [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O was heated in a Schlenk tube in an argon stream at T = 160 °C overnight. The purity of the resulting material was confirmed by XRPD (Figure 1). Attempts to synthesize larger amounts of pure [ Sc ( p F − BTC ) ] ∞ 3 failed so that no further analytical investigations could be performed.

Figure 1:

Rietveld refinement of the synchrotron diffraction pattern of [ Sc ( p F − BTC ) ] ∞ 3 (beamline BL9, DELTA, Dortmund, Germany; λ = 0.815,697 Å; T = 295(2) K; glass capillary: ∅ = 0.3 mm). Experimental data points (black crosses), the calculated profile (red solid line), the background (green solid line), and the difference curve (blue curve below) are shown. Vertical magenta bars mark the positions of Bragg reflections of [ Sc ( p F − BTC ) ] ∞ 3 .

3 Results and discussion

[ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O was crystallized from an aqueous solution containing Sc(NO₃)₃·5H₂O and K(H₂ pF-BTC), the monopotassium salt of the linker. To deprotonate the linker and to accomplish the formation of crystals of the desired product, in a desiccator a container with the volatile base triethylamine was placed next to the reaction mixture. Using this procedure, single crystals of [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O suitable for an X-ray structure analysis were obtained (Table 1). [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O crystallizes in the triclinic space group P 1 ‾ , all atoms occupy the general Wyckoff position 2i. Figure 2(a) shows an Ortep plot of the asymmetric unit of [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O . It contains one Sc³⁺ cation, one pF-BTC^3– linker, and seven water molecules. Three of them are coordinated to the Sc³⁺ cation, whereas four of them participate in a hydrogen bonding network as described later in the paper (Figure 3(a)). Sc³⁺ is surrounded by six oxygen atoms stemming from three water molecules and the carboxylate groups of three different linker anions. An almost ideal octahedron (Continuous Shape Measures: OC-6 = 0.222 [27]) is formed with Sc–O distances ranging from 2.061(2) to 2.126(2) Å and O–Sc–O angles from 84.55(8) to 94.96(8)°. Further interatomic distances and angles are given in Table 3. These ScO₆ octahedra are connected by pF-BTC^3– linkers to form double strands running along [100] (Figure 2(b)).

Figure 2:

(a) Ortep plot (50% probability) of the asymmetric unit of [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O with atomic numbering scheme; (b) double strands of ScO₆ octahedra and pF-BTC^3– linkers along [100]. Color code: Sc (white), F (green), O (red), C (dark gray), H (light gray).

Figure 3:

(a) View of the crystal structure of [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O along [100]. Hydrogen bonds are depicted as orange dashed lines; (b) Connectivity of the pF-BTC^3– linker to neighboring Sc³⁺ cations. Color code as Figure 2.

Looking along these double strands (Figure 3(a)) one can see that the four crystal water molecules are connected through hydrogen bonds. Figure 3(b) shows the connectivity of the pF-BTC^3– linker. Each carboxylate group of the linker is coordinated monodentately to its own Sc³⁺ cation. The C–O distances of these connecting groups (1.266(2)–1.271(2) Å) are significantly longer compared to the non-bonding C=O groups (1.226(2)–1.232(3) Å). Figure 3(b) also shows that the ring and its substituents are not coplanar as a result of the repulsion between the fluoro substituents and the oxygen atoms of the carboxylate groups which leads to enlarged torsion angles: ∠_Tors = 67.4° (O1–C1–O2), ∠_Tors = 58.8° (O3–C2–O4), ∠_Tors = 23.6° (O5–C3–O6).

As [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O contains four only weakly bound water molecules, we investigated its thermal behavior by DTA/TGA measurements. The results are given in Figure 4. The DTA shows a broad endothermic signal between 100 and 200 °C. It is accompanied by a mass loss of approx. 28%. For the loss of all seven water molecules a mass loss of 29.2% is calculated. Thus, quite surprisingly, [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O releases all its water molecules in one single (broad) endothermic event. An almost flat plateau around 200 °C points to the opportunity to obtain anhydrous [ Sc ( p F − BTC ) ] ∞ 3 by heating the hydrate to temperatures of ∼200 °C. At higher temperatures (>250 °C) an increasing mass loss points to a decomposition of the framework. In subsequent experiments, heating the hydrate at 160 °C in a Schlenk tube overnight using an open system with an Argon stream turned out to be the most convenient route to synthesize anhydrous [ Sc ( p F − BTC ) ] ∞ 3 . However, reproducibility was always a challenge in these experiments.

Figure 4:

DTA (black) and TGA (red) curves of [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O . Negative DTA values indicate an endothermic event.

The colorless powder obtained after dehydration of [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O was investigated by XRPD. It turned out to be highly crystalline. Using high-resolution synchrotron powder diffraction techniques, a very well-resolved diffraction pattern was obtained (Figure 1). From this data the crystal structure of [ Sc ( p F − BTC ) ] ∞ 3 was solved and refined (Table 2). It crystallizes in the orthorhombic space group Fddd with Z = 16. Sc³⁺ occupies the Wyckoff position 16 c (origin choice 2). It is surrounded by six oxygen atoms stemming from the carboxylate groups of six different linker anions. Thus, an almost ideal octahedron (Continuous Shape Measures: OC-6 = 0.069 [27]) is formed with Sc–O distances ranging from 2.013(4) to 2.102 (4) Å and O–Sc–O angles from 87.7(3) to 92.3(3)°. Three carboxylate groups bridge neighboring ScO₆ octahedra so that chains are formed (Figure 5(a)). These chains are interconnected by the pF-BTC^3– linkers to build up a 3D framework structure (Figure S2, Supplementary Material available online). Each trimesate linker is connected to six different Sc³⁺ actions (Figure 5(b)) so that the Niggli formula of UoC-6 has to be written as [ Sc ( p F − BTC ) 6 / 6 ] ∞ 3 .

Figure 5:

[ Sc ( p F − BTC ) 6 / 6 ] ∞ 3 (UoC-6): (a) connectivity of ScO₆ octahedra via carboxylate groups of bridging pF-BTC^3– ligands; (b) connectivity of a pF-BTC^3- ligand to six different Sc³⁺ cations (with atomic numbering scheme). Color code as Figure 2.

It is a remarkable feature that UoC-6 contains very small voids so that it can be classified as a MOF [4]. To the best of our knowledge, it is the very first MOF with a perfluorinated trimesate linker. In Figure 6 several views of the pores in UoC-6 are depicted.

Figure 6:

Different views of the pores within UoC-6. The large sphere in the figure on the left emphasizes a potential void.

At a first glance UoC-6 seems to be a very promising material for gas storage, as the fluorine substituents point directly into its pores. However, a closer look reveals that the opening windows to these pores are too small for any gas molecule to get through. As we failed to produce larger amounts of single-phase material, no gas sorption isotherm of UoC-6 could be recorded. However, as UoC-6 is very similar – but not isostructural! – to the recently published UoC-4 with the difluorinated dF-BTC^3– linker [12], for which gas sorption measurements revealed no permanent porosity, we conclude that UoC-6 has similar properties.

We have shown that fluoro substituents in α positions of aromatic carboxylate linkers significantly influence the crystal structures of the resulting coordination polymers and MOFs [10, 11]. This is due to the repulsion between the fluorine atoms and the carboxylate groups as well as a decreased aromaticity and thus a reduced rotational barrier leading to increased torsion angles between the phenyl ring and the carboxylate moiety [28], [29], [30], [31], [32]. In this respect, the only slight increase of the mean torsion angle in UoC-4 with the difluorinated dF-BTC^3– linker (∠_Tors(mean) = 60°) compared to ∠_Tors(mean) = 60.2° in UoC-6 with the perfluorinated linker (cp. Table 3) is smaller than expected. The torsion angle in the hydrate [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O (∠_Tors(mean) = 49.9°) is significantly smaller, but it is well-known that these effects are “system-specific” [33].

In Table 3 some relevant geometric data (interatomic distances and angles) of [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O , [ Sc ( p F − BTC ) ] ∞ 3 (UoC-6), and [ Sc ( d F − BTC ) ] ∞ 3 (UoC-4) [12] are summarized and compared. They are in a very good agreement taking into account that the crystal structure of UoC-6 was refined from (synchrotron) powder diffraction data using some soft constraints for a stable convergence. It was already mentioned that the crystal structures of UoC-4 and UoC-6 are similar, but not isostructural. A close inspection shows that UoC-6 crystallizes as a distorted variant of the UoC-4 crystal structure. A simplified group-subgroup relation scheme is given in Scheme 1. UoC-4 crystallizes in the tetragonal space group I4₁/amd, which transforms to space group Fddd by a translationengleiche transition of index [2] (t2). A detailed analysis is not given here, as this will involve too many atoms, but already the lattice parameters a _ortho and c _ortho reveal a severe distortion in the orthorhombic structure, as for an ideal transition a _ortho  = c _ortho  =  2 ·a _tet  = 12.6023 Å is calculated. It is remarkable that UoC-6 is a distortion variant of UoC-4, although it contains the pF-BTC^3– linker with all possible positions being fully occupied by fluorine atoms, whereas in the dF-BTC^3– linker forming UoC-4 two fluorine atoms are disordered over three positions: F1 (81.4%) and 2 × F2 (59.3%). However, for the carboxylate group between the two fluorine atoms with a lower occupancy (F2: 59.3%) the torsion angle is 0°. The respective torsion angle in UoC-6, where all positions are fully occupied with fluorine atoms, is 17.8° (cp. Table 3). We conclude that this increased torsion angle is the reason for the observed distortion in UoC-6. This is also obvious from different projections of the crystal structures of UoC-4 and UoC-6 given in Figure S3 (Supplementary Material available online). Thus, our proposal that fluoro substituents in aromatic carboxylate ligands significantly influence the crystal structures of the resulting coordination polymers and MOFs, is also supported by this new example.

Scheme 1:

Simplified group-subgroup relation for UoC-4 (I4₁/amd) and UoC-6 (Fddd).

4 Conclusions

By dehydrating the new coordination polymer [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O ( P 1 ‾ , Z = 2) at elevated temperatures we obtained the new MOF ∞ 3 Sc p F − BTC (Fddd, Z = 16), named UoC-6, which contains for the first time the tri-/perfluorinated trimesate linker pF-BTC^3–. The crystal structure of UoC-6 contains, determined by a powder synchrotron radiation diffraction study, sizeable pores, but the windows to these pores are too small for any guest molecules to pass through.

UoC-6 can be understood as a distortion variant of the recently published UoC-4 (I4₁/amd, Z = 8) with the difluorinated dF-BTC^3– linker [12]. The distortion can be rationalized considering the increase of the torsion angles between the carboxylate groups and the phenyl moiety with the increasing number of fluoro substituents. Thus, UoC-4 and UoC-6 are new examples of the structure directing influence of fluoro substituents in aromatic carboxylate ligands on the crystal structures of coordination polymers and MOFs.

5 Supporting information

An XRPD pattern of [ Sc ( p F − BTC ) ( H 2 O ) 3 ] ∞ 1 ⋅ 4 H 2 O as well as additional views of the crystal structures of [ Sc ( p F − BTC ) ] ∞ 3 (UoC-6) and [ Sc ( d F − BTC ) ] ∞ 3 (UoC-4) are given as supplementary material available online (https://doi.org/10.1515/znb-2021-0142).