Crystalline beryllium carboxylate frameworks with rutile-type and cubic-C₃N₄ topologies

Abstract

Two new crystalline beryllium carboxylate frameworks, formulated as (C₂NH₈)₂[Be₄(OH)₄(BTC)₂] (BCF-1) and (C₂NH₈)₆[Be₃(BTC)₄] (BCF-2), have been synthesized under solvothermal conditions, where BTC = 1,3,5-benzenetricarboxylate. Topological analyses reveal that BCF-1 has a 3,6-connected rutile topology and BCF-2 exhibits a 3,4-connected cubic-C₃N₄ topology.

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Crystalline microporous metal–organic frameworks (MOFs) have been extensively studied because of their diverse structures and chemical compositions.¹ They may find widespread applications in catalysis, ion exchange, gas storage, separation, and sensor technology.² A key factor to construct robust frameworks with permanent porosity is the use of rigid organic ligands as the linkers to connect with metal centers.³ With this synthetic strategy, a huge number of porous MOFs have been prepared during the past two decades. A notable example is the zinc benzenedicarboxylate MOF-5 with a high BET surface area of 3800 m² g⁻¹.⁴ Through functionalization and extension of the bridging dicarboxylate ligands, the pore sizes and gas storage capacity of this material can be effectively tuned.⁵

Recently, the use of lightweight main group metals (e.g., Li, Be, Mg, and Al) as framework building elements is of particular interest due to their gravimetric advantage in the synthesis of low-density structures with large surface area.⁶ It has been illustrated that the replacement of zinc atoms in the framework of MOF-5 by beryllium atoms may result in a roughly 40% increase in its surface area and gravimetric hydrogen storage capacity.⁷ As the lightest divalent metal, Be²⁺ possesses several advantages in the construction of new microporous MOFs. For example, the strong Be–O covalent bond indicates that Be-containing MOFs may have enhanced thermal and chemical stability. In addition, the small ionic radius of Be²⁺ and its tetrahedrally coordinated geometry indicate that open-framework Be-containing compounds may have specific capacity to resist hydrolysis.⁸ Unfortunately, with the exception of Be₁₂(OH)₁₂(BTB)₄ (S_BET = 4030 m² g⁻¹), we are not aware of any examples of structurally characterized Be-based MOFs.⁹

Herein we report two new crystalline beryllium carboxylate frameworks, (C₂NH₈)₂[Be₄(OH)₄(BTC)₂] (BCF-1)‡ and (C₂NH₈)₆[Be₃(BTC)₄] (BCF-2), where BTC = 1,3,5-benzenetricarboxylate. Colorless crystals of BCF-1 and BCF-2 were obtained by heating a mixture of BeSO₄·4H₂O, H₃BTC, DMF, methanol, and HF at 170 °C for 8 days.¹⁰ The pure phase of BCF-2 could be obtained by extending the reaction time to 10 d. The experimental XRD pattern of as-synthesized compound was compared with the simulated one based on single crystal data. The diffraction peaks on the two patterns corresponded well in position, indicating the phase purity of as-synthesized BCF-2 (ESI†).

A single-crystal X-ray diffraction analysis reveals that BCF-1 crystallizes in the tetragonal space groupP4₂/mnm (no. 136). The asymmetric unit contains 12 independent non-hydrogen atoms, of which O1, C2, C3 and C6 lie on a mirror plane. The Be atom is tetrahedrally coordinated to two oxygen atoms from BTC ligands and two oxygen atoms from two hydroxyl groups. The linkages between Be atom and hydroxyl groups generate a unique Be₄(OH)₄ ring structure. Prior to this work, Be₃(OH)₃ and Be₁₂(OH)₁₂ rings have been observed in beryllium-containing compounds.^9,11 However, Be₄(OH)₄ ring as the secondary building unit is not known in any open-framework inorganic solids or MOFs. The Be–O bond lengths are in the range of 1.576(5)–1.635(6) Å, and O–Be–O angles are between 104.8(3) and 116.4(3)°, in accord with other Be-containing open structures.

Each Be₄(OH)₄ ring is connected with six BTC ligands, and each BTC ligand is connected with three Be₄(OH)₄ rings. The connectivity in such a way gives rise to a three-dimensional framework with open channels. Viewed along the [001] direction, the structure displays two types of large channels (Fig. 1a). The square-like window has a pore size of 9.05 × 9.05 Å, and the vase-like window has a pore size of 6.75 × 19.98 Å (measured between the beryllium-to-beryllium distance across the windows). Intersecting the two types of channels are those channels running along the [110] direction, which have a pore size of 3.63 × 5.13 Å and 9.76 × 9.76 Å, respectively (Fig. 1b). A void space analysis using the program PLATON indicates that the framework building element of BCF-1 occupies 47.7% of the unit cell volume, leaving 52.3% as the “solvent accessible” space.¹² By considering Be₄(OH)₄ rings as 6-connected nodes and BTC ligands as 3-connected nodes in the network, the structure of BCF-1 can be described as a binodal 3,6-connected framework with rutile topology (Fig. 2).

Fig. 1 Perspective view of the framework structure of BCF-1 along (a) the [001] direction and (b) the [110] direction. Color code: beryllium, green; carbon, gray; oxygen, red; hydrogen, blue.

Fig. 2 The linkages between Be₄(OH)₄ rings and BTC ligands give rise to the three-dimensional open-framework structure of BCF-1 with a bimodal 3,6-connected rutile topology.

The single-crystal X-ray diffraction analysis reveals that BCF-2 crystallizes in the cubic space groupI4̄3d (no. 220). The asymmetric unit contains 6 independent non-hydrogen atoms, of which Be1 lies at a site with −4 imposed symmetry. Each Be atom in the three-dimensional structure is tetrahedrally coordinated to oxygen atoms from BTC ligands, and each BTC ligand links with three Be atoms (Fig. 3a). The Be–O bond lengths are 1.618(2) Å, and O–Be–O angles vary from 107.3 (8) to 113.8 (2)°. By regarding the Be atoms as 4-connected nodes and BTC ligands as 3-connected nodes in the network, the structure of BCF-2 can be described as a binodal 3,4-connected framework with cubic-C₃N₄ topology (Fig. 3b). It is worth noting that very few examples of metal–organic frameworks with cubic-C₃N₄ network are known so far.¹³ Recently, Bu and co-workers reported a microporous C₃N₄-type indium carboxylate framework, (choline)₃[In₃(BTC)₄]·2DMF, with a BET surface area of 507.8 m² g ⁻¹.¹⁴ This compound contains the same organic linker as that of BCF-2. Despite the fact that BCF-2 has smaller unit cell parameters compared to (choline)₃[In₃(BTC)₄]·2DMF, the framework density of BCF-2 (0.790 g cm⁻³) is only 83.6% of that of the indium analogue (framework density: 0.945 g cm⁻³), demonstrating the gravimetric advantage of lightweight Be atoms in the construction of low-density frameworks.

Fig. 3 (Left) Perspective view of the framework structure of BCF-2 along the [111] direction. (Right) The compound has a cubic-C₃N₄ topology by regarding Be ions as 4-connected nodes (green in color) and BTC ligands (red in color) as 3-connected nodes.

Different from the rutile network containing 4-, 6-, and 8-ring windows, only 8-ring windows are found in the cubic-C₃N₄ network. The beryllium-to-beryllium distance across the 8-ring window is about 10.79 Å, less than the indium-to-indium distance (11.40 Å) in (choline)₃[In₃(BTC)₄]·2DMF. A void space analysis using the program PLATON indicates that the space occupied by guest molecules represents 59.7% of the cell volume.

One striking structural feature of BCF-2 is its highly charged framework. The framework charge density is −2 per metal center in the structure, which is expected to be balanced by two dimethylamine cations. Elemental analysis gave C 50.40 wt%, H 5.50 wt% and N 7.23 wt%, in good agreement with the calculated values of C 50.93 wt%, H 5.34 wt% and N 7.42 wt% on the basis of the formula of (C₂NH₈)₆[Be₃(BTC)₄]. Attempts to ion-exchange the organic cations with Na⁺ ions caused the structure collapsed. By heating the exchanged solid in DMF at 150 °C for two days, the diffraction peaks associated with BCF-1 and BCF-2 in the XRD pattern reappeared, indicating that the structure of exchanged solid can be converted back to the structures of BCF-1 and BCF-2 (ESI†).

Thermogravimetric analysis, carried out in a flow of air with a heating rate of 10 °C min⁻¹, showed that BCF-2 remained stable up to 325 °C (Fig. 4a). On further heating, a two-step weight loss of 91.8% was observed, which was attributed to the decomposition of the extraframework cations and organic linkers. The fluorescent spectrum of BCF-2 was measured in solid state at room temperature (Fig. 4b). Upon excitation at 310 nm, BCF-2 displayed a violet luminescence with a peak maximum at 399 nm. According to the literature, the fluorescent emission can be tentatively assigned to the ligand-to-metal charged transfer.¹⁵

Fig. 4 (a) TGA curve and (b) solid-state fluorescent spectrum of BCF-2 (λ_ex = 310 nm) at room temperature.

In summary, two new crystalline open-framework beryllium carboxylate solids have been synthesized under solvothermal conditions. They have three-dimensional frameworks with rutile-type topology for BCF-1 and cubic-C₃N₄ topology for BCF-2. By using different dicarboxylate ligands as the cross-linkers, porous beryllium carboxylates with 4-connected zeolitic topologies are expected to be produced. Further work on this subject is in progress.

This work was supported by the NNSF of China (Grant 21171121).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: X-Ray data in CIF format, additional crystallographic figures, IR spectrum, powder XRD pattern. CCDC reference numbers 844452 and 844453. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ce06224g

‡ Data collection was performed on an Oxford Xcalibur diffractometer with graphite-monochromated MoKα (λ = 0.71073 Å) radiation at room temperature. The structure was solved by direct methods and refined on F² by full-matrix least-squares methods using the SHELXTL program package.¹⁶ The PLATON SQUEEZE program was applied to obtain better refinement for the disordered guest molecules in the two compounds. Crystal data for BCF-1: C₂₂H₂₆Be₄N₂O₁₆, M = 610.49, tetragonal, space groupP4₂/mnm (no. 136), a = 18.9009(5), c = 10.2570(3) Å, V = 3664.25(17) ų, Z = 4, D_c = 1.107 g cm⁻³, μ = 0.092 mm⁻¹, 7127 reflections measured, 1787 unique (R_int = 0.0736). Final wR₂ (all data) = 0.2676, final R₁ = 0.0799. Crystal data for BCF-2: C₄₈H₆₀Be₃N₆O₂₄, M = 1132.05, cubic, space groupI4̄3d (no. 220), a = 19.3050(2) Å, V = 7194.64(13) ų, Z = 4, D_c = 1.045 g cm⁻³, μ = 0.083 mm⁻¹, 2998 reflections measured, 964 unique (R_int = 0.0220). Final wR₂ (all data) = 0.2216, final R₁ = 0.0711.

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