Efficient iodine capture by metal–organic cubes based on hexanuclear vanadium clusters

Abstract

The cube is the only regular hexahedron among the Platonic solids, but supramolecules with a cubic configuration have rarely been reported. Herein, two isostructural metal–organic cubes VMOC-6 and VMOC-7 with large cavities are successfully synthesized by face-directed self-assembly of concave hexanuclear vanadium clusters [V₆O₆(OCH₃)₉(XO₄)(COO)₃]ⁿ⁻ (X = S, n = 2; X = V, n = 1) and large-size nitrogen-containing tetravalent ligands. VMOC-6 and VMOC-7 are the largest metal–organic cubes based on metal clusters with an outer diameter of 28.47 × 28.17 × 28.44 ų and an inner cavity volume of 7785.657 ų. Moreover, the cages can capture volatile iodine vapor with an uptake value of up to 1.83 g g⁻¹, which is superior to those of most metal–organic materials. Mechanistic studies have shown that their good adsorption capacity is closely related to the presence of large internal cavities, electron-donating pyridine groups and redox-active V sites. The present work shows the potential application of these mesoporous MOPs in the treatment of iodine-containing nuclear waste.

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Introduction

Self-assembly processes driven by non-covalent interactions are essential in the proliferation of all biological organisms and offer considerable advantages in the construction of large supramolecular assemblies.^1–6 Metal–organic polyhedra (MOPs or molecular coordination cages) constructed by coordination-driven self-assembly processes not only are aesthetically attractive but also have attracted widespread interest due to their large inner cavities and controlled cavity environments, which allow them to be used as advanced functional materials for molecular recognition, catalysis, and optoelectronics applications.^7–13 Unlike metal–organic frameworks (MOFs), MOPs are closed discrete structures, and their assembly process requires strict consideration of the harmony of chemical complementarity and spatial compatibility between organic and inorganic building units.^14,15 Therefore, the design and assembly of MOPs with large internal cavities and potential functions remains one of the challenges in the field of supramolecular chemistry and materials science.

The cube is the only regular hexahedron among the Platonic solids, consisting of congruent regular squares with three meeting at each vertex. In addition, the cube is the only zonohedron with high symmetry consisting of an even number of side faces, and only a few supramolecules with a cubic configuration have been reported.^16–19 To design and construct a molecular cube, at least three conditions must be satisfied: (i) the inorganic building block of the vertex must be trivalent; in other words, it can act as a “3-pyramidal” molecular building block (MBB); (ii) the angle η between the links in the MBB must be close to the desired ideal value, close to 90° for edge-directed models and should be close to 60° for face-directed models; (iii) eight “3-pyramidal” MBBs and the organic building blocks should be assembled into a closed shape arranged in a V₈E₁₂ or V₈F₆ geometry (V, E, and F denote vertices, edges and faces, respectively).¹⁹ In other words, the cube can consist of eight identical “3-pyramidal” MBBs connected by twelve “2-linear” or six “4-planar” ligands (Fig. 1).

Fig. 1 Schematic diagram of the cube construction guided by edge-directed (left) and face-directed (right) strategies.

Therefore, we reasoned that it is easier to construct closed polyhedra from directional metal clusters than from individual metal ions due to their abundant geometric properties. In recent years, we have been investigating polyoxovanadate-based metal–organic polyhedra and have demonstrated that polyoxovanadates are a good category of molecular building blocks (MBBs) for the construction of MOPs, including [V₆O₆(OCH₃)₉(SO₄)₄]₅ ({V₆(SO₄)₄}), [V₅O₉Cl]₆, and [WV₅O₁₁(SO₄)₆]₈.^20–25 All these concave MBBs are favorable for the construction of isolated polyhedra, but only the 3-connected {V₆(SO₄)₄} satisfies the conditions for the construction of cubes. However, in the process of trying to design MOPs with such building blocks, most of the edge-directed strategies are used as guiding principles to obtain MOPs with tetrahedral configurations.^20,24,26,27 Because of the rigidity limitation of this MBB itself, it is difficult to reach an η angle of 90°, so it also seems to be difficult to construct cubes using the edge-directed strategy. As confirmed by our previous studies, it is possible to obtain a cubic structure by guiding through a face-directed strategy, since the η angle is 60° at this point.²⁸ A series of polyoxovanadate-based metal–organic cubes constructed from three-coordinated {V₆(SO₄)(CO₂)₃} and tetracarboxylic acid ligands of different lengths and of C₄-symmetry have been obtained. The size of the cubic cage can be adjusted by changing the ligand spacer portion while keeping the structural topology unchanged. We found that in these structures, the length of the four ligands is a key determinant of the size of the cubic cage. However, increasing the length of the ligands did not increase the cage size, and the side length of the largest VMOC-4 increased by only 0.35 Å compared to the smallest VMOC-1. Therefore, in this work, wider 4,4′,4′′,4′′-(1,4-phenylbis(pyridine-4,2,6-triyl))tetrabenzoic acid (H₄PBPTA) was designed as a tetra-connected organic ligand, which was assembled with three-connected {V₆O₆(OCH₃)₉(SO₄)} or {V₆O₆(OCH₃)₉(VO₄)} into two isomorphic cubic cages VMOC-6 and VMOC-7 with large size cavities and with side lengths of about 28.36 Å. In addition, their large cavities, electron-donating pyridine groups and redox-active V sites enable them to exhibit excellent performance in iodine capture.

Experiments and methods

Synthesis of (NH₂Me₂)₁₆[(V₆O₆(OCH₃)₉(SO₄))₈(PBPTA)₆] (VMOC-6). VOSO₄·xH₂O (0.02 g, 0.12 mmol) and H₄PBPTA (0.01 g, 0.03 mmol) were dissolved in 0.5 mL of N,N-dimethylformamide (DMF) and 2.5 mL of methanol (MeOH). The mixed solution was transferred to a reaction vessel and heated to 160 °C for 48 hours. After cooling to room temperature, green rectangular crystals were obtained by filtration. Yield: 40%. Elemental analysis (%): calcd for C₃₆₈H₄₉₀N₂₈O₂₀₀S₈V₄₈: C, 39.44; H, 4.41; N, 3.50; found: C, 39.18; H, 4.56; N, 3.25. IR (KBr, cm⁻¹): 3511 (br), 3441 (m), 3385 (m), 3035 (m), 2930 (m), 2810 (m), 1711 (m), 1655 (vs), 1600 (s), 1550 (s), 1480 (s), 1403 (vs), 1242 (m),1172 (m), 1095 (m), 1067 (m), 955 (vs), 864 (m), 829 (m), 780 (m), 696 (m), 668 (m), 577 (s), 542 (s).

Synthesis of (NH₂Me₂)₂₄[(V₆O₆(OCH₃)₉(VO₄))₈(PBPTA)₆] (VMOC-7). VOSO₄·xH₂O was replaced with VO(acac)₂ (0.02 g, 0.07 mmol) in the above-mentioned procedure and other conditions were kept constant. Brown streaky crystals were obtained in 45% yield. Elemental analysis (%): calcd for C₃₈₄H₅₄₄N₃₆O₂₀₀V₅₆: C, 39.36; H, 4.68; N, 4.30; found: C, 39.18; H, 4.46; N, 4.54. IR (KBr, cm⁻¹): 3595 (br), 3518 (m), 3441 (m), 3363 (m), 3028 (m), 2915 (m), 2818 (m), 1711 (m), 1663 (s), 1600 (vs), 1550 (s), 1487 (s), 1403 (vs), 1312 (m), 1249 (s), 1179 (m), 1060 (s), 1018 (s), 984 (s), 948 (vs), 871 (s), 836 (s), 780 (s), 745 (m), 703 (m), 668 (s), 535 (s).

Results and discussion

Structural description

The strong acidity of vanadium has led to the preferential use of oxygen-donating ligands for the construction of vanadium-based metal–organic materials.^29,30 In previous work, a series of metal–organic cubes were constructed by self-assembly of {V₆S} with H₄BPTC and its derived tetradentate carboxylic acid ligands. These ligands have equal distances between adjacent carboxyl groups at both ends and have the same width. The length of the ligands is adjusted by changing the middle spacing portion of the ligands, thus adjusting the size of the cubic cage, where the longest ligand, tris-biphenyl-3,3′,5,5′-tetracarboxylic acid (H₄TCTP), constitutes the largest cubic cage. Therefore, to obtain a larger cubic cage, we chose to increase the width of the ligand while keeping the maximum length unchanged, i.e., by replacing the adjacent carboxylic acid at both ends of the original ligand with benzoic acid, as the addition of the benzene ring broadens the distance between the adjacent carboxyl groups. The larger tetracarboxylic acid ligand H₄PBPTA was synthesized (Fig. S1†), and the metal–organic cubes VMOC-6 and VMOC-7 with the same configuration were successfully obtained by reactions with VOSO₄ and VO(acac)₂, respectively. Thermogravimetric analysis and infrared spectral characterization of both are shown in Fig. S2 and S3.† Single-crystal X-ray diffraction analysis shows that both VMOC-6 and VMOC-7 crystallize in the triclinic space group P1̄ (Table S1†). The good crystallinity and phase purity of VMOC-6 and VMOC-7 were demonstrated by comparing the two powder X-ray diffraction (PXRD) patterns obtained experimentally and calculated from single crystal data (Fig. S4†).

The six {VO₆} octahedra form a ring in an edge-sharing pattern and a {XO₄} (X = S/V) tetrahedron is coordinated to the center of the unit in an angle-sharing pattern, and the sulfate at the edge is replaced by a C atom from the carboxylic acid to form a concave trigonal “3-pyramidal” MBB [V₆O₆(OCH₃)₉(XO₄)(COO)₃]ⁿ⁻ (X = S, n = 2; X = V, n = 1) (abbreviated as {V₆(XO₄)(CO₂)₃}) (Fig. 2a and Fig. S5†). Eight such concave {V₆(XO₄)(CO₂)₃} MBBs are connected by six PBPTA ligands to form a discrete metal–organic cube with the V₈F₆ geometry and with O_h symmetry, where the MBBs serve as vertices and PBPTAs serve as faces (Fig. 2b and c). The general formula of the anionic structures of VMOC-6 and VMOC-7 can be expressed as [(V₆O₆(OCH₃)₉(XO₄))₈(BPTC)₆]ⁿ⁻ (X = S, n = 16; X = V, n = 24). The negative charge is balanced by the corresponding amount of the dimethylamine cation [NH₂Me₂]⁺ generated by DMF decomposition.

Fig. 2 (a) Structure of {V₆(XO₄)(CO₂)₃} (X = S/V) and the coordination mode. The six edge-sharing {VO₆} octahedra share the {XO₄} tetrahedral vertices to form the “3-pyramidal” MBB. (b) Structures of VMOC-6 and VMOC-7. (c) The simplified cubic model, with the yellow part of H₄PBPTA simplified to “4-planar” and the blue part of the hexanuclear vanadium cluster simplified to “3-pyramidal”.

In the structures of VMOC-6 and VMOC-7, the dihedral angle (φ) between two adjacent planes formed by connecting four carboxyl carbon atoms of the tetracarboxylic acid ligands one after another of a cube is close to 90° (Fig. S6†) and the angle (η) between the links is close to 60°, which are consistent with the expected values for the cubes constructed using the face-directed strategy (Table S2†). Compared to that of the reported largest cube VMOC-4 (21.55 × 21.56 × 21.60 ų), the outer diameter (measured from the distance between the outermost carbon atoms of antipodal vertices) of VMOC-6 and VMOC-7 reached 28.47 × 28.17 × 28.44 ų due to the expansion of the width of the bridging ligands (Fig. 3a and 3c). To compare the internal space of VMOC-4 and VMOC-6, their internal cavity volumes were calculated from the crystal structure using the VOIDOO program.³¹ The internal cavity volume of VMOC-6 is 7785.657 ų, which is 3.2 times larger than that of VMOC-4 (2437.828 ų, Fig. 3b and d). Unfortunately, due to weak supramolecular interactions between the cages and also larger cavities, VMOP-6 and VMOP-7 were not stable after solvent removal and exhibited lower adsorption capacity for N₂ (Fig. S7†).^24,32,33

Fig. 3 Crystal structures of VMOC-4 (a and b) and VMOC-6 (c and d). The left column highlights the overall size of each cubic cage and the arrangement of the ligands, while the right column shows the void spaces (gray mesh) calculated using the VOIDOO program.

Iodine vapor uptake

Iodine (I₂) is a volatile and radioactive by-product of nuclear energy production, and its effective capture is particularly important because of its long isotopic lifetime, which directly affects human metabolic processes. It is widely known that I₂ is an electron acceptor, and therefore, it can form host–guest charge transfer complexes with cages containing electron donor groups, which can bind iodine molecules in the cavity. Considering that the structures of VMOC-6 and VMOC-7 contain large internal cavities and active site ligands, their potential application in iodine capture was compared to that of VMOC-4. Simulating typical nuclear waste reprocessing conditions, samples were placed in glass tubes and transferred to large sealed glass vials containing iodine with no direct contact between the solid samples and the iodine (Fig. S8†). Next, the closed containers were placed under ambient pressure and heated in an oven at 75 °C. As the reaction time increased, the iodine loading increased and was weighed at hourly intervals. Afterwards, the glass vials were placed back into the closed containers and the iodine adsorption was continued until the weight of the sample no longer changed and the adsorption equilibrium was reached. After iodine vapor capture, the iodine uptake capacity was calculated according to eqn (1):

Q = (m₂−m₁)/m₁ (1)

where Q (g g⁻¹) is the iodine uptake capacity of the sample, and m₁ (g) and m₂ (g) are the masses of the samples before and after iodine capture, respectively. During iodine capture, the color of the compounds exposed to I₂ vapor gradually darkened with time, indicating the successful adsorption of I₂ (Fig. 4a). Overall, VMOC-4 first saturated at 6 h of iodine adsorption, and then VMOC-6 and VMOC-7 also saturated in 10 h. Their equilibrium uptake capacities were 0.27 g g⁻¹, 1.83 g g⁻¹ and 1.82 g g⁻¹, respectively, and VMOC-6 has the same adsorption capacity as VMOC-7 and it is 6.8 times that of VMOC-4 (Fig. S9†). Since the adsorption curves of VMOC-6 and VMOC-7 had basically the same trend, the kinetic data of VMOC-6 and VMOC-4 can be analyzed according to a pseudo-second-order kinetic model (Fig. 4b), which can be described using eqn (2):where t is the adsorption time (h) and Q_e and Q_t (mg g⁻¹) are the amounts of iodine loaded at equilibrium and at time t, respectively. k (g mg⁻¹ h⁻¹) is the pseudo-second-order rate constant for adsorption. The high correlation coefficients (R² > 0.99) obtained indicate that the uptake of iodine by VMOC-6 and VMOC-4 conforms to a pseudo-second-order kinetic model with rate constants of 0.4484 and 0.0689 g mg⁻¹ h⁻¹, respectively. Further thermogravimetric tests were performed on VMOC-6 before and after iodine uptake, and the TGA curve of I₂@VMOC-6 after iodine uptake showed a significant mass loss before 200 °C compared to that of VMOC-6, which was due to the release of captured iodine. The loss attributed to iodine was estimated to be 63.45% of the total weight, which coincided with the iodine content of I₂@VMOC-6 (64.15%) (Fig. 4c). To the best of our knowledge, studies on the adsorption of iodine vapor by metal–organic polyhedra are relatively rare.³⁴VMOC-6 has the highest iodine adsorption capacity among MOPs³⁴ and surpasses most MOFs³⁵ (Table S3†), such as the well-known ZIF-8 (1.25 g g⁻¹),³⁶ Uio-66 (1.17 g g⁻¹)³⁷ and HKUST-1 (1.75 g g⁻¹).³⁸

Fig. 4 (a) Iodine vapor adsorption versus time for VMOC-4 and VMOC-6 at 75 °C and 1 atm (inset: photos of samples before and after I₂ vapor adsorption). (b) The pseudo-second-order kinetic data of the iodine adsorption by VMOC-4 and VMOC-6. (c) TGA curves of VMOC-6 (black line) and I₂@VMOC-6 (red line). (d) FT-IR spectra of VMOC-6 (black line) and I₂@VMOC-6 (purple line). (e) XPS spectra of I 3d for I₂@VMOC-6 derived signals. (f) Iodine release versus time (inset: photographs of the methanol solution and sample before and after I₂ release).

To investigate the determinants of iodine capture, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) mapping, Fourier transform infrared (FTIR) analysis, and X-ray photoelectron spectroscopy (XPS) were applied for characterization. SEM images showed that the prepared VMOC-6 cubes were rhombic crystals with no change in morphology after iodine adsorption. The EDS mapping patterns showed that iodine was uniformly distributed throughout VMOC-6 after iodine adsorption (Fig. S10†). Further study by FTIR spectroscopy revealed that the C=N band of VMOC-6 shifted from 1657 cm⁻¹ to 1645 cm⁻¹ after iodine loading and the C=C stretching vibration also changed, indicating that the adsorption sites may be distributed on the benzene and pyridine rings of the ligand (Fig. 4d).³⁹ As shown in Fig. 4e, in the I 3d XPS spectrum, two distinct peaks of I₂@VMOC-6 are located at 620.0 and 632.0 eV, attributed to I 3d_5/2 and I 3d_3/2 of the I₂ molecule, respectively, and the other two peaks are located at 618.6 and 630.3 eV, attributed to I₃⁻, indicating that the adsorbed iodine species are present in the I₂ and I₃⁻ forms. Examples of iodine adsorption using redox metals in some MOFs exist,^40,41 and the XPS characterization of I₂@VMOC-6 considering the redox properties of vanadium revealed the presence of V^V, suggesting that some of the V^IV was reduced to V^V (Fig. S11†). Thus, it was demonstrated that VMOC-6 and iodine have a driving force for adsorption, probably due to the weak π–π interactions between the pyridine ring and iodine. Moreover, N acts as a molecular trap for iodine due to its strong affinity for iodine.^42,43 Notably, it is due to two processes, charge transfer between I₂ and the nitrogen atoms in the pyridine ring and redox reactions between I₂ and V, that the adsorbed iodine in VMOC-6 can be further converted into the polyiodide anion (I₃⁻).^44,45 This indicates that the capture process is a combination of physical and chemical adsorption. Therefore, the high iodine uptake capacity of VMOC-6 can be attributed to the synergistic effect of the larger internal space, the high affinity of the pyridine-containing donor for iodine and the redox activity of V.

The captured iodine can be released from I₂@VMOC-6 through contact with a suitable solvent (e.g. MeOH). The iodine release process from I₂@VMOC-6 in methanol solution was observed using a UV spectrophotometer at ambient temperature. When 2 mg of the I₂@VMOC-6 solid was immersed in MeOH, the captured iodine continuously released and the color of the solution changed from colorless to pale yellow (Fig. 4f). The UV-vis spectrum of the methanol solution showed two bands at 291 and 360 nm and a shoulder at 439 nm (Fig. S12†), which was attributed to the presence of the polyiodide anion in I₂@VMOC-6.^46–49 The amount of iodine released was calculated from the UV-vis spectral data and the corresponding standard working curve of the I₂ methanol solution (Fig. S13†). The time-dependent profile of I₂@VMOC-6 in methanol showed that about 84.5% of the captured iodine rapidly released in the first 40 min, after which the release rate of iodine gradually decreased with almost complete release of iodine at 80 min (Fig. 4f).

Iodine adsorption in solution

Encouraged by its excellent performance in iodine vapor capture, the I₂ adsorption capacity of VMOC-6 in solution was investigated. As nonpolar iodine is insoluble in polar solvents such as water, cyclohexane was chosen as the solvent for the adsorption experiments and the iodine concentration in the solution was determined by UV-vis spectrometry. The standard curve of iodine in cyclohexane solution obtained using UV-vis spectroscopy showed a good linear relationship between the absorbance value and the concentration of iodine, satisfying the  Beer–Lambert law, as shown in Fig. S14.† It is well known that adsorption equilibrium isotherms can be used to describe the diffusion state of the adsorbent molecules and the adsorbent at equilibrium. Different equilibrium data were collected to determine the amount of iodine uptake versus the iodine concentration at equilibrium by varying the amount of adsorbent, that is, by exposing 5–25 mg of VMOC-6 to a solution of cyclohexane with an iodine concentration of 0.002 mmol L⁻¹. As seen in Fig. 5a, the amount of iodine adsorbed continues to increase as the equilibrium concentration increases. The adsorption isotherms can be simulated using the Langmuir and Freundlich models, where the Langmuir isotherm model assumes that the adsorption process occurs on a homogeneous monolayer surface of the adsorbent, while the Freundlich model assumes that the adsorption occurs at a non-homogeneous surface, as follows:where Q_m and Q_e (mg g⁻¹) are the maximum amount of iodine adsorbed and the amount of iodine adsorbed at equilibrium. C_e (mg L⁻¹) is the concentration of iodine in the cyclohexane solution at adsorption equilibrium. K_L and K_F are the Langmuir and Freundlich constants related to the heat of adsorption and the Freundlich constant indicates the adsorption capacity at an equilibrium concentration equal to 1, and 1/n represents the adsorption strength. The corresponding values for the two isotherm models are shown in Table 1. It can be seen from the R² values that the Langmuir model fits better than the Freundlich model, which means that the adsorption sites on the VMOC-6 surface are evenly distributed. Based on the Langmuir model, the saturation adsorption capacity was calculated to be 236.31 mg g⁻¹. The adsorption kinetics of iodine was further investigated by UV-vis monitoring of this adsorption process by immersing the treated 30 mg crystal sample in 35 mL of cyclohexane solution with an iodine concentration of 0.002 mol L⁻¹ at room temperature (Fig. 5b). The concentration of iodine in the solution decreased rapidly with increasing contact time, and the relative removal of iodine reached 53.62% within 2 h. After 24 h, the relative removal of iodine reached 77.26% and the adsorption equilibrium was reached (Fig. 5c). The rapid decrease of iodine concentration in the solution with increasing contact time was accompanied by a change of color from purple to pink. t/Q_t showed a linear relationship with t (R² = 0.9988), demonstrating that the kinetic data were in good agreement with the pseudo-secondary kinetic model.

Fig. 5 (a) Langmuir and Freundlich model fitting of the isotherm of iodine adsorption by VMOC-6 in cyclohexane solution. (b) Time evolution of the UV-vis absorption spectra of cyclohexane solutions in the presence of VMOC-6. (c) The kinetics of the adsorption process of iodine on VMOC-6 was plotted with the relative removal rate (%) of iodine (black line) and t/Q_t (red line) versus time.

Table 1 Adsorption equilibrium constants of Langmuir and Freundlich isotherm equations for iodine adsorption on VMOC-6 adsorbents

Langmuir			Freundlich
Q m (mg g^−1)	K L (L mg^−1)	R ^2	K F (mg g^−1)	n	R ^2
236.3066	0.0203	0.9965	70.3633	5.45	0.9392

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Conclusions

In summary, the tetracarboxylic acid ligand H₄PBPTA was successfully self-assembled with the 3-connected hexanuclear vanadium clusters {V₆(SO₄)(CO₂)₃} and {V₆(VO₄)(CO₂)₃} to form two isostructural metal–organic cubes VMOC-6 and VMOC-7 using the face-directed strategy under solvothermal conditions. They are the largest metal–organic cubes based on metal clusters, with a side length of about 28.3 Å and an inner cavity volume of about 7785.657 ų. The presence of electron-donating pyridine groups and redox-active V and the large inner cavity led to an iodine trapping capacity of 1.83 g g⁻¹ in the gas phase, which is the highest iodine adsorption capacity of metal–organic polyhedra to date and exceeds that of most previously reported metal–organic frameworks. This study provides a new perspective on the design of metal–organic polyhedra with tunable internal cavities and potential applications in the treatment of iodine-containing nuclear waste.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the NSFC of China (No. 22271023, 22175033).

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Footnote

† Electronic supplementary information (ESI) available: Additional characterization data, including ¹H NMR spectra, single-crystal X-ray diffraction data, IR spectra, TGA curves, PXRD patterns and UV-vis spectra. CCDC 2278526 and 2278527. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01501g

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