A Porous π-Stacked Self-Assembly of Cup-Shaped Palladium Complex for Iodine Capture

Abstract

Acquiring adsorbents capable of effective radioiodine capture is important for nuclear waste treatment; however, it remains a challenge to develop porous materials with high and reversible iodine capture. Herein, we report a porous self-assembly constructed by a cup-shaped Pd^II complex through intermolecular π···π interactions. This self-assembly features a cubic structure with channels along all three Cartesian coordinates, which enables it to efficiently capture iodine with an adsorption capacity of 0.60 g g⁻¹ for dissolved iodine and 1.81 g g⁻¹ for iodine vapor. Furthermore, the iodine adsorbed within the channels can be readily released upon immersing the bound solid in CH₂Cl₂, which allows the recycling of the adsorbent. This work develops a new porous molecular material promising for practical iodine adsorption.

1. Introduction

As a non-greenhouse energy source, nuclear energy is most likely to replace traditional fossil fuels [1,2]. Currently, nuclear energy is widely applied in many areas related to human life [3]. With the rapid development of the nuclear energy industry, the safe disposal of nuclear waste containing radioactive species, especially radioactive iodine, has become a significant concern [4,5,6,7,8]. Both ¹²⁹I and ¹³¹I, which are the main radioisotopes for iodine, are harmful to its ecological surroundings and human health. ¹²⁹I is extremely dangerous because it has a long half-life (1.57 × 10⁷ years) and can be accumulated in the human thyroid gland, causing serious diseases [6]. As for ¹³¹I, it is often combined with hydrocarbons, giving rise to harmful organic compounds such as methane iodide [9,10,11,12]. Among various possible radioactive iodine species, molecular iodine (I₂) is the main pollutant in nuclear waste disposal and the nuclear accident [13,14]. Therefore, acquiring adsorbents for effective capture of I₂ is on demand.

To date, a broad range of solid adsorbents has been found to be very promising for removing molecular iodine [15,16,17,18,19]. These adsorbents include zeolites [20,21], functionalized clays [22], activated carbon [23], metal/covalent–organic frameworks (MOFs and COFs) [24,25,26,27,28,29,30,31,32,33], supramolecular cages [34,35], supramolecular assemblies [36,37,38], etc. For example, Zheng et al. reported two amorphous MOFs exhibiting very high I₂ uptake with adsorption capacities of 2.05 and 5.04 g g⁻¹ [39], respectively. Chi et al. reported that nonporous adaptive crystals of a bipyridine cage can reversibly capture I₂ [40]. In spite of that significant progress on adsorbents for I₂ capture has been achieved, there is still much room to improve the performance of adsorbents for I₂ capture. In general, a high-performance I₂ capture material needs to simultaneously meet the following requirements: high I₂ adsorption capacity and kinetics under industrial conditions, high selectivity, a long retention time of the adsorbed I₂, and great recyclability and low-cost [14]. The search for high-performance I₂ capture adsorbents is still ongoing.

Recently, macrocycle-based supramolecular assemblies have emerged as a class of adsorbents for I₂ capture [37,41,42]. For example, Huang’s group reported perethylated pillar [6] arene, which acts as a candidate for I₂ capture [41], while Zhang and co-workers directly observed the ambiguous binding sites for I₂ in a mesoporous assembly of aluminum molecular rings [42]. Recently, we have successfully obtained a series of π-stacked porous assemblies based on metal complexes of tripodal tris(2-benzimidazolylmethyl) amine or tris(2-naphthimidazolemethyl) amine [43,44]. These achievements promoted us to synthesize porous assemblies based on metal complexes of tripodal ligands to explore high-performance adsorbents for I₂ capture.

In this work, we report a porous π-stacked self-assembly based on a cup-shaped Pd^II complex. Due to the channels in the structure, this material permits the capture of both dissolved I₂ and I₂ vapor. Furthermore, the present adsorbent can be reused several times without significant loss of I₂ uptake capacity.

2. Results and Discussion

2.1. Structure Characterizations of the π-Stacked Self-Assembly

The self-assembly of (2,2′-bipyridine) dichloropalladium (II)([Pd(bipy)]Cl₂) with tris(2-naphthimidazolemethyl) amine (H₃L) in a mixture of MeOH/acetone (v/v: 1/3) with a trace of triethylamine affords yellow crystals of [Pd₃(bipy)₃L] Cl₃·solvent (1). Single-crystal X-ray analysis (Table S1) reveals a cup-shaped trinuclear Pd^II complex in which three [Pd(bipy)]²⁺ cations are bridged by the naphthimidazolemethyl arms of L, giving rise to a macrocycle (Figure 1a). Driven by the coordination mentioned above, L is fixed into an unusual cup-shaped conformation [39,40] and the three [Pd(bipy)]²⁺ cations act as the cup holder. In the crystal structure, each [Pd₃(bipy)₃L]³⁺ associates with its six neighbors (Figure 1b) through π···π interactions between bipy and L, forming a porous non-symmetric cubic supramolecular assembly (Figure 1c). This porous structure possesses two kinds of channels along all three crystallographic axes, which are filled with Cl⁻ and solvent molecules. Determined by PLATON, the void volume is 8658 ų per unit cell, which is 48.3% of the unit volume. In the view of topology, treating [Pd₃(bipy)₃L]³⁺ as a node and the π···π interaction between bipy and L as a linker (Figure S1a), the porous assembly can be simplified as a pcu network with a Schläfli symbol of 4⁶·6⁹ (Figure S1b). Thermogravimetric (TG) analysis with the sample heated under an N₂ stream revealed a weight loss of ~15% between 30 and 200 °C, which can be attributed to the removal of solvent molecules (Figure 2a). After desolvation, the framework structure of the porous assembly collapses, as indicated by powder X-ray diffraction (PXRD) studies (Figure 2b).

2.2. Iodine Adsorption Study

The poor thermostability of compound 1 prohibits us from investigating its iodine adsorption performance at high temperatures. Therefore, the adsorption performances of compound 1 on both gaseous and dissolved iodine were investigated at room temperature. Exposing compound 1 to iodine vapor at room temperature led to a gradual color change from yellow to black (Figure S2a). The iodine uptake also gradually increased with time and attained an uptake of 1.37 g g⁻¹ after 240 h without saturation (Figure 3a). The gaseous iodine adsorption profile can be well described by the pseudo-first-order kinetic model (R² = 0.996), which gives an adsorption rate k = 1.0 × 10⁻⁴ g min⁻¹ and an equilibrium adsorption capacity Q_e = 1.81 g g⁻¹ (Table S2).

We then examined the adsorption performance of compound 1 for iodine dissolved in cyclohexane. A crystalline sample of compound 1 (0.05 g) was immersed in a 3 mM iodine–cyclohexane solution. UV–Vis spectroscopy was used to evaluate the iodine adsorption rate (Figure 3b,c and Figure S3). With the adsorption going on, the color of the iodine–cyclohexane solution gradually faded (Figure S2b). The color of the sample of compound 1 gradually deepened and turned black when the adsorption equilibrium was reached (Figure S2c). The monitoring data revealed a fast adsorption rate in the first 6 h, and then the adsorption gradually slowed down until equilibrium (Figure 3b). The experimental data can be well described by the pseudo-second-order kinetic model (R² = 0.977), which gives an adsorption rate k₂ = 3.0 × 10⁻³ g min⁻¹ and an equilibrium adsorption capacity of 0.60 g g⁻¹ (Figure 3b, Table S2). The gaseous I₂ and dissolved I₂ uptake capacities of compound 1 are comparable to those of some promising I₂ adsorbents (Table S3) [45,46,47,48]. Furthermore, the adsorbed iodine can be released from I₂@1 by soaking I₂@1 in CH₂Cl₂. When 0.50 g of solid I₂@1 was immersed in CH₂Cl₂, the solution gradually changed from colorless to dark brown in 36 h, indicating a large amount of I₂ was released (Figure S4). Therefore, this adsorbent for iodine capture can be recycled. In the third adsorption–desorption cycle, ~70% of the I₂ adsorption capability can be retained (Figure 3d).

To give insights into the I₂ adsorption mechanism, we conducted Fourier transform infrared (FT-IR) spectroscopy (Figure 4a) and X-ray photoelectron spectroscopy (XPS) (Figure 4b–d) studies on compound 1 before and after I₂ uptake. After I₂ loading, the characteristic band at ∼1634 cm⁻¹ assigned to the C=N stretching vibration decreases significantly [14,19,29,33,34]. A pair of I 3d signals can be seen from the XPS of the sample after I₂ uptake (Figure 4a,b). The signals at 617.84 and 629.37 eV can be attributed to I 3d_5/2 and I 3d_3/2, respectively. After I₂ loading, the two N 1s signals shift from 397.94 and 399.14 eV to 398.29 and 399.43 eV, respectively (Figure 4c). The two Pd 3d signals also shift from 336.31 and 341.51 eV to 337.88 and 343.78 eV, respectively (Figure 4d). These results indicate that the N and Pd atoms on compound 1 interact with the captured iodine [49]. This interaction may be rationalized in terms of that polarized bound iodine molecules favor interaction with the partly negatively charged N lone pairs, while the cylindrical electron surface of the I−I bond would favor interaction with the positively charged Pd atoms [45]. The PXRD of I₂@1 is significantly different from that of compound 1, indicating a possible significant structural change upon iodine adsorption. However, the poor crystallinity of I₂@1 prohibits us from directly observing the I₂ binding sites by single-crystal X-ray analysis. The recycled sample of compound 1 that lost crystallinity probably implies good dispersion of the adsorbed iodine molecules around the cup-shaped molecules (Figure 2b).

3. Experimental

3.1. Iodine Adsorption Study

The ligand H₃L is synthesized according to the previously reported method [50]. [Pd(bipy)] Cl₂ and 2,3-diaminonaphthalene were purchased from bidepharmatech. All other reagents were purchased from Adamas (Shanghai, China) and used directly, without purification.

3.2. Characterization

Fourier-transform infrared (FTIR, Nicolet iS 50, Thermo Fisher, Waltham, MA, USA) spectra were recorded on a Thermo Fisher Nicolet iS 50 in the range 500–4000 cm⁻¹ at room temperature. Powder X-ray diffraction (PXRD, Miniflex 600, Akishima, Rigaku, Tokyo, Japan) patterns were obtained on a Miniflex 600 diffractometer using Cu-Kα radiation with flat plate geometry. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA) studies were performed on an AXIS SUPRA Kratos system, and the C 1s line at 284.8 eV was used as the binding energy reference. TGA was performed using a thermo plus EVO2 system at a rate of 10 °C/min in the range of 30–800 °C (TGA/DSC 1, Mettler Telodo, Zurich, Switzerland). UV–Vis spectra were recorded on an Agilent Cary 5000 spectrophotometer (UV-Vis, Agilent, Santa Clara, CA, USA).

3.3. Crystallography

Single-crystal X-ray data were harvested on a Bruker D8 Venture diffractometer with Mo-Kα radiation at 200 K. Structures were solved using a direct method and refined by the full-matrix least-squares technique on F2 with the SHELXTL 2014 program [51]. All the H atoms are geometrically generated and refined using a riding model. The PLATON/SQUEEZE procedures [52] were used to treat the highly disordered solvents in the void of the porous structure. The X-ray crystallographic coordinates for structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number CCDC 2245193. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (accessed on 28 February 2023). Details of the crystallographic data are listed in Table S1.

3.4. Synthesis of Compound 1

Additionally, (2,2′-bipyridine) dichloropalladium (II) (0.030 g, 0.09 mmol), tris (2-naphthimidazole methyl) amine (H₃L) (0.020 g, 0.036 mmol), and triethylamine (0.02 mL) were added to a mixture of MeOH/acetone (v/v: 1/3), and the mixture was stirred at room temperature for 2 h. After that, the insoluble substance was removed through filtration. The resulting filtrate was kept at room temperature undisturbed for 7 days, and then pale green crystals were obtained (yield: 67.7% based on L).

3.5. Iodine Adsorption Experiments

Both the gaseous iodine and dissolved iodine uptake behaviors of compound 1 were studied at room temperature.

3.5.1. Iodine Vapor Adsorption

Air-dried compound 1 (0.050 g) was loaded into an uncapped glass vial, which was located in a sealed container with excess solid iodine kept at the bottom. After certain time intervals, the vial was taken out and weighed, and then reloaded into the vapor of iodine to continue adsorption. The iodine uptake at a certain time was calculated using Equation (1):

$$Q_t=\frac{m_2-m_1}{m_1}$$

(1)

where Q_t represents the iodine uptake at a certain time and m₁ and m₂ are the masses of the sample of compound 1 before and after iodine uptake, respectively. The pseudo-first-order model (Equation (2)) was used to fit the gaseous iodine adsorption profile, giving a set of parameters with k₁ = 1.0 × 10⁻⁴ g min⁻¹, Q_e = 1.81 g g⁻¹, and R² = 0.996.

$$Q_t=Q_e\left(1-e^{k_{1}t}\right)$$

(2)

3.5.2. Iodine Adsorption in Solution

Air-dried compound 1 (0.050 g) was immersed in a 50 mL solution of iodine in cyclohexane (3 mM). The iodine adsorption process was monitored by UV–Vis spectroscopy. The iodine uptake was calculated using Equation (3):

$$Q_t=\frac{(C_0-C_t)}{m}V$$

(3)

where Q_t represents the iodine uptake at a certain time, C₀ and C_t represent the concentration of iodine before and after adsorption, respectively, m represents the mass of compound 1, and V represents the volume of the solution. The pseudo-second-order model (Equation (4)) was used to fit the dissolved iodine adsorption profile, giving a set of parameters with k₂ = 3.0 × 10⁻³ g min⁻¹, Q_e = 0.60 g g⁻¹, and R² = 0.977.

$$Q_t=\frac{k_2{Q}_e^{2}t }{1+k_2{Q}_{e}t}$$

(4)

3.5.3. Iodine Release and Recyclability of Compound 1

I₂@1 was immersed in CH₂Cl₂ to release the adsorbed iodine. Here, I₂@1 (0.050 g) was immersed in CH₂Cl₂ (100 mL). When the release was deemed essentially complete, the resulting solid was recycled and analyzed by PXRD. Then the recycled solid of compound 1 was added to the I₂/cyclohexane solution again. After four cycles, ~70% of the I₂ adsorption capability can be retained.

4. Conclusions

In summary, we have developed a porous self-assembly of a cup-shaped Pd^II complex. This porous structure is constructed through intermolecular π···π interactions. The channels along all three crystallographic axes within the self-assembly allow for efficient reversible iodine capture, either from the vapor or solution source phases. These results demonstrate that porous crystalline materials assembled through weak intermolecular interactions can serve as a new type of promising adsorbent for I₂ capture.