Microporous Self‐Assembled Pd(II) Tetrahedral Cages for Rapid and Reversible Multi‐Phase Sequestration of Iodine and Methyl Iodide

ABSTRACT

Industrial and medical iodine leaks create serious environmental and health risks, creating the need for materials capable of efficiently capturing radioactive iodine from air and water. Herein, we report a systematic study of four tetrahedral, microporous, self‐assembled palladium‐based coordination cages (C1–C4) incorporating high‐affinity binding motifs, including naphthalene, tetrazine, benzothiadiazole, and benzene. Despite the absence of permanent porosity, these cages exhibit exceptional iodine uptake, reaching up to ~3.78 g g^‒1 in the vapor phase at 75^οC and ~3.52 g g^‒1 in aqueous media, accompanied by fast kinetics surpassing state‐of‐the‐art COFs and MOFs. Dynamic flow‐through experiments further demonstrate their efficiency, with C4 achieving a record elution volume of 9.2 L g^‒1 in a 0.5 mM I₃ ⁻ solution. The cages enable efficient iodine removal under dynamic vapor‐phase conditions (1.92 g g^‒1), reduce iodine concentrations from ppm to ppb levels in real water samples, and exhibit strong affinity toward methyl iodide vapor (up to 1.44 g g^‒1). DFT calculations indicate that the observed performance originates from cooperative π–iodine interactions and nucleophilic heteroatom binding. Combined with structural robustness, recyclability, and scalable synthesis, these coordination cages establish a new benchmark and represent the first example of dynamic vapor‐phase iodine capture by coordination cages.

This work reports a family of microporous, self‐assembled tetrahedral coordination cages that efficiently capture iodine, polyiodides, and methyl iodide from vapor, aqueous, and organic media. The cages display exceptional uptake capacities, rapid kinetics, and high selectivity, enabling the first dynamic vapor‐phase iodine capture by coordination cages and reducing iodine levels from ppm to ppb in real water samples.

1. Introduction

In the pursuit of sustainable alternatives to fossil fuels, the promotion, development, and utilization of clean and low‐carbon energy sources represent one of the most effective means to mitigate the global carbon footprint [1, 2]. While renewable energy technologies offer sustainable power generation, their inherent intermittency limits their ability to provide a stable, large‐scale energy supply [3]. In contrast, nuclear energy has gained prominence as a reliable and efficient alternative to fossil fuels with minimal greenhouse gas emissions [4, 5]. At present, nuclear power accounts for ~10% of global electricity production, a figure projected to increase to nearly 12% by 2050 [6, 7]. While advantageous, nuclear energy adoption is limited by the critical need for robust protocols for the stabilization and permanent disposal of radioactive byproducts [8]. During nuclear fuel operation and reprocessing, a broad spectrum of radionuclides, including ¹²⁹I, ¹³¹I, ³H, ¹⁴C, ⁸⁵Kr, ⁹⁰Sr, ¹³⁷Cs, as well as radioactive elements like ²³⁵U, and ⁹⁹Tc−are inevitably released [9]. Among these, radioactive iodine is regarded as one of the most hazardous volatile fission products [10]. It exists in a variety of chemical states, encompassing molecular iodine (I₂), organic derivatives such as methyl iodide (CH₃I), and inorganic forms including I⁻, I₃ ⁻, and iodate. In aqueous environments, interactions between iodide ions and I₂ readily lead to the formation of polyiodide species through dynamic equilibrium processes [11]. Owing to its chemical versatility and high mobility, radioactive iodine can readily disperse through air, water, and soil. Long‐lived ¹²⁹I (half‐life: 15.7 million years) and short‐lived yet highly toxic ¹³¹I (half‐life: 8 days) are highly volatile, with the potential to disrupt metabolic processes [12, 13]. Moreover, substantial releases of radioactive iodine from nuclear reactor cooling systems can lead to widespread contamination of aquatic ecosystems, representing serious environmental and public health concerns [14, 15]. In addition to concerns surrounding nuclear contamination, the provision of clean and safe water remains a pressing global priority [16, 17, 18]. With rapid economic development, the global demand for iodine resources continues to increase [19, 20, 21, 22, 23, 24]. Consequently, developing selective and efficient removal technologies is imperative for both nuclear safety and advanced water purification [25].

Traditional iodine remediation relies on liquid‐phase wet scrubbing using alkaline reagents such as NaOH or Hg(NO₃)₂. Alternatively, solid‐state capture is achieved through adsorption on specialized materials, including silver‐exchanged zeolites and bismuth‐based mineral frameworks [26, 27]. However, these strategies are often constrained by complex operational procedures, the formation of toxic by‐products, slow adsorption kinetics, poor selectivity, and limited reusability, primarily due to their weak iodine–adsorbent interactions and low specific surface areas [28, 29]. To address these challenges, extensive research has focused on developing advanced adsorbent materials, such as activated carbon [30, 31] silica [32], metal–organic frameworks (MOFs) [33, 34, 35, 36] covalent organic frameworks (COFs) [37, 38, 39], porous organic cages (POCs) [40, 41, 42], porous organic polymers (POPs) [43, 44, 45], as well as metal–organic cages (MOCs) [46, 47, 48, 49, 50]. While substantial progress has been achieved, the majority of reported studies focus on iodine capture in solid‐state systems or non‐polar solvents such as n‐hexane or cyclohexane. In contrast, removal of iodine from water remains far less explored, as many of these advanced materials lack the hydrolytic stability required for aqueous applications [51]. In addition, achieving fast adsorption kinetics, high uptake capacity, and strong selectivity under competitive aqueous conditions remains a significant challenge. This issue is particularly critical in nuclear waste management, where iodine is often present at trace concentrations ranging from 5 to 16 ppm in fresh and marine waters−which is significantly more dilute than the levels typically addressed in laboratory studies (>100 ppm). As a result, engineering resilient adsorbents that can effectively sequester iodine at environmentally relevant levels is a critical priority for safeguarding aquatic ecosystems and ensuring human health safety. Encouraging advances have been reported; for instance, He et al. established that an organic amine‐based cage is capable of sequestering iodine from distilled water, effectively reducing concentrations from 5 ppm to a level of 103 ppb [20], while Van Der Voort et al. showed that COFs lowered residual concentrations of iodine in seawater to as low as 0.061 ppm. [21] Nevertheless, despite these promising developments, the simultaneous removal of I₂ and I₃ ⁻ at trace concentrations remains a major unresolved problem [20].

Although considerable research has been conducted on the adsorption of molecular iodine (I₂); however, investigations into the capture of methyl iodide (CH₃I) remain relatively limited. As both I₂ and CH₃I commonly coexist in the streams of nuclear facilities, though CH₃I is typically present at lower concentrations, the development of adsorbents capable of efficiently and simultaneously removing both species continues to be a significant challenge. Therefore, the design and synthesis of novel adsorbents with high affinity, stability, and selectivity for radioactive iodine species, including both molecular and organic iodides, are urgently needed for effective off‐gas treatment and nuclear waste management. Despite ongoing efforts, only a few materials have demonstrated strong adsorption capacities toward both I₂ and CH₃I vapors [11, 38, 52, 53, 54]. While such crystalline and porous materials have dominated research in this field, non‐porous solids have traditionally been overlooked, as they were presumed to be ineffective for dual iodine species adsorption.

Over the past few decades, coordination cages [55] have been extensively explored for applications in selective host−guest interactions [56, 57], drug delivery [24, 58], molecular separation [59, 60], stabilization of reactive intermediates [61, 62], tunable photophysical properties [63, 64], and catalysis [65, 66]. Most coordination cages are self‐assembled from coordination interactions of Pd(II) centers with bi‐, tri‐, and/or tetradentate pyridyl‐ or imidazole‐based ligands; this approach allows precise control over size, geometry, and functionality through judicious selection of ligands and self‐assembly conditions. Importantly, the ligand backbone defines the chemical environment of the cage and therefore plays a decisive role in governing its structure, electronic properties, and overall function [67].

The adsorption of iodine species generally originates from the affinity between electron‐deficient iodine and the nucleophilic (electron‐rich) centers within conjugated architectures containing heteroatoms such as nitrogen, oxygen, and sulfur. Additionally, the capture of CH₃I is predominantly driven by nucleophilic substitution at the nitrogen centers. This N‐methylation process results in the transformation of neutral nitrogen sites into stable pyridinium or quaternary ammonium cations [38]. Therefore, designing a single adsorbent that can effectively trap multiple iodine forms−including molecular I₂ and CH₃I vapor, and aqueous I₃ ⁻−create a significant challenge due to the involvement of distinct adsorption mechanisms. In this regard, self‐assembled metal−organic cages offer several intrinsic advantages for iodine sequestration, including scalable synthesis under mild conditions, tunable cationic architectures that enable strong electrostatic interactions with polyiodide anions, and conformationally adaptable cavities that enhance adsorption selectivity and capacity. Their well‐defined internal cavities, combined with good solubility and processability, facilitate efficient encapsulation of iodine species. Furthermore, positively charged cages constructed from conjugated aromatic linkers (e.g., pyridine or imidazole) provide a promising platform for the dual adsorption of molecular iodine and CH₃I. The abundance of π‐electron density and nucleophilic nitrogen centers positioned at both the endohedral and exohedral binding domains of the cage establishes a favorable coordination environment. This spatial distribution of active sites facilitates the high‐affinity sequestration of I₂, I₃ ⁻, and CH₃I species.

Herein, we explore a series of four tetrahedral, microporous coordination cages (C1–C4) constructed through two‐component self‐assembly of Pd(II) nodes and multiple ditopic donors (L1–L4), and evaluate their iodine sequestration performance under a range of conditions. These architectures were engineered to integrate high‐affinity binding sites, including nucleophilic nitrogen donors (tetrazine, benzothiadiazole) and electron‐rich aromatic moieties (benzene, naphthalene), specifically to target radioactive iodine species. The resulting solids utilize strong electron‐pair interactions to facilitate exceptional adsorption performance, with C3 exhibiting an iodine uptake of 3.78 g g⁻¹ and C2 achieving 1.44 g g⁻¹ methyl iodide capture at 75°C. These tetrahedral coordination cages (C1−C4) are insoluble in aqueous media and show remarkable iodine adsorption in water, with C4 reaching capacities of 3.52 g g⁻¹ for I₂ and 3.98 g g⁻¹ for I₃ ⁻. Even in complex matrices containing a 100‐fold excess of interfering species such as F⁻, Cl⁻, Br⁻, NO₃ ⁻, and SO₄ ²⁻, C4 maintained high selectivity, highlighting its promise for real‐world water treatment. Moreover, C4 displayed very high adsorption kinetics, removing ∼98% of iodine within just 60 s, surpassing the performance of previously reported polymers, COFs, and MOFs‐based adsorbents. This study also marks the first reported instance of a coordination cage successfully performing iodine capture under dynamic vapor‐phase conditions (1.92 g g⁻¹), and demonstrated nearly complete removal efficiency in dynamic flow‐through experiments lowering iodine concentrations from ppm to the ppb range. The cage also exhibited exceptional breakthrough performance, with an elution volume surpassing 9.2 L g⁻¹ for a 0.5 mM I₃ ⁻ solution. Combined with their robust structural stability, reusability, and scalable synthesis, these coordination cages represent a promising class of nanoscale adsorbents for the effective removal of iodine and organic iodides from a wide range of environmental matrices.

2. Results and Discussion

2.1. Synthesis and Characterization

A new dipyridyl ligand, 1,4‐di(pyridin‐3‐yl)naphthalene (L1), incorporating a naphthalene core, was synthesized via a Suzuki–Miyaura cross‐coupling reaction between 1,4‐dibromonaphthalene and pyridine‐3‐boronic acid (Scheme S1). The identity and purity of L1 were confirmed through ESI‐MS and detailed NMR characterization, including ¹H NMR, ¹³C NMR, and ¹H–¹H COSY experiments (Figures S1–S5). Self‐assembly of supramolecular cage C1 was achieved by combining Pd(NO₃)₂ and the ditopic ligand L1 in DMSO in a 1:2 metal‐to‐ligand stoichiometry. The reaction mixture was heated at 70°C with continuous stirring for 12 h (Scheme 1). Upon completion, centrifugation afforded a clear solution, which was subsequently treated with excess ethyl acetate to induce precipitation. The resulting solid was isolated, washed thoroughly with diethyl ether, and dried under reduced pressure to yield C1 as a white powder. The formation of the tetrahedral coordination cage C1 was established by ¹H and ¹³C NMR spectroscopy, along with a comprehensive set of 2D NMR techniques (Figure 1, Figures S12–S18). Coordination of the pyridyl nitrogen atoms to the Pd(II) centers was evidenced by pronounced downfield shifts in the ¹H NMR spectra recorded in DMSO, where the pyridyl proton signals moved from 8.79 and 8.71 ppm in the free ligand to 9.88 and 9.33 ppm in the assembled cage (Figure 1a). Diffusion‐ordered spectroscopy (DOSY) further revealed a single diffusion species with a diffusion coefficient of approximately ~6.92 × 10⁻¹¹ m² s⁻¹, consistent with the formation of a discrete, monodisperse assembly (Figure 1b). Signal assignments in the ¹H NMR spectrum was corroborated by ¹H‐¹H COSY and NOESY analyses. Additional confirmation of the [4 + 8] cage architecture was provided by ESI‐MS, which displayed characteristic peaks at m/z = 468.09, 573.56, and 732.11 corresponding to the charged species [C1 + 2(NO₃)]⁶⁺, [C1 + 3(NO₃)]⁵⁺, and [C1 + 4(NO₃)]⁴⁺, respectively (Figure S19).

SCHEME 1.

Two‐component self‐assembly of L1–L4 with Pd(II) ions leading to the formation of discrete tetrahedral palladium cages C1−C4.

FIGURE 1.

(a) Stacked partial ¹H NMR spectra of ligand L1 (in CDCl₃) and the corresponding coordination cage C1 (in DMSO‐d ₆). (b) DOSY NMR spectrum of cage C1 in DMSO‐d ₆. (c) Crystal structure of cage C1, highlighting its tetrahedral geometry (color code: C, gray; N, blue; Pd, cyan. Solvent molecules, counter anions, and hydrogen atoms are excluded for visual clarity).

Finally, the tetrahedral nature of cage C1 was unambiguously confirmed by single‐crystal X‐ray diffraction studies (Figure 1c, Tables S1). High‐quality single crystals suitable for structural analysis were obtained through a slow vapor‐diffusion method, in which ethyl acetate was allowed to diffuse gradually into a DMSO solution of C1 over a period of approximately two weeks. The resulting block‐like crystals were obtained in the triclinic crystal system, adopting the Pī space group (CCDC 2495304). Structural analysis revealed a well‐defined tetrahedral architecture comprising four square‐planar Pd^{2 +} centers positioned at the vertices of the tetrahedron, with Pd–Pd separations within the cage ranging from 11.396 to 12.483 Å, and Pd─N bond distances falling in the narrow range of 2.003–2.050 Å. Notably, the architecture of C1 deviates significantly from conventional edge‐capped tetrahedral assemblies reported previously, which typically consist of four metal nodes connected by six donor units [68]. In contrast, C1 incorporates eight donor sites distributed across two inequivalent sets of ligands, leading to an asymmetric edge arrangement. Specifically, four edges of the tetrahedron contain a single L1 ligand, forming single‐walled connections, whereas the remaining two edges are bridged by pairs of L1 ligands, resulting in double‐walled edges.

The electron‐deficient nature of iodine facilitates the formation of stable charge‐transfer complexes with the π‐conjugated surfaces of C1. Furthermore, the presence of multiple sp²‐hybridized nitrogen and sulphur atoms enables strong electron‐pair interactions with I₂. In the case of methyl iodide sequestration, these nucleophilic sites can undergo quaternization to generate pyridinium species, effectively anchoring the organic iodide within the backbone. To explore how modifications to the aromatic and nitrogen‐rich cores influence these interactions, additional ligands and cages were prepared following reported procedures. Ligand L2 and its corresponding cage C2 were synthesized by modifying a reported literature procedure to incorporate a tetrazine core, thereby increasing the nitrogen content of the architecture [69]. Similarly, cage C3 [70], featuring a benzothiadiazole unit, was selected to enhance both the nitrogen/sulphur density and the aromatic character of the system, which together are expected to improve binding efficiency toward I₂ and CH₃I. In addition, the tetrahedral cage C4 [71], containing a simple benzene core, was prepared to evaluate the influence of an aromatic ring in the absence of additional heteroatoms. The successful assembly of cages C2–C4 has been validated through a combination of well‐established analytical techniques, including NMR, DOSY, and mass spectrometry (Figures S20−S43). Overall, increasing the heteroatoms content and tuning the electronic properties of the aromatic core were strategic design choices intended to strengthen host–guest interactions with I₂ and CH₃I, thereby improving adsorption capacity and demonstrating the versatility of self‐assembled coordination cages for selective iodine remediation.

2.2. Static and Dynamic Iodine Vapor Adsorption

The positively charged palladium‐based coordination cages (C1–C4), enriched with π‐conjugated aromatic surfaces and diverse heteroatoms, were expected to interact strongly with iodine, thereby rendering them attractive materials for I₂ capture. We performed thermogravimetric analysis (TGA) under an inert atmosphere to confirm that the tetrahedral cages remain thermally stable at 75°C, ensuring their suitability for environments analogous to nuclear‐fuel reprocessing [72]. The TGA profiles showed only a minimal weight loss (∼10%) up to 200°C, corresponding to the removal of lattice solvents and demonstrating high thermal resilience. Beyond 200°C, all four cages exhibited pronounced decomposition indicating these materials remain structurally stable throughout the operational temperature range of 25–75°C (Figure S57). Porosity of the cage material was examined through nitrogen adsorption–desorption measurements at 77 K following activation of the samples by vacuum degassing at 100°C for 12 h. All four assemblies exhibited comparable isotherm profiles (Figure S44). BET analysis of the N₂ uptake yielded surface areas of 11.53, 21.96, 65.34, and 9.18 m² g⁻¹ for C1, C2, C3, and C4, respectively. The corresponding pore‐size distributions revealed average pore diameters in the range of 1.48–1.91 nm, values that fall within the regime typically classified as microporous in the context of MOF/COF materials (Figure S45 and Table S2). Notably, the comparatively higher surface area observed for C3 and C2 can be rationalized in light of their morphological features, as evidenced by SEM analysis (Figure S46). The SEM images show that C3 consists of relatively smaller and loosely aggregated particles with a rougher surface texture, which can enhance external surface exposure and contribute to its higher measured surface area. In contrast, C2 displays moderately sized particles with somewhat irregular and coarse surfaces, consistent with its intermediate surface area. On the other hand, C1 and C4 exhibit comparatively larger, more compact, and smoother particles with a higher degree of aggregation, leading to reduced accessible external surface area and consequently lower BET values. Moreover, powder X‐ray diffraction (PXRD) patterns revealed distinct structural characteristics, with all the cages exhibiting broad, low‐intensity signals that suggest nonporous discrete cage solids with limited long‐range packing order (Figure S58). Owing to their robust thermal and chemical stability, π‐rich aromatic backbone, and heteroatom containing internal cavities, these metallo‐assemblies were anticipated to serve as efficient hosts for iodine capture. Iodine adsorption was assessed by placing the cages material in contact with I₂ vapor at 75°C under static, closed conditions (Figure S51), and their adsorption capacities were subsequently determined based on the measured mass increase at regular intervals under ambient conditions and room temperature.

Upon exposure to iodine vapor, the adsorbents exhibited a distinct and gradual color transformation from white, pink, or yellow to dark brown, and eventually dark black over time, which indicates the successful uptake of iodine vapor by the cages (Figure 2f). The time‐dependent iodine adsorption plots showed an initial rapid and nearly linear increase during the first 450 min, indicating that most adsorption sites within the cages were occupied and equilibrium was nearly reached. Over extended exposure (15–22 h), no measurable weight increase was detected, confirming that adsorption equilibrium had been completely achieved. Gravimetric analysis revealed that the maximum iodine adsorption capacities of 3.40, 3.23, 3.78, and 3.69 g g⁻¹ for C1, C2, C3, and C4 respectively, as determined from the weight difference of the vessel before and after exposure to iodine vapor (Figure 2a). Among the reported metal–organic cages, C3 exhibited the highest iodine vapor adsorption capacity under comparable conditions (Table S12).

FIGURE 2.

(a) Time‐resolved iodine vapor adsorption isotherms at 75°C. (b) Pseudo‐second‐order kinetic models with linear fitting for the uptake of iodine vapor by C1−C4. (c) Comparison of static iodine vapor capacities at different temperatures. (d) Iodine retention stability under ambient conditions over time. (e) Vapor phase dynamic iodine adsorption capacities of the cages at 75°C. (f) Digital photographs illustrating the visual color transition of the cages before (top row) and after iodine vapor exposure (bottom row).

Owing to its significantly larger accessible surface area, C3 provides a greater number of active adsorption sites for I₂ capture and shows the highest iodine adsorption capacity among the studied materials. In addition, although cages C1 and C2 exhibit comparable surface areas and would therefore be expected to display similar adsorption behavior, C4—despite its relatively lower surface area—shows markedly superior iodine uptake. These results confirm that iodine sequestration is not merely a function of surface area or pore volume, as derived from N₂‐sorption isotherms, but is fundamentally driven by the material's surface chemistry and specific architecture [9]. Despite being microporous by BET analysis, the material captures iodine through diffusion into intrinsic cage cavities, interstitial voids in the solid‐state packing, and a cooperative effect between nucleophilic nitrogen centers and π‐conjugated aromatic motifs that stabilize polyiodide species and enable efficient uptake. This uptake is further stabilized by a complex network of non‐covalent interactions, specifically halogen bonding, π−I associations, metal–iodine coordination, and dispersive van der Waals forces. Moreover, the stability of the adsorbed species is further enhanced by the strong coulombic interactions occurring between the generated polyiodide anions and the cationic Pd(II) nodes. Under ambient conditions, the iodine absorption capabilities were determined to be 1.58, 1.30, 1.80, and 1.44 g g⁻¹ for the cages C1−C4, respectively (Figure S55). Elevating the temperature resulted in a further increase in adsorption capacity. This improvement is presumably driven by the increased thermal motion of the I_2​ molecules; the higher kinetic energy allows the system to surmount activation energy barriers, resulting in a greater frequency of productive interactions with the adsorbent interface (Figure 2c). To investigate the adsorption mechanism, the kinetic data were fitted to pseudo‐first‐order and pseudo‐second‐order models. The superior correlation coefficients obtained for the pseudo‐second‐order model indicate that this model more accurately describes the iodine uptake process (Figure 2b). The calculated rate constants were found to be 0.1592, 0.1597, 0.1358, and 0.0982 g g⁻¹ h⁻¹ for C1−C4, respectively, indicating that chemisorption is the primary mechanism for iodine sequestration (Figure S54 and Table S3) [73]. Notably, C2 exhibited the fastest kinetics, a result likely driven by a high density of sp²‐hybridized nitrogen atoms, which facilitate rapid initial iodine capture (Figure S54 and Table S3). The kinetic performance of C2 is competitive with previously reported porous adsorbents, demonstrating a robust and thermally stable affinity for I₂​. Encouraged by these results, we proceeded to evaluate the performance of these cages under industry‐relevant dynamic vapor‐phase conditions [74].

In a typical measurement, 25 mg of the as‐synthesized material was packed into a glass column with cotton wool filling the void space of both ends, and a stream of N₂ containing of I₂ vapor (∼40 ± 2 mg h⁻¹) was passed through the sample at 75°C (Figures S52 and S53). The equilibrium uptake capacities were 1.58 g g⁻¹ for C1, 1.25 g g⁻¹ for C2, 1.92 g g⁻¹ for C3, and 1.71 g g⁻¹ for C4 (Figure 2e). Under these operating conditions, the adsorption performance of the C3 cage exceeds that of previously reported porous adsorbents, including industrial Ag‐based sorbents (Ag@MOR, 0.18 g g⁻¹), nitrogen‐rich COFs (4F‐COF‐TpBpy and TGDM), sulfur‐doped frameworks (JUC‐561, 0.27 g g⁻¹), porous organic cages (e.g., NKPOC‐DT, 0.63 g g⁻¹). This remarkable performance is attributed to the cooperative interaction of I₂ molecules with the multiple S/N donor sites within the coordination cage structure. Overall, the combination of high dynamic uptake capacity, chemical robustness, and long‐term durability highlights these cages as highly promising column‐packing sorbents for I₂ removal from industrial off‐gas streams.

To evaluate long‐term sequestration stability, iodine‐saturated samples were maintained under ambient conditions for seven days. The materials exhibited exceptional retention, with a negligible weight loss of less than 4%, confirming the robust binding affinity between the iodine guests and the cage architectures (Figure 2d). These findings highlight the promise of these metal–organic cages for the secure containment and transportation of radioactive iodine isotopes captured during nuclear fuel reprocessing.

2.3. Mechanistic Investigation and Insights Into the Iodine Adsorption Process

To elucidate the iodine adsorption mechanism and characterize the resulting host–guest species, a comprehensive suite of analytical techniques was employed. Structural and morphological integrity were assessed via powder X‐ray diffraction (PXRD) and scanning electron microscopy (SEM), while elemental distribution was confirmed through energy‐dispersive X‐ray spectroscopy (EDS) mapping. The chemical nature and electronic environment of the adsorbed iodine were probed using Raman, FTIR, and X‐ray photoelectron spectroscopy (XPS), complemented by electron paramagnetic resonance (EPR) and molecular modeling simulations.

PXRD analysis of the iodine‐loaded cages (I₂@C) displayed the absence of diffraction peaks corresponding to crystalline elemental iodine. This indicates that the adsorbed iodine is distributed within the cage matrix in a non‐crystalline, amorphous form (Figure S58). TGA curves of the I₂‐loaded composites exhibited pronounced mass loss in the 40–200°C range, consistent with the release of encapsulated iodine (Figure S57). The extent of mass loss aligned well with the relative uptake capacities of the cages. Importantly, the elevated desorption temperatures compared to the sublimation point of free iodine (~65°C) highlight the enhanced thermal stabilization of iodine within the host, indicating strong host–guest interactions. SEM imaging revealed that all four cages (C1−C4) possessed irregular morphologies with broad particle‐size distributions (Figure S46). Following iodine uptake, these cages retained their amorphous character; however, their surfaces became noticeably rougher, resulting in a fluffier morphology (Figure S64).

The EDAX spectra of the iodine‐saturated cages exhibited prominent signals in the 3.5−5.0 keV region, characteristic of iodine. Furthermore, the intensity of these peaks showed a direct correlation with the gravimetric iodine uptake, confirming the successful loading of the cage (Figures S65–S68) [75]. Complementary EDS elemental‐mapping analyses consistently revealed a uniform and densely distributed iodine signal across the surfaces of the host materials (Figure 2b, Figures S65–S68). The observed homogeneous dispersion suggests that robust host–guest interactions effectively suppress iodine aggregation and crystallization, thereby contributing to the structural stability and the high adsorption efficiency of the cage architectures.

Raman spectroscopy was employed to identify the chemical nature of the sequestered iodine, providing definitive evidence for the formation of polyiodide species. In contrast to the Raman‐silent pristine cages, the iodine‐loaded samples exhibited distinct bands corresponding to I₃ ⁻ (~106 cm⁻¹) and I₅ ⁻ (~166 cm⁻¹). The appearance of these characteristic vibrations confirms that iodine is strongly bound within the cage environment, forming stabilized polyiodide species. The Raman spectra of the iodine‐loaded assemblies revealed a weak, broad band at ~212 cm⁻¹, which is characteristic of the stretching vibration of molecular I₂​. The absence of this feature in the pristine material confirms that this signal originates specifically from the encapsulated iodine guests (Figure 3a, Figures S60–S63). Additional bands observed at 105–108 and 130–137 cm⁻¹ were attributed to the symmetric and asymmetric stretching modes of I₃ ⁻, respectively, while the peak at 163–169 cm⁻¹ was assigned to the stretching vibration of I₅ ⁻ (Figure 3a, Figures S60–S63) [38, 76]. Collectively, the spectroscopic analyses indicate that iodine uptake occurs predominantly through chemisorption, facilitated by strong charge‐transfer from the electron‐rich backbone to the iodine guests. Specifically, the nucleophilic nitrogen centers within the tetrazole and benzothiadiazole moieties donate electron density into the σ* orbital of I₂. This Lewis acid–base interaction, complemented by halogen bonding, promotes the polarization and subsequent transformation of molecular iodine into stabilized polyiodide species such as I₃ ⁻ and I₅ ⁻, accounting for the high iodine‐adsorption capacity of the assemblies [77, 78].

FIGURE 3.

(a) Raman spectra of pristine C1 and I₂@C1. (b) SEM images and corresponding EDS elemental mapping of iodine‐loaded C1, demonstrating the uniform distribution of sequestered iodine. (c) High‐resolution N 1s XPS spectra of C1 before and after adsorption, showing the shifts associated with host–guest charge transfer. (d) Deconvoluted I 3d XPS spectrum of I₂@C1, revealing the coexistence of molecular iodine and polyiodide anions.

EPR spectroscopy further corroborated the occurrence of electron transfer during iodine adsorption. While the pristine materials exhibited no EPR signals, the iodine‐loaded assemblies exhibited pronounced paramagnetic signals (Figure S59), indicative of radical species formed upon iodine uptake. These results are consistent with the formation of radical intermediates via electron transfer from the nucleophilic cage moieties to the electrophilic iodine molecules.

FTIR analysis showed clear spectral modifications after I₂ uptake, confirming specific interaction sites between the cages and the adsorbed iodine species. A marked decrease in the intensity of the aromatic C═C stretching band (1580–1475 cm⁻¹) was observed, consistent with the formation of iodine–π interactions. Furthermore, noticeable shifts in the C═N and C–N stretching vibrations provide evidence of iodine coordination to nitrogen‐bearing functionalities. Specifically, these bands shifted from 1607 to 1598 and from 1323 to 1318 cm⁻¹ in C1; from 1610 to 1613 and 1330 to 1327 cm⁻¹ in C2; from 1595 to 1598 and 1351 to 1349 cm⁻¹ in C3; and from 1606 to 1600 and 1326 to 1301 cm⁻¹ in C4, respectively (Figure S56). These perturbations signify the formation of Lewis acid–base adducts, confirming that both the π‐rich aromatic cores and the nitrogenous groups serve as the primary anchoring sites for iodine.

XPS was employed to probe the electronic environment and oxidation states of the sequestered iodine. Unlike the featureless spectra of the pristine cages, the XPS spectra of the iodine‐loaded cages (I₂@C1–C4) displayed two well‐resolved sets of peaks in the iodine region, confirming the coexistence of iodine in multiple oxidation states. Specifically, the I 3d region of the loaded samples exhibits peaks at 631.52 and 620.08 eV for I₂@C1, 630.44 and 619.08 eV for I₂@C2, 630.37 and 618.82 eV for I₂@C3, and 631.47 and 620.16 eV for I₂@C4, which are characteristic of the I₃ ⁻ orbitals of molecular iodine. Additionally, the associated I₅ ⁻ peaks are detected at 630.38 eV and 618.84 eV for I₂@C1, 629.96 eV and 618.49 eV for I₂@C2, 628.06 eV and 616.80 eV for I₂@C3, and 630.28 eV and 618.82 eV for I₂@C4 (Figure 3d, Figures S69–S71) [20, 79]. Quantitative deconvolution of the XPS peak areas enabled estimation of the relative polyiodide distributions within each host. The distributions of I₃ ⁻ and I₅ ⁻ were determined to be 54.8% and 45.2% for I₂@C1, 38.4% and 61.6% for I₂@C2, 10.3% and 89.7% for I₂@C3, and 84.5% and 15.5% for I₂@C4, respectively. Notably, C2 and C3, which possess a higher density of electron‐rich nitrogen centers, exhibit a larger fraction of I₅ ⁻ compared to I₃ ⁻. In contrast, the absence of abundant nitrogen‐rich sites in the cage C1 and C4 favors the formation of I₃ ⁻ relative to I₅ ⁻. This trend highlights the role of nitrogen‐mediated electron donation; higher electron density effectively stabilizes extended polyiodide chains, facilitating the conversion of I₃ ⁻ into more complex I₅ ⁻ species.

The tetrahedral cages (C1–C4) exhibit N 1s primary signals at 400.74, 400.27, 400.32, and 400.25 eV, respectively, characteristic of the C–N bonding environment. Upon iodine uptake, these peaks shift slightly towards higher and lower binding energies at 400.78 eV for I₂@C1, 400.21 eV for I₂@C2, 400.08 eV for I₂@C3, and 400.23 eV for I₂@C4, indicating subtle changes in the electronic environment of the nitrogen atoms (Figure 3c, Figures S69–S71). Additionally, new components emerged at higher binding energies at 401.59 eV for I₂@C2 and 401.10 eV for I₂@C3 (Figures S69–S70) [11]. These emergent new peaks are indicative of charge‐transfer interactions between the sp²−nitrogen sites and the adsorbed iodine. This confirms that significant electron density is transferred from the backbone to the guests, facilitating the stabilization of polyiodide species. Complementary evidence for this electronic interplay is found in the C 1s XPS spectra (Figure S72), where shifts in the signals following adsorption reflect interactions between the captured iodine and the aromatic C═C systems. Furthermore, the S 2p XPS spectrum of C3 exhibits a new peak at 168.43 eV post‐adsorption (Figure S73), providing direct evidence for S∙∙∙I₂ charge‐transfer interactions and confirming the multifaceted binding nature of the cage.

The coexistence of molecular iodine and polyiodide anions within the host indicates that iodine sequestration involves a synergistic combination of physisorption and chemisorption. We propose a two‐step adsorption pathway: initially, molecular iodine is physically sequestered within the internal voids of the coordination cages via pore confinement. Subsequently, strong electronic interactions between the guest iodine and the electron‐rich nitrogen/sulfur sites drive a charge‐transfer process. This facilitates the formation and stabilization of polyiodide anions (I₃ ⁻ and I₅ ⁻) through halogen bonding [21]. This cooperative adsorption mechanism, combining initial physical confinement with chemical stabilization enabled by the heteroatoms (nitrogen and sulphur) functionalities and aromatic units (phenyl and naphthalene moieties) of the cages, is responsible for the superior iodine uptake performance of these assemblies.

2.4. Static Iodine Adsorption in Aqueous Solution

The discharge of radioactive iodine into seawater following nuclear accidents presents significant threats to aquatic ecosystems and public health. Given that the WHO specifies 0.1 ppm as the maximum permissible iodine level in natural waters [80], the development of efficient remediation strategies for trace iodine is imperative. While iodine capture research has historically prioritized vapor‐phase adsorption, there is a growing necessity to address aqueous iodine contamination. However, aqueous treatment is complicated by the coexistence of I₂ and polyiodides (e.g., I₃ ⁻) in the presence of excess I⁻. Building on the strong vapor‐phase performance of our cages, we investigated the ability of these electron‐rich cages to sequester iodine species from aqueous media via static adsorption experiments.

Aqueous‐phase iodine uptake experiments were performed by placing 21 mg of each water‐insoluble cage material (C1−C4) into seven vials (3 mg per vial). Each vial was then treated with 3 mL of a 1.2 mM iodine solution. After the addition of the cage materials, the brown iodine solution turned colorless within 30 s, confirming rapid iodine removal (Figure S81). The remaining iodine concentration in the supernatant was quantified using UV‐vis spectroscopy following filtration (Figure 4a, Figures S75–S77). A sharp decrease in the characteristic absorption band of iodine at 450 nm was observed over time (Figure S79). The coordination cages C1, C2, C3, and C4 demonstrated exceptionally rapid iodine sequestration, with removal efficiencies reaching 95.86%, 93.02%, 93.20%, and 97.82% within the first 60 s of exposure, respectively. Notably, C4 displayed the most efficient kinetics, attaining adsorption equilibrium in just ~30 s. In comparison, C1 and C3 required approximately 120 s, while C2 reached saturation after ~180 s (Figure 4b). To determine the maximum adsorption capacities, 15 mg of each cage was dispersed in 200 mL of a 1.2 mM aqueous iodine solution and stirred for 48 h. UV–Vis spectroscopic analysis confirmed exceptional iodine loading capacities of 3.03, 2.74, 2.82, and 3.52 g g⁻¹ for C1−C4, respectively (Figure 5e, Figure S82). These values are comparable to those of the best‐performing iodine sorbents reported to date, with C4 showing the highest performance recorded for coordination cages in aqueous environments (Table S13). The adsorption kinetics were accurately described by a pseudo‐second‐order model, yielding excellent correlation coefficients (R²> 0.99), implying that chemical interaction between the iodine molecules and cage backbone (Figure S80). The calculated rate constants for C1−C4 were 0.1604, 0.1013, 0.0748, and 0.7380 g mg⁻¹ min⁻¹, respectively, with exceptional I₂ removal efficiency exceeding 99% (Table S4). Notably, the exceptionally high iodine adsorption rate constant of C4 confirms its superior kinetic performance. To further elucidate the adsorption mechanism, the data were fitted to Langmuir and Freundlich isotherms. The Freundlich model provided a better fit (R² = 0.99), relative to the Langmuir model (R² = 0.90–0.98), suggesting that the adsorption process follows a multilayer chemisorption mechanism on energetically heterogeneous surfaces (FiguresS85–S88 and Table S5).

FIGURE 4.

(a) UV‐vis spectra demonstrating the rapid depletion of iodine in water over time upon the addition of C4. (b) Adsorption kinetics of aqueous iodine for cages C1–C4. (c) Selective iodine removal efficiencies in the presence of a 100‐fold molar excess of competing anions (F⁻, Cl⁻, Br⁻, NO₃ ⁻, and SO₄ ²⁻); the “blank” entry denotes the control system without interferents. (d) Evolution of UV‐vis spectra over time during the adsorption of I₃ ⁻ by cage C4 in an I₂/KI aqueous solution (0.5 mM). e) Comparative static adsorption kinetics for I₃ ⁻​ uptake by tetrahedral cages C1–C4, in an I₂/KI solution (0.5 mM). (f) I₃ ⁻ removal efficiencies in the presence of a 100‐fold excess of competing anionic species (F⁻, Cl⁻, Br⁻, NO₃ ⁻, and SO₄ ²⁻).

FIGURE 5.

(a) Distribution coefficients (K_d ) for I₂ uptake in the presence of a 1‐equivalent mixture of competing anions. (b) Comparison of K_d values for I₃ ⁻ uptake by C1−C4 in the presence of 1 equivalent of individual interfering anions; “Blank” indicates to conditions without competing species. (c) UV‐vis absorption spectra of the eluent during dynamic flow‐through adsorption of 0.5 mM I₃ ⁻ using C4. (d) Breakthrough curves derived from the dynamic flow‐through experiments. (e) Comparative I₂ and I₃ ⁻ uptake capacities of the cages in aqueous solution. (f) I₃ ⁻ removal efficiencies (5 ppm initial concentration) across four distinct iodine‐contaminated water sources.

To assess the adsorption selectivity of the pristine cages in the presence of various interfering anions (F⁻, Cl⁻, Br⁻, NO₃ ⁻, and SO₄ ²⁻) commonly found in nuclear wastewater, competitive experiments were carried out using simulated I₂‐containing wastewater solutions. Each cage material was introduced into a 1.2 mM iodine solution comprising different concentrations (1 to 100 equivalents) of these common inorganic anions [81]. UV‐vis spectral analysis revealed that the presence of these anions had a negligible impact on performance. Despite introducing up to a 100‐fold excess of interfering anions, these cages retained over 98% removal efficiency compared to anion‐free conditions (Figure 4f), demonstrating outstanding selectivity towards iodine. This high selectivity toward iodine originates from the preferential binding within the hydrophobic cage cavities, which effectively shields the binding sites from competing aqueous anions. The affinity was further quantified by the distribution coefficient (K_d ) which remained on the order of 10⁴ mL g⁻¹ even in the presence of one equivalent of competing species, confirming the strong thermodynamic drive for iodine capture (Figure 5a, Figure S84) [82].

To assess the practical applicability of the cages, iodine sequestration was investigated using natural seawater sourced from the Bay of Bengal, India. Building on the rapid kinetics observed in static aqueous systems, the cages were tested in seawater containing 1.2 mM I₂, prepared by the addition of iodine (Figure S89). Under these high‐salinity conditions, C1−C4 maintained exceptional maximum uptake capacities of 2.91, 2.56, 2.69, and 3.43 g g⁻¹ within 48 h, respectively. These results highlight the robust performance of cage‐based sorbents in complex, real‐world matrices and their significant potential for marine iodine remediation [83].

Beyond neutral I₂​, anionic species such as I⁻ and I₃ ⁻​ are critical components of aqueous iodine chemistry. In these systems, molecular iodine readily converts to triiodide in the presence of I⁻ through the dynamic equilibrium: I₂ + I⁻ ⇌ I₃ ⁻. Since I₃ ⁻ often becomes the dominant species in contaminated waters, its efficient sequestration is essential for comprehensive remediation. We evaluated the triiodide uptake of cages C1−C4 using static adsorption experiments. Upon introducing 3 mg of each cage into a 3 mL aqueous I₃ ⁻ solution (0.5 mM), the characteristic UV‐vis absorption bands at 290 and 355 nm progressively decreased. This spectral attenuation was accompanied by a distinct color change from dark yellow to colorless, providing clear visual evidence of successful triiodide removal (Figure 4d,e, Figures S91–S93). Remarkably, C4 reduced the triiodide concentration to 1.13 ppm within only one min. After 4 min, the concentrations further declined to 0.69, 1.57, 1.04, and 0.36 ppm for C1−C4, respectively (Figure S94). The accelerated uptake kinetics observed for C4 are attributed to its comparatively larger cavity size, which facilitates rapid diffusion and efficient access to active sites throughout the cage. Kinetic modelling confirms that the I₃ ⁻ adsorption follows a pseudo‐second‐order pathway, indicating a chemisorption‐controlled mechanism (Figure S96). The calculated rate constants for C1−C4 were 0.123, 0.083, 0.045, and 0.825 g mg⁻¹ min⁻¹, respectively (Table S6). Notably, C4 exhibits the highest adsorption rate, surpassing previously reported polymer, COF, and MOF‐based adsorbents (Table S15).

To further elucidate the triiodide adsorption behavior, isotherm studies were performed at 25°C by varying the initial concentration while maintaining a constant adsorbent dosage (3 mg in 3 mL of I₃ ⁻ solution). The experimental data showed an excellent fit to the Freundlich isotherm model (Figures S101–S104 and Table S7), with a high correlation coefficient R² = 0.98–0.99 compared to the Langmuir model (R² = 0.65–0.92). This suggests that I₃ ⁻ sequestration on the cages predominantly follows a multilayer adsorption mechanism (Figures S101–S104 and Table S7) similar to aqueous I₂ adsorption. To determine the maximum uptake limits, the cages were saturated in a concentrated KI₃​ solution by immersing 6 mg of each cage in a saturated KI₃ solution (prepared by mixing 600 mg KI and 300 mg I₂ in 6 mL of water) and stirring for 48 h. Iodine quantification via sodium thiosulfate titration revealed high adsorption capacities of 3.37, 2.79, 2.96, and 3.98 g g⁻¹ for C1−C4, respectively (Figure 5e). These results collectively highlight the robust affinity of these metal–organic architectures toward polyiodide species.

Following the same approach used for the I₂ solutions, selectivity tests were performed by adding 1–100 equivalents of a wide range of interfering anions to I₃ ⁻‐contaminated water. Remarkably, the metal–organic cages maintained ~98% I₃ ⁻ removal efficiency even at high competing species concentrations, demonstrating their strong and selective affinity for I₃ ⁻, consistent with their performance in I₂‐contaminated systems (Figure 4f, Figures S97–S100). We also calculated the distribution coefficient (K_d ) values to estimate the binding affinity, and the cages exhibited K_d values on the order of 10⁴ mL g⁻¹, significantly surpassing the threshold of 10³ mL g⁻¹ typically defined as “exceptional” [84, 85]. These high values, maintained even in the presence of competing anions, substantiate the potent and selective affinity of the cages for I₃ ⁻ (Figure 5b, Figures S97–S100). Additionally, to evaluate the practical utility of these cages for the treatment of I₃ ⁻‐contaminated water, adsorption experiments were conducted in complex media containing concentrated KI/I₂ solutions (prepared by dissolving 600 mg KI and 300 mg I₂ in 100 mL of water) along with 100 equivalents of each competing anion (F⁻, Cl⁻, Br⁻, NO₃ ⁻, and SO₄ ²⁻) together. Under these challenging conditions, the cages displayed maximum iodide uptake capacities of 3.08, 2.54, 2.80, and 3.72 g g⁻¹ for C1–C4, respectively (Figure S105). The structural integrity of the architectures was verified by comparing the ¹H NMR spectra of the cages before and following iodine exposure. The lack of any significant chemical shifts or peak broadening confirmed that the molecular architectures remained entirely intact throughout the sequestration process (Figures S134–S137). Such structural durability is critical for practical implementation, positioning these metal–organic cages as reliable platforms for the continuous remediation of iodine‐contaminated aqueous streams.

2.5. Dynamic Iodine Adsorption in Aqueous Solution

Given their rapid, efficient, and highly selective removal of I₂ and I₃ ⁻ from static aqueous media, the molecular cages were further examined in a continuous flow‐through setup to assess their industrial viability [21]. While static tests demonstrate fundamental affinity, flow‐through evaluations are paramount for determining how the materials behave in large‐scale remediation systems [20]. To validate the industrial potential of the cages, dynamic flow‐through adsorption experiments were conducted using column filtration. Glass columns were packed with 15 mg of the dried cage material, secured between cotton plugs, to function as the stationary phase for continuous remediation. Aqueous solutions of I₂ (1.2 mM) and I₃ ⁻ (0.5 mM) were introduced into the columns at a controlled flow rate of 1.0 mL min⁻¹. Notably, the brown I₂ and yellow I₃ ⁻ solution underwent instantaneous decolorization upon contact with the adsorbent bed. This rapid transition to a colorless eluent provides direct visual evidence of the cages' high kinetic efficiency for sequestering aqueous iodine species (Figures S113 and S114). Quantitative analysis via UV‐vis spectroscopy confirmed the exceptional efficacy of the cages under dynamic flow conditions. The tetrahedral cages achieved near‐quantitative iodine sequestration, with removal efficiencies of 97.9% for C1, 98.7% for C2, 97.3% for C3, and 99.8% for C4 (Figures S106−S109). Based on the effluent analysis, the breakthrough volumes for a 1.2 mM I₂ solution were determined to be 5.20, 4.53, 4.92, and 6.00 L g⁻¹ for C1−C4, respectively (Figures S106−S109). Breakthrough experiments further demonstrated the exceptional triiodide sequestration performance of the series. Cages C1−C4 achieved high removal efficiencies of 99.4%, 98.9%, 99.1%, and 99.6%, respectively (Figure 5c, Figures S110–S112). UV‐vis analysis of the effluent yielded breakthrough volumes of 7.33, 5.46, 6.5, and 9.2 L g⁻¹ for cages C1−C4, respectively (Figure 5d, Figures S110–S112). The ability to sustain ∼99% efficiency prior to the breakthrough point highlights the highly accessible active sites within the architectures. To the best of our knowledge, the breakthrough volume achieved by C4 is the highest reported to date among coordination cages. This record performance is a direct result of its accelerated adsorption kinetics and a superior binding affinity for iodine species under continuous flow conditions. Together, these results confirm that the cages combine high removal efficiency with remarkable operational stability, making them highly promising for real‐world iodine capture applications.

In practical remediation scenarios, iodine pollutants are typically encountered at trace concentrations (Section S10). To evaluate the environmental viability of these cages, adsorption studies were conducted using a 5‐ppm iodine‐contaminated water across a variety of realistic matrices. These included simulated seawater (mimicking industrial effluent), distilled water (representative of groundwater), tap water, and lake water. This diverse testing suite ensures a comprehensive assessment of the cages' performance in complex, real‐world aqueous environments. To evaluate performance at environmentally relevant levels, iodine​‐contaminated solutions were processed through columns packed with 10 mg of adsorbent. Residual iodine concentrations were quantified using the leuco crystal violet method, a highly sensitive colorimetric technique (Scheme S9) [23]. All four cages demonstrated exceptional decontamination across all matrices—seawater, distilled water, drinking water, and lake water—maintaining removal efficiencies exceeding 99% (Figure 5f). Remarkably, the influent was reduced to trace ppb levels in every instance. The concentrations were lowered to 31, 38, 35, and 29 ppb in seawater; 19, 25, 22, and 17 ppb in distilled water; 18, 21, 19, and 16 ppb in drinking water; and 23, 36, 28, and 20 ppb in lake water, for C1−C4, respectively. These findings validate the robust applicability of the cages for comprehensive iodine decontamination across diverse aquatic environments. The achieved residual levels closely align with natural background iodine concentrations and remain significantly below the thresholds associated with human health risks. Given their exceptional adsorption capacity, rapid kinetics, and high selectivity, these molecular cages emerge as premier candidates for iodine remediation in both industrial wastewater treatment and domestic water purification systems.

2.6. Iodine Sequestration in Non‐Aqueous Media

To evaluate the versatility of these metal–organic cages beyond aqueous environments, their adsorption performance was examined in n‐hexane, targeting the industrial demand for iodine remediation in non‐polar organic media. In a typical experiment, 5 mg of the activated cages (C1−C4) were dispersed in 6 mL of a 1.5 mM I₂/n‐hexane solution under ambient conditions. The adsorption progress was monitored via UV‐vis spectroscopy, monitoring the attenuation of the characteristic iodine absorption band at 523 nm. Over time, the initial deep purple solution became nearly colorless, while the adsorbents transitioned to a yellow precipitate, providing a clear visual confirmation of efficient iodine extraction from the organic phase (Figure S123). This time‐dependent decrease in absorbance validates the cages' ability to serve as effective scavengers in non‐polar solvents. All four cages exhibited comparable kinetic profiles, reaching adsorption equilibrium in approximately 50 h (Figure 6b, Figures S117–S120). At this stage, the maximum uptake capacities were determined to be 404.51, 366.69, 398.60, and 416.32 mg g⁻¹ for C1–C4, respectively (Figure 6a). These results highlight the potent affinity of the cages for iodine molecules within organic phases. Furthermore, the iodine removal efficiencies reached 88% (C1), 80% (C2), 87% (C3), and 91% (C4), demonstrating the high extraction capability of these molecular cages even in non‐polar media (Figures S117–S120). Consistent with earlier investigations, the adsorption kinetics in the organic phase were analyzed using two conventional kinetic models to clarify the guest‐binding mechanism. Iodine uptake in n‐hexane was best described by the pseudo‐second‐order model, with excellent correlation coefficients (R² ≈ 0.99), indicating that the process is dominated by chemisorption. The maximum adsorption capacities predicted by this model−390.62, 362.32, 389.10, and 414.94 mg g⁻¹ for C1–C4, respectively, showed high congruency with the experimentally observed values (Figure S122 and Table S8). Furthermore, the adsorption equilibrium in organic solvents is also well described by the Freundlich isotherm model similar to that observed in aqueous systems (Figures S124 and S125 and Table S9), indicating a multilayer adsorption process on energetically heterogeneous surface sites. These findings highlight the strong affinity of the cages for iodine in both aqueous and nonpolar organic environments.

FIGURE 6.

(a) Time evolution of iodine uptake for C1−C4 in n‐hexane. (b) Time‐resolved UV‐vis absorption spectra of the iodine in hexane solution (1.5 mM) during sequestration by C4, showing the attenuation of the 523 nm band. (c) Cyclic stability of C4 over five consecutive adsorption–desorption iterations, demonstrating retained removal efficiency. (d) Stacked partial ¹H NMR spectra (DMSO‐d ₆) of C4 comparing the pristine cage (bottom), the iodine‐loaded cage (middle), and the regenerated architecture post‐desorption (top), confirming structural persistence.

2.7. Recyclability and Iodine Desorption

To evaluate the recyclability, the cages were first saturated with iodine vapor at 75°C for 24 h. TGA analysis indicated that the guest iodine desorbs between 50 and 180°C, confirming that thermal triggering is an effective method for cage regeneration. Consequently, thermal desorption was conducted at 110°C, where weight loss measurements revealed rapid release kinetics. Most of the adsorbed iodine was released within the first 300 min, and desorption equilibrium was attained within 48 h, with initial desorption efficiencies of 97–99%. After five complete cycles, the sequestration efficiency exhibited negligible decay, retaining 93%–97% of the original capacity across the series (Figure 6c, Figures S138–S140).

Iodine recovery from the cages was successfully achieved using cyclohexane as a leaching solvent. The desorption process was visually confirmed as the colorless solvent transitioned to pink, indicating the successful transfer of iodine into solution (Figure S132). UV‐vis spectroscopy at 523 nm quantified the progressive release of iodine over time (Figures S127–S130). Kinetic modeling showed that the desorption profile adheres to a pseudo‐second‐order model (Figure S133), with high rate constants of 0.360, 0.508, 0.408, and 0.631 g mg⁻¹ min⁻¹ for C1−C4, respectively (Table S10) [86]. Furthermore, ¹H NMR spectroscopy confirmed the structural integrity of the cages post‐desorption, with spectra matching those of the pristine materials. This sustained performance across multiple iterations highlights the durability of the coordination bonds and suggests significant potential for long‐term circular use in iodine remediation.

2.8. Theoretical Investigations via Density Functional Theory

To corroborate experimental observations and elucidate the fundamental binding mechanisms, Density Functional Theory (DFT) calculations were conducted (Section S14). Geometry optimizations of the energy‐minimized architectures provide molecular‐level insights into the non‐covalent interactions governing sequestration. These simulations successfully identify the most energetically favorable binding sites for both I₂ and I₃ ⁻​ within the interior and window regions of the cages [87].

The electrostatic potential (ESP) distributions of cages with I₂ and I₃ ⁻ reveal several critical features. Pronounced energy minima were observed at the preferred interaction sites within the cages. The relative ESP values associated with I₂ and I₃ ⁻ indicate cage‐dependent electrostatic environments, reflecting differences in their interaction strengths with iodine species. Furthermore, mapping the ESP surfaces identified distinct regions of high positive potential localized around the Pd(II) centers and within the aromatic ligand backbones. These domains serve as the primary sites for electrostatic stabilization, particularly for the electron‐rich triiodide species. Computational analysis reveals that both I₂ and I₃ ⁻ exhibit substantially higher binding energies when encapsulated within the cage cavity compared to surface‐level external binding. This disparity highlights the confinement effect of the internal pore environment in amplifying adsorption affinity [83, 88]. A quantitative comparison of the binding energies shows that I₂​ interacts relatively weakly with the architectures (–3 to –9 kcal mol⁻¹), whereas I₃ ⁻ exhibits significantly more potent interactions (–207 to –315 kcal mol⁻¹). These results confirm a pronounced thermodynamic preference for triiodide species, driven by the electronic environment of the cage interior (Figure 7, Figures S155–S163). This selectivity is primarily driven by the strong electrostatic attraction between the anionic I₃ ⁻ guests and the cationic cage architectures. Beyond these electrostatic contributions, the iodine species are further stabilized by an intricate network of supramolecular interactions, including C–H···I contacts, metal−halogen coordination, π···I interactions [89]. In summary, the DFT findings confirm that these metal–organic cages serve as highly effective host systems for both I₂ and I₃ ⁻, forming robust host–guest assemblies by synergizing long‐range electrostatic forces with short‐range noncovalent interactions.

FIGURE 7.

Optimized geometries and corresponding ESP distributions for iodine species within the cages: (a, b) Minimum energy binding configurations for I₂ in cages C1 and C2, respectively. (c,d) Preferential binding sites for I₃ ⁻ in cages C1 and C2 (a representative fragment of C1 was utilized for computational efficiency due to its large molecular dimensions), Color code: C (grey), N (blue), H (white), and Pd (cyan).

2.9. Kinetic Profiles and Binding Mechanisms of Organic Iodide

Building on their high efficacy in capturing iodine from vapor and aqueous phases, the cages were further evaluated for their ability to sequester methyl iodide (CH₃I), a critical radioactive organic species found in nuclear waste streams. To facilitate a direct comparison with previous iodine uptake experiments, static CH₃I adsorption studies were conducted under analogous conditions at 75°C. We expected high CH₃I uptake due to the potent nucleophilic interactions between the cages' nitrogen‐rich sites and the electrophilic methyl iodide molecules. The sequestration process was quantified gravimetrically, with the adsorption kinetics derived from the time‐dependent increase in adsorbent mass (Figure 8a). The architectures demonstrated significant CH₃​I capacities, reaching saturation values of 1.09, 1.44, 1.23, and 1.05 g g⁻¹ for C1–C4, respectively (Figure 8b), performing competitively against benchmark porous adsorbents. Notably, the data suggest that CH₃​I sequestration is primarily governed by the density of high‐affinity binding sites rather than traditional structural parameters like surface area or pore volume. While C1 and C4 reached equilibrium within 4 h, C2 and C3 required ~10 h for full saturation.

FIGURE 8.

(a) CH₃I adsorption by the cages C1−C4 over time at 75°C under static conditions. (b) Comparative CH₃I equilibrium uptake capacities across the series. (c) High‐resolution N 1s XPS spectra of C2 in its pristine state (top) and post‐CH₃I adsorption (bottom), indicating chemical interaction with nitrogen sites. (d) High‐resolution I 3d XPS spectrum of the CH₃​I‐loaded C2, confirming the successful sequestration of the organic iodide.

Kinetic modelling showed an excellent fit to the pseudo‐second‐order rate model, confirming a chemisorption‐dominated mechanism. The calculated rate constants were 0.95, 0.56, 0.96, and 1.44 g g⁻¹ h⁻¹ for C1−C4, respectively (Figure S141 and Table S11). When the adsorption experiments were performed at room temperature (25°C), the capacities underwent a moderate reduction to 0.75, 0.88, 0.80, and 0.65 g g⁻¹ for C1–C4, respectively. The superior performance of C2 is attributed to its tetrazine core, which provides enhanced nucleophilic binding affinity toward the electrophilic CH₃I molecules (Figure 8b). To verify that CH₃I molecules were securely confined within the cage interiors rather than superficial surface attachment, retention stability tests were conducted. After exposing the saturated cages to ambient conditions for seven days, a negligible mass loss of only ∼5% was recorded. This high retention rate confirms that the guest molecules are tightly bound to internal active sites and do not exist as loose surface condensates (Figure S142), highlighting the suitability of these architectures for durable CH₃​I immobilization.

To elucidate the CH₃I adsorption mechanism, the materials were subjected to comprehensive characterization, including EDS elemental mapping, FTIR, and XPS analysis. EDS mapping revealed a homogeneous distribution of iodine throughout the cage materials, providing strong evidence for the uniform encapsulation of CH₃I within the internal voids rather than surface adsorption (Figures S143–S146). FTIR spectroscopy further validated the chemical interaction between CH₃I and the cage scaffolds. Upon loading, the appearance of new vibrational bands in the 900–1000 cm⁻¹ region for CH₃I@C2 and CH₃I@C3 (Figure S147) indicates the formation of new C–N bonds [90, 91]. This spectral signature confirms the generation of ionic species [–N–CH₃]⁺I⁻ via N‐methylation, establishing that strong covalent interactions drive the sequestration process.

XPS analysis was performed to establish the CH₃I adsorption mechanism and the host–guest binding interactions. For all CH₃I‐loaded cages, prominent signals appeared at ~617–619 and ~629–630 eV, corresponding to the I 3d_5/2 and I 3d_3/2 core levels, respectively (Figure 8d, Figures S148–S150). These signals confirm the successful sequestration of iodine species within the cage scaffolds. Furthermore, high‐resolution XPS analysis of the N 1s region revealed significant binding energy shifts for both the pyridyl and sp³‐hybridized nitrogen centers following CH₃​I adsorption. The N 1s peaks shifted from 398.88 to 399.69 eV, 398.87 to 399.58 eV, 398.65 to 398.82 eV, and 398.76 to 399.92 eV, while the corresponding sp³−N peaks shifted from 400.27 to 401.81 eV, 400.74 to 401.58 eV, 400.32 to 401.64 eV, and 400.25 to 401.56 eV, for C1−C4, respectively [11]. Most importantly, the emergence of new components at higher binding energies (e.g., 402.78 eV for C2 and 402.80 eV for C3) are attributed to the formation of N–CH₃ bonds (Figure 8c, Figures S148–S150), providing definitive evidence for a covalent N‐methylation mechanism occurring within the cage interiors. These results identify the nitrogen‐ functional groups—specifically the pyridine, tetrazine, and benzothiazole units—as the primary active sites for CH₃I sequestration. In contrast, the relatively weak or absent signatures of covalent bonding in CH₃I‐loaded C1 and C4 suggest that adsorption in these backbones is governed by physical confinement within the internal cavities rather than chemical modification. Thermal stability tests at 110°C further revealed that CH₃I uptake is largely irreversible, confirming the formation of stable complexes. The combination of high gravimetric capacity and robust chemical binding highlights the significant potential of these metal–organic cages as high‐performance sorbents. Their ability to effectively capture and permanently immobilize organic iodides positions them as highly promising candidates for nuclear waste management and advanced environmental remediation strategies.

3. Conclusions

In summary, we have demonstrated that microporous, self‐assembled Pd(II) tetrahedral coordination cages constitute an exceptionally powerful yet previously overlooked class of adsorbents for radioactive iodine remediation. Despite the absence of permanent porosity, cages C1−C4 exhibit remarkably high uptake capacities, fast adsorption kinetics, and outstanding selectivity toward multiple iodine species across vapor‐phase and aqueous environments. Among them, cage C3 emerges as the most versatile and efficient sorbent, delivering record‐high performance for a coordination cage under both static and dynamic vapour conditions. The cages capture molecular iodine, methyl iodide, and polyiodide species through a cooperative adsorption mechanism involving π–iodine interactions, charge‐transfer processes, halogen bonding, electrostatic stabilization by cationic Pd(II) centers, and strong coordination with nitrogen‐ and sulfur‐rich binding sites. Spectroscopic, microscopic, and theoretical analyses confirm that iodine is stabilized within the cage matrix as polyiodide species, accounting for the exceptional uptake capacity, thermal retention, and recyclability. Importantly, iodine adsorption is shown to be governed primarily by surface chemistry and cage architecture rather than surface area or pore volume, challenging conventional design principles rooted in permanent porosity.

Beyond static adsorption, these coordination cages exhibit unprecedented dynamic performance. Cage C4 achieves rapid and nearly quantitative iodine removal under continuous aqueous flow‐through conditions, with elution volumes reaching 9.2 L g⁻¹ for I₃ ⁻−the highest reported to date for a coordination cage. The cages are capable of reducing iodine concentrations even in the presence of a large excess of competing anions and from the ppm level to the ppb regime in real water matrices, highlighting their practical applicability in nuclear waste management and water purification. Together, these findings challenge conventional porosity‐driven adsorption paradigms. The combination of scalable synthesis, structural robustness, rapid kinetics, broad‐spectrum iodine sequestration, and excellent performance under realistic dynamic conditions positions these cages as promising next‐generation materials for off‐gas treatment, radioactive iodine storage, and environmental remediation. More broadly, our findings open new avenues for exploiting supramolecular coordination architectures as functional adsorbents for challenging environmental pollutants.

4. Experimental Section

[CCDC 2495304 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/data_request/cif.]

Author Contributions

Debashree Manna: writing – review and editing, investigation, formal analysis. Monotosh Dalapati: investigation, formal analysis. Risikeshan Pradhan: investigation, formal analysis. Dipak Samanta: conceptualization, investigation, formal analysis, project administration, funding acquisition, writing – original draft, writing – review and editing, validation, methodology, supervision, data curation. Raghunath Singha: investigation, formal analysis, writing – original draft, methodology, software. Pijush De: investigation, formal analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll73642‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article.