Halogen bonding, a type of directional and highly tunable non-covalent interaction, has traditionally been harnessed via iodine and bromine atoms owing to their larger atomic sizes and strong polarizability. These heavier halogens facilitate the formation of robust halogen-bonded supramolecular architectures that have found utility across crystal engineering, sensing, and catalysis. Conversely, chlorine has been notoriously difficult to integrate into stable frameworks due to its high electronegativity and poor electron cloud polarizability, which render [N⋯Cl⁺⋯N] interactions prone to rapid dissociation and instability.

The instability of the chlorine(I) halogen bond has confined its existence to transient, low-temperature conditions—often around −80°C—and has limited its exploration in advanced materials. The team circumvented this fundamental obstacle by adopting a precise stepwise synthetic strategy paving the way to stabilize these sensitive bonds within an extended two-dimensional organic framework. This approach successfully yielded a novel halogen-bonded organic framework (XOF(Cl)-TPy-BF₄/OTf) that exhibits remarkable chemical and thermal robustness.

The synthetic route began with the assembly of a metal-organic framework (MOF) bridged by the [N⋯Ag⁺⋯N] coordination complex using the linker 4,4’,4’’-triazatriangulenium (TPy). Post assembly, chlorine gas was introduced to selectively replace silver ions with chlorine cations (Cl⁺) in situ, establishing the [N⋯Cl⁺⋯N] halogen bond that underpins the framework’s stability. Spectroscopic techniques, including ^1H NMR and UV-Vis, combined with electron microscopy and X-ray diffraction methods, confirmed the successful transformation and the high crystallinity of the resultant XOF(Cl) structure.

Distinguishing itself from fleeting small-molecule chlorine halogen bonds, the newly designed XOF(Cl)-TPy-BF₄/OTf framework maintains its structural integrity even in diverse organic solvents and under ambient conditions. This enhanced stability was further elucidated through comparative studies evaluating the influence of counterions BF₄⁻ and OTf⁻, revealing stronger electrostatic interactions in the presence of OTf⁻. Such interactions not only stabilize the framework but also introduce avenues for tunable anion exchange processes, thereby broadening the functional scope.

Capitalizing on this anion exchange capability, the research group incorporated tetrachloropalladate (PdCl₄²⁻) into the framework to generate XOF(Cl)-TPy-Pd(II), which was subsequently reduced to form palladium(0) nanoparticles uniformly distributed within the framework. A comprehensive suite of characterization tools, including X-ray photoelectron spectroscopy, ^13C NMR, and extended X-ray absorption fine structure spectroscopy (XAFS), verified the successful loading and conversion of Pd species alongside preserving the framework’s architecture.

The catalytic prowess of the palladium-loaded framework was demonstrated across a range of classical palladium-catalyzed cross-coupling reactions, namely Suzuki, Heck, and Sonogashira couplings. Impressively, these catalytic processes proceeded with high efficiency and yield under mild and aerobic conditions. In an industrial context, the catalyst achieved near-zero palladium contamination in biphenyl liquid crystal synthesis, well below stringent international standards. Equally noteworthy, the catalyst sustained its activity over multiple cycles of recovery and reuse via simple filtration methods, underscoring its practical applicability.

This milestone underlines a vital leap in understanding how to harness and stabilize inherently fragile halogen bonds within extended materials. The efficient, modular assembly strategy and comprehensive analytical verification establish a blueprint for future exploitation of chlorine(I)-based halogen bonds in multifunctional materials. Such materials could revolutionize fields ranging from green catalysis to sensor design, especially when paired with fine-tuned anion exchange properties.

The implications of this study also extend to theoretical insights into halogen bonding principles. By stabilizing the [N⋯Cl⁺⋯N] interaction within a multidimensional framework, the research enriches the conceptual framework for dynamic non-covalent interactions in chemical systems. It challenges preconceived notions surrounding chlorine’s chemical inactivity in halogen bonding and unveils a new paradigm for exploiting subtle electronic and electrostatic effects in supramolecular chemistry.

Looking forward, this work sets the stage for the rational design of halogen-bonded frameworks with tailored functionalities. The ability to combine framework stability with catalytic utility opens exciting avenues for industrially relevant processes, including pharmaceutical synthesis and advanced materials manufacturing. Additionally, manipulating counterion environments may yield customized platforms for selective adsorption and sensing applications.

The revelation of a chlorine(I)-bridged two-dimensional halogen-bonded organic framework by Prof. Chen’s team is a remarkable feat that transcends traditional halogen bonding chemistry. It demonstrates how meticulous molecular engineering and innovative synthetic chemistry can unlock the latent potential of less-explored chemical interactions, fostering advances that resonate across chemistry and materials science disciplines.

This landmark achievement has been published as an open-access article in CCS Chemistry, the flagship journal of the Chinese Chemical Society, with doctoral student Xuguan Bai serving as the first author. The publication underscores the collaborative dedication to expanding the frontiers of halogen bond chemistry and offering sustainable solutions to catalytic challenges.

As halogen bonding continues to captivate chemists worldwide, the successful integration of chlorine-centered halogen bonds into stable, functional organic frameworks offers a compelling vision for the future of supramolecular materials. This research not only enriches fundamental scientific understanding but also promises transformative impacts on technology and industry driven by non-covalent chemistry.

Subject of Research: Not applicable

Article Title: A Type of Halogen-Bonded Organic Frameworks Based on N⋯Cl⁺⋯N Bonds: Stabilizing Sensitive Species

News Publication Date: 24-Sep-2025

Web References:
https://www.chinesechemsoc.org/journal/ccschem
http://dx.doi.org/10.31635/ccschem.025.202506172

Image Credits: CCS Chemistry

Keywords

Supramolecular chemistry

Addressing this grand challenge, a recent chemical breakthrough showcases an innovative strategy to guide the assembly landscape by implementing a concept known as exo-templating. Unlike traditional endo-templating — where templates reside within the interior of the forming structure — exo-templating leverages an external molecular scaffold to direct self-assembly. Here, a charged ring molecule, designed to engage in non-covalent interactions, docks specifically at 1,5-dioxynaphthalene moieties appended exo to molecular building blocks. These non-covalent docking interactions harness the principles of pseudorotaxane formation, effectively modulating the kinetic pathways by stabilizing or destabilizing key intermediates.

The research at hand centers on designing self-assembled nanospheres composed of twelve palladium ions coordinated to twenty-four organic ligands, denoted generically as Pd₁₂L₂₄. The formation of these cuboctahedral nanospheres traditionally encounters pathway complexity and slow assembly kinetics due to competing polymerization and oligomerization processes. Introducing the exo-templating charged ring transforms this scenario: the ring selectively associates with exo-functional groups on the building blocks, effectively corralling them into preferred intermediates. This modulation results in the favored assembly of well-defined small Pd–L oligomers that subsequently converge into the target Pd₁₂L₂₄ architecture with enhanced efficiency and reduced kinetic hindrance.

Intriguingly, the presence of the exo-templating ring alters the landscape of intermediate species dramatically. Without the ring, the system tends to form a resting state comprised of larger PdₓL_y polymers, which represent kinetic traps. These species accumulate rapidly, impeding the formation of discrete nanospheres and effectively elongating the reaction timeline. Conversely, the exo-templated system circumvents this bottleneck by destabilizing these polymer resting states and promoting the assembly through manageable smaller oligomeric entities. This dynamic modulation is reminiscent of catalytic processes, where intermediate species are destabilized to chaperone the reaction toward desired products.

The methodological innovation demonstrated here is not merely a novel synthetic trick; it encapsulates a profound conceptual advance by bringing a catalytic paradigm into the realm of supramolecular chemistry. The exo-templating ring acts akin to a catalyst: it reduces pathway complexity by selectively destabilizing off-path intermediates without becoming part of the final product. This distinction is crucial because traditional endo-templating involves templates embedded within the target structure, often complicating product purification and limiting scalability. Exo-templating, situated externally, retains the functionality of the template while simplifying downstream processing.

Moreover, the study highlights the specific chemical design of the exo-functionalized building blocks, which possess strategically positioned 1,5-dioxynaphthalene stations. These sites enable selective and reversible non-covalent interactions with the positively charged ring, facilitating the formation of pseudorotaxane complexes. The robustness of this interaction under the reaction conditions is pivotal to the templating effect, allowing dynamic association and dissociation events that ensure kinetic control without permanent covalent attachment. This balance is central to achieving efficient molecular choreography during self-assembly.

This line of research also addresses a broader scientific challenge in the field of supramolecular chemistry: the scalability and complexity of artificial nanostructures. As the number of components increases, synthetic systems often suffer from combinatorial explosion, where numerous reaction pathways and intermediates hinder selective product formation. By applying exo-templating, the authors offer an elegant strategy to tame this complexity, steering the system along a simplified kinetic trajectory. This approach promises to open new vistas in the construction of large, sophisticated supramolecular assemblies that rival the precision and elegance found in nature.

In terms of potential applications, the ability to reliably and selectively assemble large nanospheres heralds exciting opportunities across catalysis, drug delivery, and materials science. Pd₁₂L₂₄ nanospheres, with their well-defined size, shape, and internal cavity, can serve as nanoreactors where encapsulated species undergo controlled transformations. The exo-templating strategy could facilitate the scalable production of such nanospheres with tailored functionalities, enabling their integration into devices and therapeutic platforms requiring high precision and uniformity.

From a kinetic viewpoint, the study illuminates the nuanced interplay between thermodynamics and pathway selection. While the thermodynamically favored product remains the Pd₁₂L₂₄ nanosphere, multiple kinetic traps impose barriers that extend reaction durations or yield less defined assemblies. The exo-templating ring, by destabilizing these traps, acts as a kinetic modulator accelerating the formation of the desired nanosphere without altering thermodynamic preferences. Such subtle intervention epitomizes next-generation molecular design strategies where pathway engineering complements energy landscape tuning.

Another remarkable aspect is the modularity and generalizability of the exo-templating strategy. The use of pseudorotaxane formation with charged rings and dioxynaphthalene-stationed ligands could be adapted to other metal–ligand systems or building block architectures. This flexibility suggests a platform technology for mediating multicomponent self-assembly processes beyond the studied Pd₁₂L₂₄ nanospheres, potentially applicable to a range of metal-organic cages, frameworks, and beyond.

Furthermore, the insights generated here contribute to the conceptual framework of kinetic templating—a paradigm where intermediate states are selectively destabilized or stabilized to guide complex assembly. This contrasts with traditional templating focused on stabilizing the product or early intermediates. By approaching the problem from an intermediate destabilization angle, the authors mimic the fundamental principle underlying enzymatic catalysis, offering a synthetic analogue that enhances control over complex assembly landscapes.

The research also prompts reconsideration of the role of external factors in self-assembly reactions. Instead of relying solely on the intrinsic properties of building blocks, the incorporation of exo-templating elements represents an interventionist approach, embedding external guidance into the assembly environment. In doing so, it fuses concepts from molecular recognition, catalysis, and supramolecular chemistry into a unified strategy for enhanced precision.

Critically, the work transcends a mere proof of concept; it provides detailed structural and kinetic analyses underpinning the mechanistic understanding of exo-templating-supported assembly. Through a combination of spectroscopic, chromatographic, and computational studies, the researchers delineate the binding interactions, intermediate populations, and assembly evolution with high resolution. These insights are essential for rationally extending the strategy to other systems and for integrating it into practical synthetic workflows.

In sum, the introduction of exo-templating through pseudorotaxane formation signals a transformative leap in managing the complexity inherent in multicomponent supramolecular assembly. By harnessing reversible, external molecular scaffolds to modulate kinetic intermediates, the approach introduces a catalytic dimension to supramolecular chemistry. It dovetails with biological inspiration while circumventing limitations of existing synthetic templating strategies, pushing the envelope toward the construction of ever more sophisticated molecular architectures.

As such, this work sets a new direction for future exploration in the design of adaptive, efficient, and selective self-assembling systems. The paradigm of exo-templating is poised to inspire novel synthetic methodologies and unlock new frontiers in materials science, nanotechnology, and chemical biology. The elegance of this approach lies in its simplicity and versatility—tools as fundamental as charged rings and carefully decorated ligands orchestrating the dance of atoms into precise, functional assemblies with remarkable fidelity.

Subject of Research: Multicomponent self-assembly and kinetic templating via exo-templating in Pd₁₂L₂₄ nanospheres.

Article Title: Exo-templating via pseudorotaxane formation reduces pathway complexity in the multicomponent self-assembly of M₁₂L₂₄ nanospheres.

Article References:
Bouwens, T., Bobylev, E.O., Antony, L.S.D. et al. Exo-templating via pseudorotaxane formation reduces pathway complexity in the multicomponent self-assembly of M₁₂L₂₄ nanospheres. Nat. Chem., (2025). https://doi.org/10.1038/s41557-025-01808-w

Image Credits: AI Generated