Construction and application of a halogen-bonded organic framework based on N⋯Cl+⋯N bond

Prof. Shigui Chen's team at the Institute of Advanced Studies at Wuhan University recently achieved, through a step-by-step construction strategy, the stabilization of the [N⋯Cl⋯N]⁺ halogen bond at the framework level, thus breaking through the bottleneck of the inability of chlorine (I) halogen bonds to form bonds under conventional conditions and successfully constructed a class of stable chlorine (I) bridged two-dimensional halogen bond organic frameworks (XOF(Cl)-TPy-BF₄/OTf). This framework not only has excellent crystallinity, chemical stability and thermal stability, but also provides an ideal platform for subsequent functionalization (such as the introduction of catalytic sites). As a cationic framework, XOF(Cl) is anion-exchanged with PdCl₄²⁻ and then reduced to generate stable Pd(0) clusters within the framework. The Pd(0) cluster-loaded XOF(Cl)-TPy-Pd⁰ exhibits excellent catalytic performance in various palladium-catalyzed coupling reactions (such as Suzuki, Heck, and Sonogashira couplings), achieving high yields even under mild conditions. In the synthesis of industrial biphenyl liquid crystal molecules, no palladium residue was detected in XOF(Cl)-TPy-Pd^0. In addition, the catalyst can be efficiently recovered by simple filtration and can be recycled many times, showing excellent practicality. These results were published as an open access article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background:
Halogen Bond (XB) are an important type of non-covalent interaction characterized by high directionality, designability, and strong binding capacity. They exhibit broad application potential in supramolecular self-assembly, crystal engineering, anion recognition, sensing, optoelectronic materials, and drug design. Depending on the participating halogen atoms, halogen bond systems primarily include those based on iodine (I), bromine (Br), and chlorine (Cl). Halogen bonds based on iodine and bromine (e.g., [N⋯I⋯N]⁺ and [N⋯Br⋯N]⁺) form relatively stable supramolecular structures due to the large size and strong polarizability of halogen atoms. These bonds have been widely used in the construction of functional materials such as catalytic systems, adsorbents, and responsive materials. However, the high electronegativity and low electron density polarizability of chlorine atoms make [N⋯Cl⋯N]⁺ halogen bonds extremely unstable. Traditional small molecule models, such as Py₂ClBF₄ , typically survive only briefly at extremely low temperatures (e.g., −80°C ), severely restricting their application in functional materials.

Although the chlorine (I) halogen bond has unique electronic structure and potential functional properties, its inherent instability makes it difficult to construct stable chlorine (I) halogen bond-based functional materials (especially multidimensional ordered frameworks).Extremely challenging. Past studies have shown that simple [N⋯Cl⋯N]⁺ complexes are extremely easy to decompose under normal conditions and are difficult to exist stably even in environments containing trace amounts of water. This has left the construction of related framework materials almost blank for a long time. In contrast, similar structures based on iodine or bromine (such as [N⋯I⋯N]⁺ or [N⋯Br⋯N]⁺) have shown significantly improved chemical and thermal stability after being assembled into multidimensional organic frameworks (XOFs), providing possibilities for functional applications (such as catalysis, adsorption, sensing, etc.). These research advances suggest that, If the stabilization of the [N⋯Cl⋯N]⁺ halogen bond can be achieved at the framework level, it will be expected to open up new application areas of chlorine (I) halogen bonds in materials science.

Highlights of this article:
In this study, the authors used TPy as a linker unit, adding Ag⁺ to form a [N⋯Ag⋯N]⁺ - bridged MOF structure. Subsequently, Cl₂ was added, replacing the Ag⁺ ions in the  MOF with Cl⁺ in situ to form an [N⋯Cl⋯N]⁺ - bridged XOF structure. This resulted in the successful construction of a new two-dimensional halogen-bonded organic framework (XOF(Cl)-TPy-BF₄_/OTf). The formation of XOF(Cl)-TPy-BF₄_/OTf was confirmed by 1H NMR, UV-Vis, XPS, IR, SEM, TEM, HRTEM, and SAED monitoring. Furthermore , the structure of XOF(Cl)-TPy-BF₄/OTf was further confirmed through PXRD and theoretical simulations.

The authors further demonstrated that, despite the model compound ClPy₂ typically existing only at extremely low temperatures, such as approximately −80°C, the resulting XOF(Cl)-TPy-BF₄/OTf  framework exhibits excellent chemical and thermal stability, maintaining its two-dimensional framework structure in a variety of organic solvents. Combining experiments with theoretical calculations, they systematically investigated the regulatory effects of counterions (BF₄⁻ and OTf⁻) on framework stability. The results revealed that the electrostatic interaction between the 2TPy@Cl⁺ species and OTf⁻ is stronger than that between BF₄⁻. Therefore, XOF (Cl)-TPy-BF₄ is more susceptible to anion exchange than XOF(Cl)-TPy-OTf, endowing it with the potential for electrostatic interactions with a variety of anionic molecules or ions. Based on the weak electrostatic interaction between Cl⁺ and BF₄⁻ in the 2TPy@Cl-BF₄ building block, the authors successfully prepared the palladium(II)-loaded framework material XOF(Cl)-TPy-PdII by anion exchange with tetrachloropalladate (PdCl₄²⁻). Subsequently, the palladium(0) nanoparticle  atalyst XOF(Cl)-TPy-Pd⁰ was obtained by hydrazine hydrate reduction. In addition, the authors systematically verified the successful construction and conversion process of XOF(Cl)-TPy-Pd^II and its reduction product XOF(Cl)-TPy-Pd⁰ through multiple characterization methods.  Multi - dimensional characterization results, including IR, XPS, ¹³C NMR, PXRD, SEM, TEM, and XAFS, all demonstrated that the palladium(II) species can be uniformly loaded through anion exchange and converted into stable and well-dispersed palladium(0) nanoparticles after hydrazine hydrate reduction. At the same time, these results further confirm the high stability of the obtained material in structure and chemical environment, laying a solid foundation for its subsequent catalytic applications.

Further experimental results demonstrated that XOF(Cl)-TPy-Pd⁰ is a highly efficient and mild catalyst, exhibiting high catalytic activity and broad substrate compatibility in various coupling reactions, including Suzuki, Heck, and Sonogashira reactions. It can also efficiently catalyze reactions in air, demonstrating excellent catalytic stability and recyclability. In industrial biphenyl liquid crystal synthesis, XOF(Cl)-TPy-Pd⁰ achieved palladium residues below the detection limit (0 ppb), well below the international standard limit (≤100 ppb), and can be reused at least eight times with simple filtration without loss of catalytic activity. This provides an efficient, green, and economical catalytic solution for applications in liquid crystal materials, pharmaceutical intermediates, and other fields.

Summary and Outlook:
In summary, in this work, Shigui Chen's research group, for the first time, constructed a stable chlorine (I)-bridged two-dimensional (2D) halogen-bonded organic framework, XOF(Cl)-TPy-BF₄/OTf, based on the highly sensitive [N⋯Cl⋯N]⁺ halogen bond. This work not only deepens our understanding of the halogen-driven self-assembly mechanism, particularly strategies for stabilizing highly sensitive halogen bonds at the framework level, but also provides new ideas and technical pathways for the development of novel chlorine (I)-based functional materials.

This achievement was recently published in CCS Chemistry, the flagship journal of the Chinese Chemical Society, with Xuguan Bai, a doctoral student at the Institute for Advanced Study at Wuhan University, as the first author of the paper.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.