Synthesis of Molecular Borromean Links Featuring Trimeric Metallocages
The Borromean link is a topological structure composed of three independent yet interlocked rings, characterized by the fact that disconnecting any one ring results in the complete separation of the other two. This structure not only possesses aesthetic value but also holds significant research importance in molecular topology and supramolecular chemistry. Traditional Borromean links are typically composed of three identical macrocycles, while those based on trimeric metallocages are relatively rare. In recent years, with the development of supramolecular chemistry and coordination chemistry, researchers have begun to explore how to construct more complex molecular topological structures through bottom-up synthetic strategies, particularly Borromean links based on metallocages.
However, existing research has mostly focused on interlocked structures of dimeric metallocages, and the synthesis of Borromean links based on trimeric metallocages still faces significant challenges. Breakthroughs in this field can not only enrich the types of Borromean links but also provide new insights for the design and synthesis of more complex molecular topological structures.
Source of the Paper
This paper was co-authored by Hai-Ning Zhang and Guo-Xin Jin from the Department of Chemistry at Fudan University and the Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials. The paper was published in April 2025 in the journal Nature Synthesis, titled “Synthesis of Molecular Borromean Links Featuring Trimeric Metallocages,” with the DOI 10.1038/s44160-024-00720-4.
Research Process
1. Design and Synthesis of Ring-Based Borromean Links
The research team first designed and synthesized a ring-based Borromean link (molecule 4). By selecting a dipyridyl ligand (ligand 2) featuring a naphthyl plane and a binuclear metal building unit (unit 3), the team successfully constructed a Borromean link composed of three identical metallarectangles. Ligand 2 served as the length side, and unit 3 served as the width side, with a length difference of approximately 7 Å, meeting the requirements of the construction strategy.
In acetonitrile, ligand 2 and unit 3 reacted at a stoichiometric ratio of 1:1 for 24 hours to yield a yellow solution. By adding diisopropyl ether and diethyl ether, an orange solid was precipitated, ultimately obtaining orange crystals with a yield of 91%. Single-crystal X-ray diffraction (SCXRD) analysis revealed that compound 4 crystallized in the triclinic system, exhibiting a Borromean link topology, with three equivalent metallarectangles stabilized by aromatic stacking interactions.
2. Synthesis of Cage-Based Borromean Links
After successfully synthesizing the ring-based Borromean link, the research team further explored the synthesis of cage-based Borromean links. By selecting a tetrapyridyl ligand based on dibenzo-18-crown-6 (ligand 6) and two binuclear metal building units (units 7 and 9), the team successfully synthesized two Borromean links composed of three identical decanuclear metallocages (molecules 8 and 10).
In a mixed solvent of methanol/nitromethane, ligand 6 reacted with unit 7 or 9 at a stoichiometric ratio of 1:2:1 for 24 hours to yield a dark-red solution. By adding diisopropyl ether and diethyl ether, a brown precipitate was obtained, ultimately yielding brown crystals with yields of 89% and 92%, respectively. SCXRD analysis showed that compound 8 crystallized in the cubic system, exhibiting a Borromean link topology, with three independent metallocages stabilized by π-π stacking and weak C-H···O interactions.
3. Structural and Property Characterization
The research team conducted detailed characterization of the synthesized Borromean links using various techniques. In addition to SCXRD analysis, nuclear magnetic resonance (NMR) spectroscopy, electrospray ionization time-of-flight mass spectrometry (ESI-TOF/MS), and elemental analysis were employed. NMR spectroscopy indicated that the synthesized Borromean links maintained stable topological structures in solution. ESI-TOF/MS spectra further confirmed the existence of the target molecules.
Key Results
Synthesis and Characterization of Ring-Based Borromean Links: The research team successfully synthesized a ring-based Borromean link (molecule 4) and confirmed its topological structure through SCXRD and NMR spectroscopy. The results showed that three equivalent metallarectangles were stabilized by aromatic stacking interactions, forming a Borromean link.
Synthesis and Characterization of Cage-Based Borromean Links: The team further synthesized cage-based Borromean links (molecules 8 and 10) and confirmed their topological structures through SCXRD and NMR spectroscopy. The results showed that three independent metallocages were stabilized by π-π stacking and weak C-H···O interactions, forming Borromean links.
Importance of Non-Covalent Interactions: Through independent gradient model (IGM) analysis, the team revealed the crucial role of π-π stacking and weak C-H···O interactions in stabilizing the Borromean link structures. These non-covalent interactions provide new insights for the design and synthesis of more complex molecular topological structures.
Conclusions and Significance
This study successfully constructed Borromean links based on trimeric metallocages through a bottom-up synthetic strategy, enriching the types of Borromean links. The results demonstrate the importance of non-covalent interactions in stabilizing molecular topological structures. This research not only provides new insights for the design and synthesis of more complex molecular topological structures but also offers significant experimental evidence for the development of supramolecular chemistry and coordination chemistry.
Research Highlights
Novel Synthetic Strategy: This study is the first to report the synthesis of Borromean links based on trimeric metallocages, providing new ideas for the design and synthesis of molecular topological structures.
Detailed Characterization Techniques: The research team conducted detailed characterization of the synthesized Borromean links, confirming their topological structures and stability.
Importance of Non-Covalent Interactions: This study highlights the crucial role of π-π stacking and weak C-H···O interactions in stabilizing molecular topological structures, offering new directions for future research.
Additional Valuable Information
The success of this study not only relies on meticulous experimental design but also benefits from the advanced equipment and technical support provided by the Department of Chemistry at Fudan University and the Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials. The research team stated that they will continue to explore more complex molecular topological structures and further investigate their potential applications in catalysis, materials science, and other fields.