News Source: School of Chemistry and Chemical Engineering

Photographer: School of Chemistry and Chemical Engineering

Editor: Duan Kailong

Reviewer: Wang Zhenhua

The construction and properties of discrete ordered assembly is a hot topic in the field of chemistry. Through the precise control of topological structure, it has wide application prospects in molecular recognition, catalysis, drug delivery, information storage and energy materials. According to the different driving forces, the construction of high-level ordered assembly structure can be divided into two categories: covalent and noncovalent interactions; noncovalent interaction is favored for its easy modification of assembly elements, rich structural diversity, predictability and high selectivity. Typical noncovalent interactions include metal coordination, hydrogen bonding, π-π interaction and so on, which have achieved great success in the construction of high-level ordered assembly structures, especially in the cationic-coordination-induced assembly of transition metals. In contrast, the anion coordination that based on hydrogen bonding system has low strength, poor selectivity and complex coordinational conformation that is hard to predict, so it was once considered difficult to construct complex ordered assembly structures. For the origin and development of the study of anion coordination chemistry, references are available: Park & Simmons, JACS 1968, 90, 2431; Lehn Acc. Chem. Res. 1978, 11 , 49; Bowman-James Acc. Chem. Res. 2005, 38, 671.

Figure 1. Anion-coordination-driven supramolecular assembly

The team of Professor Wu Biao from Beijing Institute of Technology has been focusing on the anion coordination based on oligourea ligands, and found that the rigid o-phenylene-spaced bis(urea) has unique anion coordination ability: high strength, high selectivity, and predictable structure. And the team successfully applied it to the construction of high-level ordered assembly structure. Based on the coordination between o-phenylene-spaced bis(urea) and trivalent phosphate ion (PO4 3−) (Inorg.Chem. 2013, 52, 5851), a series of supramolecular assemblies were constructed, such as the helical structures(ACIE 2011, 50, 5721; Chem. Sci. 2022, 13, 4915), tetrahedron (ACIE 2013, 52, 5096; ACIE 2022, 61, e202201789), prism (JACS 2020, 142, 21160), truncated tetrahedron and double-layered tetrahedron ( ACIE , 2022, 61 , e202115042), etc.; and have potential application prospects in object recognition (ACIE 2015, 54, 8658; Nat. Commun. 2017, 8 , 938; JACS 2017, 139 , 5946; ACIE 2018, 57, 1851), chiral induction (Chem. Commun. 2018, 54, 7378; JACS 2020, 142, 6304) and reaction catalysis (JACS 2018, 140, 5248). Recently, in response to an invitation from the Accounts of Chemical Research, a review paper has been published to systematically summarize the advances in Anion-Coordination-Driven supramolecular Assembly (ACDA) research (Figure 1).

Key points in the review:

1. Design of anion-coordination-driven supramolecular assembly strategy

Figure 2. Schematic representation of the structural resemblance between bipyridine−metal coordination and coordination of bis(urea) with PO4 3−

Compared with the classical transition metal six-coordinating system, as shown in Figure 2, the o-phenylene-spaced bis(urea) can coordinate with phosphate ions via 12 N−H···O hydrogen bonds to form an approximate octahedron. Due to the strong electron-absorbing ability of phosphate ions, the structural rigidity and the strong hydrogen bond giving ability of o-phenylene-spaced bis(urea), this coordination overcomes the shortcomings of the traditional anion coordination system, such as low binding strength, uncertain conformation, poor directivity and unpredictable structure, and opens the door for the construction of the anion-coordination-driven supramolecular assembly structure. Compared with the metal system, the anion system has the following characteristics: 1. the coordination is relatively weak and flexible, which makes the assembly structure easy to change, but at the same time gives the assembly structure stronger adaptive ability for guest encapsulation; 2. the hydrogen bonding system can realize the in-situ tracking of the coordination center and assembly mechanism through the nuclear magnetic resonance technology, making it universal; 3. compared with spherical metal ions, tetrahedral phosphate ions have lower symmetry, which can give the assembly unexpected "symmetric breaking". Therefore, the anion-coordination-driven supramolecular assembly system has broad research and application prospects.

2. Study on the assembly and properties of triple helicates

Based on the 3:1 coordination mode between o-phenylene-spaced bis(urea) and phosphate, a series of triple helicate assemblies were successfully constructed by changing the structure of the intermediate bridge segments. The first phosphate triple helicate was constructed based on the bis(biurea) ligand of the ethyl fragment (ACIE 2011, 50, 5721). By changing the linker to the 4,4′-methylenebis-(phenyl) group, as shown in Figure 3, a triple helicate with an internal cavity can be constructed to realize the encapsulation and identification of organic ammonium cations, such as tetramethylammonium ion and choline derivative molecules, and the chiral induction of helical structure can be achieved by chiral choline cations (Chem. Commun. 2018, 54, 7378). At the same time, the radial length of the helical structure can be fine-tuned by changing the size and shape of the organic ammonium ions (ACIE 2021, 60, 9389). Recently, by constructing triple helicate with different linkers, the research groups of Jia Chuandong and Yang Dong at Northwest University respectively achieved the cascaded guest delivery (ACIE 2021, 60, 9573) and the preparation of supramolecular conductive gel (ACIE 2022, 61, e202201793).

Figure 3. Schematic diagram of the highly selective binding of the triple helicate to choline molecules and its sensing to chiral choline derivatives

3. Study on the assembly and properties of tetrahedral cage structure

The transition-metal-coordination-driven tetrahedral cage assembly has been proved to have a strong application prospect in the fields of guest recognition, reaction catalysis and information storage, etc. Using the anion coordination strategy, tetrahedral cage assembly can be constructed as well. The traditional tetrahedral cage assembly can be divided into edge and plane structure according to its structural characteristics.

3.1. Edge tetrahedral cage

Based on the study of the triple helicate assembly, by enhancing the rigidity and linearity of the bis(biurea) ligand structure, the bis(biurea) ligand connected by phenyl fragments could be assembled to form a tetrahedral cage under the induction of phosphate ions. This assembly structure exhibits template, concentration and solvent dependence (ACIE 2018, 57, 1851). Tetramethylammonium cations can be used as internal and external template ions to stabilize tetrahedral cages. Interestingly, the cage's outer grooves can also be embedded with ammonium cations and exhibit unexpected "symmetric breaking" (JACS 2020, 142, 6304). Structurally similar isomers (α-methylcholine G1S /G1R and β-methylcholine G2S /G2R) can be selectively bound along the two types of edges of the cage (peripheral binding sites I and II) , resulting in different chiral induction effects (Figure 4). This kind of symmetric breaking may originate from the tetrahedral geometry of phosphate ion as coordination centers. By extending the link-subgroups from monobenzene to biphenyls, a tetrahedral cage with larger cavity and an A8L12 bicapped trigonal antiprismatic structure with 96 hydrogen bonds are obtained, which is a record number of hydrogen bonds within a discrete polyhedron ever reported (JACS 2020, 142, 21160). The tetrahedral cages, formed by the bis(biurea) ligand linked to the azobenzene fragments, can encapsulate tetramethylammonium ions bound by [18]crown-6 (ACIE 2022, 61, e202201789).

Figure 4. Interconversion between triple helicate and tetrahedron and chiral symmetric breaking properties

3.2. Planar tetrahedral cage

A planar tetrahedral cage can be constructed by introducing C3-symmetric elements.The first phosphate tetrahedral cage was successfully constructed using triphenylamine fragment linking to tris-bis(urea) ligand (ACIE 2013, 52, 5096). A tetrahedral cage with a larger cavity was constructed by using a larger triphenylbenzene linking segment, which can realize the encapsulation of tetrahedral molecules, such as the freon molecules (ACIE 2015, 54, 8658), unstable white phosphorus and yellow arsenic molecules (JACS 2017, 139, 5946). Based on the triphenyltriazine linking segments, the structural rigidity of the tetrahedral cage is further improved, and the accurate control of the methylation of DABCO (1,4-diazazbicyclo[2.2.2]octane) can be achieved (JACS 2018, 140, 5248), as shown in Figure 5.

Figure 5. Schematic diagram of the tetrahedral cage structure controlling the methylation process of DABCO

4. Conclusions and outlook

Based on the unique anion coordination of o-phenylene-spaced bis(urea), an anion-coordination-driven supramolecular assembly system has been successfully constructed, and progress has been made in the construction and application of helical and tetrahedral cage assembly structures. The flexibility of anion coordination, the adaptive ability of the assembly structure to the guest encapsulation, and the chiral induction and symmetric breaking from the coordination center of phosphate ions provide new strategies for the design, construction and application of complex and ordered supramolecular assembly. Recently, we successfully extended the coordination center from phosphate ion to organic carboxylate ion, and constructed a novel Archimedean polyhedral structure, that is the truncated tetrahedron (ACIE 2022, 61, e202115042). We believe that the structural diversity of organic carboxylate ion can inject new vitality into the development of anion-coordination-driven supramolecular assembly structures. In the future, we will continue to focus on the design and development of novel anion coordination systems, develop various supramolecular topologies, and expand their applications in catalysis, drug delivery and other fields.

These results were published in the top international journal Accounts of Chemical Research under the title "Anion-Coordinate-Driven Assembly". Liang Lin, a PhD student from the School of Chemistry and Chemical Engineering, is the first author of this paper. Professor Wu Biao and Special Researcher Zhao Wei from BIT are corresponding authors. Professor Yang Xiaojuan has done important work on the writing and revision of the paper.

Link to the paper:https://pubs.acs.org/doi/10.1021/acs.accounts.2c00435

About the corresponding authors:

Wu Biao is a professor at the School of Chemistry and Chemical Engineering of BIT. In 1998, he received his Ph. D. degree from Fujian Institute of Research on the Structure, Chinese Academy of Sciences. After his post-doctoral work, he joined Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences as a professor in 2004. He has worked at Northwestern University since 2010. From 2021 to present, he has been a professor at BIT and the director of Key Laboratory of Pharmaceutical Molecular Science and Pharmaceutical Engineering, Ministry of Industry and Information Technology. He once presided over the National Science Fund for Distinguished Young Scholars and other projects; won the provincial first prize of Science and Technology, etc. He is currently a senior member of the Chinese Chemical Society and a member of Chinese Crystallographic Society. He has achieved systematic research results in areas like anion recognition and anion-coordination-driven supramolecular assembly.

Zhao Wei is a special researcher at the School of Chemistry and Chemical Engineering of BIT. He received his Ph.D. degree from the Institute of Chemistry, University of Chinese Academy of Sciences in 2016, and then worked as a post-doctoral researcher in Amar Flood Research Group, Department of Chemistry, Indiana University, USA. In July 2020, he joined the School of Chemistry and Chemical Engineering of BIT, and presided over The National Natural Science Foundation of China Youth Project, General Program of Beijing Municipal Natural Science Foundation Project, and Beijing Institute of Technology Research Fund Program for Young Scholars. His research interests focus on the design, synthesis, object recognition and separation of novel anion ligands.