Tangle “knots” with “force”! Westlake University has made new progress in the field of molecular tension engineering

Molecular-Strain Engineering (MSE)—uses forced deformation of structures caused by incompatible geometric dimensions to exert tension within molecules, resulting in precisely adjustable strain configurations in the molecule itself; These strain configurations are expected to exhibit unique properties different from non-strain configurations in regulating the physicochemical properties and supramolecular assembly behavior of molecules, manipulating chemical reaction processes and selectivity. The key scientific problem solved by molecular tension engineering is to explore the regulation law and mechanism of tension on molecular behavior and properties.

The team of Liu Zhichang of Westlake University has been committed to the research of molecular tension engineering since its establishment. At present, a series of work has been done in the field of molecular tension engineering, including: (1) assembly of tension-regulated double-walled tetrahedron (Chem 2021, 7, 2160–2174.), (2) selective capture of phlorin intermediates by tension-regulated means (CCS Chem. 2022, DOI: 10.31635/ccschem.022.202101679) and porpodimethene intermediates (Mater. Today Chem. 2022, 24, 100868), and (3) synthesis of polycyclic aromatic hydrocarbons with tensile units to use tension to regulate the stereoelectronic structure of molecules (Angew. Chem. Int. Ed. 2022, 61, e202205658)。

Recently, on the basis of molecular tension engineering, Liu Zhichang’s team used its original nested anti-helix strategy to efficiently construct topological molecular trilobe knots with different tensions, and realized the phenomenon of chiral self-classification, and also realized the heat-induced spin crossover transformation phenomenon regulated by topological mechanical tension within the molecule. This strategy opens up a new way for the synthesis of other complex molecular topologies, and also lays a foundation for further exploration of the broad properties and applications of molecular topological junctions.

On October 20, 2022, this result was published in Nature Synthesis, with Westlake University as the only communication unit, Professor Liu Zhichang as the corresponding author, and the first author is Wu Lin, the first doctoral student of the Department of Chemistry of Westlake University.

For more than 40 years, chemists have been working on the synthesis of complex topological molecular junctions and their potential applications. Since Sauvage et al. used linear binuclear metal helices to synthesize three-leaf junctions in 1989, a large number of synthetic methods such as metal template subcomponent self-assembly, spirosilane templates, non-covalent template interactions, and hydrophobic effects have been used to synthesize molecular junctions.

At present, the more mature methods for synthesizing molecular trilobe knot mainly include the linear double helix strategy developed by Sauvage and the circular helix strategy developed by Leigh. In this paper, the authors have developed a novel nested anti-helix strategy based on multi-stranded (>2) linear helix to construct common ring junction molecules. The strategy is as follows: elongating the torus right-handed helical (Δ) trilobe knot along the C3 symmetry axis forms a tubular right-handed helical trilobe knot (Figure 1), and the outer layer is connected by a larger left-handed helical chiral (Λ) coaxial nesting. Similarly, a torus Δ-Solomon link can be implemented by a double nested Δ⊂Λ-quadruple spiral tube connection. Therefore, this universal strategy based on nested anti-helix can be achieved by using the linker in one step of appropriate coaxial nesting between the kernel and the outer core, or by pre-organizing the core multi-stranded helix first, and then using the opposite helical chiral linker to connect the outer tip of each chain with the bottom end of another nonadjacent chain. Considering that metal templates have proven their versatility in synthetic multi-chain helixes, metal template multi-stranded helix can be used as a building block to effectively implement this strategy. In the “nested anti-helix” method described by the author in this article, a multi-stranded metal helix can be constructed from only two metal template ions.

Figure 1: Nested anti-helix strategy synthesizes molecular trilobe nodes

By careful design, the authors placed 3 equivalent amounts of 3,3-bipyridine-6,6-dicarboxaldehyde (compound1Biphenylenediamine (2a,2b,2cInduced by 3 equivalent Fe(OTf)2 templates, dynamic covalent chemistry can be used to react in acetonitrile for 24 hoursTK3(dark blue),TK4(dark red),TK6(orange-red). To further verify the versatility of the strategy, the authors used p-phenylene-spaced dipyridinedicarboxaldehyde (compound 11) with benzidinediamine (2dreaction, also obtained at a high yieldTK7(Figure 2), it is also the longest trilobe junction ever synthesized (a closed-loop structure about 11 nm long with 111 atomic lengths).

Figure 2: Synthesis of three-leaf knots with carbon chains of different lengths and reduced demetallized three-leaf knots TK4D

By slowly diffusing isopropyl ether toTK3TK4andTK6acetonitrile solution andTK7The author obtains a solution of nitromethaneTK3TK4TK6andTK7crystal structure (as shown in Figure 3a). These four crystal structures exhibit a D3-symmetrical tensile spindle-shaped trilod junction topology that is consistent with the original design ideas.TK3TK4TK7There is a pair of topologically opposite enantiomers in the unit cell cell unit. So, the author used chiral HPLC to succeedTK4The enantiomer was separated and its optical purity was demonstrated by CD spectroscopy (Figure 3c). The authors further found thatTK6Chiral self-classification occurs (Figure 3b). Therefore, the two optically pure single crystals of TK6 were manually separated by single crystal X-ray diffraction technology, and their optical purity was proved by CD spectroscopy (Figure 3c).TK6The absolute configuration of Δ-(−)-TK6and Λ-(+)-TK6

Figure 3: (a) Crystal structure of different trilobe junctions; (b) Chiral self-classification of TK6; (c) CD spectra for chiral fractionation of TK4 and chiral self-classification CD spectra for TK6

Then, the author willTK4After reductive demetallization, it is separated and purified to obtain pure organic three-leaf knot at a yield of 75%.TK4D。 Confirmation of topological molecular structure by high-resolution mass spectrometry and 1H NMR and single crystal structure (Figure 4),TK4DBy intramolecular hexahydrogen bonding as well as triple[π···π]The interaction stabilizes the structure well.

Figure 4: Crystal structure of TK4D

Through variable temperature NMR experiments, the authors found that:TK3andTK4As the temperature rises, the imine protons move towards the lower field, whileTK6The imine protons are displaced to a low field (Figure 5a), as a preliminary illustrationTK3andTK4A heat-induced low-spin to high-spin crossover (SCO) occurs at the center of the ferric iron. At the same time, it is observed with the naked eyeTK3TK4andTK6The solutions are different colors at room temperature, representing their different spin states (Figure 5b). The authors further measured the variable temperature susceptibility of three trifoliated knots with different tensions, which showed that the tension was largerTK3andTK4As the temperature increases, the proportion of high spin gradually increases, and TK6, which has less tension, is always in a high spin state within the temperature measurement range (Figure 5c). Finally, measureTK3TK4andTK6The changes of Fe−N bond length and octahedral distortion angle in the variable temperature single crystal further supported the spin crossover transition phenomenon with adjustable tension (Figure 5d). This is the first example of a spin crossover transformation phenomenon that uses mechanical tension to regulate topological molecular junctions.

Figure 5: (a) Variable temperature NMR; (b) the color of the crystal solution; (c) variable temperature susceptibility curve; (d) Changes in bond length and bond angle parameters of variable temperature single crystals

Study table aboveThe nested anti-helix strategy designed by Ming researchers can efficiently construct topological molecular trilobe knots with different tensions, realize the phenomenon of chiral self-classification, and also realize the heat-induced spin crossover transition phenomenon regulated by topological mechanical tension within molecules. This strategy provides new ideas for the synthesis of other complex molecular topologies, and lays a foundation for further exploration of the broader properties and applications of molecular topological junctions.

This work was supported by the National Natural Science Foundation of China (22171232 and 21971211), the Natural Science Foundation of Zhejiang Province (2022XHSJJ007) and the Qianjiang Talent Program of Zhejiang Province (QJD1902029). (Source: Web of Science)

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