The study of noncovalent bond (intermolecular interaction) plays a very important role in many fields, such as chemistry, physics and biology. It is a very active research field that the experimental and theoretical researchers pay close attention to. Under such precursors, it is very important to explore and systematically study new noncovalent bonds, because it will have a profound impact on the discovery of new chemical reactions and physical phenomena, as well as the understanding and interpretation of life phenomena.
It can be found that noncovalent bonds such as π π、C-H… π. Halogen bonds are widely found in intermolecular and intramolecular (Fig. 1). Up to now, the experimental and theoretical research groups at home and abroad have done in-depth and systematic research on these noncovalent bonds. Due to the limitation of the bonding units of interaction, the existing non covalent bond types are mainly based on σ and π bonding molecules, but there are few reports on other bonding molecular systems, such as polyhedral boron clusters with three center two electron bonding (Fig. 2). It is worth mentioning that polyhedral boron clusters, such as carborane, have good thermal and chemical stability and special three-dimensional cage structure, which leads to the wide and important application of carborane derivatives in materials, catalysis, energy and medicine. If we can have a clear understanding of the noncovalent bond of carborane, we can rationally design the structure of carborane derivatives at the molecular level, and then realize its predictable application in materials, catalysis, energy and medicine.
Figure 1. Noncovalent interaction based on σ - and π bonding molecules
In addition to the differences in composition and bonding mode, there are many similarities in the reactivity and aromaticity between three-dimensional and two-dimensional aromatics. In 2016, Yanhong group of Nanjing University found that B-H bond of carborane can form B-H bond with aromatic ring π nonclassical hydrogen bond (J. am. Chem. SOC., 2016, 138, 4334), which indirectly indicates that three-dimensional boron clusters can form noncovalent bond. In order to further verify that the boron cluster skeleton has the same properties as the two-dimensional aromatic ring and can be used to construct noncovalent bonds, the research group obtained a new noncovalent bond Nido cage based on the three-dimensional boron cluster skeleton structure by designing the carborane pyridine system π (Figure 2-3).
Figure 2. Nido cage formed by carborane and aromatic ring system π noncovalent bond
To get Nido cage The crystal structure of carborane derivatives was measured and analyzed by X-ray single crystal diffraction. Experimental results show that Nido cage The bonding mode of π is that the c2b3 open surface of carborane and electropositive pyridine form a parallel dislocation or vertical (T-shaped) stacking mode, and the corresponding stacking distance is 4.422-5.904? (the distance from the center of three-dimensional carborane to the center of aromatic ring, Fig. 3). A similar bonding mode can also be found in the electrically neutral benzene ring or pyridine ring system (such as compounds 2-py and 2-py-cn).
Figure 3. Contains Nido cage Crystal structure of π noncovalent bond
In order to explain the nature of the noncovalent bond type, the molecular structure was optimized by quantum chemical calculation, and the noncovalent bond formed by boron cluster was analyzed by EDA (energy decomposition analysis). The results show that if the aromatic ring with positive charge is used, Nido cage The bond energy of π is - 65 – - 136 kcal mol? 1, mainly due to electrostatic interaction; however, if an electrically neutral aromatic ring is used, the bond energy of π is - 5.6 – - 12 kcal mol? 1, mainly due to dispersion and orbital interaction.
To prove the Nido cage π - noncovalent bond can be used in the development of applications. The group has also systematically studied the photophysical properties of boron cluster compounds. Because Nido cage In the presence of π action, boron cluster derivatives show obvious charge transfer state absorption in crystal state. At the same time, theoretical calculation and EPR test also prove the generation of charge transfer. In addition, the photophysical properties of the excited states show that Nido cage The accumulation of π - noncovalent bonds can induce aggregation luminescence quenching in the crystalline state, while the enhancement of aggregation induced luminescence in the amorphous state shows different luminescence behavior from the traditional fluorescent molecules. It is worth mentioning that the carborane derivative exhibits solvent polarity dependent luminescent properties in solution, which provides a way for further design of polar sensing fluorescent probes.
These results were recently published in angelw. Chem. Int. ed. Theoretical calculation was carried out in cooperation with Prof. Prof. Jordi poater (Universitat de Barcelona) and Prof. Miquel sol à (Universitat de Girona) in Spain. The research was supported by NSFC.
The Nido cage??? π Bond: a non valuable interaction between born clusters and atomic rings and its applications Deshuang Tu, Hong Yan, Jordi poater, Miquel sol à angle. Chem. Int. ed., 2020, DOI: 10.1002/anie.201915290
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