Fujian Normal University constructed the first single-crystal microporous framework material based on nitrogen-boron coordination bonds

On November 16, 2022, Beijing time, Zhang Zhangjing, a researcher at the School of Chemistry and Materials of Fujian Normal University, published a research result entitled “A crystalline and stable microporous framework based on the dative B←N bonds” in the journal Chem.

This achievement reports the first single-crystal microporous framework material BNF-1 constructed from coordinated nitrogen-boron bonds. BNF-1 remains crystalline after removing guest molecules and has high chemical stability, enabling the separation of acetylene carbon dioxide mixtures at room temperature.

The corresponding author of the paper is Zhang Zhangjing; The first author is Zhang Hao.

The establishment of network chemistry not only greatly broadened the field of porous materials, but also revolutionized the chemistry of solid materials. In network chemistry, the performance regulation of materials can be achieved by rationally designing the structure of materials. For crystalline porous materials, the establishment of permanent porosity is critical for their further development, for example, the establishment of permanent porosity in MOF-2 and later MOF-5 laid the foundation for the chemistry and function of metal-organic frameworks (MOFs), making MOFs one of the most important research topics in the past three decades. Similarly, after the permanent porosity is established, other crystalline porous materials such as covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs) have been widely used in gas storage, gas separation, catalysis, molecular identification and sensing, energy, environmental science, and biomedicine.

The construction of supramolecular compounds (BNFs) using coordination nitrogen-boron bonds for nodes has been widely reported. In 2011, Severin’s group was the first to report a potentially porosity nitrogen-boron framework compound (Angew. Chem. Int. Ed. 2011, 50, 3034-3037)。 Due to the medium bond energy (100 kJ/mol) and reversible formation/fracture properties, it is feasible to construct porous crystalline polymers with coordination nitrogen-boron bonds as nodes. In 2019, Severin’s group further established permanent porosity in nitrogen-boron coordination polymers. Unfortunately, these polymers are amorphous and prone to decomposition at high humidity (ACS Mater. Lett. 2019, 1, 3-7)。 This unstable characteristic greatly hinders the application and further development of BNFs.

In this paper, Zhang Zhangjing’s team reported the first crystalline nitrogen-boron framework BNF-1 with permanent porosity and used it for acetylene/carbon dioxide separation. The authors also studied the adsorption mechanism of acetylene by the frame through the single crystal structure and theoretical calculation of guest loading.

Figure 1: Synthetic route of BNF-1.

As shown in Figure 1, BNF-1 is formed by the pyridine monomer 3-TPPA and the borate monomer BACT connected by coordination nitrogen-boron bonds, forming a two-dimensional layered structure.

Figure 2: Crystal structure of BNF-1.

As shown in Figure 2, the single-layer structure of BNF-1 has a twisted (3,6) topology. The two cage spaces in the layer centered on Wyckoff sites 12c and 18e interspersed with each other, thus forming an interlocking structure. After interlocking, the available functional sites and spiral apertures with a radius of 4.5 angstroms remain in this site.

Figure 3: Characterization of BNF-1 porosity, stability, and acetylene and carbon dioxide adsorption separation performance at room temperature.

The two-dimensional layers in the BNF-1 structure interlock with each other to stabilize the frame, remain crystalline after removing the guest molecules, and the BET surface area is 255 m2/g. Not only that, the interlocking structure also gives BNF-1 high stability, and the porosity of the material remains unchanged after a week of immersion in acids, alkalis and common organic solvents. This stability is significantly higher than currently reported nitrogen-boron coordination compounds. In addition, BNF-1 can be prepared at the gram level by simple scale-up experiments and is easily regenerated by recrystallization. Static gas adsorption experiments show that BNF-1a has a higher adsorption capacity for acetylene than for carbon dioxide at room temperature. Dynamic gas penetration experiments show that BNF-1a can effectively separate acetylene and carbon dioxide mixtures at ambient temperature.

Figure 4: BNF-1a crystal structure of acetylene molecules and electrostatic potential near acetylene binding sites.

In order to explore the adsorption mechanism, the authors tested the single crystal structure of BNF-1a supported by acetylene guest. The guest-loaded crystal structure indicates that acetylene molecules are located at the functional site at the Wyckoff site 18e. Specifically, the acetylene molecule is located between the upper and lower layers of pyridine rings, forming a sandwich structure, and the pyridine ring and the benzene ring on the borate ester pass through the π … π role and C-H··· π action connection. Theoretical calculations show that the borate benzene ring has an electrostatic potential that is completely opposite to the pyridine ring, and this electrostatic potential is complementary to the electrostatic potential of the acetylene guest molecule in the middle. This matched spatial electrostatic potential maximizes the interaction between acetylene molecules and the skeleton.

In this work, the authors report the first crystalline and highly chemically stable nitroboron framework compound BNF-1. The authors comprehensively characterize it by their primitive, activated and acetylene-supported crystal structure, gas adsorption isotherms, and dynamic penetration experiments. Unique functional sites in the framework confer a high affinity for acetylene molecules to BNF-1, enabling efficient separation of acetylene and carbon dioxide mixtures at ambient temperature. The establishment of permanent porosity in this crystalline and stable BNF paves the way for the establishment of BNF chemical and functional materials. Given the abundance of organic molecules containing nitrogen donors and boron acceptors, the research group expects that a large number of nitrogen-boron frameworks will be reported in the future and widely used in various fields. (Source: Science Network)

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