Hybrid Triazine-Boron Two-Dimensional Covalent Organic Frameworks: Synthesis, Characterization, and DFT Approach to Layer Interaction Energies

The
conversion of 2,4,6-tris­(4′-bromophenyl)-1,3,5-triazine to
the respective triboronic acid was successfully accomplished by a
simple triple Br/Li exchange followed by boronation. Further dehydrative
condensation reactions with 2,3,6,7,10,11-hexahydroxytriphenylene
or 2,3,6,7-tetrahydroxy-9,10-dilalkylanthracenes (R = Me, Et) resulted
in materials featuring good porosity and sorption properties with
the nitrogen uptake exceeding 500 cm3/g (STP) and SBET up to 1267 m2/g (T = 77.2 K). In addition, simple dehydration of this compound was
employed for the preparation of a hybrid 2D COF composed of triazine,
boroxine, and benzene rings. The formation of materials was confirmed
by the IR analysis and NMR studies on water-decomposed samples. All
obtained COFs exhibit high thermal stability with decomposition temperatures
in the range of 400–600 °C. They also show quite different
morphology ranging from regular 0.5–4 μm spherical and
ellipsoidal clusters to 5–12 μm bent rodlike particles.
The PXRD studies supported by periodic DFT modeling in Crystal09 package
revealed the formation of crystalline 2D honeycomb-type lattices with
eclipsed stacking models. In addition, the differences between boroxine-triazine
material and related COF-1 and CTF-1 structures were investigated
by comparing layer interaction energies, work function values as well
as atomic charges and electrostatic potential maps plotted on the
electron density surfaces. It demonstrates that the interactions between
layers are enhanced by the stacking of triazine and boroxine rings.
Finally, we have investigated the upper limit to space accessible
volume using a procrystal electron density approach.

将2,4,6-三(4'-溴苯基)-1,3,5-三嗪通过简便的三步Br/Li交换反应后进行硼化，成功制备得到相应的三硼酸衍生物。进一步将其与2,3,6,7,10,11-六羟基三亚苯或2,3,6,7-四羟基-9,10-二烷基蒽（R=Me, Et）进行脱水缩合反应，所得材料展现出良好的孔隙率与吸附性能：氮气吸附量超过500 cm³/g（标准温度与压力，STP），Brunauer-Emmett-Teller比表面积（S_BET）最高可达1267 m²/g（测试温度T=77.2 K）。此外，通过该化合物的简单脱水反应，可制备得到由三嗪环、硼氧六环与苯环构成的二维杂化共价有机框架（Covalent Organic Framework, COF）。材料的成功合成通过红外光谱（Infrared Spectroscopy, IR）分析以及对水解样品的核磁共振波谱（Nuclear Magnetic Resonance, NMR）研究得以验证。所有制备得到的共价有机框架均表现出较高的热稳定性，分解温度区间为400~600 ℃。其形貌也呈现出丰富的多样性，从规则的0.5~4 μm球形与椭球形团簇，到5~12 μm弯曲棒状颗粒均有分布。

借助Crystal09程序包中的周期性密度泛函理论（Density Functional Theory, DFT）模拟对X射线粉末衍射（Powder X-ray Diffraction, PXRD）结果进行佐证，研究表明材料形成了具有重叠堆叠构型的结晶二维蜂窝型晶格。此外，通过对比层间相互作用能、功函数值、原子电荷以及绘制于电子密度面上的静电势图，本研究探究了该硼氧六环-三嗪类材料与同类COF-1和CTF-1结构之间的差异。结果证实，三嗪环与硼氧六环的堆叠作用强化了层间相互作用。

最后，本研究采用准晶体电子密度方法探究了材料可访问空间体积的上限。