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.