Synthesis and Reactions of Borazines

Given the wide array of current applications of borazine-based materials, synthetic access to these compounds is of importance. This review summarizes the many ways of preparing borazines and its carbo-substituted analogues. In addition, the functionalization of borazines is covered. The synthesis of molecules incorporating more than one borazine units as well as aspects of unsymmetrically substituted borazines are not included. The literature has been covered comprehensively until the end of 2020.

1 Introduction: Structure and Properties of Borazine

2 Synthesis of Parent Borazine

3 Synthesis of N-Substituted Borazines

4 Synthesis of B-Halo/B-Halo-N-Substituted Borazines

5 Synthesis of B-Substituted Borazines

6 Synthesis of Polycyclic Borazines Containing One Borazine Ring

7 Modifications or Hydrolysis of the Borazine Ring

8 Borazine Metal Complexes

9 Outlook and Conclusion

Graphitic supramolecular architectures based on corannulene, fullerene, and beyond

In this Feature Article, we survey the advances made in the field of fulleretic materials over the last five years. Merging the intriguing characteristics of fulleretic molecules with hierarchical materials can lead to enhanced properties of the latter for applications in optoelectronic, biomaterial, and heterogeneous catalysis sectors. As there has been significant growth in the development of fullerene- and corannulene-containing materials, this article will focus on studies performed during the last five years exclusively, and highlight the recent trends in designing fulleretic compounds and understanding their properties, that has enriched the repertoire of carbon-rich functional materials.

BN-Doped Metal-Organic Frameworks: Tailoring 2D and 3D Porous Architectures through Molecular Editing of Borazines

Building on the MOF approach to prepare porous materials, herein we report the engineering of porous BN-doped materials using tricarboxylic hexaarylborazine ligands, which are laterally decorated with functional groups at the full-carbon 'inner shell'. Whilst an open porous 3D entangled structure could be obtained from the double interpenetration of two identical metal frameworks derived from the methyl substituted borazine, the chlorine-functionalised linker undergoes formation of a porous layered 2D honeycomb structure, as shown by single-crystal X-ray diffraction analysis. In this architecture, the borazine cores are rotated by 60° in alternating layers, thus generating large rhombohedral channels running perpendicular to the planes of the networks. An analogous unsubstituted full-carbon metal framework was synthesised for comparison. The resulting MOF revealed a crystalline 3D entangled porous structure, composed by three mutually interpenetrating networks, hence denser than those obtained from the borazine linkers. Their microporosity and CO2 uptake were investigated, with the porous 3D BN-MOF entangled structure exhibiting a large apparent BET specific surface area (1091 m2  g-1 ) and significant CO2 reversible adsorption (3.31 mmol g-1 ) at 1 bar and 273 K.

Ferrocene-Based Conjugated Microporous Polymers Derived from Yamamoto Coupling for Gas Storage and Dye Removal

Conjugated microporous polymers (CMPs) have conjugated skeleton and permanent porosity, and exhibit huge potential in developing novel functional materials for resolving the challenging energy and environment issues. Metal-containing CMPs often exhibited unique properties. In the present manuscript, ferrocene-based conjugated microporous polymers (FcCMPs) were designed and synthesized with 1,1'-dibromoferrocene and 5,10,15,20-Tetrakis(4- bromophenyl) porphyrin (FcCMP-1) or Tetra (p-bromophenyl) methane (FcCMP-2) as building units via Yamamoto coupling. FcCMPs were amorphous, and exhibited excellent thermal and physicochemical stability. The BET surface area of FcCMP-1 and FcCMP-2 was 638 m2/g and 422 m2/g, respectively. In comparison with FcCMP-2, FcCMP-1 displayed better gas storage capacity due to higher porosity. FcCMPs were also used as an adsorbent for removal of methyl violet from aqueous solution, and exhibited excellent adsorption properties due to the interaction between electron-rich conjugated structure of the polymers and methyl violet with cationic groups. Moreover, FcCMPs could be extracted and regenerated by an eluent and then re-used for high efficient removal of methyl violet.

Hyper-Cross-linked Porous Organic Frameworks with Ultramicropores for Selective Xenon Capture

Exceptionally stable ultramicroporous C-C-bonded porous organic frameworks (IISERP-POF6, 7, 8) have been prepared using simple Friedel-Crafts reaction. These polymers exhibit permanent porosity with a Brunauer-Emmett-Teller surface area of 645-800 m²/g. Xe/Kr adsorptive separation has been carried out with these polymers, and they display selective Xe capture ( s(Xe/Kr) = 6.7, 6.3, and 6.3) at 298 K and 1 bar pressure. Interestingly, these polymers also show remarkable Xe/N₂ ( s(Xe/N₂) = 200, 180, and 160 at 298 K and 1 bar) and Xe/CO₂ selectivity ( s(Xe/CO₂) = 5.6, 7.4, and 5.6) for a 1:99 composition of Xe-N₂/Xe-CO₂. Selective removal of Xe at such low concentrations is extremely challenging; the observed selectivities are higher compared to those observed in porous carbons and metal-organic frameworks. Breakthrough studies were performed using the composition relevant to the nuclear off-gas mixture with the polymers, and we find that the polymers hold Xe for a longer time in the column, which illustrates the Xe/Kr separation performance under dynamic conditions.

Porous Organic Polymers via Ring Opening Metathesis Polymerization

Highly porous and solution processable organic polymers that can be structurally tailored for various applications are in great demand. Previously reported strategies to prepare porous polymers usually rely on a high level of cross-linking or structurally rigid polymer backbones. We now demonstrate that one-dimensional linear polymers with flexible backbones prepared through ring opening metathesis polymerization can be highly porous. This new strategy allows facile access to diversified porous organic polymers having tunable mechanical/chemical properties bearing different functionalities.

Theoretical study of the effects of substituents (F, Cl, Br, CH3, and CN) on the aromaticity of borazine

This paper presents a theoretical study of the effects of substituents (F, Cl, Br, CH₃, and CN) on the aromaticity of borazine (B₃N₃H₆), using density functional theory (DFT) and the Hartree-Fock (HF) method. The calculations to optimize the geometries, structural properties, and vibrational frequencies were performed using the same 6-311G(d,p) and 6-311++G(d,p) basis sets, comparing the methods with experimental results. In the analysis of the NICS_ZZ values, it was found that that replacing the hydrogen atoms by halogen atoms (F, Cl, and Br) and CH₃ reduced the aromaticity of the borazine molecule, while use of the CN group resulted in NICS_ZZ values (0.9-2.0 Å) very close to those of borazine, presenting the following order of increasing aromaticity: B₃N₃H₃-(Br)₃ < B₃N₃H₃-(Cl)₃ < B₃N₃H₃-(F)₃ < B₃N₃H₃-(CH₃)₃ < B₃N₃H₆ ~ B₃N₃H₃-(CN)₃. All the spectra of the compounds showed only the presence of transition peaks distant from the UV region, reflecting the large energy difference between the HOMO and LUMO orbitals. After the substitution of the borazine ring, all the compounds presented an intensification of the spectrum, with a shift of the maximum absorbance toward red, indicative of a bathochromic effect. There was a direct inverse relation between the energy gap and the maximum wavelength of the compounds.

Triptycene-Based Microporous Cyanate Resins for Adsorption/Separations of Benzene/Cyclohexane and Carbon Dioxide Gas

Triptycene-based cyanate monomers 2,6,14-tricyanatotriptycene (TPC) and 2,6,14-tris(4-cyanatophenyl)triptycene (TPPC) that contain different numbers of benzene rings per molecule were synthesized, from which two microporous cyanate resins PCN-TPC and PCN-TPPC were prepared. Of interest is the observation that the two polymers have very similar porosity parameters, but PCN-TPPC uptakes considerably higher benzene (77.8 wt %) than PCN-TPC (17.6 wt %) at room temperature since the higher concentration of phenyl groups in PCN-TPPC enhances the π-π interaction with benzene molecules. Besides, the adsorption capacity of benzene in PCN-TPPC is dramatically 7 times as high as that of cyclohexane. Contrary to the adsorption of organic vapors, at 273 K and 1.0 bar, PCN-TPC with more heteroatoms in the network skeleton displays larger uptake of CO₂ and higher CO₂/N₂ selectivity (16.4 wt %, 60) than those of PCN-TPPC (14.0 wt %, 39). The excellent and unique adsorption properties exhibit potential applications in the purification of small molecular organic hydrocarbons, e.g., separation of benzene from benzene/cyclohexane mixture as well as CO₂ capture from flue gas. Moreover, the results are helpful for deeply understanding the effect of porous and chemical structures on the adsorption properties of organic hydrocarbons and CO₂ gas.

Construction of porous covalent organic polymer as photocatalysts for RhB degradation under visible light

In the present work, a novel porous, and chemically stable amine-based covalent organic polymer (POP-1) was designed and synthesized under solvothermal conditions. The porosity, crystallinity, chemical stability, electrochemical properties, and diffuse reflectance of POP-1 were investigated via N₂ sorption experiment, power X-ray diffraction, thermogravimetric analysis, cyclic voltammetry, and ultraviolet visible near infrared spectrometry, respectively. POP-1 exhibits good chemical stability in both acidic and alkaline aqueous solutions, as well as in organic solvents. Undoped POP-1 can be directly used as a photocatalyst for rhodamine B irradiation degradation under light-emitting diode and natural light. The E_a of POP-1 for RhB degradation is 82.37kJ/mol. Furthermore, POP-1 can be reused as a catalyst in RhB degradation without degraded catalytic activity.

Microporous polyimide networks constructed through a two-step polymerization approach, and their carbon dioxide adsorption performance

A series of microporous polyimide networks were preparedviaa novel two-step pathway combining polymerization and crosslinking reactions.

Borazine-based inorganic–organic hybrid cyclomatrix microspheres by silicon/boron exchange precipitation polycondensation

Borazine-based inorganic–organic hybrid cyclomatrix microspheres with a mean diameter of about 900 nm have been obtained via a novel silicon/boron exchange precipitation polycondensation approach.

Highly porous photoluminescent diazaborole-linked polymers: synthesis, characterization, and application to selective gas adsorption

Highly porous and photoluminescent diazaborole-linked polymers are targeted by boron–nitrogen bond formation through simple condensation reactions. The resultant polymers exhibit remarkable gas uptake and tunable photoluminescent properties.

Phthalazinone structure-based covalent triazine frameworks and their gas adsorption and separation properties

In this work, new classes of phthalazinone-based covalent triazine frameworks (PHCTFs) were prepared by ionothermal synthesis from two full rigid dicyano building blocks with rigid, thermostable and asymmetric N-heterocycle-containing structures.

A ribbon-like ultramicroporous conjugated polycarbazole network for gas storage and separation

Ribbon-like ultramicroporous conjugated polycarbazole network P-TCzTAT was synthesized having high BET surface area and narrow pore width distribution.

Electrochemically active porous organic polymers based on corannulene

For the first time, porous organic polymers (POPs) based on the smallest buckybowl, corannulene (BB-POPs) have been synthesized.

Boron-nitrogen doped carbon scaffolding: organic chemistry, self-assembly and materials applications of borazine and its derivatives

Discovered by Stock and Pohland in 1926, borazine is the isoelectronic and isostructural inorganic analogue of benzene, where the C[double bond, length as m-dash]C bonds are substituted by B-N bonds. The strong polarity of such heteroatomic bonds widens the HOMO-LUMO gap of the molecule, imparting strong UV-emitting/absorption and electrical insulating properties. These properties make borazine and its derivatives valuable molecular scaffolds to be inserted as doping units in graphitic-based carbon materials to tailor their optoelectronic characteristics, and specifically their semiconducting properties. By guiding the reader through the most significant examples in the field, in this feature paper we describe the past and recent developments in the organic synthesis and functionalisation of borazine and its derivatives. These boosted the production of a large variety of tailored derivatives, broadening their use in optoelectronics, H2 storage and supramolecular functional architectures, to name a few.

One-step synthesis of a cyclic 2,17-dioxo[3,3](4,4') biphenylophane and first preparation of a microporous polymer network from a macrocyclic precursor by cyclotrimerization

One-step synthesis of a cyclic 2,17-dioxo[3,3](4,4')biphenylophane (MC) was achieved in high yield; its structure was verified by single crystal X-ray analysis. As a first example, a microporous polymer network was formed from macrocycle MC via acid-catalysed cyclotrimerization yielding a BET surface area of ca. 570 m(2) g(-1).

Theoretical investigation of the borazine-melamine polymer as a novel candidate for hydrogen storage applications

Ab initio calculations and molecular dynamic simulation were employed to study the interaction of molecular hydrogen with the borazine-melamine polymer (BMP) in order to explore its potential for hydrogen storage applications. The calculations were performed using the long range corrected version of density functional theory, the Coulomb-attenuating method (CAM-B3LYP) and the second order Møller-Plesset perturbation theory (MP2). The results showed that the average adsorption energy per hydrogen is about -0.7 and -0.3 kcal mol(-1) at the MP2/6-311+G(d,p) and CAMB3LYP/6-311+G(d,p) levels of theory, respectively. The adsorption energies were corrected for the basis set superposition error (BSSE) by the counterpoise method. It was found that the hydrogen storage capacity of the BMP is about 6.49 wt%, which is close to the values reported for the other selected materials for the hydrogen storage in the literature. The maximum number of hydrogen molecules, which were adsorbed by the BMP building block, is about ten. Molecular dynamic simulation was performed to assess the potential of BMP for hydrogen storage.

Preparation and characterization of novel carbon dioxide adsorbents based on polyethylenimine-modified Halloysite nanotubes

New nano-sized carbon dioxide (CO2) adsorbents based on Halloysite nanotubes impregnated with polyethylenimine (PEI) were designed and synthesized, which were excellent adsorbents for the capture of CO2 at room temperature and had relatively high CO2 adsorption capacity. The prepared adsorbents were characterized by various techniques such as Fourier transform infrared spectrometry, gel permeation chromatography, dynamic light scattering, thermogravimetry, thermogravimetry-Fourier transform-infrared spectrometry, scanning electron microscopy and transmission electron microscopy. The adsorption characteristics and capacity were studied at room temperature, the highest CO2 adsorption capacity of 156.6 mg/g-PEI was obtained and the optimal adsorption capacity can reach a maximum value of 54.8 mg/g-adsorbent. The experiment indicated that this kind of adsorbent has a high stability at 80°C and PEI-impregnated adsorbents showed good reversibility and stability during cyclic adsorption-regeneration tests.

Novel ferrocene-based nanoporous organic polymers for clean energy application

A novel ferrocene-based nanoporous organic polymer has been prepared by coupling 1,1′-ferrocene-dicarboxaldehyde with melamine.

Synthesis of covalent triazine-based frameworks with high CO2 adsorption and selectivity

PCTF-4 with benzothiadiazole exhibited the highest CO₂ uptake (20.5 wt%) and CO₂/N₂ selectivity (56) among the reported covalent triazine-based frameworks.

Electron spin-polarization and spin lattices in the boron- and nitrogen-doped organic framework COF-5

Kagome spin lattice and half-metallicity can be achieved in a COF-5 framework by substitutional doping with nitrogen and boron atoms.

Metallosalen-based microporous organic polymers: synthesis and carbon dioxide uptake

Three metallosalen-based microporous organic polymers were designed and synthesized. New materials display excellent porosity and good capacities for store and separation of carbon dioxide at 273 K and 1 bar.

The roles of dihydrogen bonds in amine borane chemistry

A dihydrogen bond (DHB) is an electrostatic interaction between a protonic hydrogen and a hydridic hydrogen. Over the past two decades, researchers have made significant progress in the identification and characterization of DHBs and their properties. In comparison with conventional hydrogen bonds (HBs), which have been widely used in catalysis, molecular recognition, crystal engineering, and supramolecular synthesis, chemists have only applied DHBs in very limited ways. Considering that DHBs and conventional HBs have comparable strength, DHBs could be more widely applied in chemistry. Over the past several years, we have explored the impact of DHBs on amine borane chemistry and the syntheses and characterization of amine boranes and ammoniated metal borohydrides for hydrogen storage. Through systematic computational and experimental investigations, we found that DHBs play a dominant role in dictating the reaction pathways (and thus different products) of amine boranes where oppositely charged hydrogens coexist for DHB formation. Through careful experiments, we observed, for the first time, a long-postulated reaction intermediate, ammonia diborane (AaDB), whose behavior is essential to mechanistic understanding of the formation of the diammoniate of diborane (DADB) in the reaction of ammonia (NH3) with tetrahydrofuran borane (THF·BH3). The formation of DADB has puzzled the boron chemistry community for decades. Mechanistic insight enabled us to develop facile syntheses of aminodiborane (ADB), ammonia borane (AB), DADB, and an inorganic butane analog NH3BH2NH2BH3 (DDAB). Our examples, together with those in the literature, reinforce the fact that DHB formation and subsequent molecular hydrogen elimination are a viable approach for creating new covalent bonds and synthesizing new materials. We also review the strong effects of DHBs on the stability of conformers and the hydrogen desorption temperatures of boron-nitrogen compounds. We hope that this Account will encourage further applications of DHBs in molecular recognition, host-guest chemistry, crystal engineering, supramolecular chemistry, molecular self-assembly, chemical kinetics, and the syntheses of new advanced materials.

Microporous Polymer Networks (MPNs) Made in Metal-Free Regimes: Systematic Optimization of a Synthetic Protocol toward N-Arylcarbazole-Based MPNs

Acidic self-condensation of 2,7-bis(N-carbazolyl)-9-fluorenone 1 produces microporous polymer networks (MPNs) with high Brunauer-Emmett-Teller (BET) surface areas of up to 2250 m²/g and hydrogen storage capacities of up to 1.7% (at 1 bar) in a fully metal-free condensation regime.