Poly(Ferrocenylene Iminoborane) and the Merging of 1,1'-Ferrocenylene with p-Phenylene Iminoborane Building Blocks into Alternating Copolymers, Oligomers, and Macrocycles

The combination of redox-active ferrocene moieties, conjugated B=N units, and p-phenylene building blocks in linearly concatenated architectural arrangements has been explored. Oligo- and poly(ferrocenylene iminoborane)s have been successfully prepared, whereby in the latter case, the formation of linear polymers vs. discrete molecular macrocycles could be largely influenced by the polymerization conditions applied. In addition, a series of well-defined co-oligomers composed of ferrocenylene and p-phenylene moieties bridged by one or two B=N units in all possible combinations have been prepared, many of which have been structurally characterized using single-crystal X-ray diffraction. Pendent triisopropylphenyl groups on boron provide pronounced kinetic stabilization. Polycondensation of complementary monomers gave copolymers comprising alternating ferrocenylene and p-phenylene iminoborane building blocks, which were found to be cyclic in nature. UV-vis absorption spectroscopic and electrochemical studies provided highly valuable insights into the interaction of the different moieties over the B=N bonds, particularly, the electronic communication between the electroactive ferrocene units.

Regioregular Poly(p-phenylene iminoborane): A Strictly Alternating BN-Isostere of Poly(p-phenylene vinylene) with Stimuli-Responsive Behavior

Incorporation of BN units into π-conjugated organic compounds, as substitutes for specific CC couples, often leads to new hybrid materials with modified physical and chemical properties. Poly(p-phenylene iminoborane)s are derived from well-known poly(p-phenylene vinylene) (PPV) by replacement of the vinylene groups by B=N linking units. Herein, an unprecedented poly(p-phenylene iminoborane) is presented that features a strictly alternating sequence of BN units along the main chain. The synthesis thereof was achieved by AB-type polymerization of a monomer featuring an N and a B terminus. Monodisperse oligomers with up to three BN units in the chain were additionally prepared and structurally characterized. Associated with the introduction of a dipole in the regioregular backbone structure, they display notable fluorescence already in solution and large Stokes shifts, distinct from their previously reported BBNN-sequenced congeners. All compounds show aggregation-induced emission enhancement (AIEE) properties. Computational studies provided evidence for emission from either locally excited (LE) or twisted intramolecular charge transfer (TICT) states. These processes can be optionally addressed by various stimuli, giving rise to dual emission, solvatochromic, thermochromic, and reversible mechanochromic behavior.

Expression of hyperconjugative stereoelectronic interactions in borazines

This paper discusses hyperconjugative stereoelectronic effects in borazines. A series of alkyl-substituted borazines were synthesized and analysed by NMR spectroscopy and X-ray diffraction. Supported by NBO analyses, the significant decreases in 1JCH coupling constant for the CH groups adjacent to the boron atoms are consistent with the presence of and interactions. These interactions lower the electrophilicity of boron atoms, enhancing moisture stability and establishing these molecules as valuable scaffolds in synthetic chemistry and materials science.

An Open-Shell Functionalization of Inorganic Benzene

A borazine derivative functionalized by nitroxide free radicals, N,N',N″-(tris(4-Bromophenyl))-B,B',B″-tris((2,6-dimethyl-4-(N-tert-butyl-N-oxyamino)phenyl) borazine (TriBNit), was synthesized as a milestone of open-shell inorganic benzene. The crystal structure determined from X-ray diffraction on a single crystal ascertains the grafting of three nitroxide radicals. The temperature dependence of the magnetic susceptibility evidences weak intramolecular antiferromagnetic interactions between the radicals with strong intermolecular antiferromagnetic interactions between two nitroxide moieties of two neighboring molecules. EPR spectroscopy at 80 K on a frozen glassy solution evidences the coexistence of S = 1/2 and S = 3/2 ground-spin state species. This is ascribed to the nitroxide radicals having different orientations with respect to the borazine core giving rise either to antiferromagnetic interaction with a low ground-spin state S = 1/2 or to ferromagnetic interaction with a high ground-spin state S = 3/2 as supported by theoretical data. At room temperature, because of nitroxide mobility, the EPR spectrum is averaged to a ground-spin state S = 1/2.

Poly(ferrocenylene iminoborane): an inorganic-organic hybrid polymer comprising a backbone of moderately interacting ferrocenes

The first poly(ferrocenylene iminoborane), that is, a polyferrocene-based metallopolymer featuring CC-isoelectronic/-isosteric BN linking units, and a series of monodisperse ferrocenylene iminoborane oligomers are presented. Our studies provide important insight into the structural and electronic nature of this novel class of hybrid materials.

Poly(thiophene iminoborane): A Poly(thiophene vinylene) (PTV) Analogue with a Fully BN-Doped Backbone

An unprecedented poly(thiophene iminoborane)-a boron-nitrogen analogue of the well-established conjugated organic polymer poly(thiophene vinylene)-is presented. The polymer synthesis is achieved by selective Si/B exchange polycondensation of a 2,5-diborylthiophene with a 2,5-diaminothiophene derivative. For the latter, a facile synthetic strategy is devised, which makes this versatile, strongly electron-releasing building block easily accessible. The novel polymer and a series of monodisperse thiophene iminoborane oligomers reveal systematic bathochromic shifts in their absorption with increasing chain length, and thus extended π-conjugation over the BN units along the backbone, which is further supported by TD-DFT calculations.

Poly(arylene iminoborane)s, Analogues of Poly(arylene vinylene) with a BN-Doped Backbone: A Comprehensive Study

Despite the great success of the concept of doping organic compounds with BN units to access new materials with tailored properties, its use in polymer chemistry has only been realized quite recently. Herein, we present a comprehensive study of oligo- and poly(arylene iminoborane)s comprising a backbone of phenylene or thiophene moieties, as well as combinations thereof, linked via B=N units. The novel polymers can be regarded as BN analogues of poly(p-phenylene vinylene) (PPV) or poly(thiophene vinylene) (PTV) or their copolymers. Our modular synthetic approach allowed us to prepare four polymers and 12 monodisperse oligomers with modulated electronic properties. Alternating electron-releasing diaminoarylene and electron-accepting diborylarylene building blocks gave rise to a pronounced donor-acceptor character. Effective π-conjugation over the arylene iminoborane backbone is evidenced by systematic bathochromic shifts of the low-energy UV-vis absorption maximum with increasing chain length, which is furthermore supported by crystallographic and computational investigations. Furthermore, all compounds investigated show emission of visible light in the solid state and aggregation-induced emission (AIE) behavior, due to the presence of partially flexible linear B=N linkages in the backbone.

Linear and macrocyclic oligo(p-phenylene iminoboranes) with ferrocenyl side groups - observation of selective, non-templated macrocyclization

A series of linear oligo(p-phenylene iminoboranes), which are BN-modified congeners of oligo(p-phenylene vinylenes), featuring pendent ferrocene groups have been prepared. Stoichiometric reaction of a bis-silylamine with a bisborane led to selective formation of an unprecedented macrocycle, without the use of a template.

Boron Nitride-Doped Polyphenylenic Organogels

Herein, we describe the synthesis of the first boron nitride-doped polyphenylenic material obtained through a [4 + 2] cycloaddition reaction between a triethynyl borazine unit and a biscyclopentadienone derivative, which undergoes organogel formation in chlorinated solvents (the critical jellification concentration is 4% w/w in CHCl₃). The polymer has been characterized extensively by Fourier-transform infrared spectroscopy, solid-state ¹³C NMR, solid-state ¹¹B NMR, and by comparison with the isolated monomeric unit. Furthermore, the polymer gels formed in chlorinated solvents have been thoroughly characterized and studied, showing rheological properties comparable to those of polyacrylamide gels with a low crosslinker percentage. Given the thermal and chemical stability, the material was studied as a potential support for solid-state electrolytes. showing properties comparable to those of polyethylene glycol-based electrolytes, thus presenting great potential for the application of this new class of material in lithium-ion batteries.

Fusion of Aza- and Oxadiborepins with Furans in a Reversible Ring-Opening Process Furnishes Versatile Building Blocks for Extended π-Conjugated Materials

A modular synthesis of both difurooxa- and difuroazadiborepins from a common precursor is demonstrated. Starting from 2,2'-bifuran, after protection of the positions 5 and 5' with bulky silyl groups, formation of the novel polycycles proceeds through opening of the furan rings to a dialkyne and subsequent re-cyclization in the borylation step. The resulting bifuran-fused diborepins show pronounced stability, highly planar tricyclic structures, and intense blue light emission. Deprotection and transformation into dibrominated building blocks that can be incorporated into π-extended materials can be performed in one step. Detailed DFT calculations provide information about the aromaticity of the constituent rings of this polycycle.

Electrophilic activation of difunctional aminoboranes: B-N coupling versus intramolecular Cl/Me exchange

Treatment of an N-silyl-B-chloro-aminoborane with substoichio-metric quantities of Me₃SiOTf afforded B-N coupling, whereas activation with 5 mol% of Ag[Al{OC(CF₃)₃}₄] led to Cl/Me exchange between the boron and the silicon center. Combined experimental and computational studies of the latter process support a chain reaction that is initiated by nucleation-limited chloride abstraction.

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

Dithiophene-Fused Oxadiborepins and Azadiborepins: A New Class of Highly Fluorescent Heteroaromatics

Access to dithiophene-fused oxadiborepins and the first azadiborepins attained via a modular synthesis route are presented. The new compounds emit intense blue light, some of which demonstrate fluorescence quantum yields close to unity. Cyclic voltammetry (CV) revealed electrochemically reversible one-electron reduction processes. The weak aromatic character of the novel 1,2,7-azadiborepin ring is demonstrated with in-depth theoretical investigations using nucleus-independent chemical shift (NICS) scans and anisotropy of the induced current density (ACID) calculations.

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.

High-Temperature Resistant Polyborosilazanes with Tailored Structures

Boron-containing organosilicon polymers are widely used under harsh environments as preceramic polymers for advanced ceramics fabrication. However, harmful chemicals released during synthesis and the complex synthesis routes have limited their applications. To solve the problems, a two-component route was adopted to synthesize cross-linked boron-containing silicone polymer (CPBCS) via a solventless process. The boron content and CPBCSs’ polymeric structures could be readily tuned through controlling the ratio of multifunctional boron hybrid silazane monomers (BSZ12) and poly[imino(methylsilylene)]. The CPBCSs showed high thermal stability and good mechanical properties. The CPBCS with Si-H/C=C ratio of 10:1 showed 75 wt% char yields at 1000 °C in argon, and the heat release capacity (HRC) and total heat release (THR) are determined to be 37.9 J/g K and 6.2 KJ/g, demonstrating high thermal stability and flame retardancy. The reduced modulus and hardness of CPBCS are 0.30 GPa and 2.32 GPa, respectively. The novel polysilazanes can be potentially used under harsh environments, such as high temperatures or fire hazards.

Bifuran-bridged bisboranes: highly luminescent B-doped oligohetarenes

Boron-doping of oligohetarenes – via classical metathesis or silicon/boron exchange routes – led to strongly luminescent and twofold reversibly reducible oligomers.

BN- and BO-Doped Inorganic-Organic Hybrid Polymers with Sulfoximine Core Units

While polysulfones constitute a class of well-established, highly valuable applied materials, knowledge about polymers based on the related sulfoximine group is very limited. We have employed functionalized diaryl sulfoximines and a p-phenylene bisborane as building blocks for unprecedented BN- and BO-doped alternating inorganic-organic hybrid copolymers. While the former were accessed by a facile silicon/boron exchange protocol, the synthesis of polymers with main-chain B-O linkages was achieved by salt elimination.

Structural Properties of Highly Doped Borazino Polyphenylenes Obtained through Condensation Reaction

Here we describe the synthesis and spectroscopic and structural characterization of various borazine-doped polyphenylenes displaying high doping dosages (16-18%). Capitalizing on the condensation reaction approach, the desired products were formed using a mixture of p-phenylendiamine and aniline with BCl₃, followed by the addition of an aryl lithium derivative. The use of mesityl lithium (MesLi) yields strained multiborazine derivatives, which proved to be unstable in the presence of moisture. However, when xylyl lithium (XylLi) was used, chemically stable multiborazines were obtained, with oligomers showing molecular weight up to 10⁴, corresponding to 16-18 monomer units. While the dimer, trimer, and tetramer could be isolated as pure products and their structure characterized by mass and NMR analysis, higher oligomers could only be isolated as mixtures of B-hydroxy-substituted derivatives and characterized by gel permeation chromatography. The structures of the dimer and trimer derivatives were confirmed by X-ray analysis, which nicely showed the presence of the two and three borazine rings spaced by one and two 1,4-aryl bridges, respectively. Notably, the trimer forms a porous crystalline clathrate. The peripheral xylyl and phenyl moieties of each molecule intramolecularly embrace each other through C-H and π-π stacking interactions. Steady-state UV-vis absorption characterization suggested that the molecules are UV absorbers, with the extinction coefficient linearly scaling with the degree of oligomerization. On the other hand, low-emission quantum yields were obtained for all derivatives (<7%), suggesting that high BN-doping dosages dramatically affect the emission properties of the doped polyphenylenes.

Easy fabrication of poly(butyl acrylate)/silicon dioxide core-shell composite microspheres through ultrasonically initiated encapsulation emulsion polymerization

In this study, instead of using the usual chemical methods, poly(butyl acrylate)/silicon dioxide (PBA/SiO₂) core-shell composite microspheres were prepared using a physical method-ultrasonically initiated encapsulation emulsion polymerization. The morphology and particle size of the PBA/SiO₂ microspheres were analysed using transmission electron microscopy (TEM) and dynamic light scattering (DLS). The encapsulation state was determined using X-ray photoelectron spectroscopy (XPS). The composition and thermogravimetric behavior were characterized using Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The TEM and DLS results show that monodisperse PBA/SiO₂ core-shell composite microspheres were successfully obtained. The diameter and shell thickness were 150 nm and 15 nm, respectively. The XPS and FTIR results show that there was no new chemical bond between the PBA shell and the SiO₂ core. They were just combined by physical adsorption. The encapsulation efficiency of SiO₂ microspheres by PBA is 8.2% through TGA. In addition, this article focuses on the formation mechanism of PBA/SiO₂ core-shell microspheres prepared through ultrasonically initiated encapsulation emulsion polymerization. Intuitive observation and the results of TEM and DLS, especially the change in zeta potential, clearly indicate an encapsulation process. Thereinto, a bilayer-structure space established by appropriate amount of cetyltrimethyl ammonium bromide (CTAB) molecules is the key to realize ultrasonically initiated encapsulation emulsion polymerization.

From Monodisperse Thienyl- and Furylborane Oligomers to Polymers: Modulating the Optical Properties through the Hetarene Ratio

The application of our newly developed B-C coupling method by catalytic Si/B exchange is demonstrated for the synthesis of a series of triarylboranes (1), monodisperse thienyl- and furylborane dimers (2) and trimers (9), extended oligomers (3) and polymers (3'), as well as mixed (oligo)thienyl-/furylboranes. The structures of 1 aa^Tip , 1 bb^Tip , and 2 bbb^Mes* , determined by X-ray crystallography, reveal largely coplanar hetarene rings and BR₃ environments, which are most pronounced in the furylborane species. Photophysical investigations, supported by TD-DFT calculations, revealed pronounced π-electron delocalization over the hetarene backbones including the boron centers. With an extended series of derivatives of varying chain lengths available, we were able to determine the effective conjugation lengths (ECL) of poly(thienylborane)s and poly(furylborane)s, which have been reached with the highest-molecular-weight derivatives of our study. Through variation of the furan-to-thiophene ratio, the photophysical properties of these materials are effectively modulated. Significantly, higher furan contents lead to considerably increased fluorescence intensities. Compounds 1 aa^Tip , 1 bb^Tip , and 3 a^Tip showed the ability to bind fluoride anions. The binding process is signaled by a distinct change in their optical absorption characteristics, thus rendering these materials attractive targets for sensory applications.

PBP bridged [3]ferrocenophane: a bisphosphanylborane with a redox trigger

Stereospecific access to the unprecedented P-B-P bridged [3]ferrocenophane Fe(C₅H₄PtBu)₂BMes is presented. In contrast to acyclic ferrocenylphosphanes, we find evidence for a shift of spin-density from Fe to P upon pyramidal inversion of the P centers in the mono-cation; the latter has a lifetime of τ = 0.31(4) s.

Difuryl(supermesityl)borane: a versatile building block for extended π-conjugated materials

B-Doping of oligo(hetarene)s led to very robust, air-stable π-conjugated materials that are strong electron-acceptors and blue light emitters.