Dehydrocoupling and Silazane Cleavage Routes to Organic-Inorganic Hybrid Polymers with NBN Units in the Main Chain

Synthesis, Structures, and Reactivities of BCN-Heterocyclic B-N Chains

The BCN-heterocyclic B-N chain compounds, the sodium and potassium salts of 3 and 4 anions (Na3, Na4, and K4), were synthesized by reactions of ethane 1,2-diamineborane (BH3NH2CH2CH2NH2BH3, 1) and propane 1,2-diamineborane (BH3NH2CH2CH2CH2NH2BH3, 2) with MH (M = Na and K). Then, the neutral B-N chain compounds 5 and 6 were prepared with dehydrogenation of [NH4]3 and [NH4]4, formed by metathesis reactions of Na3 and Na4 with NH4Cl or NH4SCN, respectively. Compounds 7 and 8, analog 5, were also prepared using pyridine and 4-methoxypyridine instead of NH3 in 5. These synthesized compounds were characterized spectroscopically, and the singe-crystal structures of the Na3·18-crown-6 and K4·18-crown-6 adducts were determined. Furthermore, the reactions of Na3 and Na4 with cationic B-N chain compounds, [NH3BH2NH3]Cl and [NH3BH2NH2BH2NH3]Cl, could not form longer BCN-heterocyclic B-N chain. The solubility of metal hydrides, the ability for proton abstracting, the basicity of Lewis bases, and the chelate effect may influence these reactions even though the reaction mechanism is not fully understood.

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.

Iron(II)-Catalyzed Activation of Si-N and Si-O Bonds Using Hydroboranes

We report the activation and functionalization of Si-N bonds with pinacol borane catalyzed by a three-coordinate iron(II) β-diketiminate complex. The reactions proceed via the mild activation of silazanes to yield useful hydrosilanes and aminoboranes. The reaction is studied by kinetic analysis, along with a detailed investigation of decomposition pathways using catecholborane as an analogue of the pinacol borane used in catalysis. We have extended the methodology to develop a polycarbosilazane depolymerization strategy, which generates hydrosilane quantitatively along with complete conversion to the Bpin-protected diamine. The analogous Si-O bond cleavage can also be achieved with heating, using silyl ether starting materials to generate hydrosilane and alkoxyborane products. Depolymerization of poly(silyl ether)s using our strategy successfully converts the polymer to 90% Bpin-protected alcohols.

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.

Synthesis, Structure, and Properties of Monodispersed and Highly Luminescent Organoborane Oligomers

Organoborane oligomers with well-defined molecular structures and high luminescence are scarce, among which those with boron not used as bridging atoms are even more so. Here, a series of well-defined ethynyl-linked or butadiynyl-linked conjugated organoborane oligomers with high fluorescence quantum yield and extinction coefficient (i.e., high brightness) were prepared by coupling different building blocks featuring dithienooxadiborepine moieties. Single crystal structures of hexyl modified dithienooxadiborepine (1a-hex) and hexyl-modified butadiynyl-linked conjugated dimer (D2-hex) not only verified the identity of the molecular structures but also revealed that the introduction of the hexyl chains distorted the molecular structures due to steric hindrance. Optical measurements showed that the absorption and emission maxima of the six oligomeric molecules bathochromic shifted with increasing numbers of repeating units. Molecules without hexyl chains emit efficient fluorescence upon photoexcitation, and the fluorescence quantum efficiency of the ethynyl-linked conjugated dimer (D1) is close to unity. Theoretical calculation results using density functional theory methods are consistent with the single crystal data, allowing a better understanding of the spectral properties. Such results indicate that the method is efficient for expanding small organoborane molecules into π-conjugated oligomers, which can be used to modulate to emit different colors with high efficiency.

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.

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.

Synthesis, Formation Mechanism, and Structure of K[BH3S(CH3)BH3] and Its Application in Preparation of KB3H8

The design, synthesis, and applications of new boranes are eternal topics in boron chemistry. A new bis(borane)alkanethiolate salt, K[BH₃S(CH₃)BH₃], was synthesized in high yield by the reaction of K with (CH₃)₂S·BH₃ at room temperature. The formation mechanism was elucidated based on experimental and theoretical studies. The single-crystal structure of the K[BH₃S(CH₃)BH₃]·18-crown-6 adduct was determined in which the B-S-B bonding information of K[BH₃S(CH₃)BH₃] was illustrated for the first time. Using K[BH₃S(CH₃)BH₃] as a starting material, KB₃H₈ was successfully synthesized.

Phenalenyl Radical: Smallest Polycyclic Odd Alternant Hydrocarbon Present in the Graphene Sheet

Phenalenyl, a zigzag-edged odd alternant hydrocarbon unit can be found in the graphene nanosheet. Hückel molecular orbital calculations indicate the presence of a nonbonding molecular orbital (NBMO), which originates from the linear combination of atomic orbitals (LCAO) arising from 13 carbon atoms of the phenalenyl molecule. Three redox states (cationic, neutral radical, and anionic) of the phenalenyl-based molecules were attributed to the presence of this NBMO. The cationic state can undergo two consecutive reductions to result in neutral radical and anionic states, stepwise, respectively. The phenalenyl-based radicals were found as crucial building blocks and attracted the attention of various research fields such as organic synthesis, material science, computation, and device physics. From 2012 onward, a strategy was devised using the cationic state of phenalenyl-based molecules and in situ generated phenalenyl radicals, which created a new domain of catalysis. The in situ generated phenalenyl radicals were utilized for the single electron transfer (SET) process resulting in redox catalysis. This emerging range of applications rejuvenates the more than six decades-old phenalenyl chemistry. This review captures such developments ranging from fundamental understanding to multidirectional applications of phenalenyl-based radicals.

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.

Unpredicted Concentration-Dependent Sensory Properties of Pyrene-Containing NBN-Doped Polycyclic Aromatic Hydrocarbons

Pyrene molecules containing NBN-doped polycyclic aromatic hydrocarbons (PAHs) have been synthesized by a simple and efficient intermolecular dehydration reaction between 1-pyrenylboronic acid and aromatic diamine. Pyrene-B (o-phenylenediamine) with a five-membered NBN ring and pyrene-B (1,8-diaminonaphthalene) with a six-membered NBN ring show differing luminescence. Pyrene-B (o-phenylenediamine) shows concentration-dependent luminescence and enhanced emission after grinding at solid state. Pyrene-B (1,8-diaminonaphthalene) exhibits a turn-on type luminescence upon fluoride ion addition at lower concentration, as well as concentration-dependent stability. Further potential applications of Pyrene-B (o-phenylenediamine) on artificial light-harvesting film were demonstrated by using commercial NiR dye as acceptor.

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.

BNB-doped phenalenyls - aromaticity switch upon one-electron reduction

Fully aromatic, luminescent, and highly robust BNB-doped phenalenyls have been prepared, which are isoelectronic to the phenalenyl cation. B-Fluoromesityl-substitution leads to fluorescence in an extremely narrow range and significant increase in the reduction potential. Detailed theoretical investigations revealed an intramolecular aromaticity switch upon one-electron reduction.

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.

Boronate Covalent and Hybrid Organic Frameworks Featuring PIII and P=O Lewis Base Sites

Two covalent organic frameworks comprising Lewis basic PIII centers and Lewis acidic boron atoms were prepared by poly-condensation reactions of newly obtained tris(4-diisopropoxyborylphenyl)phosphine with 2,3,6,7,10,11-hexahydroxytriphenylene and 2,3,6,7-tetrahydroxy-9,10-dimethylanthracene. Obtained materials exhibit significant sorption of dihydrogen (100 cm3  g-1 at 1 bar at 77 K), methane (20 cm3  g-1 at 1 bar at 273 K) and carbon dioxide (50 cm3  g-1 at 1 bar at 273 K). They were exploited as solid-state ligands for coordination of Pd0 centers. Alternatively, in a bottom-up approach, boronated phosphine was treated with Pd2 dba3 and poly-condensated, yielding hybrid materials where the polymer networks are formed by means of covalent boronate linkages and coordination P-Pd bonds. In addition, the analogous materials based on phosphine oxide were synthesized. The DFT calculations on framework-guest interactions revealed that the behavior of adjacent boron and phosphorus/phosphine oxide centers is reminiscent of that found in Frustrated Lewis Pairs and may improve sorption of selected molecules.

Synthetic Tailoring of Graphene Nanostructures with Zigzag-Edged Topologies: Progress and Perspectives

Experimental and theoretical investigations have revealed that the chemical and physical properties of graphene are crucially determined by their topological structures. Therefore, the atomically precise synthesis of graphene nanostructures is essential. A particular example is graphene nanostructures with zigzag-edged structures, which exhibit unique (opto)electronic and magnetic properties owing to their spin-polarized edge state. Recent progress in the development of synthetic methods and strategies as well as characterization methods has given access to this class of unprecedented graphene nanostructures, which used to be purely molecular objectives in theoretical chemistry. Thus, clear insight into the structure-property relationships has become possible as well as new applications in organic carbon-based electronic and spintronic devices. In this Minireview, we discuss the recent progress in the controlled synthesis of zigzag-edged graphene nanostructures with different topologies through a bottom-up synthetic strategy.

Magnesocenophane-Catalyzed Amine Borane Dehydrocoupling

The Lewis acidities of a series of [n]magnesocenophanes (1 a–d) have been investigated computationally and found to be a function of the tilt of the cyclopentadienyl moieties. Their catalytic abilities in amine borane dehydrogenation/dehydrocoupling reactions have been probed, and C[1]magnesocenophane (1 a) has been shown to effectively catalyze the dehydrogenation/dehydrocoupling of dimethylamine borane (2 a) and diisopropylamine borane (2 b) under ambient conditions. Furthermore, the mechanism of the reaction with 2 a has been investigated experimentally and computationally, and the results imply a ligand‐assisted mechanism involving stepwise proton and hydride transfer, with dimethylaminoborane as the key intermediate.

Synthesis of bis-BN-Naphthalene-Fused Oxepins and Their Photoluminescence Including White-Light Emission

A series of novel bis-BN-naphthalene-fused oxepin derivatives were synthesized via a Pd-catalyzed tandem reaction from brominated 2,1-borazaronaphthalenes and cis-bis(boryl)alkenes. X-ray crystallographic analysis revealed that bis-BN-naphthalene-fused oxepins feature a planar framework. The electronic and photophysical properties of the novel BN-naphthalene-fused oxepins were investigated by UV-vis and fluorescence spectroscopies and density functional theory (DFT) calculations, which disclosed the distinct electronic and photophysical properties of the analogous hydrocarbon system. Interestingly, dual-fluorescent emissions were observed upon dissolving N-substituted derivatives 10-14 in dimethyl sulfoxide. Tunable emission colors especially for white-light emissions can be achieved by controlling the ratio of solvents, concentration, or temperature using only a single-molecule compound.

Controllable syntheses of B/N anionic aminoborane chain complexes by the reaction of NH3BH3 with NaH and the mechanistic study

B/N anionic aminoborane linear or branched chain complexes, [BH3(NH2BH2)nH]- (n = 1, 2, and 3) and [BH(NH2BH3)3]-, have been synthesized through the controlled reactions of ammonia borane (NH3BH3) with NaH by adjusting the reactant ratios and reaction temperatures. The possible reaction mechanisms were elucidated based on experimental and theoretical studies.

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.

BN Polystyrenes: Emerging Optical Materials and Versatile Intermediates

BN polystyrenes are an emerging class of polyolefins functionalized with aromatic side chains in which at least one CC bond is replaced with a BN bond. This class of structures exhibits unusual photophysical properties relative to organic polymers. BN polystyrenes serve as intermediates in the preparation of functional polymers, including stereoregular polar polyolefins. The consequences of BN for CC bond substitution on reactivity and properties are highlighted.

Unravelling a general mechanism of converting ionic B/N complexes into neutral B/N analogues of alkanes: Hδ+⋯Hδ– dihydrogen bonding assisted dehydrogenation

Different mechanisms for [NH₃BH₂NH₃]⁺[BH₄]⁻ → NH₃BH₃ + [NH₂BH₂]_n and [NH₄]⁺[BH₃NH₂BH₃]⁻ → NH₃BH₂NH₂BH₃ have been elucidated, which leads to the proposal of a general mechanism as below.

Phosphorus-Containing Block Copolymers from the Sequential Living Anionic Copolymerization of a Phosphaalkene with Methyl Methacrylate

Although living polymerization methods are widely applicable to organic monomers, their application to inorganic monomers is rare. For the first time, we show that the living poly(methylenephosphine) (PMP_n ^- ) anion can function as a macroinitiator for olefins. Specifically, the phosphaalkene, MesP=CPh₂ (PA), and methyl methacrylate (MMA) can be sequentially copolymerized using the BnLi-TMEDA initiator system in toluene. A series of PMP_n -b-PMMA_m copolymers with narrow dispersities are accessible (Đ=1.05-1.10). Analysis of the block copolymers provided evidence for -P-CPh₂ -CH₂ -CMe(CO₂ Me)- switching groups. Importantly, this indicates that the -P-CPh₂ ^- anion directly initiates the anionic polymerization of MMA and stands in stark contrast to the isomerization mechanism followed for the homopolymerization of PA. For the first time, the glass transition of a PMP_n homopolymer has been measured (T_g =45.1 °C, n=20). The PMP_n -b-PMMA_m copolymers do not phase separate and show a single T_g which increases with higher PMMA content.

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.

Cascade Dehydrogenative Hydroboration for the Synthesis of Azaborabenzofulvenes

Tandem dehydrogenative hydroboration has been established to be highly effective in the synthesis of BN isosteres of benzofulvene and derivatives. The scope of this synthetic method is applicable to a variety of substrates. Spectroscopic and computational studies indicate that the new azaborabenzofulvenes have similar electronic properties as their carbonaceous analogues.

Catalyst-free dehydrocoupling of amines, alcohols, and thiols with pinacol borane and 9-borabicyclononane (9-BBN)

Contrary to recent reports, the dehydrocoupling of pinacol borane and 9-borabicyclononane with a variety of amines, alcohols and thiols can be achieved under mild conditions without catalyst. This process involves the formation of Lewis acid-base adducts featuring a hydridic B-H in close proximity to an acidic Nu-H.

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.

Strained azabora[2]ferrocenophanes

The first BN-bridged [2]ferrocenophanes were synthesized and their structures and strain enthalpies were investigated.

Catalytic B-C Coupling by Si/B Exchange: A Versatile Route to π-Conjugated Organoborane Molecules, Oligomers, and Polymers

Conjugated organoboranes have emerged as attractive hybrid materials for optoelectronic applications. Herein, a highly efficient, environmentally benign catalytic B-C bond formation method is presented that uses organosilicon compounds, dibromoboranes, and the metal-free organocatalyst Me₃SiNTf₂. This Si/B exchange approach has been successfully applied to the synthesis of arylborane molecules 4a-c, oligomers 8a,b, and polymers 8a',b'. Photophysical investigations, supported by TD-DFT calculations, reveal highly effective π-conjugation in thienyl- and furylborane species; the latter are also highly emissive.