BN versus CC: How Similar Are They?

Synthesis, Characterization, and Functionalization of 1-Boraphenalenes

1-Boraphenalenes have been synthesized by reaction of BBr3 with 1-(aryl-ethynyl)naphthalenes, 1-ethynylnaphthalene, and 1-(pent-1-yn-1-yl)naphthalene and they can be selectively functionalized at boron or carbon to form bench-stable products. All of these 1-boraphenalenes have LUMOs localized on the planar C12 B core that are closely comparable in character to isoelectronic phenalenyl cations. In contrast to the comparable LUMOs, the aromatic stabilization of the C5 B ring in 1-boraphenalenes is dramatically lower than the C6 rings in phenalenyl cations. This is due to the occupied orbitals of π symmetry being less delocalised in the 1-boraphenalenes.

Identifying (BN)2-pyrenes as a New Class of Singlet Fission Chromophores: Significance of Azaborine Substitution

Singlet fission converts one photoexcited singlet state to two triplet excited states and raises photoelectric conversion efficiency in photovoltaic devices. However, only a handful of chromophores have been known to undergo this process, which greatly limits the application of singlet fission in photovoltaics. We hereby identify a recently synthesized diazadiborine-pyrene ((BN)₂-pyrene) as a singlet fission chromophore. Theoretical calculations indicate that it satisfies the thermodynamics criteria for singlet fission. More importantly, the calculations provide a physical chemistry insight into how the BN substitution makes this happen. Both calculation and transient absorption spectroscopy experiments indicate that the chromophore has a better absorption than pentacene. The convenient synthesis pathway of the (BN)₂-pyrene suggests an in situ chromophore generation in photovoltaic devices. Two more (BN)₂-pyrene isomers are proposed as singlet fission chromophores. This study sets a step forward in the cross-link of singlet fission and azaborine chemistry.

9,10-Azaboraphenanthrene-containing small molecules and conjugated polymers: synthesis and their application in chemodosimeters for the ratiometric detection of fluoride ions

The introduction of main group elements into conjugated scaffolds is emerging as a key route to novel optoelectronic materials. Herein, an efficient and versatile way to synthesize polymerizable 9,10-azaboraphenanthrene (BNP)-containing monomers by aromaticity-driven ring expansion reactions between highly antiaromatic borafluorene and azides is reported, and the corresponding conjugated small molecules and polymers are developed as well. The BNP-containing small molecules and conjugated polymers showed good air/moisture stability and notable fluorescence properties. Addition of fluoride ions to the BNP-based small molecules and polymers induced a rapid change in the emission color from blue to green/yellow, respectively, accompanied by strong intensity changes. The conjugated polymers showed better ratiometric sensing performance than small molecules due to the exciton migration along the conjugated chains. Further experiments showed that the sensing process is fully reversible. The films prepared by solution-deposition of BNP-based compounds in the presence of polycaprolactone also showed good ratiometric sensing for fluoride ions.

The Dewar Isomer of 1,2-Dihydro-1,2-azaborinines: Isolation, Fragmentation, and Energy Storage

The photochemistry of 1,2-dihydro-1,2-azaborinine derivatives was studied under matrix isolation conditions and in solution. Photoisomerization occurs exclusively to the Dewar valence isomers upon irradiation with UV light (>280 nm) with high quantum yield (46 %). Further photolysis with UV light (254 nm) results in the formation of cyclobutadiene and an iminoborane derivative. The thermal electrocyclic ring-opening reaction of the Dewar valence isomer back to the 1,2-dihydro-1-tert-butyldimethylsilyl-2-mesityl-1,2-azaborinine has an activation barrier of (27.0±1.2) kcal mol-1 . In the presence of the Wilkinson catalyst, the ring opening occurs rapidly and exothermically (ΔH=(-48±1) kcal mol-1 ) at room temperature.

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.

An organoboron compound with a wide absorption spectrum for solar cell applications

Organoboron compounds offer new approaches to tune the electronic structures of π-conjugated molecules. In this work, an electron acceptor (M-BNBP4P-1) is developed by endcapping an organoboron core unit with two strong electron-withdrawing groups. M-BNBP4P-1 exhibits a unique wide absorption spectrum with two strong absorption bands in the long wavelength region (λ_max = 771 nm) and the short wavelength region (λ_max = 502 nm), which indicate superior sunlight harvesting capability. This is due to its special electronic structure, i.e. a delocalized LUMO and a localized HOMO. Prototype solution-processed organic solar cells based on M-BNBP4P-1 show a power conversion efficiency of 7.06% and a wide photoresponse from 350 nm to 880 nm.

Borenium and boronium ions of 5,6-dihydro-dibenzo[c,e][1,2]azaborinine and the reaction with non-nucleophilic base: trapping of a dimer and a trimer of BN-phenanthryne by 4,4′-di-tert-butyl-2,2′-bipyridine

6-Chloro-5,6-dihydro-dibenzo[c,e][1,2]azaborinine (1) reacts with pyridine derivatives to borenium or boronium ions. The boronium ion obtained from reaction with an excess of pyridine could be characterized structurally and transforms into the borenium ion in solution. The reaction of 1 with 2,2′-bipyridine (a) and derivatives (b: 6,6′-dimethyl; c: 4,4′-di-tert-butyl) results in spirocyclic boronium ions 8a–c of which 8b could be characterized by X-ray crystallography. Despite the chelate effect, the spirocyclic boronium ions readily undergo hydrolysis or alcoholysis. Treatment of the spirocyclic boronium ion 8c with potassium hexamethyl disilazide (KHMDS) results in neutral products that are monomers, dimers, or trimers of dibenzo[c,e][1,2]azaborinine (“BN-phenanthryne”) trapped with 4,4′-di-tert-butyl-2,2′-bipyridine by formation of B–C, B–N, or dative B–N bonds, indicative of deprotonation of NH and CH bonds of the boronium ion by KHMDS.

The State of the Art in Azaborine Chemistry: New Synthetic Methods and Applications

Boron-nitrogen heteroarenes hold great promise for practical application in many areas of chemistry. Enduring interest in realizing this potential has in turn driven perennial innovation with respect to these compounds' synthesis. This Perspective discusses in detail the most recent advances in methods pertaining to the preparation of BN-isosteres of benzene, naphthalene, and their derivatives. Additional focus is placed on the progress enabled by these syntheses toward functional utility of such BN-heterocycles in biochemistry and pharmacology, materials science, and transition-metal-based catalysis. The prospects for future research efforts in these and related fields are also assessed.

Divergent reactivity of nucleophilic 1-bora-7a-azaindenide anions

The reactions of 1-bora-7a-azaindenide anions, prepared in moderate to excellent yields by reduction of the appropriate 1-bora-7a-azaindenyl chlorides with KC₈ in THF, with alkyl halides and carbon dioxide were studied. With alkyl halides (CH₂Cl₂, CH₃I and BrCH(D)CH(D)^tBu), the anions behave as boron anions, alkylating the boron centre via a classic S_N2 mechanism. This was established with DFT methods and via experiments utilizing the neo-hexyl stereoprobe BrCH(D)CH(D)^tBu. These reactions were in part driven by a re-aromatization of the six membered pyridyl ring upon formation of the product. Conversely, in the reaction of the 1-bora-7a-azaindenide anions with CO₂, a novel carboxylation of the C-2 carbon alpha to boron was observed. Computations indicated that while carboxylation of the boron centre was kinetically feasible, the products of B-carboxylation were not thermodynamically favored relative to the observed C-2 carboxylated species, which were formed preferably due to the generation of both C-C and B-O bonds. In these products, the pyridyl ring remains non-aromatic, in part accounting for the observed reversibility of carboxylation.

Intermolecular insertion reactions of azides into 9-borafluorenes to generate 9,10-B,N-phenanthrenes

The reactions of 9-borafluorenes with organic azides result in either the insertion of the α-nitrogen and elimination of N₂ or the insertion of the γ-nitrogen to generate the corresponding phenanthrene analogues with boron and nitrogen in the 9- and 10-positions, respectively.

Synthesis of functionalized helical BN-benzo[c]phenanthrenes

A novel parent BN-benzo[c]phenanthrene, with helical chirality and remarkable structural features, has been easily obtained in three steps with a global yield of 55%.

Mono BN-substituted analogues of naphthalene: a theoretical analysis of the effect of BN position on stability, aromaticity and frontier orbital energies

An insight into the electronic structure changes driven by CC → BN substitution in a naphthalene system has been given by quantum chemical calculations.

Electronic and structural properties of N → B-ladder boranes with high electron affinity

A series of electronically and structurally diverse N → B-ladder boranes has been prepared by hydroboration.

SE2 reaction in noncarbon system: Metal-halide catalysis for dehydrogenation of ammonia borane

An electrophilic substitution (SE) reaction of BN isosteres has been investigated for the dehydrogenation of ammonia borane (AB) by metal chlorides (MCl2) using various ab initio calculations. In contrast to the typical SE reaction occurring at the carbon atom, the nitrogen atom in AB serves as the reaction center for the SE reaction with the boron moiety as the leaving group when the MCl2 approaches the AB. The SE2 backside reaction is favored as a trigger step for the dehydrogenation of AB by the MCl2 The SE2 reaction is found for 3d-transition-metal chlorides (e.g., FeCl2, CoCl2, NiCl2, CuCl2, and ZnCl2), while PdCl2 leads to the dehydrogenation of AB by a direct B-H σ-bond activation, similar to most organometallic catalysts. Interestingly, the polymerization of AB promoted by MCl2 can be explained with the similar SE2 mechanism, and the dehydrogenation of the BN derivative 3-methyl-1,2-BN-cyclopentane (CBN) bearing a carbon backbone ring also follows the SE2 reaction. In particular, the experimental observation that the use of metal-chloride catalysis decreases the by-products obtained during the hydrogenation of AB can be explained by our mechanism involving the SE2 reaction. This work is helpful for the development of novel metal-halide catalysts for practical hydrogen storage materials, including the BN moiety.

Depolymerization of Poly(phosphinoboranes): From Polymers to Lewis Base Stabilized Monomers

We report on depolymerization reactions of poly(phosphinoboranes). The cleavage of the polymers [H₂ PBH₂ ]_n (2 a), [tBuHPBH₂ ]_n (2 c), [PhHPBH₂ ]_n (2 e) and the oligomer [Ph₂ PBH₂ ]_n (2 b), with strong Lewis bases (LBs), in particular with NHCs, leads to the corresponding monomeric phosphanylboranes R¹ R² PBH₂ LB. It is observed that the depolymerization depends on the strength and stability of the LBs as well as on the substitution pattern of the poly(phosphinoboranes). The solid state structures of the monomeric phosphinoboranes H₂ PBH₂ NHC^Me (NHC=N-heterocyclic carbene) (4 a), H₂ PBH₂ NHC^dipp (5 a) and tBuHPBH₂ NHC^Me (4 c) were determined. DFT calculations support the experimentally observed reaction behavior.

A modular route to boron doped PAHs by combining borylative cyclisation and electrophilic C-H borylation

Heteroatom doping into polyaromatic hydrocarbons (PAHs) is a powerful approach for modifying key physical properties, however, there are extremely few modular routes that enable facile formation of B-, B2- and B,N-(specifically not containing direct B-N bonds) doped PAHs despite the growing importance of these materials. Sequential, one pot borylative cyclisation/intramolecular electrophilic C-H borylation of naphthyl-alkynes provides a simple new route to access novel B-, B,N- and B2-doped (PAHs). The initial products, dihydronaphthalene/dihydroquinoline B-mesityl PAHs, were reacted with [Ph3C][BF4]/pyridyl base to form the oxidised B-, and B,N-doped PAHs. However, for B-triisopropylphenyl (Trip) PAH congeners oxidation has to be performed prior to Trip installation due to preferential oxidation of an isopropylaryl moiety to the styrene. This alternative sequence enables access to Trip-B-PAHs and to structurally constrained B and B2-PAHs. Analysis of the solid state structures and optoelectronic properties of these PAHs confirm that frontier orbital energies, extended packing structures, Stokes shift and quantum yields all can be rationally modified using this methodology. The simplicity of this synthetic approach makes it a powerful tool for rapidly generating novel bench stable boron doped PAHs, which is important for facilitating further structure-property relationship studies and the wider utilisation of these materials in optoelectronic applications.

BN Doping and the Photochemistry of Polyaromatic Hydrocarbons: Photocyclization of Hexaphenyl Benzene and Hexaphenyl Borazine

Boron-nitrogen doping of polyaromatic hydrocarbon (PAH) materials can be used to tune their electronic properties while preserving the structural characteristics of pure hydrocarbons. Many multicycle PAHs can be synthesized photochemically; in contrast, very little is known about the photochemistry of their BN-doped counterparts. We present results of fs, ns, and μs time-resolved spectroscopic studies on the photoinduced dynamics of hexaphenyl benzene and hexaphenyl borazine in order to examine how BN doping alters photochemical C-C bond formation via 6π electrocyclization as well as the stability of resulting cyclized structures. Ultrafast measurements reveal photoinduced behaviors reflecting differences in excited-state decay pathways for the two molecules, with hexaphenyl borazine relaxing from its excited state with a rate that is 2 orders of magnitude faster than that of hexaphenyl benzene (3.0 vs 428 ps). Tetraphenyl dihydrotriphenylene generated from hexaphenyl benzene is observed to reopen with a ∼2 μs lifetime controlled by entropic stabilization of the cyclized structure; in contrast, photoinduced dynamics appear to be complete within 100 ps after excitation of hexaphenyl borazine. This significant difference in photochemical dynamics is reflected in the cyclodehydrogentation yields obtained for the two reactants (25 vs 0% for hexaphenyl benzene and borazine, respectively). Quantum-chemical computations predict that BN doping gives rise to energetic destabilization and increased singlet diradical character in cyclized structures. These findings indicate that the polarized BN bonds of the borazine core adversely impact photochemical bond formation relative to analogous hydrocarbons.

A Boron Protecting Group Strategy for 1,2-Azaborines

Upon reaction with either molecular oxygen or di-tert-butylperoxide in the presence of a simple copper(I) salt and an alcohol, a range of 1,2-azaborines readily exchange B-alkyl or B-aryl moieties for B-alkoxide fragments. This transformation allows alkyl and aryl groups to serve for the first time as removable protecting groups for the boron position of 1,2-azaborines during reactions that are not compatible with the easily modifiable B-alkoxide moiety. This reaction can be applied to synthesize a previously inaccessible BN isostere of ethylbenzene, a compound of interest in biomedical research. A sequence of epoxide ring opening using N-deprotonated 1,2-azaborines followed by an intramolecular version of the boron deprotection reaction can be applied to access the first examples of BN isosteres of dihydrobenzofurans and benzofurans, classes of compounds that are important to medicinal chemistry and natural product synthesis.

A DFT study on 1,4-dihydro-1,4-azaborinine annulated linear polyacenes: Absorption spectra, singlet-triplet energy gap, aromaticity, and HOMO-LUMO energy modulation

Linear polyacene (LPA) mimics containing multiple heterocycles have been computationally designed by annulating 1,4-dihydro-1,4-azaborinine moieties to benzene (aB₁ -aB₅ ), naphthalene (aN₁ -aN₅ ), anthracene (aA₁ -aA₅ ), and tetracene (aT₁ -aT₅ ) cores. DFT studies conducted on them using M06L/6-311++G(d,p) method reveal a perfect planar structure for all and suggest the utilization of nitrogen lone pairs for aromatic π-electron delocalization. The computed values of aromaticity indices such as HOMA, NICS, and dehydrogenation energy (E_dh ) of heterocycles support strong aromatic character for each six-membered ring in the LPA mimics. On the basis of the minimum value of the molecular electrostatic potential (V_min ) observed on each LPA unit in the LPA mimics, the extended delocalization of π-electrons is verified. The energetic parameter E_dh showed strong linear correlation with HOMA, NICS and V_min parameters, which strongly supports the multidimensional character of aromaticity in LPA mimics. The electronic property modification is shown by the theoretical absorption spectra data and singlet-triplet energy gap (ΔE_ST ). The bandgap and ΔE_ST tunings are achieved for LPA mimics by selecting appropriate number of azaborinine type units and the size of LPA core used for annulation. © 2017 Wiley Periodicals, Inc.

Crystalline boron-linked tetraaminoethylene radical cations

Boron-linked tetraaminoethylene radical cations has been isolated.

Single-electron oxidation of neutral boryl-linked tetraaminoethylene derivatives 4 led to the formation of radical cations 4˙⁺, which have been isolated and fully characterized. X-ray diffraction analysis, EPR spectroscopy, and computational studies revealed that the unpaired electron is delocalized over the B₂N₄C₂ skeleton and the spin density mainly resides on the carbon and boron atoms.

Excited-State Deactivation Pathways and the Photocyclization of BN-Doped Polyaromatics

Boron-nitrogen doping of polyaromatic hydrocarbons (PAH), such as borazine-core hexabenzocoronene, presents possibilities for tuning the properties of organic electronics and nanographene materials while preserving structural characteristics of pure hydrocarbons. Previous photochemical studies have demonstrated extension of a borazine-core PAH network (1,2:3,4:5,6-tris(o,o'-biphenylylene)borazine, 1) by photoinduced cyclodehydrogenation. We present steady-state and femtosecond-to-microsecond resolved spectroscopic studies of the photophysics of 1 and a related borazine-core PAH in order to characterize competing excited-state relaxation pathways that determine the efficacy of bond formation by photocyclization. Transient spectra evolve on time scales consistent with S₁ fluorescence lifetimes (1-3 ns) to features that persist onto microsecond time scales. Nanosecond-resolved oxygen-quenching measurements reveal that long-lived metastable states are triplets rather than cyclized products. Determination of fluorescence and triplet quantum yields reveal that photochemical bond formation is a minor channel in the relaxation of 1 (∼5% or less), whereas highly efficient fluorescence and intersystem crossing result in negligible photoinduced bond formation in more extended borazine-core networks. Results of computational investigations at the RICC2 level reveal sizable barriers to cyclization on the S₁ potential energy surfaces consistent with quantum yields deduced from experiment. Together these barriers and competing photophysical pathways limit the efficiency of photochemical synthesis of BN-doped polyaromatics.

Synthesis, Optical Properties, and Regioselective Functionalization of 4a-Aza-10a-boraphenanthrene

4a-Aza-10a-boraphenanthrene has been synthesized in only four steps from commercially available materials with a remarkable overall yield of 62%. In contrast to other BN-isosteres of phenathrene, this isomer is weakly fluorescent, which has been explained by means of computational studies that found a low energy conical intersection for the nonradiative deactivation of the excited state. Moreover, a completely regioselective functionalization of 4a-aza-10a-boraphenanthrene at C-1 by reaction with activated electrophiles has been achieved.

Scope of the Thermal Ring-Expansion Reaction of Boroles with Organoazides

Electronic and steric factors have been investigated in the thermal ring expansion of boroles with organic azides, a reaction that provides access to highly arylated 1,2-azaborinines, BN analogues of benzene. Reactions of a variety of boroles and organic azides demonstrate that the synthetic method is quite general in furnishing 1,2-azaborinines, but the respective reaction rates reveal a strong dependence on the substituents on the two reactants. The products have been characterized by UV/Vis, electrochemical, NMR, and X-ray diffraction methods, clarifying their constitutions and highlighting substituent effects on the electronic structure of the 1,2-azaborinines. Furthermore, analysis of several possible mechanistic pathways for 1,2-azaborinine formation, as studied by DFT, revealed that a two-step mechanism involving azide-borole adduct formation and nitrene insertion is favored.

The Least Stable Isomer of BN Naphthalene: Toward Predictive Trends for the Optoelectronic Properties of BN Acenes

The least stable isomer of the parental BN naphthalene series has been synthesized in a simple four-step sequence. Its experimental electronic structure characterization via UV-PES, cyclic voltammetry, and UV-vis spectroscopy in direct comparison with three other known BN naphthalene isomers has established two guiding principles for predicting the electronic structures of BN acene compounds: (1) Orientational BN isomers have similar HOMO-LUMO gaps. (2) For each pair of orientational BN isomers, the more thermodynamically stable compound has the lower HOMO energy. Furthermore, we demonstrate that BN/CC isosterism in the context of BN-9,1-Naph can impact crystal packing to favor a cofacial π-stack motif.