The role of 2,3-dihydro-1-H-1,3,2-diazaboroles in luminescent molecules

Synthesis, reduction and C-H activation chemistry of azaborinines with redox-active organoboryl substituents

The 2,3,4,5,6-pentaphenyl-1,2-azaborinin-1-yl (PPAB) potassium complex 1 undergoes facile salt metathesis with 9,10-dibromo-9,10-dihydroboraanthracene (DBABr2), 5-bromodibenzo[b,d]borole (DBBBr), 1-chlorotetraphenylborole (TPBCl) and dibromo(phenyl)borane (BBr2Ph) to yield the corresponding N-borylated azaborinines N-DBABr-PPAB (2, which hydrolyses and dimerises to the oxo-bridged N,N'-O(DBA)2-(PPAB)2, 3), N,N'-DBA-(PPAB)2 (4), N-DBB-PPAB (5), N-PPB-PPAB (7) and N-BBrPh-PPBA (9). Stepwise reduction of 4 yields the corresponding stable radical anion 4˙- and dianion 42-. One-electron reduction of 5 with KC8 yields the purple radical anion 5˙-, which forms a highly insoluble coordination polymer. 5˙- undergoes very slow radical intramolecular ortho-C-H activation at the C4-phenyl substituent of the PPAB moiety, yielding a BN-analogue of the 5,5'-spiro-bi[dibenzoborole] anion, [6]K. Compound 7 cannot be isolated and undergoes spontaneous and diastereoselective 2,5-anti-addition of the ortho-C-H bond of the PPAB C4-phenyl substituent to yield a novel BNB-analogue of the triply fused dihydrocyclopenta[l]phenanthrene cation, compound 8. Finally the one-electron reduction of 9 results in the ortho-C-H activation of the PPAB C4-phenyl substituent at an in situ-generated dicoordinate boryl anion (10), resulting in the formation of a BNB-analogue of 9H-fluorene, the borate 11-. DFT calculations provide a rationale for the diverse C-H activations observed in these reactions.

Boryl-substituted low-valent heavy group 14 compounds

Low valent group 14 compounds exhibit diverse structures and reactivities. The employment of diazaborolyl anions (NHB anions), isoelectronic analogues to N-heterocyclic carbenes (NHCs), in group 14 chemistry leads to the exceptional structures and reactivity. The unique combination of σ-electron donation and pronounced steric hindrance impart distinct structural characteristics to the NHB-substituted low valent group 14 compounds. Notably, the modulation of the HOMO-LUMO gap in these compounds with the diazaborolyl substituents results in novel reaction patterns in the activation of small molecules and inert chemical bonds. This review mainly summarizes the recent advances in NHB-substituted low-valent heavy Group 14 compounds, emphasizing their synthesis, structural characteristics and application to small molecule activation.

Acid/Base-Free Acyclic Anionic Oxoborane and Iminoborane Bearing Diboryl Groups

Anionic oxoboranes and neutral iminoboranes, which are isoelectronic to ketones and alkynes, respectively, have attracted much attention because of their unique structures and various reactivity. However, acid/base-free oxoboranes and iminoboranes are still limited, and readily accessible examples with diverse electronic and steric characteristics are highly desirable. Herein, we report the first syntheses of the acyclic anionic oxoborane 2 and iminoborane 4 bearing two boryl ligands, both of which are acid/base-free. Spectroscopic analysis, X-ray crystallography, and theoretical calculations reveal that 2 and 4 possess a polarized terminal B═O double bond and central B≡N triple bond, respectively.

A Benzodiphosphaborolediide

The first example of a diphosphaborolediide, the benzo-fused [C₆ H₄ P₂ BPh]^2- (1^2- ), is prepared from ortho-bis(phosphino)benzene (C₆ H₄ {PH₂ }) and dichlorophenylborane, via a sequential lithiation approach. The dilithio-salt can be obtained as an oligomeric THF solvate or discrete TMEDA adduct, both of which are fully characterized, including by X-ray diffraction. Alongside NICS calculations, data strongly suggest some aromaticity within 1^2- , which is further supported by preliminary coordination studies that demonstrate η⁵ -coordination to a zerovalent molybdenum center, as observed crystallographically for the oligomeric [{Mo(CO)₃ (η⁵ -1)}{μ-η¹ -Mo(CO)₃ (TMEDA)}₂ ] ⋅ [μ-Li(THF)][μ-Li(TMEDA)].

Construction of 1,2,3-Benzodiazaborole by Electrophilic Borylation of Azobenzene and Nucleophilic Dialkylative Cyclization

1,2,3-Benzodiazaboroles can be conveniently prepared from azobenzenes by a two-step protocol involving electrophilic ortho-borylation with BBr₃ and dialkylative cyclization with the Grignard reagent. The methodology provides a diverse range of products equipped with functionalities from azobenzenes containing substituents (Me, t-Bu, F, Cl, Br, I, and OCF₃) and a series of Grignard reagents (alkyl- and arylmagnesium reagents). Moreover, this study displays the moderate aromaticity of the B-N-N-containing five-membered ring and mechanistic investigations of the cyclization reaction.

Diazaboryl-naphthyl-ketone: A New Scaffold with Bright Fluorescence, Aggregation-Induced Emission, and Application in the Quantitation of Trace Boronic Acids in Drug Intermediates

This study describes the synthesis, structure, and photophysical properties of a new luminescent polyaromatic boronic acid scaffold, diazaboryl-naphthyl-ketones (DNKs). These stable compounds display extremely bright fluorescence, aggregation-induced emission, positive solvatochromism, and solid-state fluorescence. DFT calculations and X-ray crystallographic study revealed notable electronic and structural differences between these compounds and the parent diaminonaphthalene (DAN) adducts. Acylation of the DAN system causes a localization of both HOMO and LUMO onto the DNK unit, which validates the negligible influence of the B-aryl substituent. The LUMO energy is lowered, and its shape significantly altered. Photophysical data in solution and the solid state revealed blue-shifted, narrowed, and intense emissions for DNKs (up to 89 % quantum yield). The potential utility of the fluorogenic DNK system was demonstrated with a proof-of-concept for the determination of trace boronic acid contaminants in solid samples, down to one-ppm level, using HPLC with fluorescence detection. This method could be useful in pharmaceutical development for the quantitation of difficult-to-detect and potentially mutagenic residual boronic acid from late cross-coupling reactions in drug syntheses.

Syntheses and structures of benzo-bis(1,3,2-diazaboroles) and acenaphtho-1,3,2-diazaboroles

Colorless crystalline 2,6-dibromo-4,8-dimethyl-1,3,5,7-tetraphenylbenzobis(diazaborole) 4 resulted from the cyclocondensation of 3,6-dimethyl-1,2,4,5-tetraphenylaminobenzene 3d with two equivalents of boron tribromide in the presence of calcium hydride. Synthesis of the dark-red crystalline 2-bromo-N,N'-bis(diisopropylphenyl)acenaphtho-1,3,2-diazaborole 7 was effected by the cyclocondensation of 1,2-bis(N-2',6'-diisopropylphenylimino)acenaphthene (5) and boron tribromide with subsequent sodium amalgam reduction of the initially formed burgundy red diazaborolium salt 6. Compounds 4, 6 and 7 are characterised by elemental analyses, ¹H, ¹¹B and ¹³C NMR spectroscopy, as well as by single X-ray diffraction studies. The electronic structures of 4, 6 and 7 are subject to DFT calculations.

Near-Infrared Quadrupolar Chromophores Combining Three-Coordinate Boron-Based Superdonor and Superacceptor Units

Herein, two new quadrupolar acceptor-π-donor-π-acceptor (A-π-D-π-A) chromophores have been prepared featuring a strongly electron-donating diborene core and strongly electron-accepting dimesitylboryl (BMes₂ ) and bis(2,4,6-tris(trifluoromethyl)phenyl)boryl (B^F Mes₂ ) end groups. Analysis of the compounds by NMR spectroscopy, X-ray crystallography, cyclic voltammetry, and UV/Vis-NIR absorption and emission spectroscopy indicated that the compounds have extended conjugated π-systems spanning their B₄ C₈ cores. The combination of exceptionally potent π-donor (diborene) and π-acceptor (diarylboryl) groups, both based on trigonal boron, leads to very small HOMO-LUMO gaps, resulting in strong absorption in the near-IR region with maxima in THF at 840 and 1092 nm and very high extinction coefficients of ca. 120 000 m^-1  cm^-1 . Both molecules also display weak near-IR fluorescence with small Stokes shifts.

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.

Synthesis and reactions of 4H-1,4-telluraborine

The 4H-1,4-telluraborine is a shown to be a versatile reagent undergoing both protonolysis and hydroboration, affording a new range of 1,4-telluraborine derivatives.

Catalyst free boron carbon bond cleavage and facile formation of five-membered PNBCC heterocycles

The [3 + 2] cycloaddition reaction of phosphanyl aminoborane [N(2,6-iPr₂C₆H₃)(PPh₂)(BCy₂)] (1) with activated alkynes led to boron and phosphorus containing five-membered heterocycles [(2,6-iPr₂C₆H₃)NPPh₂(CO₂R)C-C(Cy)(CO₂R)(BCy)] [R = Me (2), Et (3) and H (4)] with facile cleavage of the B-C bond and concomitant formation of a P-C bond with an ylidic character. DFT calculations indicate that 1 can be considered as a non-conjugated 1,3-dipole having two reaction centers viz., a nucleophilic P-center and an electrophilic B-center. The reaction of 1 with the alkynes proceeds through a stepwise dipolar addition mechanism, followed by the migration of the cyclohexyl group from the B-atom to the adjacent C-atom.

Electronic structure of BN-aromatics: Choice of reliable computational tools

The importance of having reliable calculation tools to interpret and predict the electronic properties of BN-aromatics is directly linked to the growing interest for these very promising new systems in the field of materials science, biomedical research, or energy sustainability. Ionization energy (IE) is one of the most important parameters to approach the electronic structure of molecules. It can be theoretically estimated, but in order to evaluate their persistence and propose the most reliable tools for the evaluation of different electronic properties of existent or only imagined BN-containing compounds, we took as reference experimental values of ionization energies provided by ultra-violet photoelectron spectroscopy (UV-PES) in gas phase-the only technique giving access to the energy levels of filled molecular orbitals. Thus, a set of 21 aromatic molecules containing B-N bonds and B-N-B patterns has been merged for a comparison between experimental IEs obtained by UV-PES and various theoretical approaches for their estimation. Time-Dependent Density Functional Theory (TD-DFT) methods using B3LYP and long-range corrected CAM-B3LYP functionals are used, combined with the ΔSCF approach, and compared with electron propagator theory such as outer valence Green's function (OVGF, P3) and symmetry adapted cluster-configuration interaction ab initio methods. Direct Kohn-Sham estimation and "corrected" Kohn-Sham estimation are also given. The deviation between experimental and theoretical values is computed for each molecule, and a statistical study is performed over the average and the root mean square for the whole set and sub-sets of molecules. It is shown that (i) ΔSCF+TDDFT(CAM-B3LYP), OVGF, and P3 are the most efficient way for a good agreement with UV-PES values, (ii) a CAM-B3LYP range-separated hybrid functional is significantly better than B3LYP for the purpose, especially for extended conjugated systems, and (iii) the "corrected" Kohn-Sham result is a fast and simple way to predict IEs.

2,6-Bis(diazaboryl)pyridine: A Superbasic Sterically Demanding Pyridine Ligand

Herein a convenient one-pot route to a sterically demanding superbasic pyridine is presented. Functionalization of the 2- and 6-positions with the strongly σ-donating boryl-groups shifts the calculated gas phase basicity of the pyridine nitrogen atom to 1012 kJ mol^-1 , which outperforms the "proton sponge" 1,8-bis(dimethylamino)naphthalene (996 kJ mol^-1 ). The diazaboryl groups are oriented orthogonally to the pyridine ring and do not block the N-position, which resembles the geometry of commonly used N-heterocyclic carbenes. This allows the substituted pyridine to be used as a neutral N-donor ligand in coordination chemistry that is demonstrated herein with the Lewis adducts of haloboranes. Contrary to NHCs, which can form extraordinarily stable adducts, the pyridine ligand is intended to act as a weaker-coordinating alternative and could allow for alternative ligand chemistry.

Facile access to unsymmetrically substituted tellurium–boron based heterocycles

Protonolysis and alkyne exchange reactions are used to give unsymmetrically substituted Te–B based heterocycles.

Neutral Diboron Analogues of Archetypal Aromatic Species by Spontaneous Cycloaddition

Among the numerous routes organic chemists have developed to synthesize benzene derivatives and heteroaromatic compounds, transition-metal-catalyzed cycloaddition reactions are the most elegant. In contrast, cycloaddition reactions of heavier alkene and alkyne analogues, though limited in scope, proceed uncatalyzed. In this work we present the first spontaneous cycloaddition reactions of lighter alkene and alkyne analogues. Selective addition of unactivated alkynes to boron-boron multiple bonds under ambient conditions yielded diborocarbon equivalents of simple aromatic hydrocarbons, including the first neutral 6 π-aromatic diborabenzene compound, a 2 π-aromatic triplet biradical 1,3-diborete, and a phosphine-stabilized 2 π-homoaromatic 1,3-dihydro-1,3-diborete. DFT calculations suggest that all three compounds are aromatic and show frontier molecular orbitals matching those of the related aromatic hydrocarbons, C6 H6 and C4 H4 (2+) , and homoaromatic C4 H5 (+) .

Synthesis of a Naphthodiazaborinine and Its Verification by Planarization with Atomic Force Microscopy

Aiming to study new motifs, potentially active as functional materials, we performed the synthesis of a naphthodiazaborinine (the BN isostere of the phenalenyl anion) that is bonded to a hindered di-ortho-substituted aryl system (9-anthracene). We used atomic force microscopy (AFM) and succeeded in both the verification of the original nonplanar structure of the molecule and the planarization of the skeleton by removing H atoms that cause steric hindrance. This study demonstrated that planarization by atomic manipulation is a possible route for extending molecular identification by AFM to nonplanar molecular systems that are difficult to probe with AFM directly.