Engineering a Small HOMO-LUMO Gap and Intramolecular C−H Borylation by Diborene/Anthracene Orbital Intercalation

Aromatic 1,4,2,3-Diazadiborole Featuring an Unsymmetrical B=B Entity: A Versatile Synthon for Unusual Boron Heterocycles

The replacement of a CC unit with an isoelectronic BN unit in aromatic systems can give rise to molecules and materials with fascinating properties. We report here the synthesis, characterization, and reactivity of a 1,4,2,3-diazadiborole species, 2, featuring an unprecedented 6π-aromatic BN-heterocyclic moiety that is isoelectronic to cyclopentadienide (Cp-). Bearing an unsymmetrical B=B entity, 2 exhibits reactivity toward oxidants, protic reagents, electrophiles, and unsaturated substrates. This reactivity facilitates the synthesis of a variety of novel mono- and bicyclic organoboron derivatives through mechanisms including ring retention, cleavage/recombination, annulation, and expansion. These findings reveal innovative synthetic routes to BN-embedded aromatic compounds via desymmetrization, affording unique building blocks for synthetic chemistry.

Germaborene reactivity study - addition of carbon nucleophiles, cycloaddition reactions, coordination chemistry

MeNHC substituted germaborenium cation 2 was synthesized directly in reaction of bromo-substituted germaborene 1b with MeNHC. The adamantyl isonitrile substituted germaborenium cation 4 was obtained stepwise: substitution of the chloride atom against adamantyl isonitrile at the B-Cl unit in 1a, simultaneous migration of the chloride to the germanium atom followed by chloride abstraction using Na[BArF 4] gives the germaborenium cation 4. Substitution of the bromide atom in 1b against carbon monoxide followed by bromide abstraction using Ag[Al(OtBuF)4] leads to compound 6 exhibiting a B[double bond, length as m-dash]C double bond substituted at the boron atom by a germylium cation. Treating the germaborene [Ge[double bond, length as m-dash]B-Ph] (1c) with selenium, a cycloaddition product 7 was characterised featuring a GeBSe heterocycle. Carbon dioxide reacts with 1b to give a four membered ring molecule 8 as the product of a B-C and Ge-O bond formation. In reaction of 1b with dimethylbutadiene, a product 9 of a [2 + 4] cycloaddition was isolated. Transition metal fragments [Fe(CO)4 (10), CuBr (11), AuCl (12)] show coordination at the germaborene double bond. Molecular structures of the germaborene coordination compounds 10-12 are presented and the ligand properties are discussed. After treating the germaborene [Ge[double bond, length as m-dash]B-Br] (1b) with [Cp*Al]4, insertion of a Cp*Al moiety into the B-Br bond was found (13).

Unlocking Heteroaromatic Ring Systems through Chalcogen Insertion into Boroles

In this work, we report the reactivity of various annulated borole derivatives toward chalcogen (O, S, and Se) insertion. Among a series of 9-borafluorenes with different boron substituents (Ph, Br, or o-carboranyl) and a mixed thiophene-benzene-fused derivative, only the 9-o-carboranyl-substituted 9-borafluorene yielded the complete set of chalcogen-containing heteroarenes, including the first 1,2-selenaborinine derivative. To evaluate the aromaticity of this heterocyclic analogue of phenanthrene, nucleus-independent chemical shift (NICS) values were computed and compared to those of its lighter group 16 congeners.

Erasable, Rewritable, and Reprogrammable Dual Information Encryption Based on Photoluminescent Supramolecular Host-Guest Recognition and Hydrogel Shape Memory

Information encryption technologies are very important for security, health, commodity, and communications, etc. Novel information encryption mechanisms and materials are desired to achieve multimode and reprogrammable encryption. Here, a supramolecular strategy is demonstrated to achieve multimodal, erasable, reprogrammable, and reusable information encryption by reversibly modulating fluorescence. A butyl-naphthalimide with flexible ethylenediamine functionalized β-cyclodextrin (N-CD) is utilized as a fluorescent responsive ink for printing or patterning information on polymer brushes with dangling adamantane group grafted on responsive hydrogels. The photoluminescent naphthalimide moiety is bonded to β-CD and entrapped in the cavity. Its fluorescence is highly weakened in β-CD cavity and recovers after being expelled from the cavity by a competing guest molecule to emit bright green photoluminescence under UV. Experiments and theoretical calculations suggest π-π stacking and ICT as the primary mechanism for the naphthalimides assembly and fluorescence, which can be quenched through insertion of conjugated molecules and recover by removing the insert. Such reversible quenching and recovering are used to achieve repeated writing, erasing, and re-writing of information. Supramolecular recognition and hydrogel shape memory are further combined to achieve reversible dual-encryption. This study provides a novel strategy to develop smart materials with improved information security for broad applications.

Infinitene as two fused helicoidal trails of fused rings: evaluation of the magnetic behavior of [12]infinitene and anionic species displaying global aromaticity and antiaromaticity

The unique formation of an infinity-shaped carbon backbone made exclusively from fused benzene rings has recently been achieved. The structure of [12]infinitene can be viewed as two fused [6]helicene structures with a central crossover section, depicting a global aromatic behavior along with the overall structure, with deshielding regions along both helicoidal axes. In addition, the ¹³C-NMR characteristics are discussed. The formation of a cumulative region involving the shielding regions from the aromatic rings is depicted along with the overall aesthetically pleasant structural backbone, which is enhanced at the crossover section. For the evaluated dianionic counterpart, the structure shows a deshielding region above the fused-ring trail and a helicoidal shielding region, ascribed to a global antiaromatic counterpart. The aromaticity is recovered and enhanced at the tetranionic state. Thus, the neutral and tetranionic states are able to build up a long-ranged shielding region, given by the global aromatic behavior, with an enhanced shielding region at the center of the crossover section displaying π-π stacked rings.

Neutral Homoaromatic Diboradisilacyclobutene: Synthesis, Structure, and Reactivity

A formally neutral homoaromatic diboradisilacyclobutene was prepared and structurally characterized via the reduction of borylaminobromosilane with potassium graphite, which represents the first neutral boron and silicon mixed analogue of the homocyclopropenylium ion. Single-crystal X-ray analyses revealed a folded four-membered B₂Si₂ ring (26.2°) and a remarkably short transannular B-Si distance of 2.306(3) Å. DFT calculations showed that two π-electrons delocalize over the BSi₂ moiety. Both structural feature and DFT calculations show the significant 2π-homoaromatic nature of the diboradisilacyclobutene. Furthermore, diboradisilacyclobutene reacted readily with CuCl or HCl to afford the diboradisilacyclobutanes.

1,2-Dialkynyldiboranes(4): B-B versus CC bond reactivity

The reactivity of three 1,2-dialkynyl-1,2-diaminodiborane(4) derivatives, B₂(NMe₂)₂(CCR)₂ (R = H, Me, SiMe₃), towards small molecules known to react with both B-B and CC bonds was studied. With arylazides nitrene insertion into the B-B bond with concomitant loss of N₂ was kinetically favoured in all cases. While reactions with sterically unhindered hydroboranes proceeded unselectively, sterically encumbered dimesitylborane cleanly added to both alkynyl moieties, resulting in the first examples of 1,2-divinyldiboranes(4). In the presence of catalytic amounts of Pd/C room-temperature hydrogenation at 1 bar led to oxidative B-B bond cleavage and yielded the fully hydrogenated alkyl(amino)hydroborane products. These could be prevented from dimerising and isolated by complexation with an NHC ligand. Finally, stepwise halogenation of the B-B bond and the alkynyl groups afforded first the corresponding alkynyl(amino)haloboranes and then the amino(halo)(1,2-dihalovinyl)boranes.

Synthesis and hydrogenation of polycyclic aromatic hydrocarbon-substituted diborenes via uncatalysed hydrogenative B–C bond cleavage

In contrast to classical B–B bond-centred diborene hydrogenation, polycyclic aromatic hydrocarbon-substituted diborenes first undergo thermal intramolecular hydroarylation, followed by hydrogenation of the remaining B–B and endocyclic B–C bonds.

Amidinatoamidosilylene-Dibromodiborene

The amidinatoamidosilylene [LSiNMe₂] [1; L = PhC(NtBu)₂] was reacted with B₂Br₄(SMe₂)₂ in toluene at room temperature to form the bis(silylene)tetrabromodiborane [L{Me₂N}Si]₂B₂Br₄ (2). It was then reacted with excess KC₈ in tetrahydrofuran at room temperature to afford the bis(silylene)dibromodiborene [L{Me₂N}Si]₂B₂Br₂ (3).

NHC-Stabilized 1,2-Dihalodiborenes: Synthesis, Characterization, and Reactivity Toward Elemental Chalcogens

The 2-fold reduction of B₂X₄(NHC)₂ (X = Cl, Br, I; NHC = (un)saturated N-heterocyclic carbene) yields the corresponding green-colored 1,2-dihalodiborenes B₂X₂(NHC)₂, the ¹¹B NMR resonances of which are strongly upfield-shifted upon descending the halide group. The diborenes crystallize as the trans isomers, with relatively short B═B double bonds (1.513(9) to 1.568(4) Å). Cyclic voltammetric experiments with these diborenes reveal reversible one-electron oxidation processes to the corresponding diboron radical cation (E_1/2 = -1.16 to -1.50 V); the reducing power of B₂X₂(NHC)₂ increasing with the electronegativity of the halide and for the less π-accepting unsaturated NHCs. The main UV-vis absorption (393-463 nm), which corresponds mainly to a highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) transition, undergoes a blueshift upon descending the halide group and shows some dependence on the stereoelectronics of the NHC ligands. Computational analyses show that the HOMO of B₂X₂(NHC)₂ is mostly localized on the B═B π bond, with the contribution from halide p orbitals decreasing down the group, and the saturated NHCs affording some π-bonding delocalization over the B-C_NHC bonds. The calculated HOMO and LUMO energies decrease upon descending the halide group, while the HOMO-LUMO gap also decreases, correlating well with the cyclovoltammetry and UV-vis data. The reactions of B₂Br₂(NHC)₂ with elemental sulfur and red selenium lead to the formation of the corresponding diborathiiranes and seleniranes, respectively, which were characterized by NMR and UV-vis spectroscopy, cyclic voltammetry, and X-ray diffraction analyses. In one case, an additional one-electron oxidation yields a unique cyclic B₂Se radical cation. Computational analyses show that the localization of the HOMO and HOMO - 1 of the diboraseleniranes is inverted compared to the diborathiiranes.

A N-Phosphinoamidinato NHC-Diborene Catalyst for Hydroboration

The use of the N-phosphinoamidinato NHC-diborene catalyst 2 for hydroboration is described. The N-phosphinoamidine tBu₂PN(H)C(Ph)═N(2,6-iPr₂C₆H₃) was reacted with nBuLi in Et₂O to afford the lithium derivative, which was then treated with B₂Br₄(SMe₂)₂ in toluene to form the N-phosphinoamidinate-bridged diborane 1. It was reacted with the N-heterocyclic carbene IMe (:C{N(CH₃)C(CH₃)}₂) and excess potassium graphite at room temperature in toluene to give the N-phosphinoamidinato NHC-diborene compound 2. It can stoichiometrically activate ammonia-borane and carbon dioxide. It also showed catalytic capability. A 2 mol % portion of 2 catalyzed the hydroboration of carbon dioxide (CO₂) with pinacolborane (HBpin) in deuterated benzene (C₆D₆) at 110 °C (conversion >99%), which afforded the methoxyborane [pinBOMe] (yield 97.8%, TOF 33.3 h^-1) and the bis(boryl) oxide [(pinB)₂O]. In addition, 5 mol % of 2 catalyzed the N-formylation of secondary and primary amines by carbon dioxide and pinacolborane to yield the N-formamides (average yield 91.6%, TOF 25.9 h^-1). Moreover, 2 showed chemoselectivity toward catalytic hydroboration of carbonyl compounds. In mechanistic studies, the B═B double bond in compound 2 activated the substrates, the intermediates of which then underwent hydroboration with pinacolborane to yield the products and regenerate catalyst 2.

Metal-Free White Light-Emitting Fluorescent Material Based on Simple Pillar[5]arene-tripodal Amide System and Theoretical Insights on Its Assembly and Fluorescent Properties

The booming of host-guest assembly-based supramolecular chemistry provides abundant ways to construct functional systems and materials. Attracted by the important application prospect of white light emission and aggregation-induced emission (AIE) materials, herein, we report an efficient way for fabricating metal-free white light-emitting AIE materials through the supramolecular assembly of simple organic compounds: methoxyl pillar[5]arene (MP5) and tri-(pyridine-4-ylamido)benzene (TAP). By host-guest assembly, MP5 and TAP formed a supramolecular polymer (MP5-T); meanwhile, the MP5-T xerogel powder emitted white light at CIE coordinates (0.29 and 0.29). The supramolecular assembly and white light-emitting mechanisms were carefully investigated by experiments as well as quantum chemical calculations including density functional theory (DFT), reduced density gradient, electrostatic surface potential, independent gradient model, and frontier molecular orbital (highest-occupied molecular orbital-lowest-unoccupied molecular orbital) analyses. Interestingly, according to the experiments and calculations, the supramolecular assembly is critical in the white light-emitting phenomenon. Moreover, in this work, the quantum chemical calculations could not only support experimental phenomena but also provide deep understanding and visualized presentation of the assembly and emission mechanism. In addition, the obtained MP5-T solid powder could serve as a novel and easy means to make material for white light-emitting devices.

Bond-Strengthening Backdonation in Aminoborylene-Stabilized Aminoborylenes: At the Intersection of Borylenes and Diborenes

Singly NHC-coordinated (aminoboryl)aminoborenium salts react with Na₂ [Fe(CO)₄ ] to yield stable coordination complexes of aminoborylene-stabilized aminoborylenes, which exhibit exceptional σ-donor properties. Upon photolytic CO extrusion from the metal center, the diboron ligand adopts a novel η³ -BBN coordination mode, where bond-strengthening backdonation from the metal center into the vacant B-B π-orbital is observed. This bonding situation can be alternatively described as a Fe-diaminodiborene complex. In a related reduction of CAAC-stabilized (aminoboryl)aminoborenium with KC₈ , the reduced species can be captured with nucleophiles to form three-coordinate (diaminoboryl)borylenes, where both amino groups have migrated to the distal boron atom. Collectively, these reactions illustrate the isomeric flexibility imparted by amino groups on this reduced diboron system, thus opening multiple avenues of novel reactivity.

Reactivity of a Tetra(o-tolyl)diborane(4) Dianion as a Diarylboryl Anion Equivalent

Lithium and magnesium salts of tetra(o-tolyl)diborane(4) dianion, having B=B double bond character, were synthesized. It was clarified that the lithium salt of the dianion has a high-lying HOMO and a narrow HOMO-LUMO gap, which were perturbed by dissociation of Li⁺ cation, as judged by UV/Vis spectroscopy and DFT calculations. The lithium salt of the dianion reacted as two equivalents of a diarylboryl anion with CH₂ Cl₂ or S₈ to give boryl-substituted products.

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.

Phosphine-Stabilized Diiododiborenes: Isolable Diborenes with Six Labile Bonds

The lability of B=B, B-P, and B-halide bonds is combined in the syntheses of the first diiododiborenes. In a series of reactivity tests, these diiododiborenes undergo cleavage of all six of their central bonds in different ways, leading to products of B=B hydrogenation and dihalogenation as well as halide exchange.

Metallomimetic Chemistry of Boron

The study of main-group molecules that behave and react similarly to transition-metal (TM) complexes has attracted significant interest in recent decades. Most notably, the attractive idea of replacing the all-too-often rare and costly metals from catalysis has motivated efforts to develop main-group-element-mediated reactions. Main-group elements, however, lack the electronic flexibility of TM complexes that arises from combinations of empty and filled d orbitals and that seem ideally suited to bind and activate many substrates. In this review, we look at boron, an element that despite its nonmetal nature, low atomic weight, and relative redox staticity has achieved great milestones in terms of TM-like reactivity. We show how in interelement cooperative systems, diboron molecules, and hypovalent complexes the fifth element can acquire a truly metallomimetic character. As we discuss, this character is powerfully demonstrated by the reactivity of boron-based molecules with H₂, CO, alkynes, alkenes and even with N₂.

Coordination of Asymmetric Diborenes towards Cationic Coinage Metals (Au, Ag, Cu)

Synthesis of a series of cationic coinage metal (Au: 2-4, Ag: 5, and Cu: 6) complexes of asymmetric diborenes (1 a, 1 b) is reported. X-ray diffraction analysis reveals that the metal center interacts unsymmetrically with the B₂ moiety and the terminal boron atom exhibits pronounced pyramidal geometry. A computational study shows that in complexes 2-6, the bonding between the B₂ units and the metal center is dominated by electrostatic interactions concomitant with non-negligible covalent contributions.

Isolation and Characterization of Crystalline, Neutral Diborane(4) Radicals

Diaryldihalodiboranes(4) were reacted with bis(amidinato)- and bis(guanidinato)silylenes to generate the first neutral diborane-centered radicals. These formally non-aromatic 5π electron systems are stable in the solid state as well as in solution and were characterized by solid-state structure determination, high-resolution mass spectrometry, and EPR spectroscopy. The reactivity of one of these radicals with the oxidant 1,4-benzoquinone led to ring-opening and B-O bond formation.

Complexation of asymmetric diborenes with magnesium bromide

The synthesis of neutral Mg complexes of asymmetric diborenes is reported. X-ray diffraction analysis reveals that the Mg center is unsymmetrically coordinated to the B2 units of diborenes. Theoretical calculations manifest the donation of electrons from the π-type orbitals of diborenes to an empty orbital of Mg.

Triaminotriborane(3): A Homocatenated Boron Chain Connected by B-B Multiple Bonds

A triaminotriborane(3) was isolated as purple crystals through the reduction of (TMP)BCl₂ (TMP=2,2,6,6-tetramethylpiperidino) by sodium naphthalenide. Single-crystal X-ray diffraction and computational studies of the obtained triaminotriborane(3) revealed a bent structure of the [B(NR₂ )]₃ chain. The bond lengths between the central and terminal boron atoms were similar to those observed in neutral diborene species. The multiple-bonding character may be best described by a three-center two-electron π-bond along the B₃ chain. The distance between the two terminal boron atoms (2.177 Å) in the solid-state structure implies a weak interaction between them. When an excess amount of Li was used as the reducing agent, the reaction yielded an unusual dianionic species. The isolation and characterization of these two reduction products are reported herein.

Monomeric 16-Electron π-Diborene Complexes of Zn(II) and Cd(II)

Despite the prevalence of stable π-complexes of most d¹⁰ metals, such as Cu(I) and Ni(0), with ethylene and other olefins, complexation of d¹⁰ Zn(II) to simple olefins is too weak to form isolable complexes due to the metal ion's limited capacity for π-backdonation. By employing more strongly donating π-ligands, namely neutral diborenes with a high-lying π(B═B) orbital, monomeric 16-electron M(II)-diborene (M = Zn, Cd) π-complexes were synthesized in good yields. Metal-B₂ π-interactions in both the solid and solution state were confirmed by single-crystal X-ray analyses and their solution NMR and UV-vis absorption spectroscopy, respectively. The M(II) centers adopt a trigonal planar geometry and interact almost symmetrically with both boron atoms. The MB₂ planes significantly twist out of the MX₂ planes about the M-centroid(B-B) vector, with angles ranging from 47.0° to 85.5°, depending on the steric interactions between the diborene ligand and the MX₂ fragment.

CuOTf-mediated intramolecular diborene hydroarylation

CuOTf-mediated intramolecular diborene hydroarylation, followed by a σ-bond metathesis between the resulting B–H bond and CuOTf, gives a cyclic sp²–sp³ boryl-substituted boronium triflate.