Perfluoropentaphenylborole

Synthesis of phenanthrylboroles and formal nitrene insertion to access azaborapyrenes

Extended conjugation has been introduced into boroles but only on the 2,3- or 4,5-positions of the central BC4 core. In this work we synthesize phenanthrylboroles that also have conjugation on the 3,4-positions and demonstrate their insertion reactivity with azides to furnish B,N-analogues of pyrene.

A Highly Sterically Encumbered Boron Lewis Acid Enabled by an Organotellurium-Based Ligand

Lewis acidic boron compounds are ubiquitous in chemistry due to their numerous applications, yet tuning and optimizing their properties towards different purposes is still a challenging field of research. In this work, the boron-based Lewis acid B[OTeF3(C6F5)2]3 was synthesized by reaction of the teflate derivative HOTeF3(C6F5)2 with BCl3 or BCl3 ⋅ SMe2. This new compound presents a remarkably high thermal stability up to 300 °C, as well as one of the most sterically encumbered boron centres known in the literature. Theoretical and experimental methods revealed that B[OTeF3(C6F5)2]3 exhibits a comparable Lewis acidity to that of the well-known B(C6F5)3. The affinity of B[OTeF3(C6F5)2]3 towards pyridine was accessed by Isothermal Titration Calorimetry (ITC) and compared to that of B(OTeF5)3 and B(C6F5)3. The ligand-transfer reactivity of this new boron compound towards different fluorides was demonstrated by the formation of an anionic Au(III) complex and a hypervalent iodine(III) species.

Structural characterization and reactivity of a room-temperature-stable, antiaromatic cyclopentadienyl cation salt

The singlet states of cyclopentadienyl (Cp) cations are considered as true prototypes of an antiaromatic system. Unfortunately, their high intrinsic reactivity inhibited their isolation in the solid state as a salt, and controlled reactions are also scarce. Here we present the synthesis and solid state structure of the room-temperature-stable Cp cation salt [Cp(C6F5)5]+[Sb3F16]-. Although the aromatic triplet state of the [Cp(C6F5)5]+ cation is energetically favoured in the gas phase according to quantum chemical calculations, coordination of the cation by either [Sb3F16]- or C6F6 in the crystal lattice stabilizes the antiaromatic singlet state, which is present in the solid state. The calculated hydride and fluoride ion affinities of the [Cp(C6F5)5]+ cation are higher than those of the perfluorinated tritylium cation [C(C6F5)3]+. Reactions of [Cp(C6F5)5]+[Sb3F16]- with CO, which probably yields the corresponding carbonyl complex, and of radical Cp(C6F5)5∙ with selected model substrates (Cp2Fe, (Ph3C∙)2 and Cp*Al) are also presented.

One-Pot Access to Boron-Doped Fused Heterocycles via Domino Cyclization of Bis-Diazidoboranes with Isonitrile

Boron-doped fused heterocycles have shown great potential in the field of functional materials. This study reports on the synthesis of a new class of bis-diazidoboranes and the discovery of their cycloaddition reaction with isonitriles. Triply fused boron-doped heterocyclic compounds were constructed in a one-pot process through a domino cycloaddition, providing an effective route for constructing complex boron-doped heterocyclic systems.

The Effect of Carborane Substituents on the Lewis Acidity of Boranes

The Lewis acidity of primary, secondary, and tertiary boranes with phenyl, pentafluorophenyl, and all three isomers of the C-substituted icosahedral carboranes (ortho, meta, and para) was investigated by computing their fluoride, hydride, and ammonia affinities as well as their global electrophilicity indices and LUMO energies. From these calculations, it was determined that the substituent effects on the Lewis acidity of these boranes follow the trend of ortho-carborane > meta-carborane > para-carborane > C6F5 > C6H5.

Boron-Centered Lewis Superacid through Redox-Active Ligands: Application in C-F and S-F Bond Activation

A series of redox-responsive ferrocenyl-substituted boranes and boronic esters were synthesized. Oxidation of the ferrocenyl ligand to the ferrocenium resulted in a drastic increase in the Lewis acidity beyond the strength of SbF₅ , which was investigated experimentally and computationally. The resulting highly Lewis acidic boron compounds were used for catalytic C-F and S-F bond activation.

Near-Infrared-Responsive Hydrocarbons Designed by π-Extension of Indeno[1,2,3,4-pgra]perylene at the 1,2,12-Positions

The relationship between the overall electronic structure of π-conjugated molecules and the arrangement of their constituent elements is of fundamental importance. Establishing rational design guidelines for conjugated hydrocarbons with narrow HOMO-LUMO gaps is useful to develop near-infrared (NIR) responsive dyes and redox-active materials. This study describes the synthesis and properties of three conjugated hydrocarbons, i. e., an indenonaphthoperylene, an indenoterrylene, and a diindenoterrylene. These molecules exhibit NIR absorption despite the absence of significant antiaromaticity and diradical character. Notably, the indenonaphthoperylene exhibits red-to-NIR emission in the 620-850 nm region. The indenoterrylene and the diindenoterrylene exhibit NIR absorption tailing to 870 and 940 nm, respectively. Moreover, the effect of the π-extension of indenoperylene is disclosed in order to propose guidelines for achieving a narrow HOMO-LUMO gap with negligible antiaromaticity and diradical character.

Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid

The synthesis of tris(ortho-carboranyl)borane (BoCb₃ ), a single site neutral Lewis superacid, in one pot from commercially available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb₃ is a superior catalyst for promoting C-F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C₆ F₅ )₃ ].

Covalent triflates as synthons for silolyl- and germolyl cations

The synthesis of 1-silolyl and 1-germolyl triflates from the corresponding chlorides by salt metathesis reaction is reported. These covalent triflates are ideal starting materials for the preparation of ionic silolyl- and germolyl-imidazolium triflates by their reaction with N-heterocyclic carbenes. Similarily, ionic silolyl- and germolyl-oxophosphonium triflates are obtained by substitution of the triflate group by triethylphosphane oxide Et₃PO. The analysis of their ³¹P NMR chemical shifts according to the Gutmann-Beckett method reveal the high Lewis acidity of the underlying silolyl and germolyl cations. Further analysis of structural and NMR parameters of the silolyl- and germolyl-imidazolium and oxophosphonium triflates indicates that these compounds are covalently bonded silole and germole derivatives with insignificant contributions from silolyl- or germolyl cations. Silolyl and germolyl triflates are however synthetic equivalents of these cations and might serve as a source for electrophilic silolyl and germolyl units.

A free boratriptycene-type Lewis superacid

An exceptionally strong ferrocene-containing, cationic boratriptycene-type Lewis acid is stabilized by a weak Fe⋯B through-space interaction.

Azidoborolate anions and azidoborole adducts: isolable forms of an unstable borole azide

Boroles are well known to undergo ring expansion reactions with organic azides to yield 1,2-azaborinines. A synthon featuring both azide and borole moieties within the same molecule, 1-azido-2,3,4,5-tetraphenylborole, was found...

A Boratafulvene

Structurally authenticated free B-alkyl boroles are presented and electronic implications of alkyl substitution were assessed. Deprotonation of a boron-bound exocyclic methyl group in a B-methyl borole yields the first 5-boratafulvene anion-an isomer to boratabenzene. Boratafulvene was structurally characterized and its electronic structure probed by DFT calculations. The pKa value of the exocyclic B-CH3 in a set of boroles was computationally approximated and confirmed a pronounced acidic character caused by the boron atom embedded in an anti-aromatic moiety. The non-aromatic boratafulvene reacts as a C-centered nucleophile with the mild electrophile Me3 SnCl to give a stannylmethyl borole, regenerating the anti-aromaticity. As nucleophilic synthons for boroles, boratafulvenes thus open an entirely new avenue for synthetic strategies toward this highly reactive class of heterocycles. Boratafulvene reacts as a methylene transfer reagent in a bora-Wittig-type reaction generating a borole oxide.

Highly Electron-Deficient Dicyanomethylene-Functionalized Triarylboranes with Low-Lying LUMO and Strong Lewis Acidity

A series of dicyanomethylene-functionalized triarylboranes is reported in this work, with low-lying LUMO energy levels at ca. -3.66 eV for ^FMesB-ACN. The single-crystal structures of the mono- and dianion of Mes*B-ACN were obtained via chemical reduction, which revealed a conversion from a quinoidal to an aromatic structure. The strong Lewis acidity of these compounds is reflected in a fluoride-anion binding experiment. This work introduces a facile strategy for modulating the electron deficiency of boron-containing compounds.

From BN-Naphthalenes to Benzoborole Dianions

The synthesis of benzoborole dianions by alkali metal reduction of BN-naphthalene derivatives via a ring-contraction strategy has been developed. Reduction of 1-alkynyl 2,1-benzazaborine 1 a in Et₂ O led to the elimination of alkynyllithium with the formation of 1-amino-1-benzoborole trilithium salt 2 a, whereas reduction of 1-phenyl 2,1-benzazaborine 1 c in THF yielded 1-phenyl-1-benzoborole dilithium salt 2 c with the elimination of ArNHLi. The trilithium and dilithium salts 2 a and 2 c have been fully characterized. Treatment of trilithium salt 2 a with Et₃ NHCl led to the selective protonation of the amino lithium to afford the dilithium salt 2 aH, which could be cleanly oxidized to 1-amino-1-benzoborole 3 in an excellent yield. Reaction of 1-phenyl-1-benzoborole dilithium salt 2 c with MeI yielded the lithium borate 4 c, which is luminescent both in solution and in the solid state.

Synthesis, Characterization, and Density Functional Theory Studies of Three-Dimensional Inorganic Analogues of 9,10-Diboraanthracene-A New Class of Lewis Superacids

The three-dimensional inorganic analogues of 9,10-diboraanthracene, B₂X₂(C₂B₁₀H₁₀)₂ (X = Cl, 1; X = Br, 2), were attained by salt elimination of Li₂C₂B₁₀H₁₀ and trihaloboranes. The methyl- and phenyl-substituted compounds B₂Me₂(C₂B₁₀H₁₀)₂ (3) and B₂Ph₂(C₂B₁₀H₁₀)₂ (4) were obtained by treating 1 or 2 with the corresponding Grignard reagents. These compounds were fully characterized by NMR, cyclic voltammetry (CV), IR, and single-crystal X-ray diffraction analyses. Experimental (CV and Gutmann-Beckett method) and computational (fluoride ion affinity, hydride ion affinity and LUMO energy) results suggest that the order of Lewis acidity is 2 > 1 > 4 > 3 > SbF₅. Treatment of 1 or 2 with HSiEt₃ gave a rare neutral borane-silane adduct, (Et₃SiH)₂B₂H₂(C₂B₁₀H₁₀)₂ (5). The equilibrium of 5 in solution was thoroughly investigated by spectroscopy and quantum calculations.

Two derivatives of phenylpyridyl-fused boroles with contrasting electronic properties: decreasing and enhancing the electron accepting ability

Two derivatives of phenylpyridyl-fused boroles were prepared via functionalization of the pyridyl groups, namely to an electron-rich dihydropyridine moiety (compound 1) and an electron-deficient N-methylpyridinium cation (compound 2). Due to strong conjugation between the dihydropyridine moiety and the boron atom, the reduction potential of compound 1 shifts cathodically. In contrast, compound 2 exhibits three reduction processes with a first reversible reduction potential anodically shifted in comparison to its precursor (TipPBB2) or the non-borylated framework 1-methyl-2-phenylpyridin-1-ium triflate (3). The significantly anodically shifted reduction potential indicates the extreme electron deficiency of compound 2, which also leads to the reversible coordination of THF. Photophysical properties of both compounds in different solvents were investigated. Theoretical studies further support the strong conjugation in the ground state of compound 1 and the electron-deficient property of compound 2.

Synthesis, crystal structure, solid-state optical property and C–H activation of sp3 carbon of highly-stable 1-(2′,6′-dimesitylphenyl)-2,3,4,5-tetraphenylborole

We synthesized a borole having near-infrared absorption and found transformation to the unexpected fused molecule through C–H activation.

(Hetero)arene-fused boroles: a broad spectrum of applications

(Hetero)arene-fused boroles are a class of compounds containing a 5-membered boron diene-ring. Based on their molecular framework, the (hetero)arene-fused boroles can be considered as boron-doped polycyclic antiaromatic hydrocarbons and are thus of great interest. Due to the vacant p_z orbital on the 3-coordinate boron atom, the antiaromaticity and strain of the 5-membered borole ring, (hetero)arene-fused boroles possess strong electron accepting abilities and Lewis acidity. By functionalization, they can be tuned to optimize different properties for specific applications. Herein, we summarize synthetic methodologies, different strategies for their functionalization, and applications of (hetero)arene-fused boroles.

(Hetero)arene-fused boroles, ‘antiaromatic’ 2n-electron π-systems, more stable and more functionalizable than boroles, offer greater potential for a variety of applications.

Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9-Borafluorenes

Three different perfluoroalkylated borafluorenes (F Bf) were prepared and their electronic and photophysical properties were investigated. The systems have four trifluoromethyl moieties on the borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo-aryl moieties. They differ with regard to the para substituents on their exo-aryl moieties, being a proton (F XylF Bf, F Xyl: 2,6-bis(trifluoromethyl)phenyl), a trifluoromethyl group (F MesF Bf, F Mes: 2,4,6-tris(trifluoromethyl)phenyl) or a dimethylamino group (p-NMe2 -F XylF Bf, p-NMe2 -F Xyl: 4-(dimethylamino)-2,6-bis(trifluoromethyl)phenyl), respectively. All derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron-deficient derivative F MesF Bf was also chemically reduced and its radical anion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 μs; however, the underlying mechanisms responsible for this differ. The donor-substituted derivative p-NMe2 -F XylF Bf exhibits thermally activated delayed fluorescence (TADF) from a charge-transfer (CT) state, whereas the F MesF Bf and F XylF Bf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition-dipole moments.

A Cationic NHC-Supported Borole

This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF3 )3 ]4 } from the NHC-chloroborole adduct yields the first stable NHC-supported 1-(Me NHC)-2,5-(SiMe3 )2 -3,4-(Ph*)2 -borole cation. Electronically, it features both a five-membered cyclic conjugated 4 π-electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass-spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1-mesityl borole reveals strong Lewis acidity, reduced HOMO-LUMO gaps, and increased anti-aromatic character.

An Extensive Set of Accurate Fluoride Ion Affinities for p-Block Element Lewis Acids and Basic Design Principles for Strong Fluoride Ion Acceptors

The computed fluoride ion affinity (FIA) is a valuable descriptor to assess the Lewis acidity of a compound. Despite its widespread use, the varying accuracy of applied computational models hampers the broad comparability of literature data. Herein, we evaluate the performance of selected methods (like DLPNO‐CCSD(T)) in FIA computations against CCSD(T)/CBS data and guide for the choice of suitable density functionals that allow the treatment of larger Lewis acids. Based on the benchmarked methods, we computed an extensive set of gas‐phase and solvation corrected FIA, that is covering group 13–16 elements featuring moderate to strong electron‐withdrawing substituents (190 entries). It permits an unbiased comparison of FIA over a significant fraction of the periodic table, serves as a source of reference for future synthetic or theoretical studies, and allows to derive some simple design principles for strong fluoride ion acceptors. Finally, the manuscript includes a tutorial section for the computation of FIA with and without the consideration of solvation.

2,5-Bis-trimethylsilyl substituted boroles

This manuscript includes a comprehensive study of the synthesis and spectroscopic features of 2,5-disilyl boroles.

A Modular Approach to Phosphorescent π-Extended Heteroacenes

A modular route to previously inaccessible classes of ring-fused π-extended heteroacenes bearing the heavy inorganic element tellurium (Te) is presented. These new materials can be viewed as n-doped analogs of molecular graphene subunits that exhibit color tunable visible light phosphorescence in the solid state and in the presence of air. The general mechanism of phosphorescence in these systems was probed experimentally and computationally via time-dependent density functional theory (TD-DFT). The incorporation of Te into π-extended oligoacene frameworks was achieved by an efficient Zr/Te transmetalation protocol; related zirconium-element exchange reactions have been used to prepare both electron-rich and electron-deficient heterocycles containing different elements from throughout the p-block. Therefore, the current study provides a clear path to incorporate inorganic elements into heteroacenes of greater complexity and side group selectivity compared to existing synthetic routes.

Electronic Push-Pull Modulation by Peripheral Substituents in Pentaaryl Boroles

Establishing access to a bulky tetraaryl dilithiobutadiene (Ph*C)₄ Li₂ (Ph*=3,5-tBu₂ (C₆ H₃ )) allowed for the synthesis of five-membered heterocycles with incorporated main-group elements. Along with an amino borole, a set of substituted pentaaryl boroles (Ph*C)₄ BAr has been synthesized. The examination of their absorption spectra and computational studies by means of DFT granted insight into the influence of peripheral substituents on the electronic features of the parent pentaphenyl borole (PhC)₄ BPh. Introduction of the more electron-rich Ph* residue at the carbon atoms increases the HOMO energy, redshifting the visible π/π*-absorption bands compared with the parent pentaphenyl borole. The influence on the frontier orbitals of three different boron-bound aryls with electronically modulating substituents in the remote 3,5-positions Ar=3,5-R₂ -C₆ H₃ (R=Me, H, CF₃ ) was studied. The substituents were found to increase (+I effect, Me) or decrease (-I effect, CF₃ ) the LUMO energy, thus directly affecting the visible absorption spectra. This represents the first study on HOMO-LUMO-gap adjustments by a combined push-pull approach of a substituted pentaphenylborole.

Diverse Reactivity of Dienes with Pentaphenylborole and 1-Phenyl-2,3,4,5-Tetramethylborole Dimer

The reactions of a monomeric borole and a dimeric borole with 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene were investigated. The monomeric borole reacted at ambient temperature whereas heat was required to crack the dimer to form the monomer and induce reactivity. 2,3-Dimethyl-1,3-butadiene reacts to give diverse products resulting from a cycloaddition process with the B-C moiety of the boroles acting as a dienophile, followed by rearrangements to furnish bicyclic species. For 1,3-cyclohexadiene, a [4+2] process is observed in which 1,3-cyclohexadiene serves as the dienophile and the boroles as the diene partner. The experimental results are corroborated with mechanistic theoretical calculations that indicate boroles can serve as either a diene or dienophile in cycloaddition reactions with dienes.

Small molecule activation by boron-containing heterocycles

Most of the chemical and biological processes involving the fixation and transformation of small molecules have long been exclusive for metal complexes. Meanwhile, the last decades have seen a significant advance in main group chemistry that mimics transition-metal complexes, among which various boron-containing systems have been successful in mediating the small molecule activation. In this review, we focus on boron-containing heterocycles enabling the activation of σ- and π-bonds in small molecules, in conjunction with the proposed mechanisms.

Photo electron transfer induced desilylation of N,N-bis(trimethylsilyl)aminodibenzoborole to aminodibenzoborole

Photoinduced desilylation proceeds by single electron transfer and yields the first example of an unsubstituted aminoborole derivative.

A Neutral η5 -Aminoborole Complex of Germanium(II)

The synthesis of two η⁵ -aminoborole complexes of germanium(II) from the reaction of a germole dianion with aminoboron dichlorides is reported. This reaction constitutes a remarkable example of a germole-to-borole transformation. The two aminoborole complexes of germanium(II) were fully characterized by multinuclear NMR spectroscopy, IR spectroscopy, HRMS, and, in one case, by X-ray crystallography. The results of quantum-mechanical calculations favor the electronic structure of a half-sandwich complex of Ge^II over an ionic representation with a germanium dication stabilized by an aromatic aminoborole dianion.

Metallacyclopentadienes: synthesis, structure and reactivity

Metallacyclopentadienes, which possess two M-C(sp²) bonds and feature the structure of M(C[upper bond 1 start]R¹[double bond, length as m-dash]CR²-CR³[double bond, length as m-dash]C[upper bond 1 end]R⁴), are an important class of five-membered metallacycles. They are considered as both reactive intermediates in the stoichiometric and catalytic transformations of organic molecules and useful precursors to main group element compounds, and have received considerable attention in organometallic chemistry, coordination chemistry and synthetic organic chemistry over the past six decades because of their unique metallacyclic structure. This review comprehensively presents the synthesis, structure and reactivity of the s-, p-, d- and f-block metallacyclopentadienes distributed in the whole periodic table. In addition, their application in synthetic organic chemistry and polymer chemistry is summarized. This review aims to be beneficial for the design and synthesis of novel metallacyclopentadienes, and for promoting the rapid development of metallacyclic chemistry.

Abnormal Tin-Boron Exchange in the Attempted Synthesis of a Borylated Borole

Boroles are important motifs within functional materials. With the aim to prepare a pinacolboryl-substituted derivative, the metallacycle transfer from corresponding zirconium and tin precursors has been explored. We show that the reaction of 1,1-dimethyl-2,3,4,5-tetrapinacolborylstannole with dichloro(phenyl)borane does not provide the desired borole, but instead a stannyl-substituted 1-chloroboracyclopent-3-ene. Spectroscopic and structural details of this highly functionalized boracycle indicate that intramolecular interactions between the tin and oxygen atoms of the boryl substituents may account for the unexpected outcome of the tin-boron exchange reaction.

DFT Studies of Dimerization Reactions of Boroles

Boroles undergo dimerization reactions to give Diels-Alder (DA) dimers, bridged-bicyclic (BB) dimers or spiro dimers (SD) depending on the substituents on the borole. We performed DFT calculations to investigate how different substituents at the carbon atoms of the butadiene backbone as well as at the boron atom influence the dimerization reaction pathways. The DFT results show that, in general, both the DA and BB dimers are easily accessible kinetically, and the DA dimers are thermodynamically more stable than the BB dimers. When the substituent-substituent repulsive steric interactions are alleviated to a certain extent, the BB dimers are more stable than the DA dimer, and become accessible. The SD dimers are generally kinetically difficult to obtain. However, we found that aryl substituents promote the formation of the SD dimers.