Diborabutatriene: An Electron-Deficient Cumulene

Stabilization of Boron-Boron Triple Bonds by Mesoionic Carbenes

Density functional theory-based computations are carried out to analyze the electronic structure and stability of B₂(MIC)₂ complexes, where MIC is a mesoionic carbene, viz., imidazolin-4-ylidenes, pyrazolin-4-ylidene, 1,2,3-triazol-5-ylidene, tetrazol-5-ylidene, and isoxazol-4-ylidene. The structure, stability, and the nature of bonding of these complexes are further compared to those of the previously reported B₂(NHC)₂ and B₂(cAAC)₂. A thorough bonding analysis via natural bond order, molecular orbital, and energy decomposition analyses (EDA) in combination with natural orbital for chemical valence (NOCV) reveals that MICs are suitable ligands to stabilize B₂ species in its (3)¹∑_g ⁺ excited state, resulting in an effective B-B bond order of 3. Their high dissociation energy and endergonicity at 298 K for the dissociations L-BB-L → 2 B-L and L-BB-L → BB + 2 L (L = Ligand) indicate their viability at ambient condition. The donor property of MICs is comparable to that of NHC^Me. The orbital interaction plays a greater role than the coulombic interaction in forming the B-L bonds. The EDA-NOCV results show that the sum of the orbital energies associated with the (+, +) and (+, -) L→[B₂]←L σ-donations is far larger than that of L←[B₂]→L π-back donation. It also reveals that cAAC^Me possesses the largest σ-donation and π-back donation abilities among the studied ligands, and the σ-donation and π-back donation abilities of MICs are comparable to those of NHC^Me. Therefore, the present study shows that MICs would also be an excellent choice as ligands to experimentally realize new compounds having a strong B-B triple bond.

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.

Reactivity of Tetrahalo- and Difluorodiboranes(4) toward Lewis Basic Platinum(0): Bis(boryl), Borylborato, and Doubly Boryl-Bridged Platinum Complexes

The reaction of the tetrahalodiboranes(4) B₂F₄, B₂Cl₄, and B₂Br₄ with a Lewis basic platinum(0) complex led to the isolation of the cis-bis(difluoroboryl) complex cis-[(Cy₃P)₂Pt(BF₂)₂] (1) and the novel borylborato complexes trans-[(Cy₃P)₂Pt{B(X)-BX₃}] (2, X = Cl; 3, X = Br), respectively. The trans influence of the borylborato group was found to be one of the strongest ever observed experimentally. Furthermore, the reactivity of little-explored diaryldifluorodiboranes(4) F₂B-BMes₂ and the new derivative F₂B-BAn₂ (An = 9-anthryl) toward a range of platinum(0) complexes was investigated. Reactions with relatively nonbulky platinum(0) complexes led to the formation of unsymmetrical cis-bis(boryl) complexes cis-[(R₃P)₂Pt(BF₂)(BMes₂)] (6, R = Me; 7, R = Et) as well as the first example of a fourfold-unsymmetrical bis(boryl) complex, [(Me₃P)(Cy₃P)Pt(BF₂)(BMes₂)] (12). The use of a more bulky Pt complex provided access to the unprecedented dinuclear bis(boryl) complexes [{( iPr₃P)Pt}₂(μ-BF₂)(μ-BAr₂)] (8, Ar = Mes; 9, Ar = An), which feature two different μ₂-bridging boryl ligands.

Reactivity Enhancement of a Zerovalent Diboron Compound by Desymmetrization

The desymmetrization of the cyclic (alkyl)(amino)carbene-supported diboracumulene B₂(cAAC^Me)₂ (cAAC^Me = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) by monoadduct formation with IMe^Me (1,3-dimethylimidazol-2-ylidene) yields the zerovalent sp-sp² diboron compound B₂(cAAC^Me)₂(IMe^Me), which provides a versatile platform for the synthesis of novel symmetrical and unsymmetrical zerovalent sp²-sp² diboron compounds by adduct formation with IMe^Me and CO, respectively. Furthermore, B₂(cAAC^Me)₂(IMe^Me) displays enhanced reactivity compared to its symmetrical precursor, undergoing spontaneous intramolecular C-H activation and facile twofold hydrogenation, the latter resulting in B-B bond cleavage and the formation of the mixed-base parent borylene (cAAC^Me)(IMe^Me)BH.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Synthesis, Spectroscopic Characterization, and Redox Reactivity of a Cyclic (Alkyl) Amino Carbene-Derived Arsamethine Cyanine Dye

In our efforts to prepare a diarsenic allotrope supported by two cyclic alkyl amino carbene (CAAC) ligands we stumbled upon the synthesis of the first carbene-supported chloroarsinidene 3, which has been fully characterized by a combination of NMR spectroscopic and XRD methods. Although further reduction of 3 was not possible, we found that addition of a second equivalent of CAAC in refluxing toluene afforded the first example of a crystallographically characterized arsamethine cyanine dye (4). The arsenic(I) dye is structurally similar to phosphorus analogues, and contains an arsenide anion with two stereochemically active lone pairs supported by two iminium ligands. The UV/Visible spectrum and redox chemistry of 4 were also explored. Upon reduction with one equivalent of KC₈ , 3 is reduced to the originally targeted CAAC₂ As₂ allotrope 6, whereas oxidation provides access to the first example of an arsenic(II) radical dication (5).

Single and double activation of acetone by isolobal B[triple bond, length as m-dash]N and B[triple bond, length as m-dash]B triple bonds

Polar and apolar boron-based triple bonds promote the single and double C–H activation of acetone following similar coordination-deprotonation mechanisms.

B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N and B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B triple bonds induce C–H activation of acetone to yield a (2-propenyloxy)aminoborane and an unsymmetrical 1-(2-propenyloxy)-2-hydrodiborene, respectively. DFT calculations showed that, despite their stark electronic differences, both the B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N and B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B triple bonds activate acetone via a similar coordination-deprotonation mechanism. In contrast, the reaction of acetone with a cAAC-supported diboracumulene yielded a unique 1,2,3-oxadiborole, which according to DFT calculations also proceeds via an unsymmetrical diborene, followed by intramolecular hydride migration and a second C–H activation of the enolate ligand.

Direct access to a cAAC-supported dihydrodiborene and its dianion

The two-fold reduction of (cAAC)BHX₂ (cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene; X = Cl, Br) provides a facile, high-yielding route to the dihydrodiborene (cAAC)₂B₂H₂. The (chloro)hydroboryl anion reduction intermediate was successfully isolated using a crown ether. Overreduction of the diborene to its dianion [(cAAC)₂B₂H₂]^2- causes a decrease in the B-B bond order whereas the B-C bond orders increase.

CO2 Binding and Splitting by Boron-Boron Multiple Bonds

The room-temperature, ambient-pressure reactions of CO₂ with two species containing boron-boron multiple bonds led to the incorporation of either one or two CO₂ molecules. The structural characterization of a thermally unstable intermediate in one case indicates that an initial [2+2] cycloaddition is the key step in the reaction mechanism.

Isolation of diborenes and their 90°-twisted diradical congeners

Molecules containing multiple bonds between atoms-most often in the form of olefins-are ubiquitous in nature, commerce, and science, and as such have a huge impact on everyday life. Given their prominence, over the last few decades, frequent attempts have been made to perturb the structure and reactivity of multiply-bound species through bending and twisting. However, only modest success has been achieved in the quest to completely twist double bonds in order to homolytically cleave the associated π bond. Here, we present the isolation of double-bond-containing species based on boron, as well as their fully twisted diradical congeners, by the incorporation of attached groups with different electronic properties. The compounds comprise a structurally authenticated set of diamagnetic multiply-bound and diradical singly-bound congeners of the same class of compound.

Striking transformations of the hydroborylene ligand in a HB:→NiII complex with isocyanides and CO

The first reactivity of the hydroborylene ligand is described, giving access to a remarkable range of unprecedented boron-centered species.

For the first time, the reactivity of the metal- and N-heterocyclic carbene-supported monovalent hydroborylene is reported. Isocyanides react with the hydroborylene Ni^II complex [{cat(^TMSL)Si}(Cl)Ni←:BH(NHC)₂] 1 (cat = ortho-C₆H₄O₂; ^TMSL = N(SiMe₃)(Dipp); Dipp = 2,6-Prⁱ₂C₆H₃; NHC = :C[(Prⁱ)NC(Me)]₂) to form the hydride-bridged hydroborylene-Ni^II complexes 2. The reaction of 1 with isoelectronic CO, however, is reversible and furnishes the related unprecedented hydride- and CO-bridged hydroborylene Ni^II complex 2-CO, which undergoes isomerisation through silyl/NHC exchange at ambient temperature to afford the corresponding hydro(silyl)boryl Ni^II complex 3. Markedly, 2 readily and quantitatively react with one further molar equiv. of isocyanide to give, under borylene liberation and H/Cl ligand exchange, boraketiminium species, which represent cationic B^I complexes. These latter compounds are highly reactive in solution, and can undergo quantitative transformation into previously unknown cyanoborenium cations.

B-B bond activation and NHC ring-expansion reactions of diboron(4) compounds, and accurate molecular structures of B2(NMe2)4, B2eg2, B2neop2 and B2pin2

In this detailed study we report on the structures of the widely employed diboron(4) compounds bis(pinacolato)diboron (B₂pin₂) and bis(neopentyl glycolato)diboron (B₂neop₂), as well as bis(ethylene glycolato)diboron (B₂eg₂) and tetrakis(dimethylamino)diboron (B₂(NMe₂)₄), and their reactivity, along with that of bis(catecholato)diboron (B₂cat₂) with backbone saturated and backbone unsaturared N-heterocyclic carbenes (NHCs) of different steric demand. Depending on the nature of the diboron(4) compound and the NHC used, Lewis-acid/Lewis-base adducts or NHC ring-expansion products stemming from B-B and C-N bond activation have been observed. The corresponding NHC adducts and NHC ring-expanded products were isolated and characterised via solid-state and solution NMR spectroscopy and X-ray diffraction. In general, we observed B-B bond and C-N bond activation at low temperature for B₂eg₂, at room temperature for B₂neop₂ and at higher temperature for B₂cat₂. The reactivity strongly depends on steric effects of the NHCs and the diboron(4) compounds, as well as on the corresponding Lewis-basicity and Lewis-acidity. Our results provide profound insight into the chemistry of these diboron(4) reagents with the nowadays ubiquitous NHCs, the stability of the corresponding NHC adducts and on B-B bond activation using Lewis-bases in general. We demonstrate that B-B bond activation may be triggered even at temperatures as low as -40 °C to -30 °C without any metal species involved. For example, the reactions of B₂eg₂ with sterically less demanding NHCs such as Me₂Im^Me and iPr₂Im in solution led to the corresponding ring-expanded products at low temperatures. Furthermore, boronium [L₂B(OR)₂]⁺ and borenium [LB(OR)₂]⁺ cations have been observed from the reaction of the bis-borate B₂eg₃ with the NHCs iPr₂Im and Me₂Im^Me, which led to the conclusion that the activation of bis-borates with NHCs (or Lewis-bases in general) might be a facile and simple route to access such species.

Borylenes: An Emerging Class of Compounds

Free borylenes (R-B:) have only been spectroscopically characterized in the gas phase or in matrices at very low temperatures. However, in recent years, a few mono- and bis(Lewis base)-stabilized borylenes have been isolated. In both of these compounds the boron atom is in the formal oxidation state +I which contrasts with classical organoboron derivatives wherein the element is in the +III oxidation state. Mono(Lewis base)-stabilized borylenes are isoelectronic with singlet carbenes, and their reactivity mimics to some extent that of transition metals. They can activate small molecules, such as H₂ , and coordinate an additional ligand; in other words, they are boron metallomimics. Bis(Lewis base)borylene adducts are isoelectronic with amines and phosphines. In contrast to boranes, which act as electron acceptors and thus Lewis acids, they are electron-rich and act as ligands for transition metals.

Titanium Imido Complexes via Displacement of -SiMe3 and C-H Bond Activation in a Ti(III) Amido Complex, Promoted by a Cyclic (Alkyl)(Amino) Carbene (cAAC)

A strong σ-donating cyclic (alkyl)(amino) carbene (cAAC) triggers rearrangement of the silyl(aryl) amido ligand -N(SiMe3)Dipp (Dipp = 2,6-diisopropylphenyl) in the coordination sphere of titanium(III) to afford a novel zwitterionic titanium imido complex with a TiCH2SiMe2[cAAC] linkage. Reduction of this species produces a new DippN=Ti imido complex containing a cAAC-centered radical species, characterized by single crystal diffraction analysis and electron paramagnetic resonance spectroscopy.

N-Heterocyclic carbene adducts of the heavier group 15 tribromides. Normal to abnormal isomerism and bromide ion abstraction

The reactivity of the heavier group 15 tribromides, SbBr₃ and BiBr₃, towards 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) is described.

Azido- and amido-substituted gallium hydrides supported by N-heterocyclic carbenes

The synthesis of azido- and amido-gallanes supported by hindered N-heterocyclic carbene donors is reported.

Simple solution-phase syntheses of tetrahalodiboranes(4) and their labile dimethylsulfide adducts

Convenient solution-phase syntheses of tetrahalodiboranes(4) and the dimethylsulfide adducts B₂Cl₄(SMe₂)₂ and B₂Br₄(SMe₂)₂ are presented.

N-heterocyclic carbene induced reductive coupling of phosphorus tribromide. Isolation of a bromine bridged P-P bond and its subsequent reactivity

The room temperature reaction of a 1 : 1 mixture of phosphorus tribromide (PBr3) and the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) quantitatively affords the Lewis acid-base adduct (IPr)PBr3 (1). Interestingly, when 1 is heated between 55 and 65 °C for a period of several days, dark red crystals slowly begin to form in the reaction vessel accompanied by the release of bromine. The resulting crystalline sample, [P2(IPr)2Br3]Br ([2]Br), results from the reductive coupling of two equivalents of 1, and contains a cationic moiety with a P-P bond that is bridged by a bromine atom. Anion exchange reactions with Na[BArF4] (BArF4 = B(3,5-{CF3}2C6H3)4) afford [2][BArF4]. Abstraction of two equivalents of bromine allows for the isolation of the unprecedented dicationic species [P2(IPr)2Br2]2+ (3) which was isolated and structurally authenticated as two different [BArF4]- salts. Reaction of 2 with mild reductants such as SnBr2 or tetrakis(dimethylamino)ethylene (TDAE) affords [P2(IPr)2Br]+ (4) and the known radical cation [P2(IPr)2]˙+ (5), respectively. These studies show that relatively weak P-Br bonds present in compounds 1-4 can be cleaved in a straightforward manner to afford low oxidation state compounds in high yields.

Uncatalyzed Hydrogenation of First-Row Main Group Multiple Bonds

Room temperature hydrogenation of an SIDep-stabilized diboryne (SIDep=1,3-bis(diethylphenyl)-4,5-dihydroimidazol-2-ylidene) and a cAAC-supported diboracumulene (cAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) provided the first selective route to the corresponding 1,2-dihydrodiborenes. DFT calculations showed an overall exothermic (ΔG=19.4 kcal mol^-1 ) two-step asynchronous H₂ addition mechanism proceeding via a bridging hydride.

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 (+) .

Highly Strained Heterocycles Constructed from Boron-Boron Multiple Bonds and Heavy Chalcogens

The reactions of a diborene with elemental selenium or tellurium are shown to afford a diboraselenirane or diboratellurirane, respectively. These reactions are reminiscent of the sequestration of subvalent oxygen and nitrogen in the formation of oxiranes and aziridines; however, such reactivity is not known between alkenes and the heavy chalcogens. Although carbon is too electronegative to affect the reduction of elements with lower relative electronegativity, the highly reducing nature of the B-B double bond enables reactions with Se(0) and Te(0) . The capacity of multiple bonds between boron atoms to donate electron density is highlighted in reactions where diborynes behave as nucleophiles, attacking one of the two Te atoms of diaryltellurides, forming salts consisting of diboratellurenium cations and aryltelluride anions.

Quantitative structure parameters from the NMR spectroscopy of quadrupolar nuclei

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important characterization tools in chemistry, however, 3/4 of the NMR active nuclei are underutilized due to their quadrupolar nature. This short review centers on the development of methods that use solid-state NMR of quadrupolar nuclei for obtaining quantitative structural information. Namely, techniques using dipolar recoupling as well as the resolution afforded by double-rotation are presented for the measurement of spin–spin coupling between quadrupoles, enabling the measurement of internuclear distances and connectivities. Two-dimensional J-resolved-type experiments are then presented for the measurement of dipolar and J coupling, between spin-1/2 and quadrupolar nuclei as well as in pairs of quadrupolar nuclei. Select examples utilizing these techniques for the extraction of structural information are given. Techniques are then described that enable the fine refinement of crystalline structures using solely the electric field gradient tensor, measured using NMR, as a constraint. These approaches enable the solution of crystal structures, from polycrystalline compounds, that are of comparable quality to those solved using single-crystal diffraction.