Unsymmetrical, Cyclic Diborenes and Thermal Rearrangement to a Borylborylene

Doubly Base-Stabilized Diborane(4) and Borato-Boronium Species and Their Chemistry with Chalcogens

In an effort to isolate diborane(4) derivatives, we have developed an efficient and uncatalyzed approach using [BH3·THF] and the mercaptopyridine ligand. Thermolysis of 2-mercaptopyridine, in the presence of [BH3·THF], afforded a doubly base-stabilized diborane(4) species 1, [HB(μ-C5H4NS)]2, along with the formation of its isomeric species 2, [HB(μ-C5H4NS)]2, albeit in less yield. Based on the coordination of the boron with the mercaptopyridine ligand in 2 and its spectroscopic data, compound 2 has been designated as a borato-boronium species, in which the anionic borate and cationic boronium units are covalently bonded to each other. Furthermore, we have demonstrated the oxidative insertion of chalcogen atoms (S and Se) through the B-B bond of the base-stabilized diborane(4), 1, that yielded chalcogenido-diboron species, 3(S) and 4(Se).

Taming the parent oxoborane

Despite recent advancements in the chemistry of multiply bound boron compounds, the laboratory isolation of the parent oxoborane moiety, HBO has long remained an unsolved and well-recognized challenge. The reaction of 6-SIDipp·BH₃ [6-SIDipp = 1,3-di(2,6-diisopropylphenyl)tetrahydropyrimidine-2-ylidene] with GaCl₃ afforded an unusual boron-gallium 3c-2e compound (1). The addition of water to 1 resulted in the release of H₂ and the formation of a rare acid stabilized neutral parent oxoborane, LB(H)[double bond, length as m-dash]O (2). Crystallographic and density functional theory (DFT) analyses support the presence of a terminal B[double bond, length as m-dash]O double bond. Subsequent addition of another equivalent of water molecule led to hydrolysis of the B-H bond to the B-OH bond, but the 'B[double bond, length as m-dash]O' moiety remained intact, resulting in the formation of the hydroxy oxoborane compound (3), which can be classified as a monomeric form of metaboric acid.

A zwitterionic disilanylium from an unsymmetrical disilene

The reaction of PhC(NtBu)₂SiSi(SiMe₃)₃ (1) with Me₃SiCH₂Cl afforded an unsymmetrical sp²-sp³ disilene, 2, with concomitant elimination of Me₃SiCl. The analogous reaction with PhC(NtBu)₂SiCl resulted in the oxidative addition of the C-Cl bond at the Si(II) atom (3). The reactions of 2 with sulfur and selenium led to compounds with SiE (ES (4) and Se (5)) double bonds. Tellurium reacted differently with 2 and furnished a zwitterionic compound, 6.

N-Phosphinoamidinato Silylene- and Phosphine-Borylborylene Complexes

This work describes a straightforward method to synthesize a borylborylene without proceeding via the rearrangement of a diborene. An amidinato amidosilylene [LSiNMe₂] (L = PhC(NtBu)₂) and PMe₃ were reacted with an N-phosphinoamidinato diborane 1 and KC₈ to form a stable silylene-borylborylene 2 and a persistent phosphine-borylborylene 3, respectively. Compound 2 is stable as the borylene center is well stabilized by the silylene donor and boryl substituent, whereas compound 3 is unstable in solution due to labile PMe₃. The latter was illustrated by reacting compound 3 with Ar'NC (Ar' = 2,6-Me₂C₆H₃), where Ar'NC displaced PMe₃ and inserted into the N-phosphinoamidinate ligand and B-B bond to form compound 4.

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.

Synthesis and Fluorescence Studies of Diborene Coinage Metal Complexes

Nine different coinage metal (Cu, Ag) π complexes of diborenes with various anionic diborene (aryl, heteroaryl) and metal substituents (Cl, Br, C₆F₅, C₂SiMe₃), stabilizing neutral donors (N-heterocyclic carbene = NHC, phosphine), configurations (cis/trans, acyclic/cyclic diborene), and charges (neutral, cationic) were synthesized and characterized by multinuclear NMR spectroscopy and X-ray crystallographic analyses. Their optical properties were investigated by UV-vis absorption and steady-state as well as time-resolved luminescence spectroscopy in solution and the solid state to gain insights into the excited-state behavior of this unusual class of photoactive compounds and to provide structure-property relationships. The structural and electronic modification of the (B═B)···M motif greatly influences not only the visible light absorption but also the photostability and quantum yields, which can reach high values of up to f = 0.42. The lifetimes are found in the nanosecond regime, providing estimated radiative rate constants over a wide range of k_r = 1.3-14 × 10⁷ s^-1, indicative of fluorescence. Intersystem crossing (ISC) is sufficiently slow for prompt emission from the S₁ state to be observed, while the spin-orbit coupling in the T₁ state is too weak for phosphorescence to occur at room temperature. ISC can be accelerated, however, by modifying diborene ligand substitution and the coinage metal center, hinting at the potential for exploiting the properties of long-lived triplet excited states of metal diborene complexes in the future.

Metal-Free Phosphorus-Directed Borylation of C(sp2 )-H Bonds

Spectacular progress has recently been achieved in transition metal-catalyzed C-H borylation of phosphines as well as directed electrophilic C-H borylation. As shown here, P-directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine-boranes. It operates under metal-free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N-phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post-derivatizing the borane moiety). NMR monitoring supports the involvement of P-stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)₂ P→BBr₂ ⁺ species.

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.

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry

A one‐pot synthesis of a CAAC‐stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two‐electron reduction steps from an adduct of CAAC and B₂Br₄(SMe₂)₂. Theoretical studies revealed that this diborene has a considerably smaller HOMO–LUMO gap than those of reported NHC‐ and phosphine‐supported diborenes. Complexation of the diborene with [AuCl(PCy₃)] afforded two diborene–Au^I π complexes, while reaction with DurBH₂, P₄ and a terminal acetylene led to the cleavage of B−H, P−P, and C−C π bonds, respectively. Thermal rearrangement of the diborene gave an electron‐rich cyclic alkylideneborane, which readily coordinated to Ag^I via its B=C double bond.

Probing the Boundaries between Lewis-Basic and Redox Behavior of a Parent Borylene

The parent borylene (CAAC)(Me₃P)BH, 1 (CAAC=cyclic alkyl(amino)carbene), acts both as a Lewis base and one‐electron reducing agent towards group 13 trichlorides (ECl₃, E=B, Al, Ga, In), yielding the adducts 1‐ECl₃ and increasing proportions of the radical cation [1]^•+ for the heavier group 13 analogues. With boron trihalides (BX₃, X=F, Cl, Br, I) 1 undergoes sequential adduct formation and halide abstraction reactions to yield borylboronium cations and shows an increasing tendency towards redox processes for the heavier halides. Calculations confirm that 1 acts as a strong Lewis base towards EX₃ and show a marked increase in the B−E bond dissociation energies down both group 13 and the halide group.

Unsymmetrical sp2 -sp3 Disilenes

Disilenes with differently coordinated silicon atoms are not known. Here, we have shown the high yield synthesis of a range of disilenes (2-4 and 6) upon reaction of a hypersilyl silylene PhC(NtBu)₂ SiSi(SiMe₃ )₃ (1) with aliphatic chlorophosphines. The most striking characteristic of these disilenes is the presence of two differently coordinated Si atoms (one is three-coordinated, the other four-coordinated). The analogous reaction with Ph₂ PCl did not afford the desired disilene, but, surprisingly, led to the first tetraphosphinosilane (8). DFT calculations were performed to understand the bonding in disilenes and differences in reactivity of the complexes.

Reactions of diborenes with terminal alkynes: mechanisms of ligand-controlled anti-selective hydroalkynylation, cycloaddition and C[triple bond, length as m-dash]C triple bond scission

The reactions of terminal acetylenes with doubly Lewis base-stabilised diborenes resulted in different outcomes depending on the nature of the ligands at boron and the conformation of the diborene (cyclic versus acyclic). N-heterocyclic carbene (NHC)-stabilised diborenes tended to undergo anti-selective hydroalkynylation at room temperature, whereas [2 + 2] cycloaddition was observed at higher temperatures, invariably followed by a C–N bond activation at one NHC ligand, leading to the ring-expansion of the initially formed BCBC ring and formation of novel boron-containing heterocycles. For phosphine-stabilised diborenes only [2 + 2] cycloaddition was observed, followed by a rearrangement of the resulting 1,2-dihydro-1,2-diborete to the corresponding 1,3-isomer, which amounts to complete scission of both the B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B double and CC triple bonds of the reactants. The elusive 1,2-isomer was finally trapped by using a cyclic phosphine-stabilised diborene, which prevented rearrangement to the 1,3-isomer. Extensive density functional theory (DFT) calculations provide a rationale for the selectivity observed.

The outcome of reactions between diborenes and terminal alkynes can be tuned by varying the stabilising Lewis base and/or reaction conditions, to yield either the anti-hydroalkynylation product or [2 + 2] cycloaddition-derived boron heterocycles.

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.

Perspective on Organoboron Chemistry

Organoboron compounds play prominent roles in structural, synthetic, and materials chemistry because boron atoms can feature electrophilic, ambiphilic, or nucleophilic character. This perspective briefly describes the most recent progress in organoboron chemistry, focusing on new boron molecules and their applications that have attracted great interest from main-group chemists. The research hotspots arising from these pioneering results are also discussed.

1 Introduction

2 Diboron Reagents

3 Boryl Anions

4 Borylenes

5 Nucleophilic or Ambiphilic Boron-Containing N-Heterocycles

6 Conclusions and Outlook

Phosphinoborylenes as stable sources of fleeting borylenes

Base-stabilised borylenes that mimic the ability of transition metals to bind and activate inert substrates have attracted significant attention in recent years. However, such species are typically highly reactive and fleeting, and often cannot be isolated at ambient temperature. Herein, we describe a readily accessible trimethylphosphine-stabilised borylborylene which was found to possess a labile P-B bond that reversibly cleaves upon gentle heating. Exchange of the labile phosphine with other nucleophiles (CO, isocyanide, 4-dimethylaminopyridine) was investigated, and the binding strength of a range of potential borylene "ligands" has been evaluated computationally. The room-temperature-stable PMe3-bound borylenes were subsequently applied to novel bond activations including [2 + 2] cycloaddition with carbodiimides and the reduction of dichalcogenides, revealing that PMe3-stabilised borylenes can effectively behave as stable sources of the analogous fleeting dicoordinate species under mild conditions.

Neutral cyclic sp2-sp3 and sp3-sp3 diboranes from N,N'-dicyclohexylcarbodiimide insertion into 1,2-dichlorodiboranes(4)

We report the synthesis and structural characterization of new neutral cyclic sp2-sp3 and sp3-sp3 diboranes from the reaction of N,N'-dicyclohexylcarbodiimide with 1,2-dichlorodiboranes(4) at room temperature. The present study is the first example of an insertion reaction of carbodiimide into diborane(4).

Hydrophosphination of boron-boron multiple bonds

Five compounds containing boron–boron multiple bonds are shown to undergo hydrophosphination reactions with diphenylphosphine in the absence of a catalyst. With diborenes, the products obtained are highly dependent on the substitution pattern at the boron atoms, with both 1,1- and 1,2-hydrophosphinations observed. With a symmetrical diboryne, 1,2-hydrophosphination yields a hydro(phosphino)diborene. The different mechanistic pathways for the hydrophosphination of diborenes are rationalised with the aid of density functional theory calculations.

Compounds containing boron–boron double and triple bonds are shown to undergo uncatalysed hydrophosphination reactions with diphenylphosphine.

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.

Single and Double Hydroboration of B-B Triple Bonds and Convergent Routes to a Cationic Tetraborane

A compound with a boron-boron triple bond is shown to undergo stepwise hydroboration reactions with catecholborane to yield an unsymmetrical hydro(boryl)diborene and a 2,3-dihydrotetraborane. Abstraction of H^- from the latter compound produces an unusual cationic, planar tetraborane with a hydrogen atom bridging the central B₂ moiety. Spectroscopic and crystallographic data and DFT calculations support a "protonated diborene" structure for this compound, which can also be accessed via direct protonation of the corresponding diborene.

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.

Non-classical covalent bonds

The possibility of multiple bond formation between Periodic Table Group 13 – 15 elements is considered. The ways of triple bond formation between these elements are discussed; particular attention is paid to the B≡B triple bonds. New non-linear compounds with triple bonds and their molecular structures are considered. The causes are given for the formation of compounds with unusually short distances between chemically non-bonded atoms. The grounds of the theory of two-centre three-electron bonds are presented and conditions of existence of isolated square planar carbon clusters are analyzed.

The bibliography includes 181 references.

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.

Synthesis of a hydrogen-bridged tetraborane(6): a substituent effect of a diaminoboryl group toward the B–B multiple bond character

A hydrogen-bridging tetraborane(6) was synthesized from boryllithium, a boron nucleophile, in three steps.

Chelated Diborenes and Their Inverse-Electron-Demand Diels-Alder Reactions with Dienes

A doubly base-stabilised diborane based on a benzylphosphine linker was prepared by a salt elimination reaction between 2-LiC₆ H₄ CH₂ PCy₂ ⋅Et₂ O and B₂ Br₄ . This compound was reduced with KC₈ to its corresponding diborene, with the benzylphosphine forming a five-membered chelate. The diborene reacts with butadiene, 2-trimethylsiloxy-1,3-butadiene, and isoprene to form 4,5-diboracyclohexenes, which interconvert between their 1,1- (geminal) and 1,2- (vicinal) chelated isomers. The 1,1-chelated diborene undergoes a halide-catalysed isomerisation into its thermodynamically favoured 1,2-isomer, which undergoes Diels-Alder reactions more slowly than the kinetic product.

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.

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.

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.

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.