Chemistry of Boryllithium: Synthesis, Structure, and Reactivity

Mercury-Group 13 Metal Covalent Bonds: A Systematic Comparison of Aluminyl, Gallyl and Indyl Metallo-ligands

Bimetallic compounds containing direct metal-group 13 element bonds have been shown to display unprecedented patterns of cooperative reactivity towards small molecules, which can be influenced by the identity of the group 13 element. In this context, we present here a systematic appraisal of group 13 metallo-ligands of the type [(NON)E]- (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) for E=Al, Ga and In, through a comparison of structural and spectroscopic parameters associated with the trans L or X ligands in linear d10 complexes of the types LM{E(NON)} and XM'{E(NON)}. These studies are facilitated by convenient syntheses (from the In(I) precursor, InCp) of the potassium indyl species [{K(NON)In}⋅KCp]n (1) and [(18-crown-6)2K2Cp] [(NON)In] (1'), and lead to the first structural characterisation of Ag-In and Hg-E (E=Al, In) covalent bonds. The resulting structural, spectroscopic and quantum chemical probes of Ag/Hg complexes are consistent with markedly stronger σ-donor capabilities of the aluminyl ligand, [(NON)Al]-, over its gallium and indium counterparts.

Arene-, Chlorido-, and Imido-Uranium Bis- and Tris(boryloxide) Complexes

We introduce the boryloxide ligand {(HCNDipp)2BO}- (NBODipp, Dipp = 2,6-di-isopropylphenyl) to actinide chemistry. Protonolysis of [U{N(SiMe3)2}3] with 3 equiv of NBODippH produced the uranium(III) tris(boryloxide) complex [U(NBODipp)3] (1). In contrast, treatment of UCl4 with 3 equiv of NBODippK in THF at room temperature or reflux conditions produced only [U(NBODipp)2(Cl)2(THF)2] (2) with 1 equiv of NBODippK remaining unreacted. However, refluxing the mixture of 2 and unreacted NBODippK in toluene instead of THF afforded the target complex [U(NBODipp)3(Cl)(THF)] (3). Two-electron oxidation of 1 with AdN3 (Ad = 1-adamantyl) afforded the uranium(V)-imido complex [U(NBODipp)3(NAd)] (4). The solid-state structure of 1 reveals a uranium-arene bonding motif, and structural, spectroscopic, and DFT calculations all suggest modest uranium-arene δ-back-bonding with approximately equal donation into the arene π4 and π5 δ-symmetry π* molecular orbitals. Complex 4 exhibits a short uranium(V)-imido distance, and computational modeling enabled its electronic structure to be compared to related uranium-imido and uranium-oxo complexes, revealing a substantial 5f-orbital crystal field splitting and extensive mixing of 5f |ml,ms⟩ states and mj projections. Complexes 1-4 have been variously characterized by single-crystal X-ray diffraction, 1H NMR, IR, UV/vis/NIR, and EPR spectroscopies, SQUID magnetometry, elemental analysis, and CONDON, F-shell, DFT, NLMO, and QTAIM crystal field and quantum chemical calculations.

Synthesis, Isolation, and Reactivity Studies of 'Naked' Acyclic Gallyl and Indyl Anions

By exploiting the electronic capabilities of the N-heterocyclic boryloxy (NHBO) ligand, we have synthesized "naked" acyclic gallyl [Ga{OB(NDippCH)2}2]- and indyl [In{OB(NDippCH)2}2]- anions (as their [K(2.2.2-crypt)]+ salts) through K+ abstraction from [KGa{OB(NDippCH)2}2] and [KIn{OB(NDippCH)2}2] using 2.2.2-crypt. These systems represent the first O-ligated gallyl/indyl systems, are ultimately accessed from cyclopentadienyl GaI/InI precursors by substitution chemistry, and display nucleophilic reactivity which is strongly influenced by the presence (or otherwise) of the K+ counterion.

Carbene-Stabilized Phosphagermylenylidene: A Heavier Analog of Isonitrile

Phosphagermylenylidenes (R-P═Ge), as heavier analogs of isonitriles, whether in their free state or as complexes with a Lewis base, have not been previously identified as isolable entities. In this study, we report the synthesis of a stable monomeric phosphagermylenylidene within the coordination sphere of a Lewis base under ambient conditions. This species was synthesized by Lewis base-induced dedimerization of a cyclic phosphagermylenylidene dimer or via Me3SiCl elimination from a phosphinochlorogermylene framework. The deliberate integration of a bulky, electropositive N-heterocyclic boryl group at the phosphorus site, combined with coordination stabilization by a cyclic (alkyl)(amino)carbene at the low-valent germanium site, effectively mitigated its natural tendency toward oligomerization. Structural analyses and theoretical calculations have demonstrated that this unprecedented species features a P═Ge double bond, characterized by conventional electron-sharing π and σ bonds, complemented by lone pairs at both the phosphorus and germanium atoms. Preliminary reactivity studies show that this base-stabilized phosphagermylenylidene demonstrates facile release of ligands at the Ge atom, coordination to silver through the lone pair on P, and versatile reactivity including both (cyclo)addition and cleavage of the P═Ge double bond.

Redox- and Charge-State Dependent Trends in 5, 6, and 7-Membered Boron Heterocycles: A Neutral Ligand Coordination Chemistry Approach to Boracyclic Cations, Anions, and Radicals

ConspectusBoron heterocycles represent an important subset of heteroatom-incorporated rings, attracting attention from organic, inorganic, and materials chemists. The empty pz orbital at the boron center makes them stand out as quintessential Lewis acidic molecules, also serving as a means to modulate electronic structure and photophysical properties in a facile manner. As boracycles are ripe for extensive functionalization, they are used in catalysis, chemical biology, materials science, and continue to be explored as chemical synthons for conjugated materials and reagents. Neutral boron(III)-incorporated polycyclic molecules are some of the most studied types of boracycles, and understanding their redox transformations is important for applications relying on electron transfer and charge transport. While relevant redox species can often be electrochemically observed, it remains challenging to isolate and characterize boracycles where the boron center and/or polycyclic skeleton have been chemically reduced.We describe our recent work isolating 5-, 6-, and 7-membered boracyclic radicals, anions, and cations, focusing on stabilization strategies, ligand-mediated bonding situations, and reactivity. We present a versatile neutral ligand coordination chemistry approach that permits the transformation of boracycles from potent electrophiles to powerful nucleophilic heterocycles that facilitate diverse electron transfer and bond activation chemistry. Although there are a wide range of suitable stabilizing ligands, we have employed both diamino-N-heterocyclic carbenes (NHCs) and cyclic(alkyl)(amino) carbenes (CAACs), which led to boracycles with tunable electronic structures and aromaticity trends. We highlight successful isolation of borafluorene radicals and demonstrate their reversible redox behavior, undergoing oxidation to the cation or reduction to the anion. The borafluorene anion is a chemical synthon that has been used to prepare boryl main-group and transition-metal bonds, luminescent oxabora-spirocycles, borafluorenate-crown ethers, and CO-releasing molecules via carbon dioxide activation. We expanded to 6-membered boracycles and characterized neutral bis(NHC-supported 9-boraphenanthrene)s and the corresponding bis(CAAC-stabilized 9-boraphenanthrene) biradical. We detail the interconvertible multiredox states of boraphenalene, where the boraphenalenyl radical, anion, and cation mimic the charge-states of the all-hydrocarbon analogue. Reactivity studies of the boraphenalenyl anion displayed unusual nucleophilic reactivity at multiple sites on the periphery of the boraphenalenyl tricyclic scaffold. Reduced borepins, 7-membered boron containing heterocycles, have also been isolated. We used a stepwise one-pot synthesis combining the halo-borepin precursor, CAAC, and KC8 to afford the monomeric borepin radicals and anions. The π-system was extended to contain two borepin rings fused in a pentacyclic scaffold, which permitted isolation of diborepin biradicals and a diborepin containing a dibora-quinone core.Our goal is to provide a guide explaining the current structure-function trends and isolation strategies for redox-active boron-incorporated polycyclic molecules to initiate the rational design and use of these types of compounds across a vast chemical space.

Boryls, their compounds and reactivity: a structure and bonding perspective

Boryls and their compounds are important due to their diverse range of applications in the fields of materials science and catalysis. They are an integral part of boron chemistry, which has attracted tremendous research interest over the past few decades. In this perspective, we provide an in-depth analysis of the reaction chemistry of boryl compounds from a structure and bonding perspective. We discuss the reactivity of boryls in various transition metal complexes and diborane(4) compounds towards different substrate molecules, with a focus on their nucleophilic and electrophilic properties in various reaction processes. Additionally, we briefly discuss the reactivity of boryl radicals. Our analysis sheds new light on the unique properties of boryls and their potential for catalytic applications.

Boron diamide derivatives containing N-N and N-P molecular fragments

The cationic and neutral boron-diamide precursors are employed to target the inclusion of N2 and N-P molecular fragments. The species (HCN(Dipp))2BNH25, and (H2CN(Dipp))2BNH26 were prepared. While efforts to oxidize with [NO]+ gave mixtures of products, reactions with N2H4 gave the salts [(HCN(Dipp))2B(NHNH3)][O3SCF3] 7 [(H2CN(Dipp))2B(NHNH3)][O3SCF3] 8. Excess N2H4 gave the neutral species (HCN(Dipp))2B(NHNH2) 9 and (H2CN(Dipp))2B(NHNH2) 10, respectively. The species (H2CN(Dipp))2B(N3) 11 was prepared for comparative purposes. Turning to related NP species, compound 6 was converted to (HCN(Dipp))2B(NHPCl2) 12, while (HCN(Dipp))2BNK(SiMe3) 14 was used to give (HCN(Dipp))2BN(SiMe3)PCl215. Replacement of one of the chlorides gave (HCN(Dipp))2BN(SiMe3)PCl(OSO2CF3) 16 which converts to [(HCN(Dipp))2BNPCl]217. Similarly heating 15 afforded 17. The insights for the synthesis of further boron-N2 and boron-NP derivatives are discussed.

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.

Intramolecular C-N bond activation by a transient boryl anion

Using a flexible diamido framework, a bulky boron bromide has been prepared as a precusor to a boryl anion with an extremely wide N-B-N angle. Reduction of the compound with lithium metal resulted in intramolecular C-N bond activation and migration of an aryl group onto the boron centre. Reaction of the boron bromide with K[FeCp(CO)2] resulted in nucleophilic reactivity of a carbonyl oxygen and the cooperative activation of CO.

A complete series of N-heterocyclic tetrylenes (Si-Pb) with a 1,1'-ferrocenediyl backbone enabled by 1,3,2-diazaborolyl N-substituents

The use of bulky 1,3,2-diazaborolyl N-substituents has allowed the synthesis of the complete series of ferrocene-based N-heterocyclic tetrylenes fc[(N{B})2E] (fc = 1,1'-ferrocenediyl, {B} = (HCNC6H3-2,6-iPr2)2B, E = Si-Pb). The silylene fc[(N{B})2Si] is inert towards NH3, CO2 or N2O under ambient conditions and thus significantly less reactive than the N-aryl homologue fc[(NC6H3-2,6-iPr2)2Si]. In accord with its higher reactivity, computational results indicate a more pronounced ambiphilicity of fc[(NC6H3-2,6-iPr2)2Si]. Our computational investigation of the model compound fc[(NBMe2)2Si] suggests that silylenes of this type may be superior to fc[(NC6H3-2,6-iPr2)2Si] in terms of ambiphilicity.

Aluminum-Boron Bond Formation by Boron Ester Oxidative Addition at an Alumanyl Anion

The potassium diamidoalumanyl, [K{Al(SiNDipp)}]2 (SiNDipp = {CH2SiMe2NDipp}2), reacts with the terminal B-O bonds of pinacolato boron esters, ROBpin (R = Me, i-Pr), and B(OMe)3 to provide potsassium (alkoxy)borylaluminate derivatives, [K{Al(SiNDipp)(OR)(Bpin)}]n (R = Me, n = 2; R = i-Pr, n = ∞) and [K{Al(SiNDipp)(OMe)(B(OMe)2)}]∞, comprising Al-B σ bonds. An initial assay of the reactivity of these species with the heteroallene molecules, N,N'-diisopropylcarbodiimide and CO2, highlights the kinetic inaccessibility of their Al-B bonds; only decomposition at high temperature is observed with the carbodiimide, whereas CO2 preferentially inserts into the Al-O bond of [K{Al(SiNDipp)(OMe)(Bpin)}]2 to provide a dimeric methyl carbonate species. Treatment of the acyclic dimethoxyboryl species, however, successfully liberates a terminal alumaboronic ester featuring trigonal N2Al-BO2 coordination environments at both boron and aluminum.

The emerging chemistry of the aluminyl anion

The chemistry of low valent p-block metal complexes continues to elicit interest in the research community, demonstrating reactivity that replicates and in some cases exceeds that of their more widely studied d-block metal counterparts. The introduction of the first aluminyl anion, a complex containing a formally anionic Al(I) centre charge balanced by an alkali metal (AM) cation, has established a platform for a new area of chemical research. The chemistry displayed by aluminyl compounds is expanding rapidly, with examples of reactivity towards a diverse range of small molecules and functional groups now reported in the literature. Herein we present an account of the structure and reactivity of the growing family of aluminyl compounds. In this context we examine the structural relationships between the aluminyl anion and the AM cations, which now include examples of AM = Li, Na, K, Rb and Cs. We report on the ability of these compounds to engage in bond-breaking and bond-forming reactions, which is leading towards their application as useful reagents in chemical synthesis. Furthermore we discuss the chemistry of bimetallic complexes containing direct Al-M bonds (M = Li, Na, K, Mg, Ca, Cu, Ag, Au, Zn) and compounds with Al-E multiple bonds (E = NR, CR₂, O, S, Se, Te), where both classes of compound are derived directly from aluminyl anions.

Multiple stable redox states and tunable ground states via the marriage of viologens and Chichibabin's hydrocarbon†

Chichibabin's hydrocarbon and viologens are among the most famous diradicaloids and organic redox systems, respectively. However, each has its own disadvantages: the instability of the former and its charged species, and the closed-shell nature of the neutral species derived from the latter, respectively. Herein, we report that terminal borylation and central distortion of 4,4′-bipyridine allow us to readily isolate the first bis-BN-based analogues (1 and 2) of Chichibabin's hydrocarbon with three stable redox states and tunable ground states. Electrochemically, both compounds exhibit two reversible oxidation processes with wide redox ranges. One- and two-electron chemical oxidations of 1 afford the crystalline radical cation 1˙⁺ and dication 1²⁺, respectively. Moreover, the ground states of 1 and 2 are tunable with 1 as a closed-shell singlet and the tetramethyl-substituted 2 as an open-shell singlet, the latter of which could be thermally excited to its triplet state because of the small singlet-triplet gap.

Herein, we report the isolation of bis-BN-based species 1 and 2 with multiple stable redox states. Their ground states are tunable with 1 as a closed-shell singlet and 2 as an open-shell singlet with a small singlet-triplet gap.

Crystal structure of (1,4,7,10,13,16-hexaoxacyclooctadecane-κ 6O6)potassium(2-methylphenylamino)ethyl-2-methylphenylamide ammoniate (1/3.5), [K(18-crown-6)](o-CH3C6H4)NH(CH2)2N(o-CH3C6H4) 3.5 NH3, C28H53.5KN5.5O6

C28H53.5KN5.5O6, monoclinic, Pc (no. 7), a = 18.7986(12) Å, b = 8.3431(6) Å, c = 22.4638(16) Å, β = 100.554(5)°, V = 3463.6(4) Å3, Z = 4, Rgt (F) = 0.0712, wRref (F 2) = 0.2226, T = 150 K.

Systematic Electronic and Structural Studies of 9-Carbene-9-Borafluorene Monoanions and Transformations into Luminescent Boron Spirocycles

The impact of the exact spatial arrangement of the alkali metal on the electronic properties of 9-carbene-9-borafluorene monoanions is assessed, and a series of [K][9-CAAC-9-borafluorene] complexes (1-4) have been isolated (CAAC = cyclic(alkyl)(amino) carbene, (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene). Compound 1, which contains [B]-K(THF)₃ interactions, is compared to charge-separated 2-4, which were prepared by capturing the potassium cations with 18-crown-6, 2.2.2-cryptand, or 1,10-phenanthroline. Notably, the ¹¹B NMR spectra of charge-separated borafluorene monoanions 2-4 show distinct low-field signatures compared to 1. Theoretical calculations indicate that charge separation may be exploited to influence the nucleophilic and electron transfer properties of 9-carbene-9-borafluorene monoanions. When [K(2.2.2-cryptand)][9-CAAC-9-borafluorene] (3) is reacted with 9,10-phenanthrenequinone and 1,10-phenanthroline-5,6-dione, the carbene ligand is displaced, and new air-stable R₂BO₂ spirocycles are formed (5 and 6, respectively). Remarkably, compounds 5 and 6 display fluorescence under UV light in both the solid and solution phases with quantum yields of up to 20%. In addition, a drastic red-shift in the emission color is observed in 6 because of the presence of the nitrogen atoms on the phenanthroline moiety. Mechanistic insights into the formation of these spirocycles are also described based on density functional theory calculations.

Reductive Al-B σ-Bond Formation in Alumaboranes: Facile Scission of Polar Multiple Bonds

We present facile access to an alumaborane species with electron precise Al-B σ-bond. The reductive rearrangement of 1-(AlI₂ ), 8-(BMes₂ ) naphthalene (Mes=2,4,6-Me₃ C₆ H₂ ) affords the alumaborane species cyclo-(1,8-C₁₀ H₆ )-[1-Al(Mes)(OEt₂ )-8-B(Mes)] with a covalent Al-B σ-bond. The Al-B σ-bond performs the reductive scission of multiple bonds: S=C(NiPrCMe)₂ affords the naphthalene bridged motif B-S-Al(NHC), NHC=N-heterocyclic carbene, while O=CPh₂ is deoxygenated to afford an B-O-Al bridged species with incorporation of the remaining ≡CPh₂ fragment into the naphthalene scaffold. The reaction with isonitrile Xyl-N≡C (Xyl=2,6-Me₂ C₆ H₄ ) proceeds via a proposed (amino boryl) carbene species; which adds a second equivalent of isonitrile to ultimately form the Al-N-B bridged species cyclo-(1,8-C₁₀ H₆ )-[1-Al(Mes)-N(Xyl)-8-B{C(Mes)=C-N-Xyl}] with complete scission of the C≡N triple bond. The latter reaction is supported with isolated intermediates and by DFT calculations.

Activation of Carbon Dioxide by 9-Carbene-9-borafluorene Monoanion: Carbon Monoxide Releasing Transformation of Trioxaborinanone to Luminescent Dioxaborinanone

The first structurally characterized example of a trioxaborinanone (2) is produced by the reaction of a 9-carbene-9-borafluorene monoanion and carbon dioxide. When compound 2 is heated or irradiated with UV light, carbon monoxide (CO) is released, and a luminescent dioxaborinanone (3) is formed. Notably, carbon monoxide releasing molecules (CORMs) are of interest for their ability to deliver a specific amount of CO. Due to the turn-on fluorescence observed as a result of the conversion to 3, CORM 2 serves as a means to optically observe CO loss "by eye" under thermal or photochemical conditions.

Copper-Catalyzed Borylation of Acyl Chlorides with an Alkoxy Diboron Reagent: A Facile Route to Acylboron Compounds

Herein, the copper‐catalyzed borylation of readily available acyl chlorides with bis(pinacolato)diboron, (B₂pin₂) or bis(neopentane glycolato)diboron (B₂neop₂) is reported, which provides stable potassium acyltrifluoroborates (KATs) in good yields from the acylboronate esters. A variety of functional groups are tolerated under the mild reaction conditions (room temperature) and substrates containing different carbon‐skeletons, such as aryl, heteroaryl and primary, secondary, tertiary alkyl are applicable. Acyl N‐methyliminodiacetic acid (MIDA) boronates can also been accessed by modification of the workup procedures. This process is scalable and also amenable to the late‐stage conversion of carboxylic acid‐containing drugs into their acylboron analogues, which have been challenging to prepare previously. A catalytic mechanism is proposed based on in situ monitoring of the reaction between p‐toluoyl chloride and an NHC‐copper(I) boryl complex as well as the isolation of an unusual lithium acylBpinOBpin compound as a key intermediate.

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.

Diverse reactivity of an Al(I)-centred anion towards ketones

The reactivity of a seven-membered cyclic potassium diamidoalumanyl toward a variety of ketone small molecules has been assessed. Whilst acetophenone generates an aluminium pinacolate derivative, reductive C-C coupling is induced between the ketyl and ortho-carbon centres of two equivalents of benzophenone. In contrast, whereas oxidative addition of an enolisable proton is observed with 2,4-dimethyl-3-pentanone, 2,2,4,4-tetramethyl-3-pentanone undergoes an unprecedented hydroalumination process, where the reducing hydride may be traced to intramolecular oxidative addition of a (sp³)C-H bond.

Asymmetric C(sp3)-H borylation: an update

Chiral organoboronates have emerged as a key intermediate in the development of pharmaceuticals and materials science. Therefore, several attempts have been made to design various synthetic methods to easily furnish these compounds during the past few decades. Inter alia, asymmetric catalysis has been increasing rapidly as a viable, practical and beneficial strategy for their preparation. In this respect, recent years have witnessed significant progress in aliphatic C-H borylation as the generated carbon-boron bonds are largely utilized to produce other carbon-carbon, carbon-nitrogen and carbon-oxygen bonds. This review presents a detailed overview and analysis of transition metal-catalyzed asymmetric C(sp³)-H borylation strategies. Overall, it assembles all the recent developments in this particular synthetic avenue up to March 2022.

Crystalline radical cations of bis-BN-based analogues of Thiele's hydrocarbon

Two radical cations of bis-BN-based analogues of Thiele's hydrocarbons were facilely synthesized, fully characterized, and theoretically investigated. One-electron oxidation leads to the reduced bond length alternation and NICS values of the central C₄N₂ rings, suggesting the decreasing antiaromatic character. The spin density of the radical cations is significantly delocalized over the central linkers with a small contribution from two terminal N-heterocyclic boryl units.

Calix[4]pyrrolato Stannate(II): A Tetraamido Tin(II) Dianion and Strong Metal‐Centered σ‐Donor

Anionic, metal‐centered nucleophiles are emerging compounds with unique reactivities. Here, we describe the isolation and full characterization of the first tetraamido tin(II) dianion, its behavior as ligand towards transition metals, and its reactivity as a tin‐centered nucleophile. Experimental values such as the Tolman electronic parameter (TEP) and computations attest tin‐located σ‐donor ability exceeding that of carbenes or electron‐rich phosphines. Against transition metals, the stannate(II) can act as η¹‐ or η⁵‐type ligand. With aldehydes, it reacts by hydride substitution to give valuable acyl stannates. The reductive dehalogenation of iodobenzene indicates facile redox pathways mediated by halogen bond interaction. Calix[4]pyrrolato stannate(II) represents the first example of this macrocyclic ligand in low‐valent p‐block element chemistry.

Sequential Reduction of Borylstibane to an Electronically Nonsymmetric Diboryldistibene Radical Anion

Understanding the formation of metal-metal bonds and their electronic structures is still a scientific task. We herein report on the stepwise synthesis of boryl-substituted antimony compounds in which the antimony atoms adopt four different oxidation states (+III, +II, +I, +I/0). Sb-C bond homolysis of Cp*[(HCNDip)₂B]SbCl (1; Cp* = C₅Me₅; Dip = 2,6-iPr₂C₆H₃) gave diboryldichlorodistibane [(HCNDip)₂BSbCl]₂ (2), which reacted with KC₈ to form diboryldistibene [(HCNDip)₂BSb]₂ (3) and traces of cyclotetrastibane [(HCNDip)₂B]₃Sb₄Cl (5). One-electron reduction of 3 yielded the potassium salt of the diboryldistibene radical anion [(HCNDip)₂BSb]₂̇^-, [K(18-c-6)(OEt₂)][{(HCNDip)₂BSb}₂] (4), which exhibits an unprecedented inequivalent spin localization on the Sb-Sb bond and hence an unsymmetric electronic structure. Compounds 1-4 were characterized by heteronuclear nuclear magnetic resonance (NMR) (¹H, ¹³C, ¹¹B), infrared (IR), ultraviolet-visible (UV-vis) spectroscopy (3, 4), and single crystal X-ray diffraction (sc-XRD, 1-5), while the bonding nature of 3 and 4 was analyzed by quantum chemical calculations. EPR spectroscopy resolves the dissimilar Sb hyperfine tensors of 4, reflecting the inequivalent spin distribution, setting 4 uniquely apart from all previously characterized dipnictene radical anions.

Nucleophilic borylation of fluorobenzenes with reduced arylboranes

Two challenging but rewarding topics in chemical synthesis are C-F-bond activation and the development of B-centered nucleophiles. We have now tackled both subjects simultaneously by forming aryl-B bonds via S_NAr-type reactions on fluorobenzenes under mild conditions using Na₂[FluBBFlu], Li₂[HBFlu], and Li₂[Me₂DBA] (BFlu = 9-borafluorenyl, Me₂DBA = 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene).

Advances in silylation and borylation of fluoroarenes and gem-difluoroalkenes via C-F bond cleavage

Organoboron and organosilane compounds are widely used in organic synthesis and pharmaceuticals. In addition, the C-F bond functionalization is a useful tool for the construction of carbon-carbon and carbon-heteroatom bonds. In particular, the late-stage functionalization of bioactive molecules through defluoroborylation and defluorosilylation reactions will provide good opportunities for the development and diversification of new medicinal compounds. Thus, this feature article summarized the methods for the defluorosilylation and defluoroborylation of unreactive monofluoroarenes and gem-difluoroalkenes from 2000 to 2021, which might create some new ideas and will be helpful for further research in this field. These defluoroborylation and defluorosilylation strategies can be applied to synthesize silylated arenes, borylated arenes, silylated fluoroalkenes, and borylated fluoroalkenes, thus providing impressive advantages over traditional methods for the synthesis of organoboron and organosilane compounds in terms of divergent structures.

Seven-Membered Cyclic Potassium Diamidoalumanyls

The seven-membered cyclic potassium alumanyl species, [{SiN^Mes }AlK]₂ [{SiN^Mes }={CH₂ SiMe₂ N(Mes)}₂ ; Mes=2,4,6-Me₃ C₆ H₂ ], which adopts a dimeric structure supported by flanking K-aryl interactions, has been isolated either by direct reduction of the iodide precursor, [{SiN^Mes }AlI], or in a stepwise manner via the intermediate dialumane, [{SiN^Mes }Al]₂ . Although the intermediate dialumane has not been observed by reduction of a Dipp-substituted analogue (Dipp=2,6-i-Pr₂ C₆ H₃ ), partial oxidation of the potassium alumanyl species, [{SiN^Dipp }AlK]₂ , where {SiN^Dipp }={CH₂ SiMe₂ N(Dipp)}₂ , provided the extremely encumbered dialumane [{SiN^Dipp }Al]₂ . [{SiN^Dipp }AlK]₂ reacts with toluene by reductive activation of a methyl C(sp³ )-H bond to provide the benzyl hydridoaluminate, [{SiN^Dipp }AlH(CH₂ Ph)]K, and as a nucleophile with BPh₃ and RN=C=NR (R=i-Pr, Cy) to yield the respective Al-B- and Al-C-bonded potassium aluminaborate and alumina-amidinate products. The dimeric structure of [{SiN^Dipp }AlK]₂ can be disrupted by partial or complete sequestration of potassium. Equimolar reactions with 18-crown-6 result in the corresponding monomeric potassium alumanyl, [{SiN^Dipp }Al-K(18-cr-6)], which provides a rare example of a direct Al-K contact. In contrast, complete encapsulation of the potassium cation of [{SiN^Dipp }AlK]₂ , either by an excess of 18-cr-6 or 2,2,2-cryptand, allows the respective isolation of bright orange charge-separated species comprising the 'free' [{SiN^Dipp }Al]^- alumanyl anion. Density functional theory (DFT) calculations performed on this moiety indicate HOMO-LUMO energy gaps in the of order 200-250 kJ mol^-1 .

Reactions of 9-Carbene-9-Borafluorene Monoanion and Selenium: Synthesis of Boryl-Substituted Selenides and Diselenides

Reactions of 9-carbene-9-borafluorene monoanion (1) with elemental selenium and selenium-containing reagents are reported. When compound 1 is reacted with grey selenium in THF, various boryl-substituted selenides and diselenides are produced (2-6), including molecules resulting from migration of the carbene ligand Dipp group (Dipp = 2,6-diisopropylphenyl). However, when a similar reaction between 1 and grey selenium is performed in toluene in the presence of 18-crown-6, boryl-substituted selenide 7 is obtained as the sole boron-containing product. As compound 7 is the monomeric variant of organoselenide 3, 18-crown-6 promotes both product selectivity and solubility in a nonpolar solvent. Diselenide 5, which features a trans-bent B-Se-Se-B core, was directly isolated via reaction of 1 with Se₂Cl₂ in THF. Computational modeling suggests that the formation of 5 proceeds via a radical mechanism. This was supported by an experiment demonstrating that the CAAC-borafluorene radical also reacts with SeCl₂ to yield 5 [CAAC = (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene]. Energy decomposition analysis of 5 indicates a covalent borafluorene-diselenide bond (ΔE_int, -168.9 kcal mol^-1). All of the new compounds were fully characterized via single-crystal X-ray diffraction and multinuclear nuclear magnetic resonance (¹H, ¹³C, ¹¹B, and ⁷⁷Se).

Haloboration: scope, mechanism and utility

Haloboration, the addition of B–X (X = Cl, Br, I) across an unsaturated moiety e.g., C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Y or C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Y (Y = C, N, etc.), is dramatically less utilised than the ubiquitous hydroboration reaction. However, haloboration of alkynes in particular is a useful tool to access ambiphilic 1,2-disubstituted alkenes. The stereochemical outcome of the reaction is easily controlled and the resulting products have proven to be valuable building blocks in organic synthesis and materials chemistry. This review aims at providing the reader with a brief summary of the historic development and of the current mechanistic understanding of this transformation. Recent developments are discussed and select examples demonstrating the use of haloboration products are given with a focus on the major areas, specifically, natural product synthesis and the development of boron-doped polycyclic aromatic hydrocarbons (B-PAHs).

Haloboration is a mostly overlooked technique, yet it is a powerful way of transforming alkynes stereoselectively into difunctionalised ambiphilic alkenes, which readily undergo a plethora of highly useful subsequent reactions.

Probing the Extremes of Covalency in M-Al bonds: Lithium and Zinc Aluminyl Compounds

Synthetic routes to lithium, magnesium, and zinc aluminyl complexes are reported, allowing for the first structural characterization of an unsupported lithium-aluminium bond. Crystallographic and quantum-chemical studies are consistent with the presence of a highly polar Li-Al interaction, characterized by a low bond order and relatively little charge transfer from Al to Li. Comparison with magnesium and zinc aluminyl systems reveals changes to both the M-Al bond and the (NON)Al fragment (where NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), consistent with a more covalent character, with the latter complex being shown to react with CO₂ via a pathway that implies that the zinc centre acts as the nucleophilic partner.

Reactions of a diborylstannylene with CO2 and N2O: diboration of carbon dioxide by a main group bis(boryl) complex

The reactions of the boryl-substituted stannylene Sn{B(NDippCH)2}2 (1) with carbon dioxide have been investigated and shown to proceed via pathways involving insertion into the Sn-B bond(s). In the first instance this leads to formation of the (boryl)tin(ii) borylcarboxylate complex Sn{B(NDippCH)2}{O2CB(NDippCH)2} (2), which has been structurally characterized and shown to feature a κ2 mode of coordination of the [(HCDippN)2BCO2]- ligand at the metal centre. 2 undergoes B-O reductive elimination in hexane solution (in the absence of further CO2) to give the boryl(borylcarboxylate)ester {(HCDippN)2B}O2C{B(NDippCH)2} (3) i.e. the product of formal diboration of carbon dioxide. Alternatively, 2 can assimilate a second equivalent of CO2 to give the homoleptic bis(borylcarboxylate) Sn{O2CB(NDippCH)2}2 (4), which can be prepared via an alternative route from SnBr2 and the potassium salt of [(HCDippN)2BCO2]-, and structurally characterized as its DMAP (N,N-dimethylaminopyridine) adduct. Structural and reactivity studies also point to the possibility for extrusion of CO from the [(HCDippN)2BCO2]- fragment to generate the boryloxy system [(HCDippN)2BO]-, a ligand which can be generated directly from 1via reaction with N2O. The initially formed unsymmetrical species Sn{B(NDippCH)2}{OB(NDippCH)2} has been shown to be amenable to crystallographic study in the solid state, but to undergo ligand redistribution in solution to generate a mixture of 1 and the bis(boryloxy) complex Sn{OB(NDippCH)2}2.

Boron- versus Nitrogen-Centered Nucleophilic Reactivity of (Cyano)hydroboryl Anions: Synthesis of Cyano(hydro)organoboranes and 2-Aza-1,4-diborabutatrienes

Cyclic alkyl(amino)carbene-stabilized (cyano)hydroboryl anions were synthesized by deprotonation of (cyano)dihydroborane precursors. While they display boron-centered nucleophilic reactivity towards organohalides, generating fully unsymmetrically substituted cyano(hydro)organoboranes, they show cyano-nitrogen-centered nucleophilic reactivity towards haloboranes, resulting in the formation of hitherto unknown linear 2-aza-1,4-diborabutatrienes.

Electron doping of single-walled carbon nanotubes using pyridine-boryl radicals

Pyridine-boryl (py-boryl) radicals serve as efficient electron-doping reagents for single-walled carbon nanotubes (SWCNTs). The doping mechanism comprises electron transfer from the py-boryl radical to the SWCNT. The formation of a stable py-boryl cation is essential for efficient doping; the captodative effect of the py-boryl cation is important to this process.

Stabilization of the Elusive 9-Carbene-9-Borafluorene Monoanion

Two-electron reduction of carbene-supported 9-bromo-9-borafluorenes with excess KC₈ , Na, or Li-naphthalenide affords six N-heterocyclic carbene (NHC)- or cyclic(alkyl)(amino) carbene (CAAC)-stabilized borafluorene anions (3-8)-the first isolated and structurally authenticated examples of the elusive 9-carbene-9-borafluorene monoanion. The electronic structure, bonding, and aromaticity of the boracyclic anions were comprehensively investigated via computational studies. Compounds 5 and 8 react with metal halides via salt elimination to give new B-E (E=Au, Se, Ge)-containing materials (9-12). Upon reaction with diketones, the carbene ligand cleanly dissociates from 5 or 8 to yield new B-O containing spirocycles (13-14) that cannot be easily obtained using "normal" valent borafluorene compounds. Collectively, these results support the notion that carbene-stabilized monoanionic borafluorenes may serve as a new platform for the one-step construction of higher-value boracyclic materials.

Anionic oxoborane and thioxoborane molecules supported by a 1,2-bis(imino)acenaphthene ligand

The isolable anionic oxoborane 3 and thioxoborane 4 have been assembled using a 1,2-bis(imino)acenaphthene ligand (Dip-BIAN). Structural characterization and DFT calculations confirmed that two compounds contain terminal doubly bonded B[double bond, length as m-dash]E (E = O, S) groups, respectively, in which only the B[double bond, length as m-dash]O group is associated with imidazolium via a hydrogen bond.

Acyl metalloids: conformity and deviation from carbonyl reactivity

Once considered as mere curiosities, acyl metalloids are now recognized for their utility in enabling chemical synthesis. This perspective considers the reactivity displayed by acylboron, -silicon, -germanium, and tellurium species. By highlighting the role of these species in various transformations, we demonstrate how differences between the comprising elements result in varied reaction outcomes. While acylboron compounds are primarily used in polar transformations, germanium and tellurium species have found utility as radical precursors. Applications of acylsilanes are comparatively more diverse, owing to the possibility to access both radical and polar chemistry.

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

Reactivity of a magnesium diboranate with organic nitriles

A series of complexes generated through reactions of the β-diketiminato magnesium diboranate species, [(BDI)Mg{(n-Bu)pinB-Bpin}] (BDI = HC{(Me)CNDipp}2; Dipp = 2,6-di-iso-propylphenyl), and a variety of organic nitriles are reported. Although, in every case, the diboranate anion acts as a surrogate source of the {Bpin} nucleophile, resulting in B-C bond formation at the electrophilic sp-hydridised nitrile carbon, the resultant compounds display a variable propensity to undergo subsequent reaction with additional nitrile equivalents. This behaviour is rationalised to be a consequence of substituent-dependent modulation in the basicity and resultant electrophilicity of magnesium-coordinated nitrile intermediates.