Synthesis and Reactivity of a Dialane-Bridged Diradical

Radicals of the lightest group 13 element, boron, are well established and observed in numerous forms. In contrast to boron, radical chemistry involving the heavier group 13 elements (aluminum, gallium, indium, and thallium) remains largely underexplored, primarily attributed to the formidable synthetic challenges associated with these elements. Herein, we report the synthesis and isolation of planar and twisted conformers of a doubly CAAC (cyclic alkyl(amino)carbene)-radical-substituted dialane. Extensive characterization through spectroscopic analyses and X-ray crystallography confirms their identity, while quantum chemical calculations support their open-shell nature and provide further insights into their electronic structures. The dialane-connected diradicals exhibit high susceptibility to oxidation, as evidenced by electrochemical measurements and reactions with o-chloranil and a variety of organic azides. This study opens a previously uncharted class of dialuminum systems to study, broadening the scope of diradical chemistry and its potential applications.

Synthesis and Reactivity of Tricoordinate Organoberyllium Azides

A series of terminal mono- and disubstituted beryllium azides of the form [(CAAC)Be(N3)R] (R=CAACH, Dur; CAACH/CAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-yl/idene, Dur=2,3,5,6-tetramethylphenyl) and [L2Be(N3)2] (L=CAACNH=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-imine, IiPrMe=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), respectively, were synthesized and characterized by NMR spectroscopy and X-ray crystallography. Thermolysis and photolysis products of these first examples of tricoordinate azidoberyllium complexes evidence extensive ligand scrambling and the formal insertion of nitrenes into the CAAC-Be bond, generating cyclic alkyl(amino)imine (CAAI) ligands. Furthermore, the reaction with a small N-heterocyclic carbene (NHC) leads to unexpected CAAC-NHC ligand exchange, while the reaction with pentaphenylborole yields the first γ-azide adduct of a borole, long postulated to be the first step in the synthesis of 1,2-azaborinines from boroles and azides.

Bypassing Ammonia: From N2 to Nitrogen Heterocycles without N1 Intermediates or Transition Metals

Diboradiazene compounds, derived in one step from the boron-mediated reduction of dinitrogen (N2), were treated separately with sulfur and acetic anhydride, providing heterocyclic compounds that are BN isosteres of thiophene and 1,3-oxazole, respectively. These simple reactions represent the final steps in two-step routes to complex heterocycles from N2 that both circumvent the need for transition metal reagents and completely bypass the traditional intermediate ammonia.

Peter Paetzold (1935-2023): a Chemist of Many Talents

Peter Paetzold passed away on August 17, 2023 in Aachen, Germany. One of the great pioneers of low-valent boron and boron cluster chemistry, he will also be remembered for his outstanding teaching and the passion he brought to all of his many interests beyond research.

Tricoordinate Beryllium Radicals and Their Reactivity

The one-electron reduction of [(CAAC)Be(Dur)Br] (CAAC = cyclic alkyl(amino)carbene, Dur = 2,3,5,6-tetramethylphenyl = duryl) with lithium sand in diethyl ether yields the first neutral, tricoordinate, and moderately stable beryllium radical, [(CAAC)(Et2O)BeDur]• (2-Et2O), which undergoes a facile second one-electron reduction concomitant with the insertion of the beryllium center into the endocyclic C-NCAAC bond and a cyclopropane-forming C-H bond activation of an adjacent methyl group. In situ generation of 2-Et2O and addition of PMe3 yield the stable analogue, [(CAAC)(Me3P)BeDur]• (2-PMe3), which serves as a platform for PMe3-ligand exchange with stronger donors, generating the radicals [(CAAC)LBeDur]• (2-L, L = isocyanides, pyridines, and N-heterocyclic carbenes). X-ray structural analyses show trigonal-planar beryllium centers and strong π backbonding from the metal to the CAAC ligand. The EPR signals of all six isolated [(CAAC)LBeDur]• radicals display significant, albeit small, hyperfine coupling to the 9Be nucleus. DFT calculations show that the spin density is mostly delocalized over the CAAC π framework and, where present, the isocyanide CN moiety, with only a small proportion (3-6%) on the beryllium center. 2-PMe3 proved thermally unstable at 80 °C, first undergoing radical hydrogen abstraction with the solvent, followed by insertion of beryllium into the endocyclic C-NCAAC bond and PMe3 transfer to the former carbene carbon atom. The reactions with diphenyl disulfide and phenyl azide occur at the beryllium center and yield the corresponding Be(II) phenyl sulfide and amino complexes, respectively, the latter concomitant with radical transfer and hydrogen abstraction by the beryllium-bound nitrogen center.

Synthesis, reduction and C-H activation chemistry of azaborinines with redox-active organoboryl substituents

The 2,3,4,5,6-pentaphenyl-1,2-azaborinin-1-yl (PPAB) potassium complex 1 undergoes facile salt metathesis with 9,10-dibromo-9,10-dihydroboraanthracene (DBABr2), 5-bromodibenzo[b,d]borole (DBBBr), 1-chlorotetraphenylborole (TPBCl) and dibromo(phenyl)borane (BBr2Ph) to yield the corresponding N-borylated azaborinines N-DBABr-PPAB (2, which hydrolyses and dimerises to the oxo-bridged N,N'-O(DBA)2-(PPAB)2, 3), N,N'-DBA-(PPAB)2 (4), N-DBB-PPAB (5), N-PPB-PPAB (7) and N-BBrPh-PPBA (9). Stepwise reduction of 4 yields the corresponding stable radical anion 4˙- and dianion 42-. One-electron reduction of 5 with KC8 yields the purple radical anion 5˙-, which forms a highly insoluble coordination polymer. 5˙- undergoes very slow radical intramolecular ortho-C-H activation at the C4-phenyl substituent of the PPAB moiety, yielding a BN-analogue of the 5,5'-spiro-bi[dibenzoborole] anion, [6]K. Compound 7 cannot be isolated and undergoes spontaneous and diastereoselective 2,5-anti-addition of the ortho-C-H bond of the PPAB C4-phenyl substituent to yield a novel BNB-analogue of the triply fused dihydrocyclopenta[l]phenanthrene cation, compound 8. Finally the one-electron reduction of 9 results in the ortho-C-H activation of the PPAB C4-phenyl substituent at an in situ-generated dicoordinate boryl anion (10), resulting in the formation of a BNB-analogue of 9H-fluorene, the borate 11-. DFT calculations provide a rationale for the diverse C-H activations observed in these reactions.

CAAC-stabilised 9,10-diboraanthracene: an electronically and structurally flexible platform for small-molecule activation and metal complexation

A biradical cyclic alkyl(amino)carbene-stabilised 9,10-diboraanthracene (DBA) activates small molecules such as hydrogen, phenyl azide or TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) across its two boron atoms, while reactions with [(MeCN)3M(CO)3] (M = Cr, Mo, W) yield the first half-sandwich DBA complexes of the form [(η6-DBA)M(CO)3].

Boron Insertion into the N≡N Bond of a Tungsten Dinitrogen Complex

The 1,3-addition of 1,2-diaryl-1,2-dibromodiboranes (B2Br2Ar2) to trans-[W(N2)2(dppe)2] (dppe = κ2-(Ph2PCH2)2), which is accompanied by a Br-Ar substituent exchange between the two boron atoms, is followed by a spontaneous rearrangement of the resulting tungsten diboranyldiazenido complex to a 2-aza-1,3-diboraallenylimido complex displaying a linear, cumulenic B=N=B moiety. This rearrangement involves the splitting of both the B-B and N=N bonds of the N2B2 ligand, formal insertion of a BAr boranediyl moiety into the N=N bond, and coordination of the remaining BArBr boryl moiety to the terminal nitrogen atom. Density functional theory calculations show that the reaction proceeds via a cyclic NB2 intermediate, followed by dissociation into a tungsten nitrido complex and a linear boryliminoborane, which recombine by adduct formation between the nitrido ligand and the electron-deficient iminoborane boron atom. The linear B=N=B moiety also undergoes facile 1,2-addition of Brønsted acids (HY = HOPh, HSPh, and H2NPh) with concomitant Y-Br substituent exchange at the terminal boron atom, yielding cationic (borylamino)borylimido tungsten complexes.

Experimental and Computational Study of a Confirmed Borylene-to-Diborene Dimerization

While the dimerization of heavier group 13 carbene analogues to the corresponding alkene analogues is known and relatively well understood, the dimerization of dicoordinate borylenes (LRB:, L = neutral donor; R = anionic substituent) to the corresponding diborenes (LRB═BRL) has never been directly observed. In this study we present the first example of a formal borylene-to-diborene dimerization through abstraction of a labile phosphine ligand from the tricoordinate hydroborylene precursor (CAAC)(Me3P)BH (CAAC = cyclic alkyl(amino)carbene) by bulky Lewis-acidic dihaloboranes (BX2Y, X = Cl, Br, Y = aryl, boryl), generating the corresponding dihydrodiborene (CAAC)HB═BH(CAAC) and (Me3P)BX2Y as the byproduct. An in-depth experimental and computational mechanistic analysis shows that this seemingly simple process (2 LL'BH + 2 BX2Y → LHB═BHL + 2 L'BX2Y) is in fact based on a complex sequence of finely tuned processes, involving the one-electron oxidation of and PMe3 abstraction from the borylene precursor by BX2Y, multiple halide transfers between (di)boron intermediates and BX2Y/[BX3Y]-, and multiple one-electron redox processes between diboron intermediates and the borylene precursor, which make the reaction ultimately autocatalytic in [(CAAC)(Me3P)BH]•+. The findings suggest that [LBXR]• boryl radicals are more likely coupling partners than dicoordinate LRB: borylenes in the reductive coupling of base-stabilized LBX2R boranes to LRB═BRL diborenes.

Synthesis and Reactivity of Highly Electron-Rich Zerovalent Group 10 Diborabenzene Pogo-Stick Complexes

A cyclic alkyl(amino)carbene (CAAC)-stabilized 1,4-diborabenzene (DBB, 1) reacts with the group 10 precursor [Ni(CO)4] to yield the DBB pogo-stick complex [(η6-DBB)Ni(CO)] (2) as a dark-green crystalline solid. The IR-spectroscopic and X-ray crystallographic data of 2 highlight the strong π-donor properties of the DBB ligand. The reaction of 1 with [M(nbe)2] (M = Pd, Pt; nbe = norbornene) yields the unique zerovalent heavier group 10 arene pogo-stick complexes [(η6-DBB)M(nbe)] (3-M), isolated as dark-purple and black crystalline solids, respectively. 3-Pd and 3-Pt show strong near-IR (NIR) absorptions at 835 and 904 nm, respectively. Time-dependent density functional theory (TD-DFT) calculations show that these result from the S0→S1 excitation, which corresponds to a transfer of electron density from a metal d orbital aligned with the z direction (dxz or dyz) to a d orbital located in the xy plane (dxy or dx2-y2), with the redshift for 3-Pt resulting from the higher spin-orbit coupling (SOC). In complex 2, electron donation from the nickel center into the carbonyl 2π* orbital destabilizes the DBB···Ni interaction, resulting in an absorption at a higher energy. Complexes 2 and 3-M react with [Fe(CO)5] to yield the doubly CO-bridged M(0)→Fe(0) (M = Ni, Pd, Pt) metal-only Lewis pairs (MOLPs) 4-M as black (M = Ni, Pt) and dark-turquoise (M = Pd) crystalline solids. Furthermore, 3-Pt undergoes oxidative Sn-H addition with Ph3SnH to yield the corresponding Pt(II) stannyl hydride, [(η6-DBB)PtH(SnPh3)] (5).

Dibenzoberylloles: antiaromatic s-block fluorene analogues

Among a series of Lewis-base-stabilised antiaromatic dibenzoberylloles (DBBes) the cyclic alkyl(amino)carbene (CAAC) analogue undergoes a complex but highly selective thermal decomposition, involving the breaking and making of four bonds each, which yields a rare beryllium η²-alkene complex. Two-electron reduction of the CAAC-stabilised DBBe analogue yields an aromatic dianion.

Structure and Electronics of a Series of CAAC-Stabilized Diboron-Doped Acenes from 1,4-Diboranaphthalene to 6,13-Diborapentacene

This combined experimental and theoretical study examines the influence of acene elongation, boron atom position, and acene substitution pattern on the structure and electronics of cyclic alkyl(amino)carbene (CAAC)-stabilized diboraacenes and presents the first syntheses of neutral diboranaphthalene (DBN) and diborapentacene (DBP). Whereas 2,3-diethyl-substituted 1,4-(CAAC)₂-^Et2DBN is isolated as a mixture of a planar (structurally characterized) NMR-active conformer and a presumably bent EPR-active conformer, 6,13-(CAAC)₂-DBP resembles 9,10-(CAAC)₂-DBA (DBA = diboraanthracene), with a highly puckered 6,13-DBP core and a typical biradical EPR signal. Both species are easily reduced to their puckered dianions. DFT calculations confirm that 6,13-(CAAC)₂-DBP is only stable in its bent conformation, whereas 1,4-(CAAC)₂-^Et2DBN exists as both flat closed-shell and bent open-shell biradical conformers, which interchange by thermally activated ethyl and CAAC rotation/diboraacene bending processes. An in-depth computational study of the series of unsubstituted, CAAC-stabilized, symmetrically diboron-doped acenes from 1,4-(CAAC)₂-DBN to 6,13-(CAAC)₂-DBP was carried out. The results show interesting trends dependent on the position of the boron atoms within the acene framework as well as on the relative orientation of the CAAC ligands, which enable fine-tuning of the electronic and structural features.

Synthesis, Structural Characterization, and Bonding of Molecular Heavier Beryllium Chalcogenides

The reactions of a cyclic (alkyl)amino carbene (CAAC)-stabilized beryllium radical with E2Ph2 (E = S, Se, Te) and of a beryllole with HEPh (E = S, Se) yield the corresponding beryllium phenylchalcogenides, including the first structurally authenticated beryllium selenide and telluride complexes. Calculations show that their Be-E bonds are best described by the interaction between the Be+ and E- fragments, with Coulombic forces accounting for ca. 55% of the attraction and orbital interactions dominated by the σ component.

Thienyl-Substituted Diboranes(4): Electronic Stabilization of Radicals versus Increased Reactivity towards Bond Activation

Low-valent main-group chemistry involves a balancing act between steric and electronic stabilization of the electron-rich low-oxidation-state main-group centers and their desired reactivity. Herein we show that the combination of sterically-shielding mesityl and rotationally flexible 2-thienyl groups, the latter having the potential to be either electronically stabilizing or activating, at a diborane(4) provides both radical-anion stabilization and unusual bond activation and rearrangement reactions. The addition of a Lewis base to a 1,2-dimesityl-1,2-dithienyldiborane(4) (1) results in direct and unprecedented C-H borylation of one thienyl substituent with cleavage of the B-B bond. The facile one-electron reduction of 1 yields a stable diboron radical anion through delocalization of its unpaired electron over the entire planar 1,2-dithienyldiboron framework, as evidenced by EPR spectroscopy and DFT calculations. The two-electron reduction of 1 with magnesium-anthracene under more forcing conditions results in B-B-bond cleavage and replacement of one thienyl sulfur atom by a mesitylboron moiety, leading to the formation of a magnesium complex of an η5-diborafulvene dianion. Salt metathesis of the latter with [(η6-p-cymene)RuCl2] affords a mixed ruthenium sandwich complex of an η5-borylborole dianion. Calculations highlight the structural and electronic changes in the boron-substituted heterocyclic C4B dianion upon switching coordination from magnesium (diborafulvene dianion) to ruthenium (borylborole dianion).

Achieving Control over the Reduction/Coupling Dichotomy of N2 by Boron Metallomimetics

We report a detailed computational and experimental study of the fixation and reductive coupling of dinitrogen with low-valent boron compounds. Consistent with our mechanistic findings, the selectivity toward nitrogen fixation or coupling can be controlled through either steric bulk or the reaction conditions, allowing for the on-demand synthesis of nitrogen chains. The electronic structure and intriguing magnetic properties of intermediates and products of the reaction of dinitrogen with borylenes are also elucidated using high-level computational approaches.

Cyclic alkyl(amino)iminates (CAAIs) as strong 2σ,4π-electron donor ligands for the stabilisation of boranes and diboranes(4): a synthetic and computational study

Singly and doubly cyclic alkyl(amino)iminate (CAAI)-substituted boranes and diboranes(4) were synthesised by halosilane elimination between a silylimine and halo(di)borane precursors. ¹¹B NMR-spectroscopic studies show that the CAAI ligand is a much stronger electron donor than amino ligands. X-ray crystallographic analyses reveal that the degree of B-N_CAAI double bonding increases with the electron-withdrawing capacity of the other substituents at boron. The C-N-B bond angle displays a great flexibility, ranging from 131° to near-linear 176°, the narrowest angles being observed for NMe₂-substituted derivatives and the widest angles for highly sterically demanding substituents. Density functional theory (DFT) calculations on the electronic structures of the anionic CAAI ligand compared to unsaturated and saturated N-heterocyclic iminate (NHI) ligands show that the former is the best σ donor of the three but less π-donating than the unsaturated NHI. Nevertheless, the linear (CAAI)BH₂ complex displays somewhat stronger C-N and N-B π bonding than the corresponding ((S)NHI)BH₂ complexes.

C-C and C-N Bond Activation, Lewis-Base Coordination and One- and Two-Electron Oxidation at a Linear Aminoborylene

A cyclic alkyl(amino)carbene (CAAC)-stabilized dicoordinate aminoborylene is synthesized by the twofold reduction of a [(CAAC)BCl₂ (TMP)] (TMP=2,6-tetramethylpiperidyl) precursor. NMR-spectroscopic, X-ray crystallographic and computational analyses confirm the cumulenic nature of the central C=B=N moiety. Irradiation of [(CAAC)B(TMP)] (2) resulted in an intramolecular C-C bond activation, leading to a doubly-fused C₁₀ BN heterocycle, while the reaction with acetonitrile resulted in an aryl migration from the CAAC to the acetonitrile nitrogen atom, concomitant with tautomerization of the latter to a boron-bound allylamino ligand. One-electron oxidation of 2 with CuX (X=Cl, Br) afforded the corresponding amino(halo)boryl radicals, which were characterized by EPR spectroscopy and DFT calculations. Placing 2 under an atmosphere of CO afforded the tricoordinate (CAAC,CO)-stabilized aminoborylene. Finally, the twofold oxidation of 2 with chalcogens led, in the case of N₂ O and sulfur, to the splitting of the B-C_CAAC bond and formation of the 2,4-diamino-1,3,2,4-dichalcogenadiboretanes and CAAC-chalcogen adducts, whereas with selenium a monomeric boraselenone was isolated, which showed some degree of B-Se multiple bonding.

Trapping of a Transient Base-Stabilised Alumylene and Alumylene-type Reactivity of a Self-Stabilising Dialumene towards Organic Azides

The reduction of a carbene-coordinated, sterically encumbered terphenyl-substituted aluminium diiodide, (LRAlI2), yielded a "masked" dialumene (LRAl=AlRL), self-stabilised through [2+2] cycloaddition with a peripheral aromatic group. During the course of the reaction, a carbene-stabilised arylalumylene (LRAl:) was generated in situ, which was trapped using an alkyne, generating an aluminacyclopropene or a C-H activated product thereof, depending on the steric bulk of the alkyne. The masked dialumene also underwent intramolecular cycloreversion and dissociation into alumylene fragments, which reacted with various organic azides to yield monomeric or dimeric iminoalanes depending on the sterics of the azide substituent. The thermodynamics of monomeric and dimeric iminoalane formation were probed by theoretical calculations.

Aromatic 1,2-Azaborinin-1-yls as Electron-Withdrawing Anionic Nitrogen Ligands for Main Group Elements

The 2-aryl-3,4,5,6-tetraphenyl-1,2-azaborinines 1-EMe₃ and 2-EMe₃ (E=Si, Sn; aryl=Ph (1), Mes (=2,4,6-trimethylphenyl, 2)) were synthesized by ring-expansion of borole precursors with N₃ EMe₃ -derived nitrenes. Desilylative hydrolysis of 1- and 2-SiMe₃ yielded the corresponding N-protonated azaborinines, which were deprotonated with nBuLi or MN(SiMe₃ )₂ (M=Na, K) to the corresponding group 1 salts, 1-M and 2-M. While the lithium salts crystallized as monomeric Lewis base adducts, the potassium salts formed coordination polymers or oligomers via intramolecular K⋅⋅⋅aryl π interactions. The reaction of 1-M or 2-M with CO₂ yielded N-carboxylate salts, which were derivatized by salt metathesis to methyl and silyl esters. Salt metathesis of 1-M or 2-M with methyl triflate, [Cp*BeCl] (Cp*=C₅ Me₅ ), BBr₂ Ar (Ar=Ph, Mes, 2-thienyl), ECl₃ (E=B, Al, Ga) and PX₃ (X=Cl, Br) afforded the respective group 2, 13 and 15 1,2-azaborinin-2-yl complexes. Salt metathesis of 1-K with BBr₃ resulted not only in N-borylation but also Ph-Br exchange between the endocyclic and exocyclic boron atoms. Solution ¹¹ B NMR data suggest that the 1,2-azaborinin-2-yl ligand is similarly electron-withdrawing to a bromide. In the solid state the endocyclic bond length alternation and the twisting of the C₄ BN ring increase with the sterics of the substituents at the boron and nitrogen atoms, respectively. Regression analyses revealed that the downfield shift of the endocyclic ¹¹ B NMR resonances is linearly correlated to both the degree of twisting of the C₄ BN ring and the tilt angle of the N-substituent. Calculations indicate that the 1,2-azaborinin-1-yl ligand has no sizeable π-donor ability and that the aromaticity of the ring can be subtly tuned by the electronics of the N-substituent.

Modulation of the Naked‐Eye and Fluorescence Color of a Protonated Boron‐Doped Thiazolothiazole by Anion‐Dependent Hydrogen Bonding

The reaction of a cyclic alkyl(amino)carbene (CAAC)‐stabilized thiazaborolo[5,4‐d]thiazaborole (TzbTzb) with strong Brønsted acids, such as HCl, HOTf (Tf=O₂SCF₃) and [H(OEt₂)₂][BAr^F₄] (Ar^F=3,5‐(CF₃)₂C₆H₃), results in the protonation of both TzbTzb nitrogen atoms. In each case X‐ray crystallographic data show coordination of the counteranions (Cl⁻, OTf⁻, BAr^F₄⁻) or solvent molecules (OEt₂) to the doubly protonated fused heterocycle via hydrogen‐bonding interactions, the strength of which strongly influences the ¹H NMR shift of the NH protons, enabling tuning of both the visible (yellow to red) and fluorescence (green to red) colors of these salts. DFT calculations reveal that the hydrogen bonding of the counteranion or solvent to the protonated nitrogen centers affects the intramolecular TzbTzb‐to‐CAAC charge transfer character involved in the S₀→S₁ transition, ultimately enabling fine‐tuning of their absorption and emission spectral features.

Spontaneous N2-diboranylation of [W(N2)2(dppe)2] with B2Br4(SMe2)2

The 1,3-bromoboration of [W(N₂)₂(dppe)₂] (dppe = 1,2-bis(diphenylphosphino)ethane) with B₂Br₄(SMe₂)₂ in the presence of various Lewis bases L yields diboranyldiazenido complexes, with L coordinating either at the terminal or internal boron atom. The 2 : 1 reaction of [W(N₂)₂(dppe)₂] and B₂Br₄(SMe₂)₂ yields a 1,2-bis(diazenido)diborane-bridged ditungsten complex with a fully planar π-conjugated BrWN₂B₂Br₂N₂WBr core.

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

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

Stepwise reduction of a base-stabilised ferrocenyl aluminium(iii) dihalide for the synthesis of structurally-diverse dialane species

A bulky ferrocenyl-based NHC-stabilised aluminium(iii) dihalide was reduced in different solvents, leading to vastly different outcomes, including formation of a rare example of a dialane and a novel dialuminyl analogue of the Birch reduction.

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.

Harnessing the electronic differences between CAAC-stabilised 1,4-diborabenzene and 9,10-diboraanthracene for synthesis

The oxidation of doubly cyclic alkyl(amino)carbene-stabilised closed-shell 1,4-diborabenzene with sulfur or selenium yields S₄/S₅- or Se₄-bridged hexa-1,4-dienes, respectively, whereas that of the related open-shell singlet biradical 9,10-diboraanthracene with O₂, sulfur or selenium yields the endoperoxo- or S/Se-bridged bicyclic species, respectively.

Hybrid Inorganic-Organic Cross-Metathesis between Diborenes and Acetylene

The ruthenium-catalyzed cross-metathesis of alkenes and alkynes, which splits the alkene C═C double bond and couples one-half to each carbon of the alkyne C≡C triple bond, is one of the most efficient tools for the synthesis of 1,3-dienes, with wide-ranging applications, including pharmaceutical and polymer chemistry. In contrast, inorganic main-group metathesis reactions are restricted to a handful of examples of heavier p-block multiple bonds (P═P, Ge═Ge, and E≡E, E = Ge, Sn, Pb). We now report the first examples of thermally induced, transition-metal-free cross-metathesis between an organic alkyne and inorganic cyclic alkyl(amino)carbene (CAAC)-stabilized B═B double bonds, which yield fully planar, π-delocalized 1,8-diaza-3,6-diboraoctatetraenes. Density functional theory studies show that these compounds have an open-shell singlet biradical ground state with a thermally accessible closed-shell state. In-depth computational mechanistic analyses show that they are formed via a biradical cycloaddition-cycloreversion mechanism. Finally, unlike their organic counterparts, these B,N-analogues of octatetraene can undergo two-electron chemical reduction to form diamagnetic dianions.

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.

A Neutral Beryllium(I) Radical

The reduction of a cyclic alkyl(amino)carbene (CAAC)-stabilized organoberyllium chloride yields the first neutral beryllium radical, which was characterized by EPR, IR, and UV/Vis spectroscopy, X-ray crystallography, and DFT calculations.

Platinum-Templated Coupling of B=N Units: Synthesis of BNBN Analogues of 1,3-Dienes and a Butatriene

The 1:2 reaction of [μ-(dmpm)Pt(nbe)]2 (dmpm=bis(dimethylphosphino)methane, nbe=norbornene) with Cl2 BNR(SiMe3 ) (R=tBu, SiMe3 ) yields unsymmetrical (N-aminoboryl)aminoboryl PtI 2 complexes by B-N coupling via ClSiMe3 elimination. A subsequent intramolecular ClSiMe3 elimination from the tBu-derivative leads to cyclization of the BNBN unit, forming a unique 1,3,2,4-diazadiboretidin-2-yl ligand. In contrast, the analogous reaction with Br2 BN(SiMe3 )2 leads, via a twofold BrSiMe3 elimination, to a PtII 2 A-frame complex bridged by a linear BNBN isostere of butatriene. Structural and computational data confirm π electron delocalization over the entire BNBN unit.

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.

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.

Reactivity of cyano- and isothiocyanatoborylenes: metal coordination, one-electron oxidation and boron-centred Brønsted basicity

Doubly base-stabilised cyano- and isothiocyanatoborylenes of the form LL'BY (L = CAAC = cyclic alkyl(amino)carbene; L' = NHC = N-heterocyclic carbene; Y = CN, NCS) coordinate to group 6 carbonyl complexes via the terminal donor of the pseudohalide substituent and undergo facile and fully reversible one-electron oxidation to the corresponding boryl radical cations [LL'BY]˙+. Furthermore, calculations show that the borylenes have very similar proton affinities, both to each other and to NHC superbases. However, while the protonation of LL'B(CN) with PhSH yielding [LL'BH(CN)+][PhS-] is fully reversible, that of LL'B(NCS) is rendered irreversible by a subsequent B-to-CCAAC hydrogen shift and nucleophilic attack of PhS- at boron.

Synthesis and characterisation of boranediyl- and diboranediyl-bridged diplatinum A-frame complexes

A series of boranediyl-bridged diplatinum A-frame complexes, [Pt2X2(μ-BY)(μ-dmpm)2] (X = Cl, Br, I; Y = aryl, alkyl, amino, halo; dmpm = bis(dimethylphosphino)methane), were synthesised by the twofold oxidative addition of BX2Y to [Pt2(nbe)2(μ-dmpm)2] (nbe = norbornene) or to the paddlewheel complex [Pt2(μ-dmpm)3]. Similarly, the addition of B2X2(NMe2)2 (X = Cl, Br) to [Pt2(nbe)2(μ-dmpm)2] provided access to the diborane-1,2-diyl-bridged A-frame complexes [Pt2X2(μ-1,2-B2(NMe2)2)(μ-dmpm)2]. X-ray crystallographic studies of these (BY)n-bridged complexes show structural trends depending on the steric demands of Y and the nature of X. Analysis of higher-order 31P NMR satellites provided information on JP-Pt and JPt-Pt coupling constants, the latter correlating with the PtPt distance. All (di)boranediyl complexes also proved unstable towards (successive) loss of the bridging "BY" unit(s), resulting in the formation of [Pt2X2(μ-dmpm)2].

Highly Colored Boron-Doped Thiazolothiazoles from the Reductive Dimerization of Boron Isothiocyanates

Reduction of (CAAC)BBr2 (NCS) (CAAC=cyclic alkyl(amino)carbene) in the presence of a Lewis base L yields tricoordinate (CAAC)LB(NCS) borylenes which undergo reversible E/Z-isomerization. The same reduction in the absence of L yields deep blue, bis(CAAC)-stabilized, boron-doped, aromatic thiazolothiazoles resulting from the dimerization of dicoordinate (CAAC)B(NCS) borylene intermediates.

Reduction and Rearrangement of a Boron(I) Carbonyl Complex

The one-electron reduction of a cyclic (alkyl)(amino)carbene (CAAC)-stabilized arylborylene carbonyl complex yields a dimeric borylketyl radical anion, resulting from an intramolecular aryl migration to the CO carbon atom. Computational analyses support the existence of a [(CAAC)B(CO)Ar].- radical anion intermediate. Further reduction leads to a highly nucleophilic dianionic (boraneylidene)methanolate.

Oxidation, Coordination, and Nickel-Mediated Deconstruction of a Highly Electron-Rich Diboron Analogue of 1,3,5-Hexatriene

The reductive coupling of an N-heterocyclic carbene (NHC) stabilized (dibromo)vinylborane yields a 1,2-divinyldiborene, which, although isoelectronic to a 1,3,5-triene, displays no extended π conjugation because of twisting of the C2 B2 C2 chain. While this divinyldiborene coordinates to copper(I) and platinum(0) in an η2 -B2 and η4 -C2 B2 fashion, respectively, it undergoes a complex rearrangement to an η4 -1,3-diborete upon complexation with nickel(0).

Boranediyl- and Diborane(4)-1,2-diyl-Bridged Platinum A-Frame Complexes

Diplatinum A-frame complexes with a bridging (di)boron unit in the apex position were synthesized in a single step by the double oxidative addition of dihalo(di)borane precursors at a bis(diphosphine)-bridged Pt0 2 complex. While structurally analogous to well-known μ-borylene complexes, in which delocalized dative three-center-two-electron M-B-M bonding prevails, theoretical investigations into the nature of Pt-B bonding in these A-frame complexes show them to be rare dimetalla(di)boranes displaying two electron-sharing Pt-B σ-bonds. This is experimentally reflected in the low kinetic stability of these compounds, which are prone to loss of the (di)boron bridgehead unit.

Synthesis of polyheterocyclic 1,1-diboryltriazenes by γ-nitrogen insertion of azides into activated B-B single bonds

The γ-nitrogen insertion of arylazides into the B-B bond of electron-rich cyclic μ-hydridodiboranes stabilised by one N-heterocyclic carbene (NHC) ligand leads to the expansion of the central C3B2 ring, yielding unsymmetrical polyheterocyclic 1,1-diboryltriazenes. The 2-benzyl-bridged analogues undergo further NHC ring expansion and thermally induced loss of N2.

trans-Selective Insertional Dihydroboration of a cis-Diborene: Synthesis of Linear sp3 -sp2 -sp3 -Triboranes and Subsequent Cationization

The reaction of aryl- and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3-trans-dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis-1,2-μ-H-3-hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base-stabilized B₃ H₄ ⁺ analogues.