Taming a silyldiium cation and its reactivity towards sodium phosphaethynolate

A dicationic bis(NHC)-stabilised silyldiium complex, [bis(NHC)-SiPh₂]²⁺ (7²⁺) (bis(NHC) = [CH₂(NC₃H₂NDipp)₂], Dipp = 2,6-ⁱPr₂C₆H₃), was synthesised for the first time. It reacts with sodium phosphaethynolate (NaOCP) as a source of monoanionic phosphorus to give the P-insertion product [bis(NHC)-PSiPh₂]⁺ (8⁺). The latter comprises a seven-membered heterocycle containing a Si-P moiety which can easily be desilylated when exposed to dichlorophosphanes as exemplified by the synthesis of the diphosphanide cations [bis(NHC)-PPCy]⁺ (9⁺) and [bis(NHC)-PPPh]⁺ (10⁺).

Molecular Manipulations of a Utility Nitrogen-Heterocyclic Carbene by Sodium Magnesiate Complexes and Transmetallation Chemistry with Gold Complexes

Expanding the scope and applications of anionic N-heterocyclic carbenes (NHCs), a novel series of magnesium NHC complexes is reported using a mixed sodium-magnesium approach. Sequential reactivity of classical imidazol- 2-ylidene carbene IPr with NaR and MgR₂ (R=CH₂ SiMe₃ ) affords [(THF)₃ Na(μ-IPr^- )MgR₂ (THF)] (2) [IPr^- =:C{[N(2,6-iPr₂ C₆ H₃ )]₂ CHC] containing an anionic NHC ligand, whereas surprisingly sodium magnesiate [NaMgR₃ ] fails to deprotonate IPr affording instead the redistribution coordination adduct [IPr₂ Na₂ MgR₄ ] (1). Compound 2 undergoes selective C2-methylation when treated with MeOTf furnishing novel abnormal NHC complex [{aIPr^Me MgR₂ }₂ ] (3). Dissolving 3 in THF led to the dissociation of this complex into MgR₂ and aIPr^Me with the latter isomerizing to the olefinic NHC IPr=CH₂ . The ability of 2 and 3 to transfer their anionic and abnormal NHC ligands, respectively to Au^I metal fragments has been investigated allowing the isolation and structural characterization of [RAu(μ-IPr^- )MgR(THF)₂ ] (4) and [aIPr^Me AuR] (5) respectively. In both cases transfer of an alkyl R group is observed. However while 3 can also transfer its abnormal NHC ligand to give 5, in 4 the anionic NHC still remains coordinated to Mg via its C4 position, whereas the {AuR} fragment occupies the C2 position previously filled by a donor-solvated {Na(THF)₃ }⁺ cation.

NHCs in Main Group Chemistry

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

Dative versus electron-sharing bonding in N-oxides and phosphane oxides R3EO and relative energies of the R2EOR isomers (E = N, P; R = H, F, Cl, Me, Ph). A theoretical study

Quantum chemical calculations using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory using BP86 and M06-2X functionals in conjunction with def2-TZVPP basis sets have been carried out on the title molecules.

A New Area in Main-Group Chemistry: Zerovalent Monoatomic Silicon Compounds and Their Analogues

Monoatomic zerovalent main-group element complexes emerged very recently and attracted increasing attention of both theoretical and experimental chemists. In particular, zerovalent silicon complexes and their congeners (metallylones) stabilized by neutral Lewis donors are of significant importance not only because of their intriguing electronic structure but also because they can serve as useful building blocks for novel chemical species. Featuring four valence electrons as two lone pairs at the central atoms, such complexes may form donor-acceptor adducts with Lewis acids. More interestingly, with the central atoms in the oxidation state of zero, they could pave a way to new classes of compounds and functional groups that are otherwise difficult to realize. In this Account, we mainly describe our contributions in the chemistry of monatomic zerovalent silicon (silylone) and germanium (germylone) supported by a chelate bis-N-heterocyclic carbene (bis-NHC) ligand in the context of related species developed by other groups in the meantime. Utilizing the bis-NHC stabilized chlorosilyliumylidene [:SiCl]⁺ and chlorogermyliumylidene [:GeCl]⁺ as suitable starting materials, we successfully isolated silylone (bis-NHC)Si and germylone (bis-NHC)Ge, respectively. The electronic structures of the latter complexes established by theoretical calculations and spectroscopic data revealed that they are genuine metallylone species with electron-rich silicon(0) and germanium(0) centers. Accordingly, they can react with 1 molar equiv of GaCl₃ to form Lewis adducts (bis-NHC)E(GaCl₃) (E = Si, Ge) and with 2 molar equiv of ZnCl₂ to furnish (bis-NHC)Si(ZnCl₂)₂. Conversion of the metallylones with elemental chalcogens affords isolable monomeric silicon(II) and germanium(II) monochalcogenides (bis-NHC)EX(GaCl₃) (X = Se, Te), representing molecular heavier congeners of CO. Moreover, their reaction with elemental chalcogens can also yield monomeric silicon(IV) and germanium(IV) dichalcogenides (bis-NHC)EX₂ (X = S, Se, Te) as the first isolable complexes of the molecular congeners of CO₂. Moreover, (bis-NHC)Si could even activate CO₂ to afford the monomolecular silicon dicarbonate complex (bis-NHC)Si(CO₃)₂ via the formation of SiO and SiO₂ complexes as intermediates. Furthermore, starting with a chelate bis-N-heterocyclic silylene supported [:GeCl]⁺, we developed two bis-N-heterocyclic silylene stabilized germylone→Fe(CO)₄ complexes. Our achievements in the chemistry of metallylones demonstrate that the characteristic of monatomic zerovalent silicon and its analogues can provide novel reaction patterns for access to unprecedented species and even extends the series of functional groups of these elements. With this, we can envision that more interesting zerovalent complexes of the main-group elements with unprecedented reactivity will follow in the near future.

Computational Study on the Mechanisms of Multiple Complexation of CO and Isonitrile Ligands to Boron

The recent experimental realization of compound Tripp-B(CO)₂ (denoted as 2a), where Tripp is 2,6-di(2,4,6-triisopropylphenyl)-phenyl), breaks through conventional knowledge that only transition metals can bind more than one CO to form multicarbonyl adducts. Compound 2a is stable in air but liberates CO under light. The B-CO bonds of 2a are considered to be similar to donor-acceptor bonds of transition metal complexes. To address the formation mechanism and chemical bonding of this novel type of boron compounds, we present a density functional theory study on the formation and photolysis of 2a and similar compounds. The results suggest that the formation of 2a is facile by three consecutive additions of CO to the terminal borylene metal complex, that is, the boron source of the synthesis. These CO additions can be practically accomplished via two different paths: CO direct addition and CO migration followed by addition. Such mechanisms can be excellently rationalized by the donor-acceptor bonding model of the terminal borylene complex, which in turn suggests that using donor-acceptor bonds for 2a is natural for understanding the mechanisms. Liberation of CO from 2a and its similar compounds has higher energy barriers at the ground states than that at the triplet states by 40 kcal/mol. These energy barriers explain the experimentally observed air stability and photolysis of these compounds. The results for the first time provide mechanistic insights for the unprecedented chemical processes; they allow evaluation of the applicability of donor-acceptor bonding in main-group compounds from the new perspective of chemical reactions.

Metal-like Boronic-Organic Frameworks: A Design and Computation

Because of the lack of strong π-interaction in their bonds connecting building units, most of the metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) achieved so far are insulators or wide-bandgap semiconductors. The design of metal-like frameworks based on known chemical components is a challenge. This work reports that aryl borons can be linked together through isocyanides to form stable and easily accessible low-dimensional boronic-organic frameworks (BOFs). Particularly, the boron atoms in the BOFs behave like transition metals, forming the combined σ-donation and π-backdonation bonds instead of the usual electron-sharing bonds with the isocyanide linkers. This peculiar bonding endows BOFs with semimetal and narrow-bandgap semiconductor features, which are different from MOFs and COFs and may be found to be useful in future nanoelectronics. The results open a door to integrating the knowledge of the donor-acceptor chemistry in the main group into materials science.

Carbones as Ligands in Novel Main-Group Compounds E[C(NHC)2 ]2 (E=Be, B+ , C2+ , N3+ , Mg, Al+ , Si2+ , P3+ ): A Theoretical Study

Quantum chemical calculations of the main-group compounds E[C(NHC^Me )₂ ]₂ (E=Be, B⁺ , C²⁺ , N³⁺ , Mg, Al⁺ , Si²⁺ , P³⁺ ) have been carried out using density functional theory at the BP86/def2-TZVPP and BP86-D3(BJ)/def2-TZVPP levels of theory. The geometry optimization at BP86/def2-TZVPP gives equilibrium structures with two-coordinated species E and bending angles C-E-C between 152.5° (E=Be) and 110.5° (E=Al). Inclusion of dispersion forces at BP86-D3(BJ)/def2-TZVPP yields a three-coordinated beryllium compound Be[C(NHC^Me )₂ ]₂ as the only energy minimum form. Three-coordinated isomers are found besides the two-coordinated energy minima for the boron and carbon cations B[C(NHC^Me )₂ ]₂⁺ and C[C(NHC^Me )₂ ]₂²⁺ . The three-coordinated form of the boron compound is energetically lower lying than the two-coordinated form, while the opposite trend is calculated for the carbon species. The theoretically predicted bond dissociation energies suggest that all compounds are viable species for experimental studies. The X-ray structure of the benzoannelated homologue of P[C(NHC^Me )₂ ]₂³⁺ that was recently reported by Dordevic et al. agrees quite well with the calculated geometry of the molecule. A detailed bonding analysis using charge and energy decomposition methods shows that the two-coordinated neutral compounds Be[C(NHC^Me )₂ ]₂ and Mg[C(NHC^Me )₂ ]₂ possess strongly positively charged atoms Be and Mg. The carbodicarbene groups C(NHC^Me )₂ serve as acceptor ligands in the compounds and may be sketched with dative bonds (NHC^Me )₂ C←E→C(NHC^Me )₂ (E=Be, Mg). Dative bonds in which the carbones C(NHC^Me )₂ are donor ligands are suggested for the cations (NHC^Me )₂ C→E←C(NHC^Me )₂ (E=B⁺ , Al⁺ ). The dications and trications possess electron-sharing bonds in which the bonding situation is best described with the formula [(NHC^Me )₂ C]⁺ -E-[C(NHC^Me )₂ ]⁺ (E=C, Si, N⁺ , P⁺ ).

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

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

Bonding analysis of ylidone complexes EL2 (E = C–Pb) with phosphine and carbene ligands L

Quantum chemical calculations using DFT and ab initio methods have been carried out of the compounds EL2 with atoms E = C–Pb and the ligands L = PPh3, N-heterocyclic carbene (NHC), bicyclic NHC, and cyclic alkyl-amino carbene (cAAC). The equilibrium structures are reported and the bonding situation was analyzed with a variety of charge- and energy decomposition methods. Some of the molecules are experimentally known, but most molecules have not yet been prepared. The bonding analysis suggests that all but one molecule should be considered as ylidones that possess dative bonds L→E←L. The sole exception is C(cAAC)2, which has a linear structure and classical double bonds (cAAC)=C=(cAAC). The ylidones EL2 have two lone-pair orbitals at atom E with σ and π symmetry. The π lone pair is somewhat delocalized due to L←E→L π-backdonation. The contribution of L←E→L π-backdonation relative to L→E←L σ-donation increases for the heavier elements. The compounds have rather large first and second proton affinities, which is a characteristic feature of ylidones. They are thus double Lewis bases that could be utilized in chemical reactions.

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

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

The Structure of the Carbene Stabilized Si2H2 May Be Equally Well Described with Coordinate Bonds as with Classical Double Bonds

The cyclic alkyl(amino) carbene stabilized Si2H2 has been isolated in the molecular form of composition (Me-cAAC:)2Si2H2 (1) and (Cy-cAAC:)2Si2H2 (2) at room temperature. Compounds 1 and 2 were synthesized from the reduction of HSiCl3 using 3 equiv of KC8 in the presence of 1 equiv of Me-cAAC: and Cy-cAAC:, respectively. These are the first molecular examples of Si2H2 characterized by single crystal X-ray structural analysis. Moreover, electrospray ionization mass spectrometry and (1)H as well as (29)Si NMR data are reported. Furthermore, the structure of compound 1 has been investigated by theoretical methods. The theoretical analysis of 1 explains equally well its structure with coordinate bonds as with classical double bonds of a 2,3-disila-1,3-butadiene.

Transition metal-mediated donor-acceptor coordination of low-oxidation state Group 14 element halides

The reactivity of tungsten carbonyl adducts of Group 14 element (Ge, Sn and Pb) dihalides towards the metal-based donors (η(5)-C5H5)Rh(PMe2Ph)2 and Pt(PCy3)2 was examined. When (η(5)-C5H5)Rh(PMe2Ph)2 was treated with the Lewis acid supported Ge(ii) complex, THF·GeCl2·W(CO)5, cyclopentadienyl ring activation occurred, whereas the analogous Lewis acidic units SnCl2·W(CO)5 and PbCl2 form direct adducts with the Rh complex to yield Rh-Sn and Rh-Pb dative bonds. Attempts to prepare metal coordinated element(ii) hydrides by adding hydride sources to the above mentioned rhodium-E(ii) halide complexes were unsuccessful; in each case insoluble products were formed along with regeneration of free (η(5)-C5H5)Rh(PMe2Ph)2. In a parallel study, ECl2·W(CO)5 (E = Ge or Sn) groups were shown to participate in E-Cl oxidation addition chemistry with (Cy3P)2Pt to give the formal Pt(ii) complexes ClPt(PCy3)2ECl·W(CO)5.

Abnormal carbene–silicon halide complexes

Reaction of the anionic NHDC ligand, [:C{[N(2,6-Prⁱ₂C₆H₃)]₂CHCLi}]_n (1), with SiCl₄ gives the trichlorosilyl-substituted NHC ligand (7). Abnormal carbene–SiCl₄ complex (8) can be conveniently synthesized by combining 7 with HCl·NEt₃. Meanwhile, 7 may react with CH₂Cl₂ in warm hexane, giving the abnormal carbene-complexed SiCl₃⁺ cation (9). The structure and bonding of 9 have also been probed by DFT computations.

Donor–acceptor bonding in novel low-coordinated compounds of boron and group-14 atoms C–Sn

Donor–acceptor complexes of one, two or three atoms E = B, Si–Sn which are stabilized by σ-donor ligands L are discussed.

Group 14 inorganic hydrocarbon analogues

This Review article deals with the synthesis and properties of inorganic hydrocarbon analogues: binary chemical species that contain heavier Group 14 elements (Si, Ge, Sn or Pb) and hydrogen as components. Rapid advances in our general knowledge of these species have enabled the development of industrially relevant processes such as the hydrosilylation of unsaturated substrates and the chemical vapor deposition of semi-conducting films.

25 years of N-heterocyclic carbenes: activation of both main-group element-element bonds and NHCs themselves

N-Heterocyclic carbenes (NHCs) are widely used ligands and reagents in modern inorganic synthesis as well as in homogeneous catalysis and organocatalysis. However, NHCs are not always innocent bystanders. In the last few years, more and more examples were reported of reactions of NHCs with main-group elements which resulted in modification of the NHC. Many of these reactions lead to ring expansion and the formation of six-membered heterocyclic rings involving insertion of the heteroatom into the C-N bond and migration of hydrides, phenyl groups or boron-containing fragments. Furthermore, a few related NHC rearrangements were observed some decades ago. In this Perspective, we summarise the history of NHC ring expansion reactions from the 1960s till the present.

Design, Synthesis, and Structural Analysis of Divalent N(I) Compounds and Identification of a New Electron-Donating Ligand

The dative-bond representation (L→E) in compounds with main group elements (E) has triggered extensive debate in the recent past. The scope and limits of this nonclassical coordination bond warrant comprehensive exploration. Particularly compounds with (L→N←L')(+) arrangement are of special interest because of their therapeutic importance. This work reports the design and synthesis of novel chemical species with the general structural formula (L→N←L')(+) carrying the unusual ligand cyclohexa-2,5-diene-4-(diaminomethynyl)-1-ylidene. Four species belonging to the (L→N←L')(+) class carrying this unconventional ligand were synthesized. Quantum chemical and X-ray diffraction analyses showed that the electronic and geometric parameters are consistent with those of already reported divalent N(I) compounds. The molecular orbital analysis, geometric parameters, and spectral data clearly support the L→N and N←L' interactions in these species. The newly identified ligand has the properties of a reactive carbene and high nucleophilicity.

Reactivity in the periphery of functionalised multiple bonds of heavier group 14 elements

Heavier group 14 multiple bonds have intrigued chemists since more than a century. The synthesis of stable compounds with double and triple bonds with silicon, germanium, tin and lead had considerable impact on modern ideas of chemical bonding. These developments were made possible by the use of bulky substituents that provide kinetic and thermodynamic protection. Since about a decade the compatibility of heavier multiple bonds with various functional groups has moved into focus. This review covers multiply bonded group 14 species with at least one additional reactive site. The vinylic functionalities of groups 1 and 17, resulting in nucleophilic and electrophilic disila vinyl groups, respectively, are the most prevalent and well-studied. They have been employed repeatedly for the transfer of heavier multiple bonds to yield low-valent group 14 compounds with novel structural motifs. Vinylic functionalities of groups 2 to 16 and a few σ-bonded transition metal complexes are experimentally known, but their reactivity has been studied to a lesser extent. Donor-coordinated multiple bonds are a relatively new field of research, but the large degree of unsaturation as isomers of alkynes (as well as residual functionality in some cases) offers considerable possibility for further manipulation, e.g. for the incorporation into more extended systems. Heavier allyl halides constitute the major part of heavier multiple bonds with a functional group in allylic position and some examples of successful transformations are given. At present, remote functionalities are basically limited to para-phenylene functionalised disilenes. The reported use of the latter for further derivatisation might encourage investigations in this direction. In summary, the study of peripherally functionalised multiple bonds with heavier group 14 elements is already well beyond its infancy and may be an instrumental factor in awakening the potential of group 14 chemistry for applications in polymers and other materials.

Rational synthesis of normal, abnormal and anionic NHC-gallium alkyl complexes: structural, stability and isomerization insights

Advancing the rational design of main-group N-heterocyclic carbene complexes, this study reports the synthesis, X-ray crystallographic and NMR spectroscopic characterisation of a novel series of Ga complexes containing neutral or anionic NHC ligands using the unsaturated carbene IPr (IPr = 1,3-bis-(2,6-di-isopropylphenyl)imidazol-2-ylidene). Starting from normal adduct GaR3·IPr (1) (R = CH2SiMe3), the addition of polar LiR led to the formation of NHC-stabilised gallate species IPr·LiGaR4 (2), resulting from co-complexation of the single-metal species. Contrastingly, reversing the order of addition of these organometallic reagents, by treating unsaturated free IPr, first with LiR followed by GaR3, furnished novel heteroleptic gallate (THF)2Li[:C{[N(2,6-iPr2C6H3)]2CHCGa(CH2SiMe3)3}] (3), which contains an anionic NHC ligand acting as an unsymmetrical bridge between the two metals, coordinating through its abnormal C4 position to Ga and through its normal C2 position to Li. Electrophilic interception studies of 3 using methyl triflate (MeOTf), methanol and imidazolium salt (IMes·HCl) led to the isolation and structural elucidation of the two novel neutral abnormal NHC (aNHC) complexes [CH3C{[N(2,6-iPr2C6H3)]2CHCGa(CH2SiMe3)3}] (4) and aIPr·GaR3 (5) (aIPr = HC{[N(2,6-iPr2C6H3)]2CHC}). These studies disclose the preference of the anionic IPr ligand present in 3 to react with electrophiles via its C2 position, leaving its Ga-C4 bond intact. Abnormal complex 5 can also be accessed by a thermally induced rearrangement of its normal isomer 1. Combining NMR spectroscopic and kinetic studies with DFT calculations, new light has been shed on this intriguing transformation, which suggests that it occurs via a dissociative mechanism, highlighting the importance of the donor ability of the solvent used in these thermal isomerizations as well as the steric bulk of the substituents on the NHC and the Ga reagent. These findings intimate that relief of the steric hindrance around Ga by forming an abnormal complex is a key driving force behind these rearrangements.

Phosphine and carbene azido-cations: [(L)N3]+ and [(L)2N3]. Chem Sci 2015;6:6367-6372. [PMID: 30090255 PMCID: PMC6054069 DOI: 10.1039/c5sc02336j]

The cationic N₃-species [(p-HC₆F₄)₃PN₃]⁺ (1) featuring a perfluoro-arene phosphonium group serves as a N₃⁺-source in stoichiometric reactions with several Lewis bases (L) allowing for the stepwise formation of [(L)N₃]⁺ and [(L)₂N₃]⁺ cations (L = phosphine, carbene) with liberation of (p-HC₆F₄)₃P.

The cationic N₃-species [(p-HC₆F₄)₃PN₃]⁺ (1) featuring a perfluoro-arene phosphonium group serves as a N₃⁺-source in stoichiometric reactions with several Lewis bases (L) allowing for the stepwise formation of [(L)N₃]⁺ and [(L)₂N₃]⁺ cations (L = phosphine, carbene) with liberation of (p-HC₆F₄)₃P. X-Ray diffraction analysis and computational studies provide insight into the bonding in these remarkably stable azido-cations.

Donor-acceptor chemistry in the main group

This Perspective article summarizes recent progress from our laboratory in the isolation of reactive main group species using a general donor-acceptor protocol. A highlight of this program is the use of carbon-based donors in combination with suitable Lewis acidic acceptors to yield stable complexes of parent Group 14 element hydrides (e.g. GeH2 and H2SiGeH2). It is anticipated that this strategy could be extended to include new synthetic targets from throughout the Periodic Table with possible applications in bottom-up materials synthesis and main group element catalysis envisioned.

Generation of 1,2-azaboretidines via reduction of ADC borane adducts

ADC borane adducts RBX₂·ADC (R = Mes, Dur; X = Cl, Br; ADC = :C(NiPr₂)₂) have been prepared and reduced by KC₈ to afford air stable 1,2-azaboretidines with high selectivity.

Reaction of the acyclic (diamino)carbene (ADC) :C(NiPr₂)₂ (1) with different dihaloboranes of the type RBX₂ (R = Mes, Dur; X = Cl, Br) smoothly afforded a novel class of ADC-stabilized borane adducts. For MesBBr₂ however, the reaction did not stop at the adduct level, but an uncommon rearrangement process occurred, which eventually resulted in the formation of a 5-membered boracycle after elimination of mesitylene. Chemical reduction of the ADC borane adducts by KC₈ selectively yielded air stable 1,2-azaboretidines. Detailed DFT studies suggest a reduction mechanism involving a highly reactive borylene intermediate, which is converted into the boracycles via a rearrangement/C–H activation sequence.