NHC aluminum chemistry on the rise

This perspective highlights recent developments of the use of N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (cAACs) in alane and aluminum organyl chemistry. Especially in the last few years this flourishing research field led to some remarkable discoveries including various substitution patterns at the central aluminum atom, different oxidation states, neutral and charged compounds with varying coordination numbers and unique reactivities. Thereby NHCs play a vital role in the stabilization of these otherwise highly reactive compounds, which would not be realizable without the use of this intriguing class of ligands. Nevertheless, main group hydrides and especially NHC ligated alanes also tend to undergo NHC decomposition reactions, which are part of ongoing research and provide important information for NHC research in general.

Carbene-Decorated Geometrically Constrained Borylenes for Bond Activations

While metal-ligand cooperativity is well-known, studies on element-ligand cooperativity involving main group species are comparatively much less explored. In this study, we computationally designed a few geometrically constrained borylenes supported by different carbenes. Our density functional theory studies indicate that they possess enhanced nucleophilicity as well as electrophilicity, thus rendering them promising candidates for exhibiting borylene-ligand cooperativity. The cooperation between the boron and adjacent carbene centers facilitates different bond activation processes, including the cycloaddition of acetylene across the boron-carbene bond as well as B-H/Si-H bond activation reactions, which have been analyzed in detail. To the best of our knowledge, the borylenes proposed in this study represent the first examples of theoretically proposed geometrically constrained bis(carbene)-stabilized borylenes capable of cooperative activation of enthalpically strong bonds.

Nickel-Catalyzed Cross-Coupling Reaction of Aryl Bromides/Nitriles with Imidazolium Salts Involving Inert C-N Bond Cleavage

We herein present a nickel-catalyzed cross-coupling reaction of aryl halides and nitriles with imidazolium salts. A series of 2-arylated imidazoles could be obtained in moderate to good yields through inert C-N bond cleavage. The imidazolium salt in this reaction acts as both a coupling partner and N-heterocyclic carbene (NHC) ligand precursor. Mechanistic studies reveal that consecutive steps of migratory insertion of the NHC into the aryl C-Ni bond and β-C elimination might be involved in the proposed reaction mechanism.

Ring Contraction by Rearrangement of Sterically Congested Cyclic (Amino)(aryl)carbenes

The rearrangement of sterically congested cyclic (amino)(aryl)carbenes (CAArCs) by the reaction of related iminium salts with potassium bis(trimethylsilyl)amide is reported, allowing for forming benzocyclobutanimines via a ring contraction process. Mechanistic studies by theoretical calculations indicate that the formation of conjugated ketenimines as intermediates could be considered, in which steric hindrance caused by N-alkyl motifs of CAArCs plays an important role in promoting the ring-opening by the cleavage of C-N bond.

How to Decarbonize N-Heterocyclic Carbenes (NHCs): The simple Alane Adducts (NHC) ⋅ AlR3 (R=H, Me, Et)

The reaction of the amine-stabilized alane (NMe3) ⋅ AlH3 1 with the backbone-saturated N-heterocyclic carbene (NHC) SIDipp (SIDipp=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolidin-2-ylidene) at 0 °C yielded the NHC alane adduct (SIDipp) ⋅ AlH3 2. Reaction at elevated temperatures or prolonged reaction at room temperature gave the product of a ring expansion reaction (RER) of the NHC, (NMe3) ⋅ AlH(RER-SIDippH2) 3 ⋅ (NMe3). Subsequent reaction of the latter with sterically less hindered NHCs (IMeMe {=1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPrMe {=1,3-di-iso-propyl-4,5-dimethyl-imidazolin-2-ylidene}, and IiPr {=1,3-di-iso-propyl-imidazolin-2-ylidene}) afforded the NHC-stabilized RER-products (NHC) ⋅ AlH(RER-SIDippH2) 3 ⋅ (NHC) (NHC=IMeMe, IiPrMe, IiPr), while no reaction was observed with the sterically more demanding NHCs IDipp (=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolin-2-ylidene), SIDipp and ItBu (=1,3-di-tert-butyl-imidazolin-2-ylidene). The compounds 3 ⋅ (NHC) were also obtained starting from (SIDipp) ⋅ AlH3 2 and NHC at room temperature. Heating solutions of (SIDipp) ⋅ AlH3 2 without additional base to 95 °C resulted in decarbonization of the NHC and substitution of the carbene carbon atom with aluminum hydride under loss of ethene. Subsequent dimerization afforded cis-[AlH{μ-N(Dipp)CH2CH2N(Dipp)}]2 4_dimer. Heating solutions of the NHC-ligated aluminum alkyls (SIDipp) ⋅ AlR3 2R (R=Me, Et) to 145 °C instead led to complete scission of the NHC backbone with evolution of ethene and isolation of the dialkylaluminium(III) amidinates {DippNC(R)NDipp}AlR2 5R (R=Me, Et).

Coordination chemistry and FLP reactivity of 1,1- and 1,2-bis-boranes

Coordination chemistry and frustrated Lewis pair (FLP) chemistry have been most commonly studied using monodentate Lewis acids. In this paper, we examine the corresponding reactions employing the 1,1- and 1,2-bis-boranes, PhCH2CH(B(C6F5)2)21 and Me3SiCH(B(C6F5)2)CH2B(C6F5)22, respectively. Coordination of isocyanide to these species results in the formation of the products RCH(B(C6F5)2CNtBu)CH2(B(C6F5)2CNtBu) (R = Ph 3, Me3Si 4). The rearrangement of 1 to give the 1,2-bis-borane adduct 3 was probed and attributed to a donor-induced retrohydroboration and subsequent hydroboration. The analogous reaction of 1 is evident in efforts to use the Gutman-Beckett method to assess its Lewis acidity. However, in combination with tBu3P, bis-boranes 1 and 2 form FLPs and react with H2 to give [tBu3PH][PhCH2CH(B(C6F5)2)2(μ-H)] 5a and [tBu3PH][Me3SiCH(B(C6F5)2)CH2(B(C6F5)2)(μ-H)] 6, respectively. Reactions of 1 and 2 with various donors and PhCCH were shown to give deprotonation and addition products, depending on the nature of the base. However, in the case of 1, products resulting from retrohydroboration, and subsequent hydroboration are evident. Several of these alkyne products are crystallographically characterized.

Ring-Opening of 1,3-Imidazole Based Mesoionic Carbenes (iMICs) and Ring-Closing Clicks: Facile Access to iMIC-Compounds

Herein, ring-opening of mesoionic carbenes (iMICs) (iMIC=[ArC{N(Dipp)}2 C(SiMe3 )C:) (Dipp=2,6-iPr2 C6 H3 , Ar=Ph, 4-Me2 NC6 H4 or 4-PhC6 H4 ) based on an 1,3-imidazole scaffold to yield N-ethynylformimidamide (eFIM) derivatives as crystalline solids (eFIM={(Dipp)N=C(Ar)N(Dipp)}C≡CSiMe3 ) is reported. eFIMs are thermally stable under inert gas atmosphere and show moderate air stability (t1/2= 3 h for Ar=Ph). eFIMs are excellent surrogates of iMICs, which generally have a limited shelf-life, and readily undergo ring-closing click reactions with a variety of main-group as well as transition metal Lewis acids to form hitherto challenging iMIC-compounds in good to excellent yields. In addition to the relevance of eFIMs in the synthesis of iMIC-compounds, quantification of the stereoelectronic properties of a representative iMIC (Ar=Ph) by experimental and theoretical methods suggests remarkably σ-donor property and steric profile of these new ligand sets.

Mono(Lewis Base)-Stabilized Gallium Iodide: An Unexplored Class of Promising Ligands

Quantum-chemical (DFT) calculations on hitherto unknown base(carbene)-stabilized gallium monoiodides (LB→GaI) suggest that these systems feature one lone pair of electrons and a formally vacant p-orbital - both centered at the central gallium atom - and exhibit metallomimetic behavior. The calculated reaction free energies as well as bond dissociation energies suggest that these LB→GaI systems are capable of forming stable donor-acceptor complexes with group 13 trichlorides. Examination of the ligand exchange reactions with iron and nickel complexes indicates their potential use as ligands in transition metal chemistry. In addition, it is found that the title compounds are also able to activate various enthalpically robust bonds. Further, a detailed mechanistic investigation of these small molecule activation processes reveals the non-innocent behavior of the carbene (base) moiety attached to the GaI fragment, thereby indicating the cooperative nature of these bond activation processes. The energy decomposition analysis (EDA) and activation strain model (ASM) of reactivity were also employed to quantitatively understand and rationalize the different activation processes.

Phosphino-Phosphination Reactions: Frustrated Lewis Pair Reactivity of Phosphino-Phosphonium Cations with Alkynes

The phosphino-phosphonium cations of the form [R3 PPR'2 ]+ are labile and provide access to the constituent Lewis acidic and Lewis basic fragments. This permits frustrated Lewis pair-type addition reactions to alkynes, affording unprecedented phosphino-phosphination reactions and giving cations of the form [cis-R3 PCHC(R'')PR'2 ]+ . This reactivity is further adapted to prepare several examples of a rare class of dissymmetric cis-olefin-linked bidentate phosphines.

Activation of Ge-H and Sn-H Bonds with N-Heterocyclic Carbenes and a Cyclic (Alkyl)(amino)carbene

A study of the reactivity of several N-heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene (cAACMe ) with the group 14 hydrides GeH2 Mes2 and SnH2 Me2 (Me=CH3 , Mes=1,3,5-(CH3 )3 C6 H2 ) is presented. The reaction of GeH2 Mes2 with cAACMe led to the insertion of cAACMe into one Ge-H bond to give cAACMe H-GeHMes2 (1). If 1,3,4,5-tetramethyl-imidazolin-2-ylidene (Me2 ImMe ) was used as the carbene, NHC-mediated dehydrogenative coupling occurred, which led to the NHC-stabilized germylene Me2 ImMe ⋅GeMes2 (2). The reaction of SnH2 Me2 with cAACMe also afforded the insertion product cAACMe H-SnHMe2 (3), and reaction of two equivalents Me2 ImMe with SnH2 Me2 gave the NHC-stabilized stannylene Me2 ImMe ⋅SnMe2 (4). If the sterically more demanding NHCs Me2 ImMe , 1,3-di-isopropyl-4,5-dimethyl-imidazolin-2-ylidene (iPr2 ImMe ) and 1,3-bis-(2,6-di-isopropylphenyl)-imidazolin-2-ylidene (Dipp2 Im) were employed, selective formation of cyclic oligomers (SnMe2 )n (5; n=5-8) in high yield was observed. These cyclic oligomers were also obtained from the controlled decomposition of cAACMe H-SnHMe2 (3).

N-heterocyclic carbene and cyclic (alkyl)(amino)carbene adducts of plumbanes and plumbylenes

Lewis-acid/base adducts of N-heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene cAAC^Me (1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) with selected lead(II) and lead(IV) compounds are presented. The reaction of the NHCs Me₂Im^Me (1,3,4,5-tetramethyl-imidazolin-2-ylidene), iPr₂Im^Me (1,3-di-isopropyl-4,5-dimethyl-imidazolin-2-ylidene), Dipp₂Im (1,3-bis-(2,6-di-isopropylphenyl)-imidazolin-2-ylidene) and cAAC^Me (1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) with PbI₂ yielded the NHC-containing plumbylenes NHC·PbI₂ (NHC = Me₂Im^Me (1), iPr₂Im^Me (2), Dipp₂Im (3) and cAAC^Me·PbI₂ (4)). Using the Pb(IV) compound PbCl₂Ph₂, the plumbane adducts NHC·PbCl₂Ph₂ (NHC = Me₂Im^Me (5), iPr₂Im^Me (6), Dipp₂Im (7)) and cAAC^Me·PbCl₂Ph₂ (8)) were isolated in high yields. Reduction of the lead(IV) adducts 5 and 6 with excess KC₈ afforded the diaryl substituted plumbylenes Me₂Im^Me·PbPh₂ (9) and iPr₂Im^Me·PbPh₂ (10), which are stable in the solid state but decompose in solution.

A Silylene Stabilized by a σ‐Donating Nickel(0) Fragment

A donor‐stabilized silylene 4 featuring a Ni⁰‐based donating ligand was synthesized. Complex 4 exhibits a pyramidalized and nucleophilic Si^II center and shows a peculiar behavior due to the cooperative reactivity of Si and Ni centers. Calculations indicate that the orientation of Ni‐ligands with respect to the silylene moiety is crucial in determining the role of the Ni‐fragment (Lewis acid or Lewis base) towards silylene. Indeed, a simple 90° rotation of the Si−Ni bond, reverses the role of Ni, and transforms a classical silylene→Ni⁰ complex into an unprecedented Ni⁰→silylene complex.

Reactions of a Four‐Membered Borete with Carbon, Silicon, and Gallium Donor Ligands: Fused and Spiro‐Type Boracycles

Donor‐acceptor cyclopropanes or cyclobutanes are dipolar reagents, which are widely used in the synthesis of complex organic (hetero)cycles in ring expansion reactions. Applying this concept to boron containing heterocycles, the four‐membered borete cyclo‐iPr₂N‐BC₁₀H₆ reacted with the carbon donor ligands 2,6‐xylylisonitrile and the carbene IMes :C(NMesCH)₂ with ring expansion and ring fusion, respectively. In particular, the tetracyclic structure formed with IMes displays zwitterionic character and absorption in the visible region. In contrast to the carbene IMes, the heavier carbenoids :Si(NDippCH)₂ and :Ga(AmIm) with a two‐coordinate donor atom afford spiro‐type bicyclic compounds, which display four‐coordinate geometry at silicon or gallium. (TD‐)DFT calculations provide deeper insight into the mechanism of formation and the absorption properties of these new compounds.

Versatile chemistry of six-membered NHC with boranes: bromination at sp3 borane, activation of B–H bond of HBpin, and ring expansion of NHC

The NHC•borane chemistry has been majorly restricted to imidazol-2-ylidene classes of carbenes. In our previous communication, we have reported the synthesis of 6-SIDipp•BH3 [6-SIDipp = 1,3-di(2,6-diisopropylphenyl) tetrahydropyrimidine-2-ylidene] and its electrophilic...

Leveraging the Domino Skeletal Expansion of Thia-/Selenazolidinones via Nitrogen-Atom Transfer in Hexafluoroisopropanol: Room Temperature Access to Six-Membered S/Se,N-Heterocycles

Herein, a highly regioselective domino skeletal-expansion process that transforms 2-aminothiazolidinone into six-membered S,N-heterocycle is developed with the aid of TMS-azide in hexafluoroisopropanol (HFIP) at ambient temperature. Functioning of the C2 tertiary amine as latent reactive group on thiazolidinone moiety was the key to this development, which allowed relay substitution with azide and imparted subsequent ring-expansion under metal/acid free-conditions. The reaction also underscored an intermolecular nitrogen-atom transfer process from TMS-azide leading to final products, where any intermediary azidothiazolidinone was absent. The strategy was extendable to analogous synthesis of Se,N-heterocycles, and furthermore, late-stage drug-modification and follow-up transformations were also performed. Density functional theory calculations and control experiments provided important mechanistic insights and highlighted potential roles of HFIP in the transformation.

Latent Nucleophilic Carbenes

Using DFT and ab initio calculations, we demonstrate that noncyclic formamidines can undergo thermal rearrangement into their isomeric aminocarbenes under rather mild conditions. We synthesized the silylformamidine, for which the lowest activation energy in this process was predicted. Experimental studies proved it to serve as a very reactive nucleophilic carbene. The reactions with acetylenes, benzenes, and trifluoromethane proceeded via insertion into sp, sp², and sp³ CH bonds. The carbene also reacted with the functional groups, such as CHO, COR, and CN at double or triple bonds, displaying high mobility of the trimethylsilyl group. The obtained silylformamidine can be considered as a latent nucleophilic carbene. It can be prepared in bulk quantities, stored, and used when the need arises. Calculation results predict similar behavior for some other silylated formamidines and related compounds.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

C-N and C-H Activation of an N-Heterocyclic Carbene by Magnesium(II) Hydride and Magnesium(I) Complexes

Reactions of the hindered N-heterocyclic carbene, :C{(MesNCH)₂} (IMes; Mes = mesityl), with a series of β-diketiminatomagnesium(II) hydride and dimagnesium(I) complexes were carried out at 80 °C. The reactions involving the magnesium hydrides, [{(^ArNacnac)Mg(μ-H)}₂] [^ArNacnac = [(ArNCMe)₂CH]^-, where Ar = 2,6-diethylphenyl (Dep) or 2,6-diisopropylphenyl (Dip)], proceeded via activation of an exocyclic C-N bond of IMes, giving magnesium imidazolyl compounds [(^ArNacnac)Mg(μ-H)(μ-Imid)Mg(^ArNacnac)] (Imid = [NC₂H₂N(Mes)C]^-) and mesitylene. A low-yield IMes methyl C-H activation product, [(^DepNacnac)Mg(IMes^-H)], was also obtained, via H₂ elimination, from the reaction between IMes and [{(^DepNacnac)Mg(μ-H)}₂]. Reactions between IMes and dimagnesium(I) compounds [{(^ArNacnac)Mg}₂] [Ar = 2,6-dimethylphenyl (Xyl) or Mes] afforded isostructural C-H activation products [(^ArNacnac)Mg(IMes^-H)] but in higher yields. Density functional theory calculations suggest that the reactions do not progress via stable adduct complex intermediates, which are sterically inaccessible.

Nickel-Catalyzed Intramolecular 1,2-Aryl Migration of Mesoionic Carbenes (iMICs)

Intramolecular 1,2-Dipp migration of seven mesoionic carbenes (iMICAr ) 2 a-g (iMICAr =ArC{N(Dipp)}2 CHC; Ar=aryl; Dipp=2,6-iPr2 C6 H3 ) under nickel catalysis to give 1,3-imidazoles (IMDAr ) 3 a-g (IMDAr =ArC{N(Dipp)CHC(Dipp)N}) has been reported. The formation of 3 indicates the cleavage of an N-CDipp bond and the subsequent formation of a C-CDipp bond in 2, which is unprecedented in NHC chemistry. The use of 3 in accessing super-iMICs (5) (S-iMIC=ArC{N(Dipp)N(Me)C(Dipp)}C) has been shown with selenium (6), gold (7), and palladium (8) compounds. The quantification of the stereoelectronic properties reveals the superior σ-donor strength of 5 compared to that of classical NHCs. Remarkably, the percentage buried volume of 5 (%Vbur =45) is the largest known amongst thus far reported iMICs. Catalytic studies show a remarkable activity of 5, which is consistent with their auspicious stereoelectronic features.

N-Heterocyclic silylenes as ambiphilic activators and ligands

Recent developments of the use of N-heterocyclic silylenes (NHSis), higher homologues of Arduengo-carbenes, as ambiphilic activators and ligands are highlighted and a comparison of NHSi ligands with NHC and phosphine ligands is provided.

Divergent reactivity of divinylsilanes toward sulfonamides in different oxidative systems

Oxidative sulfonamidation of divinylsilanes with various sulfonamides in different solvents is reported. With t-BuOI as an oxidant, halogenation is the main process, whereas aziridines are the minor products. With NBS in CH₂Cl₂ the products of bromination or bromosulfonamidation were obtained, whereas in MeCN or THF the Ritter-type solvent interception products are formed. The obtained bromosulfonamidation products undergo base-induced cyclization to various heterocycles, including imidazolines, 1,4-oxazocanes, or Si,N-containing heterocycles of a new type, 1,3,5-diazasilinanes, in up to quantitative yield.

Insight into the Decomposition Mechanism of Donor-Acceptor Complexes of EH2 (E = Ge and Sn) and Access to Germanium Thin Films from Solution

Electron-donating N-heterocyclic carbenes (Lewis bases, LB) and electron-accepting Lewis acids (LA) have been used in tandem to yield donor-acceptor complexes of inorganic tetrelenes LB·EH₂·LA (E = Si, Ge, and Sn). Herein, we introduce the new germanium (II) dihydride adducts ImMe₂·GeH₂·BH₃ (ImMe₂ = (HCNMe)₂C:) and ImⁱPr₂Me₂·GeH₂·BH₃ (ImⁱPr₂Me₂ = (MeCNⁱPr)₂C:), with the former complex containing nearly 40 wt % germanium. The thermal release of bulk germanium from ImMe₂·GeH₂·BH₃ (and its deuterated isotopologue ImMe₂·GeD₂·BD₃) was examined in solution, and a combined kinetic and computational investigation was undertaken to probe the mechanism by which Ge is liberated. Moreover, the thermolysis of ImMe₂·GeH₂·BH₃ in solution cleanly affords conformal nanodimensional layers of germanium as thin films of variable thicknesses (20-70 nm) on silicon wafers. We also conducted a computational investigation into potential decomposition pathways for the germanium(II)- and tin(II)-dihydride complexes NHC·EH₂·BH₃ (NHC = [(HCNR)₂C:]; R = 2,6-ⁱPr₂C₆H₃ (Dipp), Me, and H; and E = Ge and Sn). Overall, this study introduces a mild and convenient solution-only protocol for the deposition of thin films of Ge, a widely used semiconductor in materials research and industry.

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.

Ge(0) Compound Stabilized by a Diimino-Carbene Ligand: Synthesis and Ambiphilic Reactivity

The germylone dimNHCGe (5, dimNHC = diimino N-heterocyclic carbene) was successfully prepared via the reduction of the germanium cation [dimNHCGeCl]⁺ with KC₈. The molecular structure of 5 was unambiguously established by both NMR spectroscopy and single-crystal X-ray diffraction. The reactivity of 5 was investigated, revealing that it undergoes oxidative addition of HCl, CH₃I, and PhI, accompanied by an unusual migration of the H, Me, and Ph groups from germanium to the carbene ligand. Related chemistry was also observed with C₅F₅N, which results in the migration of the fluorinated pyridine moiety to the carbene ligand. Compound 5 also undergoes cycloaddition with tetrachloro-o-benzoquinone to afford a Ge(IV) adduct.

Theoretical Investigation of Main-Group Element Hydride Insertion into Phosphorus-Heterocyclic Carbenes (PHCs)

Initial reports of ring expansion reactions (RER) of N-heterocyclic carbenes (NHCs) with main-group element hydrides have led to several synthetic and theoretical investigations, which include reports of insertion by Be, B, Al, Si, and Zn hydrides. The RERs generally lead to insertion of the heteroatom into the endocyclic C–N bond with formation of an expanded heterocyclic ring. Following the recent isolation of a P-heterocyclic carbene (PHC), here we report results from a computational study (RI-SCS-MP2/def2-TZVP//M06–2X/def2-TZVP) of RERs with a series of PHCs for the ring-insertion of silicon (SiH4, SiH2Ph2) and boron (BH3, BH2NMe2) hydrides. In order to explore the roles of both electronic and steric effects on PHCs and their reactivity, a series of P-substituent PHCR (R=H, Me, Ph, and bulky Ar groups) were investigated. Bulky R groups serve to maximise ring planarity and the σ-donating capability of the PHC. For RER, the PHC analogues exhibit facile initial hydride transfer from the main-group hydrides to the carbene carbon, with barriers that are substantially lower than with NHCs. However, the full ring insertion mechanisms for PHCs are, in general, kinetically unfavourable due to a large barrier associated with the ring-expansion step. While bulky P-substituents maximise heterocycle planarity towards that of NHCs, the RER reactivity with bulky PHCs does not reflect that of an NHC.

Reaction Mechanism of Li and Mg Carbenoid Cyclopropanations: Metal-π and σ Interactions

The mechanism of the reaction of lithium and magnesium carbenoids with ethylene to give cyclopropane has been explained in detail in all the steps at the G4 level of theory. We explored the lithium and magnesium interaction toward π_C=C and σ_C-C bonds in the reactants and the products. We have also investigated the reaction path by means of the force profile formalism in order to highlight the electronic and the structural rearrangements along the potential energy surface of the cyclopropanation. The results indicate that all of the reactions are stepwise, exoenergetic, with low barriers. All our findings were confirmed by dynamic simulations for chlorometal carbenoids. Furthermore, from the intrinsic reaction coordinate procedure, we were able to find out the intermediates that can take place when the reaction is descending from the transition state to the products or reactants. The reaction force analysis at B3LYP/6-311G(d,p) indicates that the energy barriers are mostly due to structural rearrangements which are produced by the approach of the carbenoid to ethylene. Quantum theory of atoms in molecules and electron localization function analyses indicate that products, reactants, and intermediates form complexes stabilized by attractive forces between Li/Mg and single/double bonds.

Interconversion and reactivity of manganese silyl, silylene, and silene complexes

Interconversions between manganese silylene and silene complexes are reported, including those involving the first spectroscopically observed silene complexes with an SiH substituent, and their involvement in ethylene hydrosilylation is discussed.

Manganese disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) reacted with ethylene to form silene hydride complexes [(dmpe)₂MnH(RHSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,H: R = Ph, 6^Bu,H: R = ⁿBu). Compounds 6^R,H reacted with a second equivalent of ethylene to generate [(dmpe)₂MnH(REtSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,Et: R = Ph, 6^Bu,Et: R = ⁿBu), resulting from apparent ethylene insertion into the silene Si–H bond. Furthermore, in the absence of ethylene, silene complex 6^Bu,H slowly isomerized to the silylene hydride complex [(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiEtⁿBu)] (3^Bu,Et). Reactions of 4^R with ethylene likely proceed via low-coordinate silyl {[(dmpe)₂Mn(SiH₂R)] (2^Ph: R = Ph, 2^Bu: R = ⁿBu)} or silylene hydride {[(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR)] (3^Ph,H: R = Ph, 3^Bu,H: R = ⁿBu)} intermediates accessed from 4^R by H₃SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 4^R with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)₂Mn(SiH₂R)(CNR′)] (7a–d: R = Ph or ⁿBu; R′ = o-xylyl or ^tBu), and NHC-stabilized silylene hydride complexes [(dmpe)₂MnH{ Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR(NHC)}] (8a–d: R = Ph or ⁿBu; NHC = 1,3-diisopropylimidazolin-2-ylidene or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene), respectively, all of which were crystallographically characterized. Silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe)₂MnH(C₂H₄)] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C₂H₄ and C₂D₄) hydrosilylation was investigated using [(dmpe)₂MnH(C₂H₄)] (1) as a pre-catalyst, resulting in stepwise conversion of primary to secondary to tertiary hydrosilanes. Various catalytically active manganese-containing species were observed during catalysis, including silylene and silene complexes, and a catalytic cycle is proposed.

B-B Cleavage and Ring-Expansion of a 1,4,2,3-Diazadiborinine with N-Heterocyclic Carbenes

A 1,4,2,3-diazadiborinine derivative was found to form Lewis adducts with strong two-electron donors such as N-heterocyclic and cyclic (alkyl)(amino)carbenes. Depending on the donor, some of these Lewis pairs are thermally unstable, converting to sole B,N-embedded products upon gentle heating. The products of these reactions, which have been fully characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction, were identified as B,N-heterocycles with fused 1,5,2,4-diazadiborepine and 1,4,2-diazaborinine rings. Computational modelling of the reaction mechanism provides insight into the formation of these unique structures, suggesting that a series of B-H, C-N, and B-B bond activation steps are responsible for these "intercalation" reactions between the 1,4,2,3-diazadiborinine and NHCs.

Intramolecular Ring-Expansion Reaction (RER) and Intermolecular Coordination of In Situ Generated Cyclic (Amino)(aryl)carbenes (cAArCs)

Cyclic (amino)(aryl)carbenes (cAArCs) based on the isoindoline core were successfully generated in situ by α-elimination of 3-alkoxyisoindolines at high temperatures or by deprotonation of isoindol-2-ium chlorides with sodium or copper(I) acetates at low temperatures. 3-Alkoxy-isoindolines 2 a,b-OR (R=Me, Et, iPr) have been prepared in high yields by the addition of a solution of 2-aryl-1,1-diphenylisoindol-2-ium triflate (1 a,b-OTf; a: aryl=Dipp=2,6-diisopropylphenyl; b: Mesityl-, Mes=2,4,6-trimethylphenyl) to the corresponding alcohol (ROH) with NEt3 at room temperature. Furthermore, the reaction of 2 a,b-OMe in diethyl ether with a tenfold excess of hydrochloric acid led to the isolation of the isoindol-2-ium chlorides 1 a,b-Cl in high yields. The thermally generated cAArC reacts with sulfur to form the thioamide 3 a. Without any additional trapping reagent, in situ generation of 1,1-diphenylisoidolin-3-ylidenes does not lead to the isolation of these compounds, but to the reaction products of the insertion of the carbene carbon atom into an ortho C-H bond of a phenyl substituent, followed by ring-expansion reaction; namely, anthracene derivatives 9-N(H)aryl-10-Ph-C14 H8 4 a,b (a: Dipp; b: Mes). These compounds are conveniently synthesized by deprotonation of the isoindol-2-ium chlorides with sodium acetate in high yields. Deprotonation of 1 a-Cl with copper(I) acetate at low temperatures afforded a mixture of 4 a and the corresponding cAArC copper(I) chloride 5 a, and allowed the isolation and structural characterization of the first example of a cAArC copper complex of general formula [(cAArC)CuCl].

Pseudodiborenes: hydride-bridged diboranes(5) as two-electron reductants of chalcogens

The reactivity of two nucleophilic neutral sp2-sp3 diboranes towards chalcogens is presented herein. Both diboranes(5) serve as two-electron reductants, incorporating oxygen, sulfur and selenium atoms. Treatment with chalcogen sources results in the oxidative insertion of one chalcogen atom into the B-B single bond, while depending on the negative inductive effect of the chalcogen and the boron bound aryl substituent further N-heterocyclic carbene (NHC) ring expansion and hydride migration can occur. These reactions provide access to unprecedented six- or seven-membered heterocycles and help to illuminate the pseudo-multiple bonding character of hydrogen-bridged B-B single bonds.

N-Heterocyclic Carbenes in Materials Chemistry

N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

Silylene induced cooperative B–H bond activation and unprecedented aldehyde C–H bond splitting with amidinate ring expansion

Silylene mediated B–H and aldehyde C–H bond splitting were realized under ambient conditions.

Synthesis of fused B,N-heterocycles by alkyne cleavage, NHC ring-expansion and C-H activation at a diboryne

The addition of alkynes to a saturated N-heterocyclic carbene (NHC)-supported diboryne results in spontaneous cycloaddition, with complete B[triple bond, length as m-dash]B and C[triple bond, length as m-dash]C triple bond cleavage, NHC ring-expansion and activation of a variety of C-H bonds, leading to the formation of complex mixtures of fused B,N-heterocycles.