Silyliumylidene Ion Stabilized by Two σ-Donating Ni(0)- and Pd(0)-Fragments

A silyliumylidene ion 2 stabilized by two σ-donating Ni(0)- and Pd(0)-fragments was successfully synthesized. Due to the σ-donation of M→Si interactions, 2 presents a pyramidalized cationic silicon center with a localized lone pair. The additional coordination of basic Pd(0) fragment to the mono-Ni(0)-stabilized silyliumylidene 1 results in a higher HOMO level and an unchanged HOMO-LUMO gap and thus, 2 remains highly reactive. Interestingly, the coordination mode at the Si center is closely related to the nature of M-ligands. Indeed, the donor/donor-stabilized silyliumylidene ion 2 has been transformed into a donor/acceptor-stabilized ion 13, featuring a trigonal planar Si center with a vacant orbital, just via a ligand exchange reaction from PCy3/NHC toward PMe3.

Tetryliumylidene ions in synthesis and catalysis

Tetryliumylidene ions ([R-E:]+), recognised for their intriguing electronic properties, have attracted considerable interest. These positively charged species, with two vacant p-orbitals and a lone pair at the E(ii) centre (E = Si, Ge, Sn, Pb), can be viewed as the combination of tetrylenes (R2E:) and tetrylium ions ([R3E]+), which makes them potent Lewis ambiphiles. Such electronic features highlight the potential of tetryliumylidenes for single-site small molecule activation and transition metal-free catalysis. The effective utilisation of the electrophilicity and nucleophilicity of tetryliumylidenes is expected to stem from appropriate ligand choice. For most of the isolated tetryliumylidenes, electron donor- and/or kinetic stabilisation is necessary. This minireview highlights the developments in tetryliumylidene syntheses and the progress of research towards their reactivity and applications in catalytic reactions.

NHC-Silyliumylidene Cation-Catalyzed Hydroboration of Isocyanates with Pinacolborane

The low-oxidation-state silicon-catalyzed hydroboration of isocyanates with pinacolborane (HBpin) using the NHC-silyliumylidene cation catalyst [(IMe)2SiH]I (1, IMe = :C{N(Me)C(Me)}2) is described. In the catalysis, the Si lone pair electrons activate isocyanates, and the latter react with HBpin to form N-boryl formamides at room temperature. Catalyst 1 further activates N-boryl formamides at 70 °C, the intermediates of which react with HBpin to form N-boryl methylamines and (pinB)2O.

A donor-supported silavinylidene and silylium ylides: boroles as a flexible platform for versatile Si(ii) chemistry

Electron-deficient, anti-aromatic 2,5-disilyl boroles are shown to be a flexibly adaptive molecular platform with regards to SiMe3 mobility in their reaction with the nucleophilic donor-stabilised precursor dichloro silylene SiCl2(IDipp). Depending on the substitution pattern, selective formation of two fundamentally different products of rivalling formation pathways is achieved. Formal addition of the dichlorosilylene gives the 5,5-dichloro-5-sila-6-borabicyclo[2.1.1]hex-2-ene derivatives. Under kinetically controlled conditions, SiCl2(IDipp) induces 1,3-trimethylsilyl migration and adds exocyclically to the generated carbene fragment giving an NHC-supported silylium ylide. In some cases interconversion between these compound classes was triggered by temperature or NHC-addition. Reduction of silaborabicyclo[2.1.1]hex-2-ene derivatives under forcing conditions gave clean access to recently described nido-type cluster Si(ii) half-sandwich complexes of boroles. Reduction of a NHC-supported silylium ylide gave an unprecedented NHC-supported silavinylidene which rearranges to the nido-type cluster at elevated temperatures.

Chelating Bis-silylenes As Powerful Ligands To Enable Unusual Low-Valent Main-Group Element Functions

ConspectusSilylenes are divalent silicon species with an unoccupied 3p orbital and one lone pair of electrons at the Si^II center. Owing to the excellent σ-donating ability of amidinato-based silylenes, which stems from the intramolecular imino-N donor interaction with the vacant 3p orbital of the silicon atom, N-heterocyclic amidinato bis(silylenes) [bis(NHSi)s] can serve as versatile strong donating ligands for cooperative stabilization of central atoms in unusually low oxidation states. Herein, we present our recent achievement on the application of bis(NHSi) ligands with electronically and spatially different spacers to main-group chemistry, which has allowed the isolation of a variety of low-valent compounds consisting of monatomic zero-valent group 14 E⁰ complexes (named "metallylones", E = Si, Ge, Sn, Pb); monovalent group 15 E^I complexes (E = N, P, isoelectronic with metallylones); and diatomic low-valent E₂ complexes (E = Si, Ge, P) with intriguing electronic structures and chemical reactivities.The role of the Si^II···Si^II distance was revealed to be crucial in this chemistry. Utilizing the pyridine-based bis(NHSi) (Si···Si distance: 7.8 Å) ligand, germanium(0) complexes with additional Fe(CO)₄ protection at the Ge⁰ site have been isolated. Featuring a shorter Si···Si distance of 4.3 Å, the xanthene-based bis(NHSi) has allowed the realization of the full series of heavy zero-valent group 14 element E⁰ complexes (E = Si, Ge, Sn, Pb), while the o-carborane-based bis(NHSi) (Si···Si distance: 3.3 Å) has enabled the isolation of Si⁰ and Ge⁰ complexes. Remarkably, reduction of the o-carborane-based bis(NHSi)-supported Si⁰ and Ge⁰ complexes induces the movement of two electrons into the o-carborane core and provides access to Si^I-Si^I and Ge^I-Ge^I species as oxidation products. Additionally, the o-carborane-based bis(NHSi) reacts with adamantyl azide, leading to a series of nitrogen(I) complexes as isoelectronic species of a carbone (C⁰ complex). Moreover, cooperative activation of white phosphorus gives bis(NHSi)-supported phosphorus complexes with varying and unexpected electronic structures when employing the xanthene-, o-carborane-, and aniline-based bis(NHSi)s. With the better kinetic protection provided by the xanthene-based bis(NHSi), small-molecule activation and functionalization of the bis(NHSi)-supported central E or E₂ atoms (E = Si, Ge, P) are possible and furnish several novel functionalized silicon, germanium, and phosphorus compounds.With knowledge of the ability of chelating bis(NHSi)s in coordinating and functionalizing low-valent group 14 and 15 elements, the application of these ligand systems to other main-group elements such as group 2 and 13 is quite promising. To fully understand the role of the NHSi in a bis(NHSi) ligand, introducing a mixed ligand, i.e., the combination of an NHSi with other functional groups, such as Lewis acidic borane or Lewis basic borylene, in one chelating ligand could lead to new types of low-valent main-group species. Furthermore, the development of a genuine acyclic silylene, without an imino-N interaction with the vacant 3p orbital at the silicon(II) atom, as part of a chelating bis(acyclic silylene) has the potential to form very electronically different main-group element complexes that could achieve even more challenging bond activations such as N₂ or unactivated C-H bonds.

Labile Base-Stabilized Silyliumylidene Ions. Non-Metallic Species Capable of Activating Multiple Small Molecules

Several base-stabilized silyliumylidene ions (2 and 3) with different ligands were synthesized. Their behaviour appeared strongly dependent on the nature of ligand. Indeed, in contrast to the poorly reactive silyliumylidene ions 3 c,d stabilized by strongly donating ligands (DMAP, NHC), the silylene- and sulfide-supported one (2-H and 3 a) exhibits higher reactivity toward various small molecules. Furthermore, their capability to successively activate multiple small molecules was clearly demonstrated by processes involving successive reactions with silane/formamide, CO2 and H2 . Moreover, HBPin adduct of 3 a (8-C) catalyzes the hydroboration of pyridine. Of particular interest, silylene-supported silyliumylidene complex 2-H is one of the rare species able to activate two H2 molecules.

Neutral and Cationic Complexes of Silicon(IV) Halides with Phosphine Ligands

The reaction of SiI4 and PMe3 in n-hexane produced the yellow salt, [SiI3(PMe3)2]I, confirmed from its X-ray structure, containing a trigonal bipyramidal cation with trans-phosphines. This contrasts with the six-coordination found in (the known) trans-[SiX4(PMe3)2] (X = Cl, Br) complexes. The diphosphines o-C6H4(PMe2)2 and Et2P(CH2)2PEt2 form six-coordinate cis-[SiI4(diphosphine)], which were also characterized by X-ray crystallography, multinuclear NMR, and IR spectroscopy. Reaction of trans-[SiX4(PMe3)2] (X = Cl, Br) with Na[BArF] (BArF = [B{3,5-(CF3)2C6H3}4]) produced five-coordinate [SiX3(PMe3)2][BArF], but while Me3SiO3SCF3 also abstracted chloride from trans-[SiCl4(PMe3)2], the reaction products were six-coordinate complexes [SiCl3(PMe3)2(OTf)] and [SiCl2(PMe3)2(OTf)2] with the triflate coordinated. X-ray crystal structures were obtained for [SiCl3(PMe3)2][BArF] and [SiCl2(PMe3)2(OTf)2]. The charge distribution across the silicon species was also examined by natural bond orbital (NBO) analyses of the computed density functional theory (DFT) wavefunctions. For the [SiX4(PMe3)2] and [SiX3(PMe3)2]+ complexes, the positive charge on Si decreases and the negative charge on X decreases going from X = F to X = I. Upon going from [SiX4(PMe3)2] to [SiX3(PMe3)2]+, i.e., removal of X-, there is an increase in positive charge on Si and a decrease in negative charge on the X centers (except for the case X = F). The positive charge on P shows a slight decrease.

Synthesis, Characterization and Reactivity of a σ-Donating Ni0 -Stabilized Silyliumylidene Ion

The synthesis of a silyliumylidene cation complex 2 stabilized by a Ni0 -based donating ligand is reported. Experimental and theoretical studies demonstrate that the highly electrophilic SiII center is stabilized by a dative Ni→Si σ-interaction and π-donations from the amino- and Ni-moieties. Due to the energetically close frontier orbitals localized on the Si and Ni atoms, complex 2 presents a competitive reactivity at Si and Ni sites.

Halogen-Exchange Reactions of Iminophosphonamido-Chlorosilylenes with Alkali Halides: Convenient Synthesis of Heavier Halosilylenes

Halogen-substituted silylenes are an important building block for synthesizing silicon-based low-valent and multiple-bond species. However, the number of reports on heavier halosilylenes that contain bromine and iodine is still limited. Here, we present a convenient synthesis for bromo- and iodosilylenes supported by an iminophosphonamide ligand. The heavier halosilylenes [Ph₂P(^tBuN)₂]SiX (2: X = Br, 3: X = I) were successfully synthesized via the halogen-exchange reaction of chlorosilylene 1 with alkali halides in THF. As a demonstration of the reactivity of 2 and 3, oxidative addition reactions of 2 and 3 with elemental selenium in C₆D₆ afforded the corresponding bromo- (5) or iodosilylene-selone (6) as colorless crystals. The molecular structures of 2, 3, 5, and 6 were fully characterized by spectroscopic means and single-crystal X-ray diffraction analysis. Furthermore, the effects of the halogen atom on the electronic state of halosilylenes 1-3 and halosilylene-selones 4-6 were investigated using density functional theory (DFT) calculations.

An NHC-Stabilized H2 GeBH2 Precursor for the Preparation of Cationic Group 13/14/15 Hydride Chains

The synthesis, characterization and reactivity studies of the NHC-stabilized complex IDipp ⋅ GeH2 BH2 OTf (1) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) are reported. Nucleophilic substitution of the triflate (OTf) group in 1 by phosphine or arsine donors provides access to the cationic group 13/14/15 chains [IDipp ⋅ GeH2 BH2 ERR1 R2 ]+ (2 E=P; R, R1 =H; R2 =t Bu; 3 E=P; R=H; R1 , R2 =Ph; 4 a E=P; R, R1 , R2 =Ph; 4 b E=As; R, R1 , R2 =Ph). These novel cationic chains were characterized by X-ray crystallography, NMR spectroscopy and mass spectrometry. Moreover, the formation of the parent complexes [IDipp ⋅ GeH2 BH2 PH3 ][OTf] (5) and [IDipp ⋅ GeH3 ][OTf] (6) were achieved by reaction of 1 with PH3 . Accompanying DFT computations give insight into the stability of the formed chains with respect to their decomposition.

Synthesis, electronic nature, and reactivity of selected silylene carbonyl complexes

Room-temperature stable main group element carbonyl complexes are rare. Here we report on the synthesis of two such complexes, namely gallium-substituted silylene-carbonyl complexes [L(X)Ga]2SiCO (X = I 2, Me 3;...

Entering new chemical space with isolable complexes of single, zero-valent silicon and germanium atoms

Monatomic zero-valent silicon and germanium complexes (silylones and germylones), stabilised by neutral donating ligands, emerged only recently as a new class of low-valent group 14 element compounds. Featuring four valence electrons in the form of two lone pairs at a single site, silylones and germylones represent a molecular resting state of single Si and Ge atoms, which are typically only observed at high temperature in the gas phase or in interstellar matter. These species are capable of transferring single Si and Ge atoms to unsaturated substrates and acting as building blocks for novel group 14 species. After introducing this type of compound and the examples known to date, this feature article highlights some chelating bis N-heterocyclic carbene (bis(NHC)) and bis N-heterocyclic silylene (bis(NHSi)) supported Si⁰ and Ge⁰ complexes, for which a range of unprecedented reactivity has been discovered. The characteristic behaviour of these silylones and germylones discussed here consists of (i) coordination to Lewis acids, (ii) oxidation with elemental chalcogens, (iii) bond activation of common organic substrates and inert small molecules; and (iv) homocoupling of the Si⁰ and Ge⁰ centres. This wealth of reactivity has opened the door to a series of Si and Ge compounds, which would be otherwise difficult to realise.

(NHC)Si═C═N-R: A Two-Coordinated Si0-Isocyanide Compound as Si(NHC) Transfer Reagent

Experimental and theoretical studies are reported of the first two-coordinated Si⁰-isocyanide compound (SIDipp)Si═C═N-Ar^Mes (1: SIDipp (NHC) = C[N(Dipp)CH₂]₂, Ar^Mes = 2,6-dimesitylphenyl), supported by an N-heterocyclic carbene (NHC). A Si atom economic two-step synthesis of 1 involves a 2e reduction of the isocyanide-stabilized silyliumylidene salt [SiBr(CNAr^Mes)(SIDipp)][B(Ar^F)₄] (2[B(Ar^F)₄], Ar^F = B(C₆H₃-3,5-(CF₃)₂)₄) with KC₈. 2[B(Ar^F)₄] was obtained from SiBr₂(SIDipp) after bromide abstraction with an equimolar mixture of Na[B(Ar^F)₄] and Ar^MesNC. Exact adherence to the stoichiometry is crucial in the latter reaction, since 2[B(Ar^F)₄] reacts with SiBr₂(SIDipp) via isocyanide exchange to afford the disilicon(II) salt [Si₂Br₃(SIDipp)₂)][B(Ar^F)₄] (3[B(Ar^F)₄]), the reaction leading to an equilibrium that favors 3[B(Ar^F)₄] (K_eq(298 K) = 10.6, ΔH° = -10.6 kJ mol^-1; ΔS^° = -16.0 J mol^-1 K^-1). 3[B(Ar^F)₄] was obtained selectively from the 2:1 reaction of SiBr₂(SIDipp) with Na[B(Ar^F)₄] and fully characterized. Detailed studies of 1 reveal an intriguing structure featuring a planar C_NHC-Si-C-N skeleton with a V-shaped geometry at the dicoordinated Si⁰ center, a slightly bent Si═C═N core, a C_NHC-Si-C_CNR 3c-2e out of plane π-bond (HOMO), and an anticlinal conformation of the SIDipp and Ar^Mes substituents leading to axial chirality and the presence of two enantiomers, (R_a)-1 and (S_a)-1. Compound 1 displays structural dynamics in solution, rapidly interconverting the enantiomers. The silacumulene 1 is a potent Si(SIDipp) transfer agent as demonstrated by the synthesis and full characterization of the NHC-supported germasilyne (Z)-(SIDipp)(Cl)Si═GeAr^Mes (4) from 1 and Ge(Ar^Mes)Cl.

An isolable, crystalline complex of square-planar silicon(IV)

The structure and reactivity of silicon(IV), the second most abundant element in our Earth's crust, is determined by its invariant tetrahedral coordination geometry. Silicon(IV) with a square-planar configuration (ptSi ^IV ) represents a transition state. Quantum theory supported the feasibility of stabilizing ptSi ^IV by structural constraint, but its isolation has not been achieved yet. Here, we present the synthesis and full characterization of the first square-planar coordinated silicon(IV). The planarity provokes an extremely low-lying unoccupied molecular orbital that induces unusual silicon redox chemistry and CH-agostic interactions. The small separation of the frontier molecular orbitals enables visible-light ligand-element charge transfer and bond-activation reactivity. Previously, such characteristics have been reserved for d-block metals or low-valent p-block elements. Planarization transfers them, for the first time, to a p-block element in the normal valence state.

Si(II) Cation-Promoted Formation of an Abnormal NHC-Bound Silylene and a cAAC-Silanyl Radical Ion

The reaction of amidinatosilylene LSi(:)Cl [L = PhC(NtBu)₂] with N-heterocyclic carbene I_Ar [:C{N(Ar)CH}₂, where Ar = 2,6-iPr₂C₆H₃] and NaOTf in tetrahydrofuran (THF) facilely afforded a silicon(II) cation [LSi(:)-_aI_Ar]⁺OTf^- (1⁺OTf^-), where I_Ar isomerizes to abnormal N-heterocyclic carbene _aI_Ar, coordinating to the silicon(II) center. Its Ge homologue, [LGe(:)-_aI_Ar]⁺OTf^- (2⁺OTf^-), was also accessed via the same protocol. For the formation of 1⁺, we propose that an in situ-generated Si(II) cation [LSi(:)]⁺ under the treatment of LSi(:)Cl with NaOTf may isomerize I_Ar in THF. In contrast, the replacement of I_Ar with cyclic alkyl(amino) carbene (cAAC) furnished a cAAC-silanyl radical ion [LSi(H)-cAAC]^•+(LiOTf₂)^- [3^•+(LiOTf₂)^-], which may undergo an abstraction of the H radical from THF. All of the products were characterized by nuclear magnetic resonance spectroscopy, electron paramagnetic resonance, and X-ray crystallography, and their bonding scenarios were investigated by density functional theory calculations. These studies provide new perspective on carbene-silicon chemistry.

A Mono-Substituted Silicon(II) Cation: A Crystalline "Supersilylene"

Mono-coordinated silicon(II) cations are predicted to be reactive ambiphiles, combining the typically high Lewis acidity of silicon cations with nucleophilicity due to the presence of an electron pair at the same atomic centre. Here, a carbazole-derived scaffold was used to isolate salts with a mono-coordinated silicon(II) cation, [RSi]+ (R=bulky carbazolyl substituent), by halide abstraction from a base-free halosilylene, RSiI, with Ag[Al(Ot BuF )4 ]. Despite the bulk of the carbazolyl moiety, the silylenylium cation [RSi]+ retains high reactivity. It was shown to react with an amine to form three bonds at the silicon atom in one reaction which conforms with the notion of a "supersilylene". The resulting silylium cation [RSi(H)NR'2 ]+ (in the formal oxidation state SiIV ) obtained by oxidative addition of an NH bond at [RSi]+ is even more acidic than the silylenylium cation (SiII ) due to the absence of a lone pair of electrons the silicon atom.

Small Molecule Activation by Two-Coordinate Acyclic Silylenes

In recent decades, the chemistry of stable silylenes (R2Si:) has evolved significantly. The first major development in this chemistry was the isolation of a silicocene which is stabilized by the Cp* (Cp* = η5-C5Me5) ligand in 1986 and subsequently the isolation of a first N-heterocyclic silylene (NHSi:) in 1994. Since the groundbreaking discoveries, a large number of isolable cyclic silylenes and higher coordinated silylenes, i.e. Si(II) compounds with coordination number greater than two, have been prepared and the properties investigated. However, the first isolable two-coordinate acyclic silylene was finally reported in 2012. The achievements in the synthesis of acyclic silylenes have allowed for the utilization of silylenes in small molecule activation including inert H2 activation, a process previously exclusive to transition metals. This minireview highlights the developments in silylene chemistry, specifically two-coordinate acyclic silylenes, including experimental and computational studies which investigate the extremely high reactivity of the acyclic silylenes.

Facile Access to Dative, Single, and Double Silicon-Metal Bonds Through M-Cl Insertion Reactions of Base-Stabilized SiII Cations

Silicon(II) cations can offer fascinating reactivity patterns due to their unique electronic structure: a lone pair of electrons, two empty p orbitals and a positive charge combined on a single silicon center. We now report the facile insertion of N-heterocyclic carbene (NHC)-stabilized silyliumylidene ions into M-Cl bonds (M=Ru, Rh), forming a series of novel chlorosilylene transition-metal complexes. Theoretical investigations revealed a reaction mechanism in which the insertion into the M-Cl bond with concomitant 1,2-migration of a silicon-bound NHC to the transition metal takes place after formation of an initial silyliumylidene transition-metal complex. The mechanism could be verified experimentally through characterization of the intermediate complexes. Furthermore, the obtained chlorosilylene complexes can be conveniently utilized as synthons to access Si-M and Si=M bonding motifs bonds through reductive dehalogenation.

Lewis base-stabilized silyliumylidene ions in transition metal coordination chemistry

An overview of the progress made in the transition metal chemistry of isolable base-stabilized silyliumylidene ions.

NHC-Stabilized Silyl-Substituted Chlorosilylene

The first N-heterocyclic carbene (NHC) stabilized silyl-substituted chlorosilylene (1) was isolated via selective dehydrochlorination by NHC from silyl-based Si(IV) precursor tBu₃SiSiHCl₂. Compound 1 can form an iron chlorosilylene complex (2) with an iron carbonyl dimer and undergoes chloride/hydride metathesis to yield a stable NHC-silylene hydride borane adduct (3). Upon treatment with additional NHC, chlorosilylene 1 was converted into silyl-substituted silyliumylidene ions (4).

NHC-Stabilised Silyliumylidene Ions

Donor-stabilised silyliumylidene ions, from the parent [R-Si:]⁺ , are a class of low-valent silicon species which have received increasing research interest in the last several years. This interest began in the fundamental synthesis and characterisation of these compounds, but has since started to include more investigation into their further reactivity after several stable NHC-stabilised silyliumylidene ions were reported. This personal account briefly discusses the history of the still-young field of silyliumylidene ions followed by a more detailed discussion of published work from our group on the further development of silyliumylidene chemistry over the last four years.

Influence of N-Substitution on the Formation and Oxidation of NHC-CAAC-Derived Triazaalkenes

We have studied the effect of N-substitution on the course of the reaction of imidazolium triflate. The reaction of N-heterocyclic carbene with N-tBu-substituted pyrrolinium triflate afforded 2-(pyrrolidin-2-yl)-imidazolium triflate, 3^R. Treatment of 3^R with potassium bis(trimethylsilyl)amide (KHMDS) leads to either the dealkylation product 4 or the deprotonation product, triazaalkene 5, depending on the N-substitution at the imidazolium moiety. Density functional studies using the B3LYP/TZVP setup have been employed to explore various pathways for the dealkylation reaction and the calculated energies support the dealkylation by a large energy margin compared to the deprotonatation process. Theoretical calculations revealed that dealkylation reaction is thermodynamically more favorable than deprotonation. The triazaalkene 5 could be oxidized by AgOTf to the corresponding radical cation 6 and dication 7 in-situ. While 6 and 7 could not be isolated, the formation of the former is inferred by electron paramagnetic resonance spectroscopy and its abstraction of a H-atom to afford 3^Me. Similarly, the formation of the dication 7 is inferred by its ready elimination of isobutylene affording 8.

Equilibrium Coordination of NHCs to Si(IV) Species and Donor Exchange in Donor-Acceptor Stabilized Si(II) and Ge(II) Compounds

We report the reversible coordination of the N-heterocyclic carbene (NHC), NHC ^iPr₂Me₂ (NHC ^iPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), to silicon(IV)-halides, SiCl₄, MeSiCl₃, Me₂SiCl₂, and Me₃SiCl. Predicted as well as experimentally determined thermodynamic parameters of these equilibria confirm that the complexation constant increases with the Lewis acidity of the silicon halides. In contrast, the more σ-donating N-heterocyclic carbene, NHC^Me₄ (NHC^Me₄ = 1,3,4,5-tetramethylimidazol-2-ylidene), does not show any signs of dissociation from the corresponding SiCl₄ and Me₂SiCl₂ adducts even at higher temperatures. As a consequence, NHC ^iPr₂Me₂ in donor-acceptor stabilized Si(II)- and Ge(II)-dimethyl complexes, NHC ^iPr₂Me₂·GeMe₂·Fe(CO)₄ and NHC ^iPr₂Me₂·SiMe₂·Fe(CO)₄, is readily replaced by NHC^Me₄.

Synthesis of Cyclic Alkyl(amino) Carbene Stabilized Silylenes with Small N-Donating Substituents

Lewis base cAACs stabilized monomeric silylenes with halogen or methyl substituents at the silicon center have not been reported due to the strong σ-donor and π-acceptor character of cAAC. To prepare these monomeric silylenes, we used the silicon(IV) precursors 5 and 6 with a nitrogen donor group L (L=o-C₆ H₄ NMe₂ ). The cAAC-stabilized (cAAC=C(CH₂ )(CMe₂ )₂ N-Ar, Ar=2,6-iPr₂ C₆ H₃ ) silylenes LSiCl(cAAC) (7) and LSiMe(cAAC) (8) were synthesized by reduction of LSiCl₃ and LSiMeCl₂ with two equivalents of KC₈ in the presence of one equivalent of cAAC, respectively. Compounds 7 and 8 were characterized by single-crystal X-ray crystallography, NMR spectroscopy, and elemental analysis. Compounds 7 and 8 are stable in the solid state as well as in solution at room temperature for at least four months under inert conditions.

NHC-stabilized silyl-substituted silyliumylidene ions

The first silyl-substituted silyliumylidene ions are reported. The scope of a previously described convenient one-step procedure for the preparation of N-heterocyclic carbene (NHC) stabilized silyliumylidene ions via dehydrochlorination from a Si(iv) precursor with NHCs is expanded to silyl-based substituents as well as aryl substituents with reduced steric bulk.

"Abnormal" Addition of NHC to a Conjugate Acid of CAAC: Formation of N-Alkyl-Substituted CAAC

The addition reactions of N-heterocyclic carbenes (NHCs) are mostly known to occur through the carbenic centre (C2), which leads to a "normal" adduct. Herein, we report the "abnormal" addition of NHC^Dip 1 (1,3-(2,6-iPr₂ C₆ H₃ )-imidazole-2-ylidene) to a conjugate acid of cyclic (alkyl)(amino)carbene 2 (CAAC^iPr =1-iPr-3,3,5,5-Me₄ -pyrrolinium triflate). Mechanistic study revealed that this reaction proceeded through the in situ formation of 1,3-(2,6-iPr₂ C₆ H₃ )-imidazolium cation 4 and N-iPr-substituted CAAC 5 followed by the oxidative addition of compound 5 across the C4-H bond (alias backbone C-H) of compound 4. The in situ formation of compound 5 was also proven by the oxidative addition of it to the N-H group of iPrNH₂ . DFT calculations also supported the mechanistic findings. A different methodology for the in situ generation of compound 5 by using TMPLi is also described.

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.

Low-valent group 14 element hydride chemistry: towards catalysis

The chemistry of group 14 element(ii) hydride complexes has rapidly expanded since the first stable example of such a compound was reported in 2000. Since that time it has become apparent that these systems display remarkable reactivity patterns, in some cases mimicking those of late transition-metal (TM) hydride compounds. This is especially so for the hydroelementation of unsaturated organic substrates. Recently, this aspect of their reactivity has been extended to the use of group 14 element(ii) hydrides as efficient, "TM-like" catalysts in organic synthesis. This review will detail how the chemistry of these hydride compounds has advanced since their early development. Throughout, there is a focus on the importance of ligand effects in these systems, and how ligand design can greatly modify a coordinated complex's electronic structure, reactivity, and catalytic efficiency.

An isolable β-diketiminato chlorosilylene

The first β-diketiminate ligated chlorosilylene has been synthesised and isolated from the corresponding β-diketiminato dichlorohydrosilane through dehydrochlorination with an N-heterocyclic carbene.

Silicon chemistry in zero to three dimensions: from dichlorosilylene to silafullerane

As one of the simplest examples of functionalized Si(ii) species, the SiCl₂/[SiCl₃]⁻ system is not only fundamentally interesting, but also an important starting point for the assembly of oligosilane chains, rings, and clusters.

Coinage metal complexes of NHC-stabilized silyliumylidene ions

The first coinage metal complexes (M = Cu, Ag, Au) of an N-heterocyclic carbene-stabilized silyliumylidene ion are reported.