Efficient Synthesis of a NHC-Coordinated Trisilacyclopropylidene and Its Coordination Behavior

Reactions of main group compounds with azides forming organic nitrogen-containing species

Since the seminal discovery of phenyl azide by Grieß in 1864, a variety of organic azides (R-N3) have been developed and extensively studied. The amenability of azides to a number of reactions has expanded their utility as building blocks not only in organic synthesis but also in bioorthogonal chemistry and materials science. Over the decades, it has been demonstrated that the reactions of main group compounds with azides lead to diverse N-containing main group molecules. In view of the pronounced progress in this area, this review summarizes the reactions of main group compounds with azides, emphatically introducing their reaction patterns and mechanisms. The reactions of forming inorganic nitrogen species are not included in this review.

A strongly twisted SiSi bond with resemblance to a buckled dimer in an unexpected isomer of hexasilabenzene

The reductive debromination of {N(SiMe₃)Ph}SiBr₃1 with Rieke magnesium yields the six-vertex amido-substituted silicon cluster 2 with zwitterionic character that represents an unprecedented isomer of hexasilabenzene. The topology of Si1 and Si2 in 2 has bonding features of a highly twisted disilene and resembles that of a buckled dimer of Si(100)2 × 1 reconstructed surfaces. Cluster 2 forms the adducts 3 and 4 with NHC^Me4 and DMAP, respectively. The NHC adduct 4 additionally coordinates to BH₃ which affords the saturated cluster BH₃NHC^Me4Si₆{N(SiMe₃)Ph}₆ (5). Furthermore, 2 undergoes addition with MeI and iodine to form the halogenated silicon clusters 6 and 7, respectively.

An NHC-Mediated Metal-Free Approach towards an NHC-Coordinated Endocyclic Disilene

A convenient metal‐free approach towards an N‐heterocyclic carbene (NHC)‐coordinated disilene 2 is described. Compound 2, featuring the disilene incorporated in cyclopolysilane framework, was obtained in good yield and characterized using NMR spectroscopy and X‐ray crystallography. Density functional theory (DFT) calculations of the reaction mechanism provide a rationale for the observed reactivity and give detailed information on the bonding situation of the base‐stabilized disilene. Compound 2 undergoes thermal or light‐ induced (λ=456 nm) NHC loss, and a dimerization process to give a corresponding dimer with a Si₁₀ skeleton. In order to shed light on the dimerization mechanism, DFT calculations were performed. Moreover, the reactivity of 2 was examined with selected examples of transition metal carbonyl compounds.

Reactivity of the Bicyclic Amido-Substituted Silicon(I) Ring Compound Si4 {N(SiMe3 )Mes}4 with FLP-Type Character

The bicyclic amido‐substituted silicon(I) ring compound Si₄{N(SiMe₃)Mes}₄2 (Mes=Mesityl=2,4,6‐Me₃C₆H₂) features enhanced zwitterionic character and different reactivity from the analogous compound Si₄{N(SiMe₃)Dipp}₄1 (Dipp=2,6‐ⁱPr₂C₆H₃) due to the smaller mesityl substituents. In a reaction with the N‐heterocyclic carbene NHCMe4 (1,3,4,5‐tetramethyl‐imidazol‐2‐ylidene), we observe adduct formation to give Si₄{N(SiMe₃)Mes}₄ ⋅ NHCMe4 (3). This adduct reacts further with the Lewis acid BH₃ to yield the Lewis acid–base complex Si₄{N(SiMe₃)Mes}₄ ⋅ NHCMe4  ⋅ BH₃ (4). Coordination of AlBr₃ to 2 leads to the adduct 5. Calculated proton affinities and fluoride ion affinities reveal highly Lewis basic and very weak Lewis acidic character of the low‐valent silicon atoms in 1 and 2. This is confirmed by protonation of 1 and 2 with Brookharts acid yielding 6 and 7. Reaction with diphenylacetylene only occurs at 111 °C with 2 in toluene and is accompanied by fragmentation of 2 to afford the silacyclopropene 8 and the trisilanorbornadiene species 9.

Amidinatoamidosilylene-Dibromodiborene

The amidinatoamidosilylene [LSiNMe₂] [1; L = PhC(NtBu)₂] was reacted with B₂Br₄(SMe₂)₂ in toluene at room temperature to form the bis(silylene)tetrabromodiborane [L{Me₂N}Si]₂B₂Br₄ (2). It was then reacted with excess KC₈ in tetrahydrofuran at room temperature to afford the bis(silylene)dibromodiborene [L{Me₂N}Si]₂B₂Br₂ (3).

An intermolecular FLP System derived from an NHC-coordinated trisilacyclopropylidene

The NHC-coordinated trisilacyclopropylidene (A) is shown to behave as the basic component of an FLP used in combination with the Lewis acid B(C6F4H)3 (i.e. B(2,3,5,6-C6F4H)3). This FLP cleaves dihydrogen highly selectively at room temperature giving rise to the ionic compound [(NHC)SiH(Mes2SiSiMes2)][HB(C6F4H)3] 1 in 90% isolated yield. Further reaction of the FLP with Ph2NH and acetone yielded compounds [(NHC)SiH(Mes2SiSiMes2)][Ph2NB(C6F4H)3] 2 and [(NHC)SiH(Mes2SiSiMes2)][MeC(CH2)OB(C6F4H)3] 3 in 75% and 80% yield, respectively. Reaction of the FLP with N2O results in the oxidation of the silylene center affording [((NHC)SiOB(C6F4H)3)(Mes2SiSiMes2)] 4 in 53% yield. These products are spectroscopically characterized and an X-ray structure of 4 is reported.

An Air-Stable Heterobimetallic Si2 M2 Tetrahedral Cluster

Air- and moisture-stable heterobimetallic tetrahedral clusters [Cp(CO)2 MSiR]2 (M=Mo or W; R=SitBu3 ) were isolated from the reaction of N-heterocyclic carbene (NHC) stabilized silyl(silylidene) metal complexes Cp(CO)2 M=Si(SitBu3 )NHC with a mild Lewis acid (BPh3 ). Alternatively, treatment of the NHC-stabilized silylidene complex Cp(CO)2 W=Si(SitBu3 )NHC with stronger Lewis acids such as AlCl3 or B(C6 F5 )3 resulted in the reversible coordination of the Lewis acid to one of the carbonyl ligands. Computational investigations revealed that the dimerization of the intermediate metal silylidyne (M≡Si) complex to a tetrahedral cluster instead of a planar four-membered ring is due to steric bulk.

DMAP-stabilized bis(silyl)silylenes as versatile synthons for organosilicon compounds

DMAP-stabilized silylenes 1a–c are obtained from the reductive debromination of the corresponding dibromosilanes in the presence of DMAP. Their distinctly different thermal isomerization reactions via C–H bond activation, dearomative ring expansion and silyl migration are discussed. Furthermore, complexes 1 dissociate at elevated temperatures, providing the corresponding free silylenes in situ, which are even capable of single-site activation of H₂. Additionally, a potassium-substituted silicon-centered radical 2 is isolated from overreduction of (^tBu₃Si)₂SiBr₂.

DMAP-stabilized silylenes 1a–c, which are convenient, room temperature stable synthetic equivalents for the corresponding highly reactive free bis(silyl)silylenes are reported.

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).

Facile Access to an NHC-Coordinated Silicon Ring Compound with a Si=N Group and a Two-Coordinate Silicon Atom

Reaction of the bicyclo[1.1.0]tetrasilatetraamide Si₄ {N(SiMe₃ )Dipp}₄ 1 (Dipp=2,6-diisopropylphenyl) with 5 equiv of the N-heterocyclic carbene NHC^Me4 (1,3,4,5-tetramethylimidazol-2-ylidene) affords a bifunctional carbene-coordinated four-membered-ring compound with a Si=N group and a two-coordinate silicon atom Si₄ {N(SiMe₃ )Dipp}₂ (NHC^Me4 )₂ (NDipp) 2. When 2 reacts with 0.25 equiv sulfur (S₈ ), two sulfur atoms add to the divalent silicon atom in plane and perpendicular to the plane of the Si₄ ring, which confirms the silylone character of the two-coordinate silicon atom in 2.

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.

Formation of an NHC-stabilized heterocyclic housane and its isomerization into a cyclopentenyl anion analogue

:PC–^tBu reacts with Si₃Mes₄NHC^iPr2Me2E to afford a so-called “housane” 1, that rearranges upon irradiation with a high-pressure mercury lamp or at 60 °C to an NHC-stabilized cyclopentenyl anion analogue 2 with a three-center four-electron π-bond.