Stable? You can bottle it! The base‐stabilized dichlorosilylene L1SiCl2 (see picture; L1=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) is stable at room temperature. L1SiCl2 can undergo a reaction with diphenylacetylene to form a trisilacyclopentene derivative. These compounds have been characterized by X‐ray crystallography and computational studies.

Stabil? Man kann es abfüllen! Das basenstabilisierte Dichlorsilylen L1SiCl2 (siehe Bild; L1=1,3‐Bis(2,6‐diisopropylphenyl)imidazol‐2‐yliden) ist bei Raumtemperatur stabil und reagiert mit Diphenylacetylen zu einem Trisilacyclopentenderivat. Beide Verbindungen wurden röntgenographisch und mithilfe von Computerstudien charakterisiert.

Isolating stable compounds with low-valent main group elements have long been an attractive research topic, because several of these compounds can mimic transition metals in activating small molecules. In addition, compounds with heavier low-valent main group elements have fundamentally different electronic properties when compared with their lighter congeners. Among group 14 elements, the heavier analogues of carbenes (R(2)C:) such as silylenes (R(2)Si:), germylenes (R(2)Ge:), stannylenes (R(2)Sn:), and plumbylenes (R(2)Pb:) are the most studied species with low-valent elements. The first stable carbene and silylene species were isolated as N-heterocycles. Among the dichlorides of group 14 elements, CCl(2) and SiCl(2) are highly reactive intermediates and play an important role in many chemical transformations. GeCl(2) can be stabilized as a dioxane adduct, whereas SnCl(2) and PbCl(2) are available as stable compounds. In the Siemens process, which produces electronic grade silicon by thermal decomposition of HSiCl(3) at 1150 °C, chemists proposed dichlorosilylene (SiCl(2)) as an intermediate, which further dissociates to Si and SiCl(4). Similarly, base induced disproportionation of HSiCl(3) or Si(2)Cl(6) to SiCl(2) is a known reaction. Trapping these products in situ with organic substrates suggested the mechanism for this reaction. In addition, West and co-workers reported a polymeric trans-chain like perchloropolysilane (SiCl(2))(n). However, the isolation of a stable free monomeric dichlorosilylene remained a challenge. The first successful attempt of taming SiCl(2) was the isolation of monochlorosilylene PhC(NtBu)(2)SiCl supported by an amidinate ligand in 2006. In 2009, we succeeded in isolating N-heterocyclic carbene (NHC) stabilized dichlorosilylene (NHC)SiCl(2) with a three coordinate silicon atom. (The NHC is 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes).) Notably, this method allows for the almost quantitative synthesis of (NHC)SiCl(2) without using any hazardous reducing agents. Dehydrochlorination of HSiCl(3) with NHC under mild reaction conditions produces (NHC)SiCl(2). We can separate the insoluble side product (NHC)HCl readily and recycle it to form NHC. The high yield and facile access to dichlorosilylene allow us to explore its chemistry to a greater extent. In this Account, we describe the results using (NHC)SiCl(2) primarily from our laboratory, including findings by other researchers. We emphasize the novel silicon compounds, which supposedly existed only as short-lived species. We also discuss silaoxirane, silaimine with tricoordinate silicon atom, silaisonitrile, and silaformyl chloride. In analogy with N-heterocyclic silylenes (NHSis), oxidative addition reactions of organic substrates with (NHC)SiCl(2) produce Si(IV) compounds. The presence of the chloro-substituents both on (NHC)SiCl(2) and its products allows metathesis reactions to produce novel silicon compounds with new functionality. These substituents also offe...

Formyl chloride (H(Cl)C=O) is unstable at room temperature and decomposes to HCl and CO. Silicon analogue of formyl chloride, silaformyl chloride IPr·SiH(Cl)=O·B(C(6)F(5))(3) (3) (IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazol-2-ylidene), was stabilized by Lewis donor-acceptor ligands. Compound 3 is not only the first stable acyclic silacarbonyl compound but also the first silacarbonyl halide reported so far.

One-electron reduction of C2-arylated 1,3-imidazoli(ni)um salts (IPr )Br (Ar=Ph, 3 a; 4-DMP, 3 b; 4-DMP=4-Me NC H ) and (SIPr )I (Ar=Ph, 4 a; 4-Tol, 4 b) derived from classical NHCs (IPr=:C{N(2,6-iPr C H )} CHCH, 1; SIPr=:C{N(2,6-iPr C H )} CH CH , 2) gave radicals [(IPr )] (Ar=Ph, 5 a; 4-DMP, 5 b) and [(SIPr )] (Ar=Ph, 6 a; 4-Tol, 6 b). Each of 5 a,b and 6 a,b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid-state characterization of NHC-derived carbon-centered radicals 6 a,b by single-crystal X-ray diffraction is reported. DFT calculations indicate that the unpaired electron is mainly located at the original carbene carbon atom and stabilized by partial delocalization over the adjacent aryl group.