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

Abstract

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 GaCl to form Lewis adducts (bis-NHC)E(GaCl) (E = Si, Ge) and with 2 molar equiv of ZnCl to furnish (bis-NHC)Si(ZnCl). Conversion of the metallylones with elemental chalcogens affords isolable monomeric silicon(II) and germanium(II) monochalcogenides (bis-NHC)EX(GaCl) (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)EX (X = S, Se, Te) as the first isolable complexes of the molecular congeners of CO. Moreover, (bis-NHC)Si could even activate CO to afford the monomolecular silicon dicarbonate complex (bis-NHC)Si(CO) via the formation of SiO and SiO 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) 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.

NHC‐Stabilised Silyliumylidene Ions

“…Silyliumylidene ions are silicon(II) cations of the type [RÀSi:] + , and can be considered to be related to both silylenes, R 2 Si:, [5,6] and silylium ions, [R 3 Si] + . [7,8] They possess only four valence electrons, two as a localised lone pair, and two degenerate vacant p-orbitals on the silicon centre ( Figure 1, left).…”

Tetrylones: An Intriguing Class of Monoatomic Zero‐valent Group 14 Compounds

“…Moreover, 15 can serve as two‐fold donor towards Lewis acids. For example, treatment of 15 with two molar equivalents of ZnCl 2 in THF furnished germinal dimetallated complex 32 (Scheme ) . In the molecular structure of 32 , the coordination environment of both Zn atoms is different (Figure ) .…”

“…For example, treatment of 15 with two molar equivalents of ZnCl 2 in THF furnished germinal dimetallated complex 32 (Scheme ) . In the molecular structure of 32 , the coordination environment of both Zn atoms is different (Figure ) . One Zn adopts a trigonal planar geometry with a Si−Zn bond length of 2.3735(8) Å, whereas other Zn atom adopts a tetrahedral coordination geometry with a Si‐Zn bond length of 2.4396(9) Å, whereby one THF molecule coordinates to the Zn atom.…”

“…The intensive red solution of 15 underwent a color change and bis(NHC)Si(CO 3 ) 2 37 was obtained as white solid in 75 % yield (Scheme ) . It is important to note that the analogous reaction of germylone 13 with CO 2 led to the formation of the Lewis adduct bis(NHC)(CO 2 ) and the precipitation of GeO 2 . The 29 Si{ 1 H} NMR spectrum in [D 8 ]THF displayed a singlet at δ =−195.0 ppm, while the solid‐state 29 Si NMR spectrum showed a singlet at δ =−196.5 ppm.…”

N‐Phosphine Oxide‐Substituted Imidazolylidenes (PoxIms): Multifunctional Multipurpose Carbenes

“…Much effort was devoted to understand their electronic and steric natures, which contributed to the design of other types of stable carbenes, as well as to the functionalization of NHCs . Today, NHCs are widely employed as for example ligands for transition metals, organocatalysts, and Lewis basic partners for the stabilization of reactive main‐group species ,…”