A Dimer of Silaisonitrile with Two‐Coordinate Silicon Atoms

Abstract

Dedicated to Professor Raymundo Cea-Olivares on the occasion of his 60th birthday Carbenes and silylenes are among the most important reactive intermediates of carbon [1] and silicon. [2,3] These species play key roles in numerous thermal and photochemical reactions and are extremely important in synthetic chemistry. [1][2][3] The first N-heterocyclic carbene (NHC) that is stable at room temperature was isolated by Arduengo et al. in 1991. [4] The silicon analogue of NHC was reported three years later by West and co-workers [5] as an N-heterocyclic silylene (NHSi) and using the same stabilization concept. Since then, a number of stable carbene [6, 7] and silylene [8] compounds have been isolated with various electronic structures and properties. Synthesis and stabilization of the reactive species are not just a simple desire of academic interest to realize highly reactive molecules at normal laboratory conditions; they have also found many useful applications. [7][8][9][10] The extraordinary electronic properties of NHCs as strong s-donor and weak p-acceptor ligands have been shown to bind more strongly with transition metals than classical ligands. [7, 9] The unusual stability of transition-metal complexes supported by NHC as ligands has led to a major breakthrough in the second generation of Grubbs catalysts. [10] The properties of silicon and carbon are rather different from each other although both have the same number of valence electrons. These differences are most pronounced for compounds with low-coordinate silicon and carbon atoms. Among unsaturated compounds, a wide range of carbon compounds are known that are stable, whereas analogous silicon compounds are very limited and quite reactive. The syntheses of stable silicon compounds have been inspired almost exclusively by their relationship with organic congeners. Silicon analogues of alkenes, [11] alkynes, [12] allenes, [13] and other unsaturated organic compounds with a C=X (X = O or S) functional group [14,15] have been prepared. The molec-ular and electronic structures of unsaturated compounds of silicon are very different from those of the corresponding carbon compounds in which s/p hybridization models are valid. [16,17] To understand the physical and chemical properties of unsaturated silicon compounds, additional experimental investigations are required. Further progress and understanding of the field depend on the availability of appropriate stable silicon compounds.Organic nitriles and isonitriles are very stable, but their silicon analogues are only detected as transient species in a low-temperature argon matrix. [18][19][20] The first transient silaisonitrile, HN = Si, was described in 1966 by Ogilvie and Cradock; [19] this species was generated by photolysis of H 3 SiN 3 in an argon matrix at 4 K. The formation of silaisonitrile [20] was shown by PE and IR spectroscopic studies in an argon matrix. Quantum chemical calculations have shown that silaisonitriles are more stable than silanitriles. [21] This result is in contrast to the ca...

Mit N‐heterocyclischem Gallylen stabilisierte niedervalente Ge2‐ und Ge4‐Spezies

“…[19] Wir weisen darauf hin, dass die Si-Atome im viergliedrigen Ring Si 2 (NAr) 2 an N-Atomen gebunden sind, die über freie p-Elektronenpaare verfügen und daher elektronische Ladung in die formal leeren p(p)-AOs des Si donieren kçnnen. 2 1(r) der Elektronendichte des Ge 2 Ga 2 -Rings von 1M.…”

“…[19] Der Ge 2 Ga 2 -Ring in 1M hat nur ein besetztes s-"lone-pair"-Orbital an den Germaniumatomen, nämlich das HOMOÀ2, während im Si 2 (NAr) 2 zwei besetzte s-"lone-pair"-Orbitale am Si vorliegen. [19] Der Ge 2 Ga 2 -Ring in 1M hat nur ein besetztes s-"lone-pair"-Orbital an den Germaniumatomen, nämlich das HOMOÀ2, während im Si 2 (NAr) 2 zwei besetzte s-"lone-pair"-Orbitale am Si vorliegen.…”

Recent Advances in the N-Heterocyclic Carbene-Supported Chemistry of Silicon

“…were determined by X-ray crystallography. Calculated Laplacian distributions support the presence of a lone pair of electrons at each Si center, and accordingly, Staudinger type reactivity between 69 and Me 3 SiN 3 was noted to form the oxidized silaimine Me 3 SiN¼Si(μ-NAr 0 ) 2 Si¼NSiMe 3 (70) [103]. Compound 74 was structurally authenticated by X-ray crystallography, and this study revealed the presence of a short P¼Si multiple bond [2.0718(6) Å ] and second-order perturbation theory identified a significant N-Si negative hyperconjugative interaction between the lone pair on N and the low-lying Si-P π* orbital in 74 [105].…”

“…Moreover, substantial pyramidalization of the Si center adjacent to the carbene-bound Si was noted due to the acquisition of non-bonding (lone pair) character upon coordination of the NHC [95]. In a conceptually related fashion, the Si(IV)imido adduct IPr•SiCl 2 ¼NAr 0 (35) [102] was reduced with KC 8 to give the silaisonitrile dimer (:SiNAr 0 ) 2 (69) as a yellow crystalline solid in a 21% yield (13) [103]. The two terphenyl ligands in 69 form a steric pocket which surrounds the Si 2 N 2 core and intraring Si-N single bonds of 1.755(1) Å avg.…”

Niedervalentes Silicium in formal antiaromatischen Vierring‐Systemen

“…[6] Die Reaktion des Aryl(tribrom)silans (EMind)SiBr 3 mit drei ¾quivalenten Lithiumnaphthalenid führte zum luft-und feuchtigkeitsempfindlichen, orangefarbenen Tetrasilacyclobutadien Si 4 Ar 4 (1; Schema 1). Dieses zersetzt sich bei Raumtemperatur langsam, in Lçsung und auch im festen Zustand, konnte aber trotzdem durch Rçntgenstrukturana-lyse sowie durch NMR-und UV/Vis-Spektroskopie charakterisiert werden.…”