Nachweis des Lewis‐amphoteren Charakters von Tris(pentafluorethyl)silanid, [Si(C₂F₅)₃]⁻

Abstract

Bei Betrachtung der geometrischen und elektronischen Struktur des Tris(pentafluorethyl)silanids erscheint das Anion auf den ersten Blick als Lewis‐Base. Quantenmechanischen Rechnungen zufolge liegen die HOMO‐ und LUMO‐Energien von Perfluoralkylsilaniden deutlich niedriger als die nicht fluorierter Derivate. Dies hat eine geringere Lewis‐Basizität und zugleich eine erhöhte Lewis‐Acidität des [Si(C2F5)3]−‐Ions zur Folge. Mit diesen Erkenntnissen und einer der N‐heterocyclischen Silylenen (NHSis) ähnlichen HOMO‐LUMO‐Lücke von 4.80 eV sind Perfluoralkylsilanide dazu prädestiniert, Lewis‐amphoteres Verhalten ähnlich dem der Silylene aufzuweisen. Die Deprotonierung von Si(C2F5)3H mit sterisch anspruchsvollen Phosphazenbasen liefert thermisch stabile Phosphazenium‐Salze des [Si(C2F5)3]−‐Anions, welche in unterschiedlicher Weise an Benzaldehyd, Benzophenon, CS2 und CO2 addieren. Dieses Verhalten spiegelt die Reaktivität von Silylenen gegenüber Ketonen sowie Heterocumulenen wider und wird durch die Lewis‐Amphoterie des Silanids begründet.

A Striking Mode of Activation of Carbon Disulfide with a Cooperative Bis(silylene)

“…Compound 4 crystallizes in the monoclinic space group Pc with two independent molecules in the unit cell. Its molecular structure (Figure 5) contains at hree-membered SiCS-ring as central structural motif with aS i À Cb ond distance of 1.855(6) ,w hich is in accordance with the reported SiÀC bond distance of 1.843 in the thiasilirane N reported by Hoge et al [27] and the typical Si-C distance of about 1.86 . [28] However,the Si À Cbond distance in 4 is shorter compared to that in the NHSi!CS 2 adduct F (see Figure 1) reported by Tacke et al (d(Si-C) = 1.865 ).…”

“…[28] However,the Si À Cbond distance in 4 is shorter compared to that in the NHSi!CS 2 adduct F (see Figure 1) reported by Tacke et al (d(Si-C) = 1.865 ). [15] Notably,t he Si1•••S2 distance of 2.707 is significantly longer than those in the related thiasiliranes K-N (see Figure 6) (2.09-2.21 ) [27,[29][30][31] and typical Si-S bond distances (2.13-2.15 ). [32] However,the Si1-S2 distance of 4 is similar to that in the anionic, pentacoordinated thiasiliranide N (d(Si-S) = 2.660 ), [27] suggesting aw eak S2!Si1 bonding interaction.…”

“…[15] Notably,t he Si1•••S2 distance of 2.707 is significantly longer than those in the related thiasiliranes K-N (see Figure 6) (2.09-2.21 ) [27,[29][30][31] and typical Si-S bond distances (2.13-2.15 ). [32] However,the Si1-S2 distance of 4 is similar to that in the anionic, pentacoordinated thiasiliranide N (d(Si-S) = 2.660 ), [27] suggesting aw eak S2!Si1 bonding interaction. TheC-S bond distances in 4 (1.657(7) and 1.667( 7) )a re in close agreement with those in N with d(C-S) = 1.658(7) and 1.684(6) ,r espectively.…”

“…Chemie by the p*(C-N) orbital. [27,36] Additional theoretical calculations revealed that the formation of the proposed sileneintermediate found at DG = 30.1 kcal mol À1 is most unlikely to occur.…”

“…[15] With compound 4 in hand, we wondered whether the postulated silene can be trapped upon reaction with NHSi 5 containing a(methylamino)pyridine group with an additional intramolecular N-donor site. [34] However, we learned that 4 acts as aC ,S-transfer reagent, [15,27,[29][30][31] affording the first mesoionic heterobicyclic species of this type,c ompound 6, which could be isolated in 41 %yields as orange crystals from concentrated Et 2 Os olutions at À30 8 8C( Scheme 3). [35] In-situ NMR analysis of the reaction mixture revealed the selective formation of 6 within minutes as indicated by acolor change from dark red to yellow.…”