Abstract
According to a first view on the geometrical and electronic structure of the tris(pentafluoroethyl)silanide, this anion appears as a Lewis base. Quantum chemical calculations on perfluoroalkylated silanides show significantly lower HOMO and LUMO energy levels in comparison to their non‐fluorinated counterparts, which implies reduced Lewis basicity and increased Lewis acidity of the [Si(C2F5)3]− ion. With these findings and a HOMO–LUMO gap of 4.80 eV similar to N‐heterocyclic silylenes (NHSis), perfluoroalkyl silanides are predestined to exhibit Lewis‐amphoteric character similar to silylenes. Deprotonation of Si(C2F5)3H with sterically demanding phosphazene bases afforded thermally stable phosphazenium salts of the [Si(C2F5)3]− anion, which add to benzaldehyde, benzophenone, CS2, and CO2 in various manners. This behavior also mirrors the reactivity of silylenes towards ketones as well as heterocumulenes and is rationalized by Lewis amphotericity being inherent in these silanides.
Highlights
In keeping with the Lewis definition, amphoteric species possess electron-deficient (Lewis acidic) and electron-rich (Lewis basic) centers
The Lewis acidic as well as the Lewis basic position are located at the same center, namely the divalent tetrel atom
The latter species are the higher homologues of N-heterocyclic carbenes (NHCs), the first of which has been presented by Arduengo III in 1991.[6]. Denks seminal discovery stimulated further intense research on silylene chemistry.[7,8,9,10,11]
Summary
In keeping with the Lewis definition, amphoteric species possess electron-deficient (Lewis acidic) and electron-rich (Lewis basic) centers. Quantum chemical calculations on the tris(trifluoromethyl)silanide anion, [Si(CF3)3]À, disclose a HOMO–LUMO gap of 4.80 eV as well as lower HOMO and LUMO energy levels (À7.06 eV; À2.26 eV) in comparison to the nonfluorinated analogue, [Si(CH3)3]À (À6.88 eV; À2.10 eV) (Figure 1).[16] This is clearly due to the strong electron withdrawing effect of the perfluoroalkyl groups.[17] These computations point to a reduced Lewis basicity and in turn an increased Lewis acidity of the perfluorinated species. To Lewis amphoteric silylenes,[26,27,28] the [Siabove À80 8C.[18,21] As demonstrated recently, phosphazenium cations, [R3P=N(H)tBu]+ with R being Me2N- ([MeP1H]+), (Me2N)2C=N- ([tmgP1H]+) and (Et2N)3P=N- ([EtP4H]+), are weakly coordinating and predestined to stabilize reactive (C2F5)3]À ion undergoes a side-on addition with carbonyl compounds like benzaldehyde and benzophenone to afford a phosphazenium salt of the corresponding oxasiliranide anion 1 a and 1 b (Scheme 3).
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