Reactivity of the Bicyclic Amido-Substituted Silicon(I) Ring Compound Si-4{N(SiMe3)Mes}(4) with FLP-Type Character

Hepp, A; Lips, F; Hansen, M.R.

Abstract

The bicyclic amido-substituted silicon(I) ring compound Si4{N(SiMe3)Mes}4 2 (Mes=Mesityl=2,4,6-Me3C6H2) features enhanced zwitterionic character and different reactivity from the analogous compound Si4{N(SiMe3)Dipp}4 1 (Dipp=2,6-iPr2C6H3) due to the smaller mesityl substituents. In a reaction with the N-heterocyclic carbene NHC (1,3,4,5-tetramethyl-imidazol-2-ylidene), we observe adduct formation to give Si4{N(SiMe3)Mes}4 ⋅ NHC (3). This adduct reacts further with the Lewis acid BH3 to yield the Lewis acid–base complex Si4{N(SiMe3)Mes}4 ⋅ NHC ⋅ BH3 (4). Coordination of AlBr3 to 2 leads to the adduct 5. Calculated proton affinities and fluoride ion affinities reveal highly Lewis basic and very weak Lewis acidic character of the low-valent silicon atoms in 1 and 2. This is confirmed by protonation of 1 and 2 with Brookharts acid yielding 6 and 7. Reaction with diphenylacetylene only occurs at 111 °C with 2 in toluene and is accompanied by fragmentation of 2 to afford the silacyclopropene 8 and the trisilanorbornadiene species 9.