Reactivity of the Bicyclic Amido‐Substituted Silicon(I) Ring Compound Si(4){N(SiMe(3))Mes}(4) with FLP‐Type Character

The bicyclic amido‐substituted silicon(I) ring compound Si(4){N(SiMe(3))Mes}(4) 2 (Mes=Mesityl=2,4,6‐Me(3)C(6)H(2)) features enhanced zwitterionic character and different reactivity from the analogous compound Si(4){N(SiMe(3))Dipp}(4) 1 (Dipp=2,6‐( i )Pr(2)C(6)H(3)) due to the smaller mesityl substituents. In a reaction with the N‐heterocyclic carbene NHC [Formula: see text] (1,3,4,5‐tetramethyl‐imidazol‐2‐ylidene), we observe adduct formation to give Si(4){N(SiMe(3))Mes}(4) ⋅ NHC [Formula: see text] (3). This adduct reacts further with the Lewis acid BH(3) to yield the Lewis acid–base complex Si(4){N(SiMe(3))Mes}(4) ⋅ NHC [Formula: see text] ⋅ BH(3) (4). Coordination of AlBr(3) 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.