The bicyclic amido-substituted silicon(I) ring compound Si₄ {N(SiMe₃ )Mes}₄ 2 (Mes=Mesityl=2,4,6-Me₃ C₆ H₂ ) features enhanced zwitterionic character and different reactivity from the analogous compound Si₄ {N(SiMe₃ )Dipp}₄ 1 (Dipp=2,6-ⁱ Pr₂ C₆ H₃ ) due to the smaller mesityl substituents. In a reaction with the N-heterocyclic carbene NHC Me 4 (1,3,4,5-tetramethyl-imidazol-2-ylidene), we observe adduct formation to give Si₄ {N(SiMe₃ )Mes}₄ ⋅ NHC Me 4 (3). This adduct reacts further with the Lewis acid BH₃ to yield the Lewis acid-base complex Si₄ {N(SiMe₃ )Mes}₄ ⋅ NHC Me 4 ⋅ 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.

Michael Quest

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