Neutral 4π-electron tetrasilacyclobutadiene contains unusual features of a Möbius-type aromatic ring

The search for stable compounds containing an antiaromatic cyclic 4π system is a challenge for inventive chemists that can look back on a long history. Here we report the isolation and characterization of the novel 4π-electron tetrasilacyclobutadiene, an analogue of a 4π neutral cyclobutadiene that exhibits surprising features of a Möbius-type aromatic ring. Reduction of RSiCl₃ (R = (ⁱPr)₂PC₆H₄) with KC₈ in the presence of cycloalkyl amino-carbene (cAAC) led to the formation of corresponding silylene 1. Compound 1 shows further reactivity when treated with RSiCl₃ under reducing conditions resulting in the formation of unsymmetrical bis-silylene 2, which was isolated as a dark red crystalline solid. Compound 3 was obtained when chlorosilylene was reduced with potassium graphite in the presence of 2. Although 3 is, according to Hückel's rule an antiaromatic system it possesses significant aromatic character due to the unusual delocalization of the HOMO-1 and the loss of degeneracy of the higher-lying π MOs. The aromatic character of 3 is indicated by the structural stability of the compound by the very similar Si-Si bond lengths and by the NICS values. There is an unusual π conjugation between the 4 π electrons in the nearly square-planar Si₄ ring where the delocalization of the HOMO-1 occurs at two opposite sides of the ring. The reversal of the π conjugation resembles the twisting of the π conjugation in Möbius aromatic systems and it contributes to the stability of the compound.