A Tricyclic Aromatic Isomer of Hexasilabenzene

摘要

Benzene represents the showcase of Hueckel aromaticity. The silicon analog, hexasilabenzene, has consequently been targeted for decades. We now report an intensely green isomer of Si_6R_6 (R being 2,4,6-triisopropylphenyl) with a tricyclic structure in the solid state featuring silicon atoms with two, one, and no substituents outside the ring framework. The highly dispersed ~(29)Si nuclear magnetic resonance shifts in solution ranging from +125 to -90 parts per million indicate an inhomogeneous electron distribution due to the dismutation of formal oxidation numbers as compared with that of benzene. Theoretical analysis reveals nonetheless the cyclic delocalization of six mobile electrons of the π-, σ- and non-bonding type across the central four-membered ring. For this alternative form of aromaticity, in principle applicable to many Hueckel aromatic species, we propose the term dismutational aromaticity.