Synthesis, Structure and Electronic Properties of a Stable π‐Type 3‐Electron‐2‐Center‐Bonded Species: A Silicon Analogue of a Bicyclo[1.1.0]butane Radical Anion

Abstract

σ‐Type 3‐electron‐2‐center (3e‐2c) bonds have been extensively studied as one of the key bonding motifs in radical chemistry and some biological systems. “π‐Type 3e‐2c‐bonded species” that contain a 3e‐2c π‐bond without an underlying σ‐bond framework, however, have been unexplored so far both theoretically and experimentally. Herein, we report the synthesis of the first stable π‐type 3e‐2c‐bonded species, a silicon analogue of a bicyclo[1.1.0]butane radical anion. This compound exhibits an extremely long bridgehead Si−Si bond (3.0638(8) Å) and a strong near‐IR absorption at 922 nm in solution which arises from a HOMO→SOMO [π(Si−Si)→π*(Si−Si)] transition. DFT calculations revealed a π‐type bonding interaction between the two bridgehead silicon atoms with an unpaired electron mainly delocalized across the corresponding π*‐type orbital, which introduces a novel bonding motif for constructing π‐electron systems.