From a (Silatrigerma)cyclobutenylium Ion to a (Silatrigerma)cyclobutenyl Radical and Back

(Silatrigerma)­cyclobutenylium
ion salt 2+·[B­(C6F5)4]– was readily
prepared by the reaction of cyclotrigermene 1 with an
equimolar amount of [Et3Si­(benzene)]+·[B­(C6F5)4]− in
benzene. The homoaromatic nature of 2+ was firmly established by its crystallographic analysis, which
revealed a highly folded SiGe3 four-membered ring (40.4°)
and a remarkably short transannular Ge–Ge distance of 2.9346(3)
Å. The homoaromaticity of 2+ was supported by DFT calculations, which confirmed an extensive
transannular bonding orbital interaction. One-electron reduction of 2+·[B­(C6F5)4]– with potassium graphite resulted in the selective formation
of (sila­tri­germa)­cyclo­butenyl free radical 2·, which has an allylic-type structure as confirmed
by its X-ray and EPR studies. Cation 2+ and free radical 2· can be readily interconverted,
thus constituting a fully reversible redox pair.