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.