Benzodisilacyclobutadienes: 8π-Electron Systems with an Antiaromatic Silicon Ring

Benzodisilacyclobutadienes 2 a-c were isolated as blue to green crystalline solids from the reaction of stable disilyne 1 and 1,2-dibromobenzenes in the presence of potassium graphite. In the solid state, substantial bond alternation was observed within the benzene rings of 2 a-c. In hexane, 2 a-c showed remarkable bathochromic shifts of the π→π* (HOMO→LUMO) absorption bands at 625-670 nm. NMR spectra and theoretical calculations indicated that the diamagnetic ring currents of the benzene rings of 2 a-c are considerably reduced by contributions from the antiaromatic 1,2-disilacyclobutadienes. In their entirety, the obtained results indicate that 2 a-c represent 8π-electron systems that contain an antiaromatic 1,2-disilacyclobutadiene.