Reversible Electron Transfer in the Formation of Radical Anionic and Dianionic 1,4-Diborabutatrienes

One- and two-electron reductions of diborylacetylene 1 with potassium graphite (KC₈) yielded radical anionic and dianionic 1,4-diborabutatrienes 1^•-K⁺(thf)₂ and 1^2-(K⁺)₂, respectively. X-ray diffraction, electron paramagnetic resonance, and computational studies indicated the π electron delocalization over the B-C-C-B cumulene skeleton. 1^2-(K⁺)₂ possesses a relatively small HOMO-LUMO gap and exhibits a significantly red-shifted UV-vis absorption at λ_max = 578 nm, compared to the all-carbon tetraarylbutatrienes. 1^2-(K⁺)₂ could undergo a formal 1,4-addition with the C_sp-H bond of phenylacetylene to produce 2, while the reaction with an organic azide led to the cleavage of B═C bonds with the formation of anionic iminoborane 3. This work showcases a potential method for the facile construction of boracumulenes.