Highly Strained Arene-Fused 1,2-Diborete Biradicaloid.

The stepwise reduction of a doubly cyclic alkyl(amino)carbene (CAAC)-stabilized 2,3-bis(dibromoboryl)naphthalene enables the isolation of the corresponding mono- and bis(boryl) radicals (one- and two-electron reduction), a 2π-aromatic 1,2-diborete (four-electron reduction), which shows biradical character in the solid-state EPR spectrum, and its cyclic bis(alkylidene)diboron dianion (six-electron reduction). The X-ray crystallographic analysis of the diborete shows a highly strained and twisted four-membered ring with a formal -diborene motif featuring a very elongated B-B double bond. Calculations based on DFT and multireference approaches reveal that the diborete possesses an open-shell singlet biradicaloid ground state, which is slightly energetically preferred to its EPR-active triplet-state congener. The addition of CO to the diborete resulted in B-B bond splitting and the formation of the corresponding closed-shell singlet, doubly Lewis base-stabilized bis(borylene), whereas a twofold γ insertion of phenyl azide generates a 1,3-bis(diazenyl)-1,3,2,4-diazadiboretidine.