The Triboracyclopropenyl Dianion: The Lightest Possible Main‐Group‐Element Hückel π Aromatic

Hückel π aromaticity is typically a domain of carbon‐rich compounds. Only very few analogues with non‐carbon frameworks are currently known, all involving the heavier elements. The isolation of the triboracyclopropenyl dianion is presented, a boron‐based analogue of the cyclopropenyl cation, which belongs to the prototypical class of Hückel π aromatics. Reduction of Cl₂BNCy₂ by sodium metal produced [B₃(NCy₂)₃]^2?, which was isolated as its dimeric Na⁺ salt (Na₄[B₃(NCy₂)₃]₂?2 DME; 1 ) in 45 % yield and characterized by single‐crystal X‐ray diffraction. Cyclic voltammetry measurements established an extremely high oxidation potential for 1 (E_pc=?2.42 V), which was further confirmed by reactivity studies. The Hückel‐type π aromatic character of the [B₃(NCy₂)₃]^2? dianion was verified by various theoretical methods, which clearly indicated π aromaticity for the B₃ core of a similar magnitude to that in [C₃H₃]⁺ and benzene.