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 Cl2 BNCy2 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 (Epc =-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 [C3 H3 ](+) and benzene.