Reversible Assembly and Disassembly of σ-Aromatic B₃ Rings

Electron-deficient, planar oligoboron ring compounds fascinate due to their peculiar bonding properties (σ-aromaticity) and reactivity; however, the directed synthesis of such compounds is challenging. In this work we report on the reaction of cationic diboranes with boranes to give cationic, σ-aromatic B₃ ring compounds, with three-center-two-electron bond between the three boron atoms, being analogues of B₃H₈ ^-, but with opposite charge. Base-induced, reversible disassembly of the cationic B₃ ring leads back to cationic diboranes and borane adducts. Quantum-chemical calculations show that the thermodynamics of these reactions can be tuned by choice of the base. The reactions interconvert electron-deficient boron three-center bonds and electron-precise two-center bonds and disclose new synthetic access routes to cationic diboranes and σ-aromatic B₃ rings.