Isolation of a Diborane(6) Dication: Formation and Cleavage of an Electron-Precise B(sp³)-B(sp³) Bond

One-electron oxidation of organoboron L₂PhB: 1 (L = oxazol-2-ylidene) afforded a dicationic diborane(6) species [L₂PhB-BPhL₂]·2X (X = OTf, BF₄, AlCl₄) 3, representing a new strategy to construct a B(sp³)-B(sp³) covalent bond. Each boron atom in 3 is in the formal oxidation state +II, and tetracoordinate with a Ph group and two oxazol-2-ylidenes. The cyclic voltammetry of 3 shows irreversible reduction and oxidation. Indeed, two-electron reduction of 3 with potassium graphite (KC₈) afforded 1, making a fully reversible 1 ↔ 3 redox system, whereas two-electron oxidation with AuCl produced a boronium [L₂PhBCl]OTf 4. Moreover, the reactions of 3 with isonitrile derivatives RNC: under heating conditions gave a cyano-substituted boronium [L₂PhBCN]BF₄ 5 and a 2-boranyl-indole derivative 6, depending on the substituent R. The proposed reaction mechanism involves a borinylium radical 1^•+ which is generated via a homolytic cleavage of the B-B bond of 3.