Three-Coordinate Boron(III) and Diboron(II) Dications

The enhanced electron-donor properties of the bulky bisimino ligand 1,2-(L^MesN)₂-C₂H₄ (1; L^Mes=1,3-bis(mesityl)-imidazolin-2-ylidene), mesityl=2,4,6-trimethylphenyl) were exploited for the stabilization of elusive electron-deficient and low-coordinate boron dication species. The reaction of 1 with PhBBr₂ or (B(Cl)NMe₂)₂ afforded a dicationic mononuclear boron(III) complex or a dicationic dinuclear boron(II) complex, respectively (4²⁺, 6²⁺). The bonding situations of 4²⁺ and 6²⁺ were examined by means of single crystal X-ray diffraction analysis, as well as theoretical methods. Significant allocation of positive charge density into the ligand system was diagnosed for both dications. However, the metalloid-centered Lewis acidity of the dications was confirmed via hydride transfer reactions.