A Crystalline Diazadiborinine Radical Cation and Its Boron-Centered Radical Reactivity.

One-electron oxidation of 1,4,2,5-diazadiborinine 1 has been studied. While the reaction of 1 a bearing phenyl groups on the B atoms with AgAl{OC(CF3 )3 }4 afforded a complex mixture, the same oxidation reaction with 1 b featuring bulky mesityl substituents on the B atoms rendered the corresponding cation radical 2 b as an isolable species. X-ray diffraction analysis, EPR spectroscopy, and DFT calculations of 2 b revealed the delocalization of the unpaired electron over the entire π-system of 2 b, as well as a large spin density (0.76 in total) on the two equivalent boron atoms. The chemical trapping reaction of 2 b with p-benzoquinone and triphenyltin hydride afforded the dicationic species 3 containing two newly formed B-O bonds and the monocationic product 2b-H containing a B-H bond, respectively, thus confirming the boron-centered radical reactivity of 2 b.