CAAC-Coordinated Boron Dication: The Effect of π-Acidic Ligand on the Reactivity of Boron Electrophile

CAAC-coordinated dichloroborane adduct 1a, chloroborenium [2a]+ and boron dication [3a]2+ have been synthesized and characterized. The structure of [3a]2+ is confirmed by X-ray diffraction analysis and is similar to the previously reported NHC-coordinated boron dication. The enhanced Lewis acidity of [3a]2+ compared to its NHC-congener was demonstrated by experimental observation and theoretical calculation. Reactivity study revealed that [3a]2+ possesses three electrophilic sites (the boron center, the Cp* ring and the CAAC carbene center) according to the nature of the nucleophile. This is a rare case that three electrophilic sites reside on a single small molecule. Chemical reduction of [3a]2+ and [2a]+ yielded the corresponding radical cation [6]+• and neutral radical [7]•, respectively. The exceptionally large boron hyperfine coupling constant of [6]+• is attributed to the enhanced electron-deficiency of the boron center.