Cp*-Substituted Boron Cations: The Effect of NHC, NHO, and CAAC Ligands

The effect of a ligand on the electron deficiency and Lewis acidity of the Cp*-substituted boron dication has been investigated experimentally and theoretically. In addition to the reported IMes- and N-heterocyclic olefin (NHO)-stabilized boron dications, the related cyclic alkylamino carbene (CAAC)-coordinated boron mono- and dications have also been synthesized and structurally characterized. An electrochemical study of dications [3a-3c]²⁺ confirms the higher electron deficiency of the dicationic system than the related boron monocations. Moreover, the presence of a π-acidic CAAC ligand is critical for realizing stable radical species generated from the chemical reduction of boron cations. The nature of the axial ligand also significantly affects the selectivity of the hydride addition reaction of boron dications. While bulky superhydride reacts with [3a-3c]²⁺ in the same manner to give the cyclic boreniums, [BH₄]^- attacks three different electrophilic sites of boron dications: the sp² carbon of Cp* of the IMes-coordinated system ([3a]²⁺), the central boron atom of the NHO-stabilized analogue ([3b]²⁺), and the ylidene carbon of the CAAC-containing boron dication ([3c]²⁺).