A two-coordinate boron cation featuring C–B+–C bonding

Two-coordinate boron cations (R 2 B + ), referred to as borinium ions, are chemical species in which the boron bears only four valence electrons, and that are isoelectronic with hypothetical carbon dications (R 2 C 2+ ). Although lone-pair-donating substituents such as amino groups have enabled the isolation of several borinium ions, diarylated and dialkylated borinium derivatives remain entirely unexplored. Here, we present the synthesis, structure and reactivity of the dimesitylborinium ion, which displays unexpectedly high thermal stability. X-ray crystallography and 11 B NMR spectroscopy, supported by density functional theory calculations, reveal that the borinium ion adopts a linear two-coordinate structure in both the solid state and in solution. The boron centre is stabilized by p π bonding from the mesityl groups and is free from coordination by the counterion or solvent molecules. This diarylborinium ion possesses exceptional Lewis acidity, accepting a pair of electrons from CO 2 to cause an unusual deoxygenation reaction. Borinium ions are two-coordinate boron cations that contain only four valence electrons on boron, and are difficult to isolate without electron donation from adjacent heteroatoms. Now, diarylborinium salts with no lone-pair donation from heteroatoms have been isolated, characterized and found to participate in an unusual reaction with CO 2 .