Crystalline neutral and cationic oxoboranes

The isolation of oxoboranes (RB≡O) featuring a genuine B-O triple bond has remained an outstanding challenge owing to the electron deficiency of boron and the intrinsic instability of the B≡O linkage. So far, direct evidence for such species has been largely limited to gas-phase and matrix-isolation studies and only recently inferred in solution. Here we report the isolation of a stable, free oxoborane generated via a carbon-monoxide-triggered transformation of a heterocumulene. Single-crystal X-ray diffraction establishes a terminal B≡O unit, while electronic structure analyses reveal two orthogonal π bonds consistent with a highly polarized triple bond. Mechanistic studies support a carbon monoxide coordination-insertion-isomerization pathway. The resulting oxoborane exhibits ambiphilic reactivity, enabling access to both neutral and cationic derivatives. These findings establish free oxoboranes as isolable species under condensed-phase conditions and provide a platform for exploring low-coordinate boron-oxygen multiple bonds.