A Base‐Free Two‐Coordinate Oxoborane

Oxoboranes (R-BO) are transient species that rapidly trimerise to form boroxines. To date, the only method used to stabilise oxoboranes is to add a Lewis base, but this forms a three-coordinate at boron oxoborane that has a different bonding/reactivity profile. Herein we report a base-free, two-coordinate oxoborane that is isolated as a Lewis adduct with AlCl₃. This species, Mes*BO-AlCl₃ (Mes*=2,4,6-^tBu-C₆H₂), has a ν¹¹ΒΟ stretching frequency of 1843 cm^-1, indicating a strong BO bond. Computational analysis indicates this is due to a highly polarised BO bonding interaction combined with modest BO multiple bond character. While the polarisation of the BO bond on AlCl₃ coordination enhances the Lewis acidity at boron it also reduces the basicity at oxygen and the latter is key to accessing a base-free oxoborane. Finally, this oxoborane reacts with PhN₃ in a unique way to form an unprecedented boron heterocycle.