A Base‐Free Two‐Coordinate Oxoborane

Abstract 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 AlCl 3 . This species, Mes*BO‐AlCl 3 (Mes*=2,4,6‐ t Bu‐C 6 H 2 ), has a ν 11 ΒΟ 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 AlCl 3 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 PhN 3 in a unique way to form an unprecedented boron heterocycle.