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.