B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO(2), Isocyanates, or Carbodiimides

The intriguing (μ‐hydrido)diboranes(4) with their prominent pristine representative [B(2)H(5)](−) have mainly been studied theoretically. We now describe the behavior of the planarized tetraaryl (μ‐hydrido)diborane(4) anion [1H](−) in cycloaddition reactions with the homologous series of heterocumulenes CO(2), iPrNCO, and iPrNCNiPr. We show that a C=O bond of CO(2) selectively activates the B−B bond of [1H](−), while the μ‐H ligand is left untouched ([2H](−)). The carbodiimide iPrNCNiPr, in contrast, neglects the B−B bond and rather adds the B‐bonded H(−) ion to its central C atom to generate a formamidinate bridge across the B(2) pair ([3](−)). As a hybrid, the isocyanate iPrNCO combines the reactivity patterns of both its congeners and gives two products: one of them ([4H](−)) is related to [2H](−), the other ([5](−)) is an analog of [3](−). We finally propose a mechanistic scenario that rationalizes the individual reaction outcomes and combines them to a coherent picture of B–B vs. B–H bond activation.