B-B vs. B-H Bond Activation in a (μ-Hydrido)diborane(4) Anion upon Cycloaddition with CO2 , Isocyanates, or Carbodiimides.

The intriguing (μ-hydrido)diboranes(4) with their prominent pristine representative [B2 H5 ]- 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 CO2 , iPrNCO, and iPrNCNiPr. We show that a C=O bond of CO2 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 B2 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.