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 [B 2 H 5 ] - have mainly been studied theoretically. We now describe the behavior of the planarized tetraaryl (μ-hydrido)diborane(4) anion [ 1 H] - in cycloaddition reactions with the homologous series of heterocumulenes CO 2 , i PrNCO, and i PrNCN i Pr. We show that a C=O bond of CO 2 selectively activates the B-B bond of [ 1 H] - , while the μ-H ligand is left untouched ([ 2 H] - ). The carbodiimide i PrNCN i Pr, 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 i PrNCO combines the reactivity patterns of both its congeners and gives two products: one of them ([ 4 H] - ) is related to [ 2 H] - , the other ([ 5 ] - ) is an analog of [ 3 ] - . We finally propose a mechanistic scenario, which rationalizes the individual reaction outcomes and combines them to a coherent picture of B-B- vs. B-H-bond activation.