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 [ 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 that rationalizes the individual reaction outcomes and combines them to a coherent picture of B–B vs. B–H bond activation.