Addition of dihydrogen to a borylborenium center.

The activation of a H-H bond, the simplest covalent bond, is a fundamentally important process. Addition of H(2) to an elemental center typically occurs on low valent transition metal or main group complexes through oxidative addition to afford metal dihydride complexes. In contrast, activation of H(2) on a high valent center generally results in heterolytic cleavage of the H-H bond to a proton and hydride. Here, we report experimental and computational evidence for the addition of H(2) to a borenium center in an N-heterocyclic carbene (NHC) coordinated borylborenium cation, which leads to the formation of a dihydroborenium complex accompanied by the elimination of two σ-bonded substituents, namely mesityl (Mes) and pinacolboryl (Bpin) groups, as mesitylboronic ester.