Reversible Dihydrogen Activation by Reduced Aryl Boranes as Main‐Group Ambiphiles

A new approach to main‐group H 2 activation combining concepts of transition‐metal and frustrated Lewis pair chemistry is reported. Ambiphilic, metal‐like reactivity toward H 2 can be conferred to 9,10‐dihydro‐9,10‐diboraanthracene (DBA) acceptors by the injection of two electrons. The resulting [DBA] 2− ions cleave the H−H bond with the formation of hydridoborates under moderate conditions ( T= 50–100 °C; p <1 atm). Depending on the boron‐bonded substituents R, the addition is either reversible (R=C≡C t Bu) or irreversible (R=H). The reaction rate is strongly influenced by the nature and the coordination behavior of the countercation (Li + slower than K + ). Quantum‐chemical calculations support the experimental observations and suggest a concerted, homolytic addition of H 2 across both boron atoms. As proven by the successful conversion of Me 3 SiCl into Me 3 SiH, the system Li 2 [DBA]/H 2 appears generally relevant for the hydrogenation of element–halide bonds.