Bond‐strengthening backdonation in aminoborylene‐stabilized aminoborylenes: At the intersection of borylenes and diborenes

Singly NHC‐coordinated (aminoboryl)aminoborenium salts react with Na${}_2$[Fe(CO)${}_4$] to yield stable coordination complexes of aminoborylene‐stabilized aminoborylenes, which exhibit exceptional σ‐donor properties. Upon photolytic CO extrusion from the metal center, the diboron ligand adopts a novel η${}^3$‐BBN coordination mode, where bond‐strengthening backdonation from the metal center into the vacant B−B π‐orbital is observed. This bonding situation can be alternatively described as a Fe‐diaminodiborene complex. In a related reduction of CAAC‐stabilized (aminoboryl)aminoborenium with KC${}_8$, the reduced species can be captured with nucleophiles to form three‐coordinate (diaminoboryl)borylenes, where both amino groups have migrated to the distal boron atom. Collectively, these reactions illustrate the isomeric flexibility imparted by amino groups on this reduced diboron system, thus opening multiple avenues of novel reactivity. (© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)