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 sigma-donor properties. Upon photolytic CO extrusion from the metal center, the diboron ligand adopts a novel eta(3)-BBN coordination mode, where bond-strengthening backdonation from the metal center into the vacant B-B pi-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 KC8, 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.