Complexation and Release of N‑Heterocyclic Carbene-Aminoborylene Ligands from Group VI and VIII Metals

The coordination chemistry and stability of aminoborylene ligands bearing different N-heterocyclic carbene (NHC) stabilizing groups has been investigated with Group VI and VIII metals. NHC-aminoborylene complexes have been accessed via reduction of NHC-dihaloaminoborane adducts with Na2[M­(CO)x] species (M = Fe, Ru, Cr, W). Imidazol-2-ylidene-stabilized aminoborylene ligands were found to afford thermally robust metal-borylene complexes, which are inert to oxidation, hydrolysis, and insertion of unsaturated substrates. Such ligands have additionally been demonstrated to be significantly more electron releasing than NHCs and other carbon-based ligands by infrared spectroscopy, and can be regarded as unique examples of highly nucleophilic borylene ligands isolobal to classical NHCs. In contrast, cyclic alkylaminocarbene (CAAC)-bound dihaloaminoboranes were found to be reduced by one or two electrons upon reaction with Na2[M­(CO)x] species to form either a stable borane-centered radical, or the free CAAC-aminoborylene complex, which further reacts to form a carbonyl-stabilized aminoborylene. Borylene-to-CO migration was also observed upon reaction of a ruthenium imidazol-2-ylidene aminoborylene complex with B­(C6F5)3, where the product borylene remains trapped by the Ru center.