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.