Isolation of a Bis(oxazol-2-ylidene)-phenylborylene adduct and its reactivity as a boron-centered nucleophile

An isolable phenylborylene species supported by two oxazol-2-ylidene ligands was synthesized and structurally characterized. Computational studies revealed the presence of lone-pair electrons on the boron atom in this molecule; therefore, there are eight electrons around the three-coordinate boron center. The nucleophilic property was confirmed by the reactions with trifluoromethanesulfonic acid and [(thf)Cr(CO)₅], which gave the corresponding conjugate acid and a chromium-borylene complex, respectively. Two oxazol-2-ylidene ligands effectively stabilize a phenylborylene fragment that features a lone pair of electrons on the boron atom. There are eight electrons around the tricoordinate boron center (8-B-3), and the nucleophilic boron center readily reacts with an electrophile, such as a proton or a transition-metal species, to form a boronium species and a metal-borylene complex, respectively.