Difluorination at Boron Leads to the First Electrophilic Ligated Boryl Radical (NHC-BF₂^. )

1,3-Dimethylimidazol-2-ylidene difluoroborane (NHC-BF₂ H) was prepared in a one-pot, two-step reaction from the parent ligated borane (NHC-BH₃ ). The derived difluoroboryl radical (NHC-BF₂^. ) was generated by laser flash photolysis experiments and characterized by UV spectroscopy and rate-constant measurements. It is transient and reacts quickly with O₂ . Unusually, it also reacts more rapidly with ethyl vinyl ether than with methyl acrylate. By this measure, it is the first electrophilic ligated boryl radical. Both NHC-BH₃ and NHC-BF₂ H serve as co-initiators in bulk photopolymerizations, converting both electron-poor and electron-rich monomers at roughly similar rates. However, the difluorinated coinitiator provides polymers with dramatically increased chain lengths from both monomers.