N-Heterocyclic Carbenes−Borane Complexes: A New Class of Initiators for Radical Photopolymerization

Newly discovered N-heterocyclic carbene-boryl radicals (NHC-BH2•) derived from readily available N-heterocyclic carbene−boranes are found to be efficient initiators for acrylate photopolymerization. Laser flash photolysis (LFP) experiments were used to generate three carbene-boryl radicals, which were characterized by their transient absorption spectra with the aid of DFT calculations. Rate constants were measured for the generation of the carbene-boryl radicals by hydrogen abstraction with tert-butoxyl radical, a ketophosphonyl radical, and triplet benzophenone. Rate constants were also measured for the reactions of the carbene-boryl radicals with oxygen, three alkenes, two alkyl chlorides, and diphenyliodonium hexafluorophosphate. The observed trends were interpreted with the aid of measured oxidation potentials of the carbene−boranes and calculated ionization potentials of the carbene-boryls. N-Heterocyclic carbene−boranes show excellent potential as both photopolymerization co-initiators and mediators of small molecule radical reactions, and these results will help guide further development in both fields.