Persistent Borafluorene Radicals.

N-Heterocyclic carbene (NHC)- and cyclic (alkyl)(amino)carbene (CAAC)-stabilized borafluorene radicals have been isolated and characterized by elemental analysis, single-crystal X-ray diffraction, UV/Vis absorption, cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and theoretical studies. Both the CAAC-borafluorene radical (2) and the NHC-borafluorene radical (4) have a considerable amount of spin density localized on the boron atoms (0.322 for 2 and 0.369 for 4). In compound 2, the unpaired electron is also partly delocalized over the CAAC ligand carbene C and N atoms. However, the unpaired electron in compound 4 mainly resides throughout the borafluorene π-system, with significantly less delocalization over the NHC ligand. These results highlight the Lewis base dependent electrostructural tuning of materials-relevant radicals. Notably, this is the first report of crystalline borafluorene radicals, and these species exhibit remarkable solid-state and solution stability.