Persistent Borafluorene Radicals

Abstract

Abstract 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.