Photochemically- and Thermally-Generated BN- and BNP-Doped Borafluorenate Heterocycles via Intramolecular Staudinger-Type Reactions

We report a series of BN- and BNP-borafluorenate heterocycles, bis(borafluorene-phosphinimine) (11-15), formed via intramolecular Staudinger-type reactions. The reactions were promoted by light or heat using monodentate phosphine-stabilized 9-azido-9-borafluorenes (R3P-BF-N3) (6-10) and involve the release of dinitrogen (N2), phosphine migration from boron to nitrogen, and oxidation of the phosphorus center (PIII to PV). Compounds 11-15 are blue emissive in the solution and solid states with solution phase absolute quantum yields ranging from 13-67%. Our studies were extended by comparing the reactivity of borafluorene azides supported by monodentate phosphorus ligands with those of bidentate phosphorus ligands, diphenyl-2-pyridylphosphine, trans-1,2-bis(diphenylphosphino)ethylene, and bis(diphenylphosphino)methane. A number of five-membered BNP-doped heterocycles were obtained which served as molecular precursors to access a new class of 4-membered BN-borafluorenate heterocycles via ring contraction and dimerization. All compounds discussed in this study have been fully characterized by single crystal X-ray diffraction and multinuclear NMR spectroscopy (1H, 13C, 11B, and 31P).