Dynamic N→B coordination and anion-selective turn-on fluorescence in oxadiazole-functionalized organoboranes

A versatile route for the preparation of chemically and electronically diverse Mes₂BPh-based boranes (Mes = mesityl, 2,4,6-trimethylphenyl) is presented that allows the conversion of tetrazolyl rings in a common borane precursor (2H) into boranes bearing variously substituted oxadiazolyl groups. A series of eight boranes (4a-4h) was prepared with functional groups on the 5-position of the oxadiazole ranging from electron donating (4a: 4-Me-phenyl, 4b: 4-MeO-phenyl, etc.) to strongly electron withdrawing (4d: 4-O₂N-phenyl, 4e: 3,5-bis(CF₃)-phenyl, 4h: CF₃) and also including a bifunctionalized example bearing two Mes₂B moieties (4g). A full characterization study of the optical, electrochemical and electronic properties, both experimentally and by DFT calculations, was carried out. Our investigation shows that the boranes exhibit dynamic equilibria between closed intramolecularly N→B-coordinated and open non-coordinated conformers, as indicated by variable temperature NMR, ¹¹B NMR and anion binding studies with F^- and CN^-. The anion binding studies reveal substantial differences in the fluorescence response of the compounds ranging from differing degrees of quenching to fluorescence shifts (4g) and enhanced emission (4c) (4-OMe-phenyl). These results show that this synthetic strategy allows easy creation of a series of compounds with incrementally varied optical properties and Lewis acidities.