Dynamic N-->B Coordination and Anion-selective Turn-On Fluorescence in Oxadiazole-functionalized Organoboranes

Abstract

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 a 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,..) 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 of the optical, electrochemical and electronic properties, both experimentally and by DFT calculations was carried out. Our investigation shows the boranes to 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 shift (4g) and enhanced emission 4c (4-OMe-phenyl). These results show that this synthetic strategy allows to easily create series of compounds with incrementally varied optical properties and Lewis acidities.