Synthesis, Characterization, and Properties of Three-Dimensional Analogues of 9-Borafluorenes

Three-dimensional (3D) analogues of 9-borafluorenes, (C2B10H10)2BR (R = Cl (1), Br (2), H (3, merely as ether or silane adduct), phenyl (4), mesityl (5)), were synthesized and fully characterized. Gutmann-Beckett and computational fluoride/hydride ion affinity (FIA/HIA) studies confirmed the Lewis superacidity of 1-4, with the Lewis acidity of 1-3 being higher than that of the corresponding 3D analogues of 9,10-diboraanthracenes (DBA). The phenyl group and the C4B borole unit are nearly coplanar in the solid state, while the rotation of the mesityl group of 5 is hindered, forcing 5 to adopt a large C4B/Mes dihedral angle, which leads to distinct properties: air-stable and bluish-violet fluorescence arising from the intramolecular charge-transfer (ICT) transition. The 29Si NMR spectra indicated a high silylium cation character of 3·Et3SiH. The reaction of 4 with Ph3CBr yielded (Ph3C)+[(C2B10H10)2B(Ph)Br]- (6). The 4/PPh3 Lewis pair was capable of splitting dihydrogen and Si-H bond of triethylsilane.