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

Three-dimensional (3D) analogues of 9-borafluorenes, (C 2 B 10 H 10 ) 2 BR (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 C 4 B 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 C 4 B/Mes dihedral angle, which leads to distinct properties: air-stable and bluish-violet fluorescence arising from the intramolecular charge-transfer (ICT) transition. The 29 Si NMR spectra indicated a high silylium cation character of 3 ·Et 3 SiH. The reaction of 4 with Ph 3 CBr yielded (Ph 3 C) + [(C 2 B 10 H 10 ) 2 B(Ph)Br] - ( 6 ). The 4 /PPh 3 Lewis pair was capable of splitting dihydrogen and Si-H bond of triethylsilane.