High-Temperature Reactivity of the Strongly Electrophilic Pristine 9H-9-Borafluorene

Temperature treatment (120 °C, C 6 H 6 or toluene, sealed glass ampule) of the C 1 -symmetric dimer of 9 H -9-borafluorene ( 1 ) 2 leads to the ring-opened dimer 1,2:1,2-bis(2,2′-biphenylylene)diborane(6) ( 2 ), 2,2′-bis(9-borafluoren-9-yl)biphenyl ( 3 ), and 1,2-(2,2′-biphenylylene)diborane(6) ( 4 ). 2 readily crystallizes from the thermolysis mixture in 34% yield. The ditopic borane 3 does not form in preparatively useful quantities but has been made accessible through salt metathesis between 9-bromo-9-borafluorene and donor-free 2,2′-dilithiobiphenyl in almost quantitative yields. Even at 330 K, the 1 H and 13 C{ 1 H} NMR spectra of 3 are broadened almost beyond interpretability. X-ray crystallography and molecular modeling studies on 3 indicate a highly restricted conformational space. 3 binds THF and SMe 2 only weakly but readily forms a stable B–N adduct upon addition of 2 equiv of pyridine (py). The resulting 3 ·py 2 (X-ray crystal structure) shows perfectly well resolved NMR spectra.