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

Temperature treatment (120 °C, C6H6 or toluene, sealed glass ampule) of the C1-symmetric dimer of 9H-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 1H and 13C­{1H} 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 SMe2 only weakly but readily forms a stable B–N adduct upon addition of 2 equiv of pyridine (py). The resulting 3·py2 (X-ray crystal structure) shows perfectly well resolved NMR spectra.