Synthesis and Characterization of a Sterically Encumbered Unsymmetrical 9-Borafluorene, Its Pyridine Adduct, and Its Dilithium Salt

The reaction of 2,6-(4- t -BuC 6 H 4 ) 2 C 6 H 3 Li with BH 2 Cl·SMe 2 in hexane or Et 2 O solution affords the terphenyl-substituted unsymmetrical 9-borafluorene 1-(4- tert -butylphenyl)-7- tert -butyl-9-(bis-2,6-(4- tert -butylphenyl)phenyl)-9-borafluorene ( 1 ) in good to moderate yields. Addition of pyridine gives the colorless crystalline adduct 1 · py . Compound 1 is readily reduced to the deep red heteroaromatic dianionic (μ 2 -η 5,η 5 -1-(4- tert -butylphenyl)-7- tert -butyl-9-(bis-2,6-(4- tert -butylphenyl)phenyl)-9-borafluorenyl)bis(diethyl ether)dilithium ( 2 ) with excess lithium powder in Et 2 O solution. Reactions of the dianionic 2 with various metal salts leads to reduction of these salts, and bright yellow 1 is recovered in essentially quantitative yields. Compounds 1, 1 · py, and 2 were characterized by 1 H, 13 C, and 11 B NMR spectroscopy and compounds 1 · py and 2 also by single-crystal X-ray diffraction.