Synthesis and structural characterization of pentaarylboroles and their dianions

A series of 1-aryl-2,3,4,5-tetraphenylboroles [Ph₄C ₄BAr] [Ar = p-MeC₆H₄ (2), p-Me ₃SiC₆H₄ (3), p-FC₆H₄ (4)] were synthesized. The X-ray crystallography of 2-4 revealed that they have short distances between the borole boron atom and the phenyl rings of the neighboring molecules, suggesting the existence of certain intermolecular interaction. Despite tins interaction, the borole rings in 2-4 remain planar and contain substantial bond alternation in the butadiene skeleton, which is consistent with the theoretically optimized structures of the singlet boroles. These results demonstrated that the boroles still have antiaromatic character in the crystalline state. Compound 3 and pentaphenylborole [Ph₄C ₄BPh] (5) were further reacted with potassium or potassium graphite to form potassium borole dianion salts [K₂[Ph₄C ₄B(p-Me₃SiC₆H₄)]] (6) and [K ₂(Ph₄C₄BPh)] (7), respectively. In their X-ray crystal structures, compounds 6 and 7 are polymeric and have two K+ ions lying on both sides of the borole plane with η⁵-coordination.