Synthesis and Structural Characterization of Pentaarylboroles and Their Dianions

A series of l-aryl-2,3,4,5-tetraphenylboroles [Ph4C4BAr] [Ar = p-MeC6H4 (2), p-Me3SiC6H4 (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 this 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 [Ph4C4BPh] (5) were further reacted with potassium or potassium graphite to form potassium borole dianion salts [K2{Ph4C4B(p-Me3SiC6H4)}] (6) and [K2(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.