Steric Shielding vs Structural Constraint in a Boron-Containing Polycyclic Aromatic Hydrocarbon

The highly reactive boron-doped polycyclic aromatic hydrocarbon 8H-8-bromo-8-borabenzo­[gh]­tetraphene has been synthesized via a multistep sequence encompassing a Peterson olefination, a stilbene-type photocyclization, and an Si/B exchange reaction. The compound was subsequently treated with mesityllithium to give the derivative 3 or with [2,6-bis­(propen-2-yl)­phenyl]lithium to furnish the intermediate 4. A scandium­(III) triflate mediated Friedel–Crafts reaction transformed 4 into the rigid, planarized triarylborane 5. Compounds 3 and 5 are both inert toward air and moisture but are still able to bind fluoride ions, where the affinity of 3 in CHCl3 is higher than that of 5. Both arylboranes are yellow solids, and their solutions exhibit an intense blue fluorescence (ΦPL = 85% (3), 89% (5)). According to cyclic voltammetry, reversible reduction occurs at half-wave potentials of E1/2 = −2.05 V (3) and −2.14 V (5; in THF, vs FcH/FcH+). The crystal lattices consist of π-stacked dimers, arranged in herringbone patterns. Importantly, 3 and 5 share nearly identical properties, despite possessing fundamentally different structures.