Planarity Is Not Plain: Closed- vs Open-Shell Reactivity of a Structurally Constrained, Doubly Reduced Arylborane toward Fluorobenzenes

The ability to activate small molecules is imparted to 9,10-dihydro-9,10-diboraanthracenes (DBAs) through the injection of two electrons. We report on the activation of fluorobenzenes C₆F_<i>n</i>H_6-<i>n</i> by the doubly reduced, structurally constrained DBA [<b>1</b>]^2- in THF (<i>n</i>: 1,3,4,5,6). Compound <b>1</b> is a 9,10-diphenyl DBA, forced into planarity by methylene bridges between the phenyl substituents and the DBA core. This rigidity results in enhanced stability under ambient conditions and an elevated planar-to-pyramidal reorganization energy upon boron tetracoordination, unlocking new reactivity. The dianion salts M₂[<b>1</b>] were synthesized in excellent yields by stirring neutral <b>1</b> with alkali metals M in THF (M: Li, Na, K); comproportionation of Li₂[<b>1</b>] with <b>1</b> generates the blue radical salt Li[<b>1</b>], characterized by EPR spectroscopy and X-ray diffraction. While Li₂[<b>1</b>] is inert toward C₆FH₅ up to 120 °C, it reacts with 1,3,5-C₆F₃H₃ at 100 °C to yield a B(sp²)/B(sp³) adduct with a difluorophenyl ligand (Li[<b>2</b>]). Treatment of Li₂[<b>1</b>] with 1 eq. of C₆F₅H or C₆F₆ induces selective monohydrodefluorination, occurring in parallel with the formation of a unique B(sp²)/B(sp³) tetrahydrofuran-2-yl adduct (Li[<b>3</b>]). The three isomers of C₆F₄H₂ represent intermediate cases, where the competition between trifluorophenyl- and tetrahydrofuran-2-yl-adduct formation is governed by the relative positions of the F substituents and the nature of the countercation (M⁺: Li⁺, K⁺). Through experimental and quantum-chemical studies, we unveil the underlying reaction mechanisms and show that Li₂[<b>1</b>] acts either as a B-centered nucleophile in an S_NAr-type conversion (low benzene fluorination) or as a reducing agent in a single-electron transfer/H atom abstraction sequence (high benzene fluorination).