Bis(boryl anion)-Substituted Pyrenes: Syntheses, Characterizations, and Crystal Structures

The two new diboranes 1 and 2 connected by a pyrene moiety at the 1,6- and 1,3-positions, respectively, were synthesized, and their two-electron-reduction reactions were investigated. The doubly reduced species 1••2– is silent in electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopic measurements, suggesting a quasi-quinoidal structure with a diradical character of 1••2–, which has a singlet–triplet gap of 6.6 kcal mol–1 as determined by theoretical calculations. In contrast, the reduction product 2••2– is EPR active and theoretical calculations indicate that 2••2– has an open-shell singlet ground state with a singlet–triplet energy gap of 4.9 kcal mol–1.