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

Abstract

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.

Crystalline boron-linked tetraaminoethylene radical cations

“…4a ). The 11 B hyperfine coupling constant is smaller than those of I (6.43 G) 4 and isolable boron radical anions, 2 but comparable to those of II (≤1.18 G) 5 and reported stable neutral boron radicals (0.96–8.5 G). 3 The moderate a ( 11 B) coupling constant together with relative small a ( 14 N) values suggests delocalization of the unpaired electron over the B 2 N 4 C 2 core.…”

“…Boron-containing radicals have been extensively studied due to their importance in fundamental chemistry and promising applications in various organic syntheses involving radical reactions. 1 Thus far, a number of stable anionic 2 and neutral 3 boron radicals have been reported. By stark contrast, isolable boron radical cations are extremely rare probably due to the intrinsic electron deficient nature of boron.…”

Controlling the Optoelectronic Properties of Pyrene by Regioselective Lewis Base‐Directed Electrophilic Aromatic Borylation

“…Related 2,7‐diborylated pyrenes with fused phenoxy bridges were recently reported by Müllen, X.‐Y. Wang et al . The π‐annulation to phenyl rings in the 1,3,6,8‐positions led to a narrower HOMO–LUMO gap and a strong green fluorescence with a maximum intensity at 525 nm.…”

“…Indeed, Tan, X. Wang et al. later showed that the doubly reduced species adopts a quasi‐quinoidal structure and is EPR silent because it adopts a singlet ground state . In contrast, the isomeric 1,3‐diborylated species ( B ) adopts an open‐shell singlet state in its doubly reduced form with a small singlet–triplet gap and is therefore EPR active .…”

“…later showed that the doubly reduced species adopts a quasi‐quinoidal structure and is EPR silent because it adopts a singlet ground state . In contrast, the isomeric 1,3‐diborylated species ( B ) adopts an open‐shell singlet state in its doubly reduced form with a small singlet–triplet gap and is therefore EPR active . Marder and co‐workers showed that for 2,7‐bis(dimesitylboryl)pyrene ( C ), despite the substituent attachment to positions that lie on a nodal plane in the parent pyrene molecule, mixing of the pyrene LUMO+1 and empty p z ‐orbital on boron results in a low‐lying delocalized LUMO .…”